This glossary ofaerospace engineering terms pertains specifically toaerospace engineering, its sub-disciplines, and related fields includingaviation andaeronautics. For a broad overview of engineering, seeglossary of engineering.

• Above ground level – Inaviation,atmospheric sciences andbroadcasting, aheight above ground level (AGL^[1]) is aheight measured with respect to the underlyingground surface. This is as opposed to altitude/elevationabove mean sea level (AMSL), or (inbroadcast engineering)height above average terrain (HAAT). In other words, these expressions (AGL, AMSL, HAAT) indicate where the "zero level" or "reference altitude" is located.
• Absolute humidity – describes the water content of air and is expressed in either grams per cubic meter^[2] or grams per kilogram.^[3]
• Absolute value – Inmathematics, the absolute value ormodulus|x| of areal number x is thenon-negative value of x without regard to itssign. Namely,|x| =x for apositive x,|x| = −x for anegative x (in which case−x is positive), and|0| = 0. For example, the absolute value of 3 is 3, and the absolute value of −3 is also 3. The absolute value of a number may be thought of as itsdistance from zero.
• Acceleration – Inphysics, acceleration is therate of change ofvelocity of an object with respect to time. An object's acceleration is the net result of any and allforces acting on the object, as described byNewton's second law.^[4] TheSI unit for acceleration ismetre per second squared(m s⁻²). Accelerations arevector quantities (they havemagnitude anddirection) and add according to theparallelogram law.^[5][6] As avector, the calculated netforce is equal to the product of the object's mass (ascalar quantity) and its acceleration.
• Acquisition of signal – Apass, inspaceflight andsatellite communications, is the period in which asatellite or otherspacecraft is above thelocal horizon and available forradio communication with a particularground station,satellite receiver, or relay satellite (or, in some cases, for visual sighting). The beginning of a pass is termedacquisition of signal; the end of a pass is termedloss of signal.^[7] The point at which a spacecraft comes closest to a ground observer is thetime of closest approach.^[7]
• Action – Inphysics, action is an attribute of thedynamics of aphysical system from which theequations of motion of the system can be derived. It is amathematical functional which takes thetrajectory, also calledpath orhistory, of the system as its argument and has areal number as its result. Generally, the action takes different values for different paths.^[8] Action has thedimensions of[energy]⋅[time] or[momentum]⋅[length], and itsSI unit isjoule-second.
• ADF – Automatic direction finder
• Advanced Space Vision System – The Advanced Space Vision System (also known as the Space Vision System or by its acronym SVS) is acomputer vision system designed primarily forInternational Space Station (ISS) assembly.^[9] The system uses regular 2D cameras in theSpace Shuttle bay, on theCanadarm, or on the ISS along with cooperative targets to calculate the 3D position of an object.^[9]
• Aeroacoustics – is a branch ofacoustics that studies noise generation via eitherturbulent fluid motion oraerodynamic forces interacting with surfaces. Noise generation can also be associated with periodically varying flows. A notable example of this phenomenon is theAeolian tones produced by wind blowing over fixed objects.
• Aerobraking – is aspaceflight maneuver that reduces the high point of anelliptical orbit (apoapsis) by flying the vehicle through theatmosphere at the low point of theorbit (periapsis). The resultingdrag slows thespacecraft. Aerobraking is used when a spacecraft requires a low orbit after arriving at a body with an atmosphere, and it requires less fuel than does the direct use of arocket engine.
• Aerocapture – is anorbital transfer maneuver used to reduce the velocity of a spacecraft from ahyperbolic trajectory to an elliptical orbit around the targeted celestial body.
• Aerodynamics – is the study of the motion ofair, particularly with respect to its interaction with a solid object, such as anairplane wing. Aerodynamics is a sub-field ofgas dynamics, which in turn is a sub-field offluid dynamics. Many aspects and principles of aerodynamics theory are common to these three fields.
• Aeroelasticity – is the branch ofphysics andengineering that studies the interactions between theinertial,elastic, andaerodynamic forces that occur when an elastic body is exposed to afluid flow. Although historical studies have been focused on aeronautical applications, recent research has found applications in fields such asenergy harvesting^[10] and understandingsnoring.^[11] The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity, which deals with the static orsteady response of an elastic body to a fluid flow; and dynamic aeroelasticity, which deals with the body'sdynamic (typicallyvibrational) response. Aeroelasticity draws on the study offluid mechanics,solid mechanics,structural dynamics anddynamical systems. The synthesis of aeroelasticity withthermodynamics is known as aerothermoelasticity, and its synthesis withcontrol theory is known as aeroservoelasticity.
• Aeronautics – is the science or art involved with the study,design, and manufacturing of airflight capable machines, and the techniques of operatingaircraft and rockets within theatmosphere.^[12]
• Aerospace architecture – is broadly defined to encompassarchitectural design of non-habitable and habitable structures and living and working environments in aerospace-related facilities, habitats, and vehicles. These environments include, but are not limited to: science platform aircraft and aircraft-deployable systems;space vehicles,space stations, habitats and lunar andplanetary surface construction bases; and Earth-based control, experiment, launch, logistics, payload, simulation and test facilities. Earth analogs to space applications may include Antarctic, desert, high altitude, underground, undersea environments and closed ecological systems.
• Aerospace bearing – Aerospace bearings are the bearings installed inaircraft andaerospace systems including commercial, private, military, or space applications.
• Aerospace engineering – is the primary field ofengineering concerned with the development ofaircraft andspacecraft.^[13] It has two major and overlapping branches: Aeronautical engineering and Astronautical Engineering.Avionics engineering is similar, but deals with theelectronics side of aerospace engineering.
• Aerospace materials – are materials, frequentlymetal alloys, that have either been developed for, or have come to prominence through, their use foraerospace purposes. These uses often require exceptional performance, strength or heat resistance, even at the cost of considerable expense in their production or machining. Others are chosen for their long-term reliability in this safety-conscious field, particularly for their resistance tofatigue.
• Aerospike engine – is a type ofrocket engine that maintains itsaerodynamic efficiency across a wide range ofaltitudes. It belongs to the class ofaltitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low altitudes, where most missions have the greatest need forthrust.
• Aerostat – is alighter than air aircraft that gains its lift through the use of a buoyant gas. Aerostats include unpoweredballoons and poweredairships.
• Aerostructure – is a component of anaircraft's airframe. This may include all or part of thefuselage,wings, or flight control surfaces.
• Aft-crossing trajectory – is an alternate flight path for a rocket. The rocket's rotation (induced by the deployment from the aircraft) is slowed by a smallparachute attached to its tail, then ignited once the carrier aircraft has passed it. It is ignited before it is pointing fully vertically, however it will turn to do so, and accelerates to pass behind the carrier aircraft.
• AGL – Above ground level
• Aileron – is a hingedflight control surface usually forming part of thetrailing edge of eachwing of afixed-wing aircraft. Ailerons are used in pairs to control the aircraft inroll (or movement around the aircraft'slongitudinal axis), which normally results in a change in flight path due to the tilting of thelift vector. Movement around this axis is called 'rolling' or 'banking'.
• Air-augmented rocket –
• Aircraft – is amachine that is able tofly by gaining support from theair. It counters the force of gravity by using eitherstatic lift or by using thedynamic lift of anairfoil,^[14] or in a few cases thedownward thrust fromjet engines. Common examples of aircraft includeairplanes,helicopters,airships (includingblimps),gliders, andhot air balloons.^[15]
• Aircraft flight control systems – A conventionalfixed-wingaircraft flight control system consists offlight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight.Aircraft engine controls are also considered as flight controls as they change speed.
• Aircraft flight mechanics –
• Airfoil – An airfoil (American English) oraerofoil (British English) is the cross-sectional shape of awing, blade (of apropeller,rotor, orturbine), orsail (as seen incross-section).
• Airlock – is a device which permits the passage of people and objects between apressure vessel and its surroundings while minimizing the change ofpressure in the vessel and loss ofair from it. The lock consists of a small chamber with twoairtightdoors in series which do not open simultaneously.
• Airship – An airship ordirigible balloon is a type ofaerostat or lighter-than-air aircraft that can navigate through the air under its own power.^[16] Aerostats gain their lift from large gas bags filled with alifting gas that is less dense than the surrounding air.
• Albedo – is the measure of thediffuse reflection ofsolar radiation out of the totalsolar radiation received by anastronomical body (e.g. aplanet likeEarth). It isdimensionless and measured on a scale from 0 (corresponding to ablack body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation).
• Anemometer – is a device used for measuringwind speed, and is also a commonweather station instrument.^[17] The term is derived from the Greek wordanemos, which meanswind, and is used to describe any wind speed instrument used inmeteorology.
• Angle of attack – Influid dynamics, angle of attack (AOA, or${\displaystyle \alpha }$) is theangle between areference line on a body (often thechord line of anairfoil) and thevector representing the relative motion between the body and the fluid through which it is moving.^[18] Angle of attack is the angle between the body's reference line and the oncoming flow.
• Angular momentum – Inphysics, angular momentum (rarely,moment of momentum orrotational momentum) is the rotational equivalent oflinear momentum. It is an important quantity in physics because it is aconserved quantity—the total angular momentum of a system remains constant unless acted on by an externaltorque.
• Angular velocity – Inphysics, the angular velocity of a particle is the rate at which it rotates around a chosen center point: that is, the time rate of change of itsangular displacement relative to the origin (i.e. in layman's terms: how quickly an object goes around something over a period of time – e.g. how fast the earth orbits the sun). It is measured in angle per unit time,radians per second inSI units, and is usually represented by the symbolomega (ω, sometimesΩ). By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.
• Anticyclone – An anticyclone (that is, opposite to acyclone) is aweatherphenomenon defined by the United StatesNational Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere".^[19]
• Antimatter rocket – is a proposed class ofrockets that useantimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of therest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higherenergy density andspecific impulse than any other proposed class of rocket.
• Apsis – is an extreme point in theorbit of anobject. The word comes via Latin from Greek and iscognate withapse.^[20] Forelliptic orbits about a larger body, there are two apsides, named with the prefixesperi- (from περί (peri) 'near') andap-/apo- (from ἀπ(ό) (ap(ó)) 'away from') added to a reference to thebody being orbited.
• Arcjet rocket – orarcjet thruster is a form ofelectrically powered spacecraft propulsion, in which an electrical discharge (arc) is created in a flow of propellant^[21][22] (typicallyhydrazine orammonia). This imparts additional energy to the propellant, so that one can extract more work out of each kilogram of propellant, at the expense of increased power consumption and (usually) higher cost. Also, thethrust levels available from typically used arcjet engines are very low compared with chemical engines.
• Areal velocity – Inclassical mechanics, areal velocity (also calledsector velocity orsectorial velocity) is the rate at whicharea is swept out by a particle as it moves along acurve.
• Argument of periapsis – (also calledargument of perifocus orargument of pericenter), symbolized asω, is one of theorbital elements of anorbiting body. Parametrically,ω is the angle from the body'sascending node to itsperiapsis, measured in the direction of motion.
• ARP4761 –
• Aspect ratio (aeronautics) – Inaeronautics, the aspect ratio of awing is the ratio of itsspan to its meanchord. It is equal to the square of the wingspan divided by the wing area. Thus, a long, narrow wing has a high aspect ratio, whereas a short, wide wing has a low aspect ratio.^[23] Aspect ratio and other features of theplanform are often used to predict the aerodynamic efficiency of a wing because thelift-to-drag ratio increases with aspect ratio, improvingfuel economy in aircraft.
• Asteroid – Asteroids areminor planets, especially of theinner Solar System. Larger asteroids have also been calledplanetoids. These terms have historically been applied to any astronomical object orbiting theSun that did not resemble a planet-like disc and was not observed to have characteristics of an activecomet such as a tail. Asminor planets in the outer Solar System were discovered they were typically found to havevolatile-rich surfaces similar to comets. As a result, they were often distinguished from objects found in the mainasteroid belt.^[24]
• Astrodynamics –Orbital mechanics or astrodynamics is the application ofballistics andcelestial mechanics to the practical problems concerning the motion ofrockets and otherspacecraft.
• Atmospheric entry – is the movement of an object fromouter space into and through the gases of anatmosphere of aplanet,dwarf planet ornatural satellite. There are two main types of atmospheric entry: uncontrolled entry, such as the entry ofastronomical objects,space debris orbolides; and controlled entry (or reentry) of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing the controlled atmosphericentry, descent and landing of spacecraft are collectively termed asEDL.
• Attitude control – is controlling the orientation of an object with respect to aninertial frame of reference or another entity like thecelestial sphere, certain fields, and nearby objects, etc. Controlling vehicle attitude requiressensors to measure vehicle orientation,actuators to apply the torques needed to re-orient the vehicle to a desired attitude, andalgorithms to command the actuators based on (1) sensor measurements of the current attitude and (2) specification of a desired attitude. The integrated field that studies the combination of sensors, actuators and algorithms is called "Guidance, Navigation and Control" (GNC).
• Automatic direction finder – (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station.^[25][26]
• Avionics – are theelectronic systems used on aircraft,artificial satellites, andspacecraft. Avionic systems include communications, navigation, the display and management of multiple systems, and the hundreds of systems that are fitted to aircraft to perform individual functions.
• Axial stress – a normal stress parallel to the axis of cylindrical symmetry.

• Balloon – Inaeronautics, a balloon is an unpoweredaerostat, which remains aloft or floats due to itsbuoyancy. A balloon may be free, moving with the wind, ortethered to a fixed point. It is distinct from anairship, which is a poweredaerostat that can propel itself through the air in a controlled manner.
• Ballute – (aportmanteau ofballoon andparachute) is a parachute-like braking device optimized for use at high altitudes andsupersonic velocities. Invented byGoodyear in 1958, the original ballute was a cone-shaped balloon with atoroidal burble fence fitted around its widest point. A burble fence is an inflated structure intended to ensureflow separation.^[27]

This stabilizes the ballute as it decelerates through different flow regimes (from supersonic to subsonic).

• Beam-powered propulsion – also known as directed energy propulsion, is a class ofaircraft orspacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either amicrowave or alaser beam and it is either pulsed or continuous. A continuous beam lends itself tothermal rockets, photonic thrusters andlight sails, whereas a pulsed beam lends itself to ablative thrusters andpulse detonation engines.^[28]
• Bearing – Innavigation, bearing is the horizontal angle between the direction of an object and another object, or between it and that of true north.Absolute bearing refers to the angle between the magnetic North (magnetic bearing) or true North (true bearing) and an object. For example, an object to the East would have an absolute bearing of 90 degrees.Relative bearing refers to the angle between the craft's forward direction, and the location of another object. For example, an object relative bearing of 0 degrees would be dead ahead; an object relative bearing 180 degrees would be behind.^[29] Bearings can be measured inmils or degrees.
• Bernoulli's principle – Influid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease inpressure or a decrease in thefluid'spotential energy.^{[30]: Ch.3 [31]: 156–164, § 3.5}
• Bi-elliptic transfer – is anorbital maneuver that moves aspacecraft from oneorbit to another and may, in certain situations, require lessdelta-v than aHohmann transfer maneuver. The bi-elliptic transfer consists of two half-elliptic orbits. From the initial orbit, a first burn expends delta-v to boost the spacecraft into the first transfer orbit with anapoapsis at some point${\displaystyle r_b}$ away from thecentral body. At this point a second burn sends the spacecraft into the second elliptical orbit withperiapsis at the radius of the final desired orbit, where a third burn is performed, injecting the spacecraft into the desired orbit.^[32]
• Big dumb booster – (BDB), is a general class oflaunch vehicle based on the premise that it is cheaper to operate large rockets of simple design than it is to operate smaller, more complex ones regardless of the lower payload efficiency.^[33]
• Bleed air – produced bygas turbine engines iscompressed air that is taken from the compressor stage of those engines, which is upstream of the fuel-burning sections.
• Booster – A boosterrocket (or engine) is either the first stage of amultistagelaunch vehicle, or else a shorter-burning rocket used in parallel with longer-burningsustainer rockets to augment thespace vehicle's takeoff thrust and payload capability.^[34][35]
• Boundary layer – Inphysics andfluid mechanics, a boundary layer is an important concept and refers to the layer offluid in the immediate vicinity of abounding surface where the effects of viscosity are significant. In theEarth's atmosphere, theatmospheric boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface. On anaircraftwing the boundary layer is the part of the flow close to the wing, whereviscousforces distort the surrounding non-viscous flow.
• Buoyancy – Inphysics, buoyancy orupthrust, is an upwardforce exerted by afluid that opposes theweight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. This pressure difference results in a net upwards force on the object. The magnitude of that force exerted is proportional to that pressure difference, and (as explained byArchimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. thedisplaced fluid.

• Cabin pressurization – is a process in which conditioned air is pumped into thecabin of an aircraft orspacecraft, in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usuallybled off from thegas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, oftencryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary, before it is distributed to the cabin by one or moreenvironmental control systems.^[36] The cabin pressure is regulated by the outflow valve.
• Cable lacing – is a method for tyingwiring harnesses and cable looms, traditionally used intelecommunication, naval, and aerospace applications. This oldcable management technique, taught to generations oflinemen,^[37] is still used in some modern applications since it does not create obstructions along the length of the cable, avoiding the handling problems of cables groomed by plastic orhook-and-loopcable ties.
• Camber – the asymmetric curves on the top and bottom, or front and back, of an aerofoil
• Canard – is anaeronautical arrangement wherein a small forewing or foreplane is placed forward of the main wing of afixed-wing aircraft. The term "canard" may be used to describe the aircraft itself, thewing configuration or the foreplane.^[38][39][40]
• Centennial challenges –
• Center of gravity – A body's center of gravity is the point around which theresultant torque due to gravity forces vanishes. Where a gravity field can be considered to be uniform, the mass-center and the center-of-gravity will be the same. However, for satellites in orbit around a planet, in the absence of other torques being applied to a satellite, the slight variation (gradient) in gravitational field between closer-to (stronger) and further-from (weaker) the planet can lead to a torque that will tend to align the satellite such that its long axis is vertical. In such a case, it is important to make the distinction between the center-of-gravity and the mass-center. Any horizontal offset between the two will result in an applied torque.
• Center of mass – Inphysics, thecenter of mass of a distribution ofmass in space is the unique point where theweighted relativeposition of the distributed mass sums to zero, or the point where if a force is applied it moves in the direction of the force without rotating. The distribution of mass is balanced around the center of mass and the average of the weighted position coordinates of the distributed mass defines its coordinates.
• Center of pressure – is the point where the total sum of apressure field acts on a body, causing aforce to act through that point.
• Centrifugal compressor –Centrifugal compressors, sometimes calledradial compressors, are a sub-class of dynamic axisymmetric work-absorbingturbomachinery.^[41] They achieve a pressure rise by addingkinetic energy/velocity to a continuous flow offluid through the rotor orimpeller. This kinetic energy is then converted to an increase inpotential energy/static pressure by slowing theflow through a diffuser. The pressure rise in the impeller is in most cases almost equal to the rise in the diffuser.
• Chord – is the imaginary straight line joining the leading and trailing edges of anaerofoil. Thechord length is the distance between thetrailing edge and the point on the leading edge where the chord intersects theleading edge.^[42][43]
• Clean configuration – is the flight configuration of afixed-wing aircraft when its external equipment is retracted to minimize drag and thus maximizeairspeed for a given power setting.
• Cockpit – orflight deck, is the area, usually near the front of anaircraft orspacecraft, from which apilot controls the aircraft.
• Collimated beam – Acollimated beam oflight or otherelectromagnetic radiation has parallelrays, and therefore will spread minimally as it propagates. A perfectly collimatedlight beam, with nodivergence, would not disperse with distance. Such a beam cannot be created, due todiffraction.^[44]
• Comet – is an icy,small Solar System body that, when passing close to theSun, warms and begins to release gases, a process calledoutgassing. This produces a visible atmosphere orcoma, and sometimes also atail.
• Compressibility – Inthermodynamics andfluid mechanics,compressibility (also known as the coefficient of compressibility^[45] or isothermal compressibility^[46]) is ameasure of the relative volume change of afluid orsolid as a response to apressure (or meanstress) change. In its simple form, the compressibility${\displaystyle \beta }$ may be expressed as

${\displaystyle \beta =-\frac{1}{V}\frac{\mathrm{\partial }V}{\mathrm{\partial }p}}$, whereV isvolume andp is pressure. The choice to define compressibility as theopposite of the fraction makes compressibility positive in the (usual) case that an increase in pressure induces a reduction in volume. t is also known as reciprocal of bulk modulus(k) of elasticity of a fluid.

• Compression – Inmechanics,compression is the application of balanced inward ("pushing") forces to different points on a material or structure, that is, forces with no net sum ortorque directed so as to reduce its size in one or more directions.^[47] It is contrasted withtension or traction, the application of balanced outward ("pulling") forces; and withshearing forces, directed so as to displace layers of the material parallel to each other. Thecompressive strength of materials and structures is an important engineering consideration.
• Compressor map – is a diagram showing significant performance parameters for a rotating compressor, and how they vary with changing ambient conditions of pressure and temperature.
• Computational fluid dynamics – (CFD), is a branch offluid mechanics that usesnumerical analysis anddata structures to analyze and solve problems that involvefluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids andgases) with surfaces defined byboundary conditions. With high-speedsupercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems.
• Conservation of momentum – The total momentum of objects involved in a collision remains constant regardless of friction and permanent deformation that may occur during the collision. The law of conservation of momentum can be used to analyse the interactions between objects, even in the presence of friction and other non-conservative forces. Conservation of momentum is a consequence of Newton's laws of motion.
• Constant speed drive – (CSD), is a type oftransmission that takes an input shaft rotating at a wide range of speeds, delivering this power to an output shaft that rotates at a constant speed, despite the varying input. They are used to drive mechanisms, typicallyelectrical generators, that require a constant input speed. The term is most commonly applied tohydraulic transmissions found on theaccessory drives ofgas turbine engines, such as aircraftjet engines. On modern aircraft, the CSD is often combined with a generator into a single unit known as anintegrated drive generator (IDG).
• Control engineering – orcontrol systems engineering, is anengineering discipline that appliesautomatic control theory to design systems with desired behaviors incontrol environments.^[48] The discipline of controls overlaps and is usually taught along withelectrical engineering at many institutions around the world.^[48]
• Controllability –
• Crew Exploration Vehicle –
• Critical mach – Inaerodynamics, thecritical Mach number (Mcr or M* ) of anaircraft is the lowestMach number at which the airflow over some point of the aircraft reaches thespeed of sound, but does not exceed it.^[49] At thelower critical Mach number, airflow around the entire aircraft is subsonic. At theupper critical Mach number, airflow around the entire aircraft is supersonic.^[50]
• Cylinder stress – Inmechanics, acylinder stress is astress distribution withrotational symmetry; that is, which remains unchanged if the stressed object is rotated about some fixed axis.

• Damage tolerance – is a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach to engineering design to account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage.
• Decalage – Decalage on afixed-wing aircraft is the angle difference between the upper and lower wings of abiplane, i.e. the acute angle contained between thechords of the wings in question. Decalage is said to be positive when the upper wing has a higherangle of incidence than the lower wing, and negative when the lower wing's incidence is greater than that of the upper wing. Positive decalage results in greater lift from the upper wing than the lower wing, the difference increasing with the amount of decalage.^[51]
• De Laval nozzle – (orconvergent-divergent nozzle,CD nozzle orcon-di nozzle), is a tube that is pinched in the middle, making a carefully balanced, asymmetrichourglass shape. It is used to accelerate a hot, pressurizedgas passing through it to a highersupersonic speed in the axial (thrust) direction, by converting the heat energy of the flow intokinetic energy. Because of this, thenozzle is widely used in some types ofsteam turbines androcket engine nozzles. It also sees use in supersonicjet engines.
• Dead reckoning – Innavigation, dead reckoning is the process of calculating one's current position by using a previously determined position, orfix, and advancing that position based upon known or estimated speeds over elapsed time and course.
• Deflection – is the degree to which a structural element is displaced under aload. It may refer to an angle or a distance.
• Deformation (engineering) – Inmaterials science, deformation refers to any changes in the shape or size of an object due to an appliedforce (the deformation energy, in this case, is transferred through work) or a change in temperature (the deformation energy, in this case, is transferred through heat).
• Deformation (mechanics) – incontinuum mechanics is the transformation of a body from areference configuration to acurrent configuration.^[52] A configuration is a set containing the positions of all particles of the body. A deformation may be caused byexternal loads,^[53]body forces (such asgravity orelectromagnetic forces), or changes in temperature, moisture content, or chemical reactions, etc.
• Delta-v – (literally "change invelocity"), symbolised as∆v and pronounceddelta-vee, as used inspacecraft flight dynamics, is a measure of theimpulse that is needed to perform a maneuver such as launch from, or landing on a planet or moon, or in-spaceorbital maneuver. It is ascalar that has the units ofspeed. As used in this context, it isnot the same as thephysical change in velocity of the vehicle.
• Delta-v budget – is an estimate of the totaldelta-v required for aspace mission. It is calculated as the sum of the delta-v required for thepropulsivemaneuvers during the mission, and as input to theTsiolkovsky rocket equation, determines how much propellant is required for a vehicle of given mass and propulsion system.
• Delta wing – is awing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letterdelta (Δ). Although long studied, it did not find significant applications until thejet age, when it proved suitable for high-speed subsonic and supersonic flight.
• Density –
• Departure resistance – is a quality of anaircraft which enables it to remain in controlled flight and resist entering potentially dangerous less-controlled maneuvers such asspin.
• Derivative – The derivative of afunction of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool ofcalculus. For example, the derivative of the position of a moving object with respect totime is the object'svelocity: this measures how quickly the position of the object changes when time advances.
• Digital Datcom – TheUnited StatesAir Force Stability and Control Digital DATCOM is a computer program that implements the methods contained in theUSAF Stability and Control DATCOM to calculate the static stability, control and dynamic derivative characteristics offixed-wing aircraft. Digital DATCOM requires an input file containing a geometric description of an aircraft, and outputs its corresponding dimensionless stability derivatives according to the specified flight conditions. The values obtained can be used to calculate meaningful aspects offlight dynamics.
• Dihedral – Dihedral angle is the upward angle from horizontal of the wings or tailplane of afixed-wing aircraft. "Anhedral angle" is the name given to negative dihedral angle, that is, when there is adownward angle from horizontal of the wings or tailplane of a fixed-wing aircraft.
• Disk loading – Influid dynamics, disk loading or disc loading is the averagepressure change across anactuator disk, such as an airscrew. Airscrews with a relatively low disk loading are typically called rotors, includinghelicoptermain rotors andtail rotors;propellers typically have a higher disk loading.^[54]
• Displacement (vector) –
• Distance measuring equipment – (DME), is a radio navigation technology that measures theslant range (distance) between an aircraft and a ground station by timing thepropagation delay of radio signals in the frequency band between 960 and 1215 megahertz (MHz). Line-of-visibility between the aircraft and ground station is required. An interrogator (airborne) initiates an exchange by transmitting a pulse pair, on an assigned 'channel', to the transponder ground station. The channel assignment specifies the carrier frequency and the spacing between the pulses. After a known delay, the transponder replies by transmitting a pulse pair on a frequency that is offset from the interrogation frequency by 63 MHz and having specified separation.^[55]
• DME – distance measuring equipment.
• DO-178B –
• DO-254 –
• Drag (physics) – Influid dynamics, drag (sometimes called air resistance, a type offriction, or fluid resistance, another type of friction or fluid friction) is aforce acting opposite to the relative motion of any object moving with respect to a surrounding fluid.^[56] This can exist between two fluid layers (or surfaces) or a fluid and asolid surface. Unlike other resistive forces, such as dryfriction, which are nearly independent of velocity, drag forces depend on velocity.^[57][58] Drag force is proportional to the velocity for alaminar flow and the squared velocity for aturbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent ofviscosity.^[59] Drag forces always decrease fluid velocity relative to the solid object in the fluid'spath.
• Drag coefficient – Influid dynamics, the drag coefficient (commonly denoted as:${\displaystyle C_{\mathrm{d}}}$,${\displaystyle C_{\mathrm{x}}}$ or${\displaystyle C_{\mathrm{w}}}$) is adimensionless quantity that is used to quantify thedrag or resistance of an object in a fluid environment, such as air or water. It is used in thedrag equation in which a lower drag coefficient indicates the object will have lessaerodynamic orhydrodynamic drag. The drag coefficient is always associated with a particular surface area.^[60]
• Drag equation – Influid dynamics, the drag equation is a formula used to calculate the force ofdrag experienced by an object due to movement through a fully enclosingfluid. The equation is:

${\displaystyle F_D=\frac{1}{2}\rho u^2{C}_{D}A}$

${\displaystyle F_D}$ is the dragforce, which is by definition the force component in the direction of the flow velocity,

${\displaystyle \rho }$ is themass density of the fluid,^[61]

${\displaystyle u}$ is theflow velocity relative to the object,

${\displaystyle A}$ is the referencearea, and

${\displaystyle C_D}$ is thedrag coefficient – adimensionlesscoefficient related to the object's geometry and taking into account bothskin friction andform drag. In general,${\displaystyle C_D}$ depends on theReynolds number.

• Drop test – is a method of testing the in-flight characteristics ofprototype orexperimentalaircraft andspacecraft by raising the test vehicle to a specific altitude and then releasing it. Test flights involving powered aircraft, particularlyrocket-powered aircraft, may be referred to as drop launches due to thelaunch of the aircraft's rockets after release from its carrier aircraft.
• Dual mode propulsion rocket – Dual modepropulsion systems combine the high efficiency ofbipropellant rockets with the reliability and simplicity ofmonopropellant rockets. It is based upon the use of tworocket fuels,liquid hydrogen and more dense hydrocarbon fuels, like RP, which are all burned withliquid oxygen.^[62]
• Ductility – is a measure of a material's ability to undergo significant plastic deformation before rupture, which may be expressed as percent elongation or percent area reduction from a tensile test.

• Earth's atmosphere – The atmosphere of Earth is the layer ofgases, commonly known asair, that surrounds the planetEarth and is retained byEarth's gravity. The atmosphere of Earth protectslife on Earth by creatingpressure allowing forliquid water to exist on the Earth'ssurface, absorbingultravioletsolar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes betweenday andnight (thediurnal temperature variation).
• Eccentric anomaly – Inorbital mechanics, the eccentric anomaly is anangular parameter that defines the position of a body that is moving along anellipticKepler orbit. The eccentric anomaly is one of three angular parameters ("anomalies") that define a position along an orbit, the other two being thetrue anomaly and themean anomaly.
• Eccentricity vector – Incelestial mechanics, the eccentricity vector of aKepler orbit is thedimensionlessvector with direction pointing fromapoapsis toperiapsis and with magnitude equal to the orbit's scalareccentricity. For Kepler orbits the eccentricity vector is a constant of motion. Its main use is in the analysis of almost circular orbits, as perturbing (non-Keplerian) forces on an actual orbit will cause theosculating eccentricity vector to change continuously. For the eccentricity andargument of periapsis parameters, eccentricity zero (circular orbit) corresponds to a singularity. The magnitude of the eccentricity vector represents the eccentricity of the orbit. Note that the velocity and position vectors need to be relative to the inertial frame of the central body.
• Eigenvector slew – In aerospace engineering, especially those areas dealing withspacecraft, the eigenvector slew is a method to calculate a steering correction (called aslew) by rotating the spacecraft aroundone fixed axis, or agimbal. This corresponds in general to the fastest and most efficient way to reach the desired target orientation as there is only one acceleration phase and one braking phase for the angular rate. If this fixed axis is not aprincipal axis a time varying torque must be applied to force the spacecraft to rotate as desired, though. Also thegyroscopic effect ofmomentum wheels must be compensated for.
• Electrostatic ion thruster – is a form ofelectric propulsion used forspacecraft propulsion. It createsthrust by acceleratingions usingelectricity.
• Elevator – is aflight control surface, usually at the rear of anaircraft, which control the aircraft'spitch, and therefore theangle of attack and the lift of the wing. The elevators are usually hinged to thetailplane or horizontalstabilizer.
• Elliptic partial differential equation –
• Empennage – The empennage (/ˌɑːmpɪˈnɑːʒ/ or/ˈɛmpɪnɪdʒ/), also known as thetail ortail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on anarrow.^[63][64][65] The term derives from theFrench language verbempenner which means "to feather an arrow".^[66] Most aircraft feature an empennage incorporating vertical and horizontal stabilising surfaces which stabilise theflight dynamics ofyaw andpitch,^[63][64] as well as housingcontrol surfaces.
• Enstrophy – Influid dynamics, the enstrophyE can be interpreted as another type ofpotential density; or, more concretely, the quantity directly related to thekinetic energy in the flow model that corresponds todissipation effects in the fluid. It is particularly useful in the study ofturbulent flows, and is often identified in the study ofthrusters as well as the field ofcombustion theory.

Given a domain${\displaystyle \mathrm{\Omega }\subseteq {\mathbb{R}}^n}$ and a once-weakly differentiable vector field${\displaystyle u\in H^1({\mathbb{R}}^n{)}^n}$ which represents a fluid flow, such as a solution to theNavier-Stokes equations, its enstrophy is given by:^[67]

${\displaystyle \mathcal{E}(u):={\int }_{\mathrm{\Omega }}|\mathrm{\nabla }\mathbf{u}{|}^{2}dx}$

Where${\displaystyle |\mathrm{\nabla }\mathbf{u}{|}^2=\sum _{i,j=1}^n{\left|{\mathrm{\partial }}_i{u}^j\right|}^2}$. This is quantity is the same as the squaredseminorm${\displaystyle |\mathbf{u}{|}_{H^1(\mathrm{\Omega }{)}^n}^2}$of the solution in theSobolev space ::::${\displaystyle H^1(\mathrm{\Omega }{)}^n}$.

In the case that the flow isincompressible, or equivalently that${\displaystyle \mathrm{\nabla }\cdot \mathbf{u}=0}$, the enstrophy can be described as the integral of the square of thevorticity${\displaystyle \omega }$,^[68]

${\displaystyle \mathcal{E}(\mathit{\omega })\equiv {\int }_{\mathrm{\Omega }}|\mathit{\omega }{|}^{2}dx}$

or, in terms of theflow velocity,

${\displaystyle \mathcal{E}(\mathbf{u})\equiv {\int }_S|\mathrm{\nabla }\times \mathbf{u}{|}^{2}dS.}$

In the context of the incompressible Navier-Stokes equations, enstrophy appears in the following useful result^[20]

${\displaystyle \frac{d}{dt}\left(\frac{1}{2}{\int }_{\mathrm{\Omega }}|\mathbf{u}{|}^2\right)=-\nu \mathcal{E}(\mathbf{u})}$

The quantity in parentheses on the left is the energy in the flow, so the result says that energy declines proportional to thekinematic viscosity${\displaystyle \nu }$ times the enstrophy.

• Equations of motion – Inphysics, equations of motion areequations that describe the behavior of aphysical system in terms of itsmotion as afunction of time.^[69] More specifically, the equations of motion describe the behavior of a physical system as a set of mathematical functions in terms of dynamic variables. These variables are usually spatial coordinates and time, but may includemomentum components. The most general choice aregeneralized coordinates which can be any convenient variables characteristic of the physical system.^[70] The functions are defined in aEuclidean space inclassical mechanics, but are replaced bycurved spaces inrelativity. If thedynamics of a system is known, the equations are the solutions for thedifferential equations describing the motion of the dynamics.
• ESA – European Space Agency
• ET –(Space Shuttle) external tank
• Euler angles – are three angles introduced byLeonhard Euler to describe theorientation of arigid body with respect to a fixedcoordinate system.^[71] They can also represent the orientation of a mobileframe of reference in physics or the orientation of a generalbasis in3-dimensional linear algebra. Alternative forms were later introduced byPeter Guthrie Tait andGeorge H. Bryan intended for use in aeronautics and engineering.
• European Space Agency –
• Expander cycle (rocket) – is a power cycle of abipropellant rocketengine. In this cycle, the fuel is used to cool the engine's combustion chamber, picking up heat and changing phase. The now heated and gaseous fuel then powers the turbine that drives the engine's fuel and oxidizer pumps before being injected into the combustion chamber and burned for thrust.

• Fatigue – Inmaterials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximumstress values that cause such damage may be much less than the strength of the material typically quoted as theultimate tensile stress limit, or theyield stress limit.
• Field-emission electric propulsion – (FEEP), is an advanced electrostatic space propulsion concept, a form ofion thruster, that uses a liquidmetal as a propellant – usually eithercaesium,indium, ormercury.
• Fixed-wing aircraft – is a heavier-than-airflying machine, such as anairplane, which is capable offlight usingwings that generatelift caused by the aircraft's forwardairspeed and theshape of the wings. Fixed-wing aircraft are distinct fromrotary-wing aircraft (in which the wings form arotor mounted on a spinning shaft or "mast"), andornithopters (in which the wingsflap in a manner similar to that of abird). The wings of a fixed-wing aircraft are not necessarily rigid; kites,hang gliders,variable-sweep wing aircraft and airplanes that usewing morphing are all examples of fixed-wing aircraft.
• Flange –
• Flap – is ahigh-lift device used to reduce thestalling speed of anaircraftwing at a given weight. Flaps are usually mounted on the wing trailing edges of afixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase indrag so they are retracted when not needed.
• Flight control surfaces – are aerodynamic devices allowing a pilot to adjust and control the aircraft's flightattitude.
• Flight control system (aircraft) – A conventionalfixed-wingaircraft flight control system consists offlight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight.Aircraft engine controls are also considered as flight controls as they change speed.
• Flight control system (helicopter) – Ahelicopterpilot manipulates thehelicopter flight controls to achieve and maintain controlled aerodynamicflight.^[72] Changes to theaircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a deliberate way. To tilt forward and back (pitch) or sideways (roll) requires that the controls alter theangle of attack of the main rotor bladescyclically during rotation, creating differing amounts oflift (force) at different points in the cycle. To increase or decrease overall lift requires that the controls alter theangle of attack for all bladescollectively by equal amounts at the same time, resulting in ascent, descent, acceleration and deceleration.
• Flight dynamics – is the study of the performance, stability, and control of vehiclesflying through the air or inouter space.^[73] It is concerned with how forces acting on the vehicle determine itsvelocity andattitude with respect to time. For afixed-wing aircraft, its changingorientation with respect to the local air flow is represented by two critical angles, the angle of attack of the wing ("alpha") and the angle of attack of the vertical tail, known as thesideslip angle ("beta"). A sideslip angle will arise if an aircraft yaws about its centre of gravity and if the aircraft sideslips bodily, i.e. the centre of gravity moves sideways.^[74] These angles are important because they are the principal source of changes in the aerodynamic forces and moments applied to the aircraft. Spacecraft flight dynamics involve three main forces: propulsive (rocket engine), gravitational, and atmospheric resistance.^[75] Propulsive force and atmospheric resistance have significantly less influence over a given spacecraft compared to gravitational forces.
• Flight management system – A flight management system (FMS) is a fundamental component of a modern airliner'savionics. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that modern civilian aircraft no longer carryflight engineers ornavigators. A primary function is in-flight management of the flight plan. Using various sensors (such asGPS andINS often backed up byradio navigation) to determine the aircraft's position, the FMS can guide the aircraft along the flight plan. From the cockpit, the FMS is normally controlled through aControl Display Unit (CDU) which incorporates a small screen and keyboard or touchscreen. The FMS sends the flight plan for display to theElectronic Flight Instrument System (EFIS), Navigation Display (ND), or Multifunction Display (MFD). The FMS can be summarised as being a dual system consisting of theFlight Management Computer (FMC), CDU and a cross talk bus.
• Floatstick – is a device to measurefuel levels in modern largeaircraft. It consists of a closed tube rising from the bottom of the fuel tank. Surrounding the tube is a ring-shaped float, and inside it is a graduated rod indicating fuel capacity. The float and the top of the rod containmagnets. The rod is withdrawn from the bottom of thewing until the magnets stick, the distance it is withdrawn indicating the level of the fuel. When not in use, the stick is secured within the tube.
• Fluid – Inphysics, a fluid is aliquid,gas, or other material that continuouslydeforms (flows) under an appliedshear stress, or external force.^[76] They have zeroshear modulus, or, in simpler terms, aresubstances which cannot resist anyshear force applied to them.
• Fluid dynamics – Inphysics andengineering, fluid dynamics is a subdiscipline offluid mechanics that describes the flow offluids—liquids andgases. It has several subdisciplines, includingaerodynamics (the study of air and other gases in motion) andhydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculatingforces andmoments onaircraft, determining themass flow rate ofpetroleum throughpipelines,predicting weather patterns, and understandingnebulae ininterstellar space.
• Fluid mechanics – is the branch ofphysics concerned with themechanics offluids (liquids,gases, andplasmas) and theforces on them.^[77] It has applications in a wide range of disciplines, includingmechanical,civil,chemical andbiomedical engineering,geophysics,oceanography,meteorology,astrophysics, andbiology. It can be divided intofluid statics, the study of fluids at rest; andfluid dynamics, the study of the effect of forces on fluid motion.^[77]
• Fluid statics – orhydrostatics, is the branch offluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids athydrostatic equilibrium^[78] and the pressure in a fluid, or exerted by a fluid, on an immersed body".^[79]
• FMS – Flight management system.
• Force – Inphysics, a force is any influence that, when unopposed, will change themotion of anobject. A force can cause an object withmass to change itsvelocity (which includes to begin moving from astate of rest), i.e., toaccelerate. Force can also be described intuitively as a push or a pull. A force has bothmagnitude anddirection, making it avector quantity. It is measured in theSI unit ofnewton (N). Force is represented by the symbolF (formerlyP).
• Freefall – InNewtonian physics, free fall is any motion of abody wheregravity is the onlyforce acting upon it. In the context ofgeneral relativity, where gravitation is reduced to aspace-time curvature, a body in free fall has no force acting on it. An object in the technical sense of the term "free fall" may not necessarily be falling down in the usual sense of the term. An object moving upwards might not normally be considered to be falling, but if it is subject to only the force of gravity, it is said to be in free fall. TheMoon is thus in free fall around theEarth, though itsorbital speed keeps it invery far orbit from theEarth's surface. In a roughly uniformgravitational field, in the absence of any other forces, gravitation acts on each part of the body roughly equally. When there is nonormal force exerted between a body (e.g. anastronaut in orbit) and its surrounding objects, it will result in the sensation ofweightlessness, a condition that also occurs when the gravitational field is weak (such as when far away from any source of gravity).
• Fuselage – Inaeronautics, the fuselage (/ˈfjuːzəlɑːʒ/; from theFrenchfuselé "spindle-shaped") is anaircraft's main body section. It holdscrew, passengers, orcargo. In single-engine aircraft, it will usually contain anengine, as well, although in someamphibious aircraft the single engine is mounted on apylon attached to the fuselage, which in turn is used as a floatinghull. The fuselage also serves to position thecontrol andstabilization surfaces in specific relationships tolifting surfaces, which is required for aircraft stability and maneuverability.
• Future Air Navigation System – (FANS), is anavionics system which provides direct data link communication between the pilot and theair traffic controller. The communications include air traffic control clearances, pilot requests and position reporting.^[80]
• Flying wing – is ataillessfixed-wing aircraft that has no definitefuselage, with its crew, payload, fuel, and equipment housed inside the main wing structure. A flying wing may have various small protuberances such as pods,nacelles, blisters, booms, orvertical stabilizers.^[81]

• Galaxy – is agravitationally bound system ofstars,stellar remnants,interstellar gas,dust, anddark matter.^[82][83] The word is derived from theGreekgalaxias (γαλαξίας), literally "milky", a reference to theMilky Way. Galaxies range in size fromdwarfs with just a few hundred million (10⁸) stars togiants with one hundredtrillion (10¹⁴) stars,^[84] each orbiting its galaxy'scenter of mass. Galaxies are categorized according to their visualmorphology aselliptical,^[85]spiral, orirregular.^[86]
• Gas-generator cycle (rocket) – is a power cycle of a pumped liquidbipropellant rocketengine. Part of the unburned propellant is burned in agas generator (or preburner) and the resulting hot gas is used to power the propellant pumps before being exhausted overboard, and lost. Because of this loss, this type of engine is termedopen cycle.
• Geostationary orbit – also referred to as ageosynchronous equatorial orbit^[a] (GEO), is acirculargeosynchronous orbit 35,786 kilometres (22,236 miles) in altitude above Earth'sequator (42,164 kilometers in radius from Earth's center) and following thedirection ofEarth's rotation. An object in such an orbit has anorbital period equal to the Earth's rotational period, onesidereal day, and so to ground observers it appears motionless, in a fixed position in the sky.
• Geosynchronous orbit – (sometimes abbreviated GSO) is an Earth-centeredorbit with anorbital period that matchesEarth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (onesidereal day). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky may remain still or trace out a path,typically in a figure-8 form, whose precise characteristics depend on the orbit'sinclination andeccentricity. A circular geosynchronous orbit has a constant altitude of 35,786 km (22,236 mi), and all geosynchronous orbits share that semi-major axis. A special case of geosynchronous orbit is thegeostationary orbit, which is a circular geosynchronous orbit in Earth'sequatorial plane. A satellite in a geostationary orbit remains in the same position in the sky to observers on the surface.
• Glide ratio – As the aircraftfuselage and control surfaces will also add drag and possibly some lift, it is fair to consider the lift-to-drag ratio (or L/D ratio) of the aircraft as a whole. As it turns out, theglide ratio, which is the ratio of an (unpowered) aircraft's forward motion to its descent, is (when flown at constant speed) numerically equal to the aircraft's L/D. This is especially of interest in the design and operation of high performancesailplanes, which can have glide ratios almost 60 to 1 (60 units of distance forward for each unit of descent) in the best cases, but with 30:1 being considered good performance for general recreational use. Achieving a glider's best L/D in practice requires precise control of airspeed and smooth and restrained operation of the controls to reduce drag from deflected control surfaces. In zero wind conditions, L/D will equal distance traveled divided by altitude lost. Achieving the maximum distance for altitude lost in wind conditions requires further modification of the best airspeed, as does alternating cruising and thermaling. To achieve high speed across country, glider pilots anticipating strong thermals often load their gliders (sailplanes) withwater ballast: the increasedwing loading means optimum glide ratio at greater airspeed, but at the cost of climbing more slowly in thermals. The maximum L/D is not dependent on weight or wing loading, but with greater wing loading the maximum L/D occurs at a faster airspeed. Also, the faster airspeed means the aircraft will fly at greaterReynolds number and this will usually bring about a lowerzero-lift drag coefficient.
• Glider – is afixed-wing aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whosefree flight does not depend on an engine.^[87] Most gliders do not have an engine, althoughmotor-gliders have small engines for extending their flight when necessary by sustaining the altitude (normally a sailplane relies on rising air to maintain altitude) with some being powerful enough to take offself-launch.
• Global Positioning System – (GPS), originallyNavstar GPS,^[88] is asatellite-based radionavigation system owned by theUnited States government and operated by theUnited States Space Force.^[89] It is one of theglobal navigation satellite systems (GNSS) that providesgeolocation andtime information to aGPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.^[90] Obstacles such as mountains and buildings can block the relatively weakGPS signals.
• Goddard problem – Inrocketry, the Goddard problem is to optimize the peakaltitude of a rocket, ascending vertically, and taking into accountatmospheric drag and thegravitational field. This was first posed byRobert H. Goddard in his 1919 publication, "A Method of Reaching Extreme Altitudes".^[91][92]
• GPS – Global Positioning System
• Gravitational constant – The gravitational constant (also known as theuniversal gravitational constant, theNewtonian constant of gravitation, or theCavendish gravitational constant),^[b] denoted by the letterG, is anempiricalphysical constant involved in the calculation ofgravitational effects inSir Isaac Newton'slaw of universal gravitation and inAlbert Einstein'sgeneral theory of relativity. In Newton's law, it is the proportionality constant connecting thegravitational force between two bodies with the product of theirmasses and theinverse square of theirdistance. In theEinstein field equations, it quantifies the relation between the geometry of spacetime and the energy–momentum tensor (also referred to as thestress–energy tensor). The measured value of the constant is known with some certainty to four significant digits. InSI units, its value is approximately6.674×10⁻¹¹ m³⋅kg⁻¹⋅s⁻².^[93] The modern notation of Newton's law involvingG was introduced in the 1890s byC. V. Boys. The first implicit measurement with an accuracy within about 1% is attributed toHenry Cavendish in a1798 experiment.^[c]
• Gravitational slingshot – Inorbital mechanics andaerospace engineering, a gravitationalslingshot,gravity assist maneuver, orswing-by is the use of the relative movement (e.g. orbit around theSun) andgravity of aplanet or otherastronomical object to alter thepath andspeed of aspacecraft, typically to savepropellant and reduce expense. Gravity assistance can be used toaccelerate a spacecraft, that is, to increase or decrease its speed or redirect its path. The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft.^[94]
• Gravity – (from Latin gravitas 'weight'^[95]), orgravitation, is anatural phenomenon by which all things withmass orenergy—includingplanets,stars,galaxies, and evenlight^[96]—are attracted to (orgravitate toward) one another.On Earth, gravity givesweight tophysical objects, and theMoon's gravity causes thetides of the oceans. The gravitational attraction of the original gaseous matter present in theUniverse caused it to begincoalescing andforming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become weaker as objects get further away.

• Hall effect thruster – Inspacecraft propulsion, a Hall-effect thruster (HET) is a type ofion thruster in which thepropellant is accelerated by anelectric field. Hall-effect thrusters (based on the discovery byEdwin Hall) are sometimes referred to asHall thrusters orHall-current thrusters.Hall-effect thrusters use amagnetic field to limit the electrons' axial motion and then use them to ionize propellant, efficiently accelerate theions to producethrust, and neutralize the ions in the plume. The Hall-effect thruster is classed as a moderatespecific impulse (1,600 s) space propulsion technology and has benefited from considerable theoretical and experimental research since the 1960s.^[97]
• Heat shield – A heat shield is designed to protect an object from overheating by dissipating, reflecting, absorbing heat, or simply gradually burn and fall away from the aircraft, pulling the excess heat with it. The term is most often used in reference toexhaust heat management and to systems for dissipation of heat due to friction.
• Helicopter – is a type ofrotorcraft in whichlift andthrust are supplied by horizontally-spinningrotors. This allows the helicopter to take off and land vertically, tohover, and to fly forward, backward and laterally. These attributes allow helicopters to be used in congested or isolated areas wherefixed-wing aircraft and many forms ofVTOL (Vertical TakeOff and Landing) aircraft cannot perform.
• High-hypersonic –
• Hohmann transfer orbit – Inorbital mechanics, the Hohmann transfer orbit (/ˈhoʊmən/) is anelliptical orbit used to transfer between twocircular orbits of different radii around a central body in the sameplane. The Hohmann transfer often uses the lowest possible amount ofpropellant in traveling between these orbits, butbi-elliptic transfers can use less in some cases.
• Hybrid rocket – A hybrid-propellant rocket is arocket with arocket motor that usesrocket propellants in two different phases:one solid and the other either gas orliquid. The hybrid rocket concept can be traced back to at least the 1930s.^[98]
• Hydrodynamics – Inphysics andengineering,fluid dynamics is a subdiscipline offluid mechanics that describes the flow offluids—liquids andgases. It has several subdisciplines, includingaerodynamics (the study of air and other gases in motion) andhydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculatingforces andmoments onaircraft, determining themass flow rate ofpetroleum throughpipelines,predicting weather patterns, understandingnebulae ininterstellar space andmodelling fission weapon detonation.
• Hydrostatics –Fluid statics orhydrostatics is the branch offluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids athydrostatic equilibrium^[78] and the pressure in a fluid, or exerted by a fluid, on an immersed body".^[79] It encompasses the study of the conditions under which fluids are at rest instable equilibrium as opposed tofluid dynamics, the study of fluids in motion. Hydrostatics is a subcategory of fluid statics, which is the study of all fluids, both compressible or incompressible, at rest.
• Hyperbolic partial differential equation – Inmathematics, a hyperbolic partial differential equation of order${\displaystyle n}$ is apartial differential equation (PDE) that, roughly speaking, has a well-posedinitial value problem for the first${\displaystyle n-1}$ derivatives. More precisely, theCauchy problem can be locally solved for arbitrary initial data along any non-characteristichypersurface. Many of the equations ofmechanics are hyperbolic, and so the study of hyperbolic equations is of substantial contemporary interest. The model hyperbolic equation is thewave equation. In one spatial dimension, this is

${\displaystyle \frac{{\mathrm{\partial }}^{2}u}{\mathrm{\partial }{t}^2}=c^2\frac{{\mathrm{\partial }}^{2}u}{\mathrm{\partial }{x}^2}}$

The equation has the property that, ifu and its first time derivative are arbitrarily specified initial data on the linet = 0 (with sufficient smoothness properties), then there exists a solution for all timet.

• Hypersonic speed – Inaerodynamics, a hypersonic speed is one that greatly exceeds thespeed of sound, often stated as starting at speeds ofMach 5 and above.^[99] The preciseMach number at which a craft can be said to be flying at hypersonic speed varies, since individual physical changes in the airflow (like moleculardissociation andionization) occur at different speeds; these effects collectively become important around Mach 5–10. The hypersonic regime can also be alternatively defined as speeds where specific heat capacity changes with the temperature of the flow as kinetic energy of the moving object is converted into heat.^[100]
• Hypoxia – is a condition^[101] in which the body or a region of the body is deprived of adequateoxygen supply at thetissue level. Hypoxia may be classified as eithergeneralized, affecting the whole body, orlocal, affecting a region of the body.^[102] Although hypoxia is often apathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, duringhypoventilation training or strenuous physical exercise.

• Impulse – Specific impulse (usually abbreviatedI_sp) is a measure of how efficiently arocket uses propellant or ajet engine uses fuel. For engines whose reaction mass is only the fuel they carry, specific impulse is exactly proportional to exhaust gas velocity.
• Indicated airspeed – (IAS), is theairspeed read directly from theairspeed indicator (ASI) on an aircraft, driven by thepitot-static system.^[103] It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measuredynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume thesea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably fromtrue airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass.Calibrated airspeed (CAS) is the IAS corrected for instrument andposition error.^[103] An aircraft's indicated airspeed in knots is typically abbreviatedKIAS for "Knots-Indicated Air Speed" (vs.KCAS forcalibrated airspeed andKTAS fortrue airspeed).
• Instrument landing system – Inaviation, the instrument landing system (ILS) is aradio navigation system that provides short-range guidance toaircraft to allow them to approach arunway at night or in bad weather. In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a1⁄2 mile (800 m) of the runway. At that point the runway should be visible to the pilot; if it is not, they perform amissed approach. Bringing the aircraft this close to the runway dramatically improves the weather conditions in which a safelanding can be made. Later versions of the system, or "categories", have further reduced the minimum altitudes.
• Interplanetary Transport Network – (ITN)^[104] is a collection ofgravitationally determined pathways through theSolar System that require very littleenergy for an object to follow. The ITN makes particular use ofLagrange points as locations wheretrajectories throughspace can be redirected using little or no energy. These points have the peculiar property of allowing objects toorbit around them, despite lacking an object to orbit. While it would use little energy, transport along the network would take a long time.^[105]
• Interplanetary travel –Interplanetary spaceflight or interplanetary travel is thecrewed oruncrewed travel betweenstars andplanets, usually within a singleplanetary system.^[106]
• Interstellar travel – refers to the currently theoretical idea ofinterstellar probes or crewed spacecraft moving betweenstars orplanetary systems in a galaxy. Interstellar travel would be much more difficult thaninterplanetary spaceflight. Whereas the distances between theplanets in theSolar System are less than 30astronomical units (AU), the distances between stars are typically hundreds of thousands of AU, and usually expressed inlight-years. Because of the vastness of those distances, practical interstellar travel based on known physics would need to occur at a high percentage of thespeed of light; even so, travel times would be long, at least decades and perhaps millennia or longer.^[107]
• Ion thruster – An ion thruster,ion drive, orion engine is a form ofelectric propulsion used forspacecraft propulsion. It createsthrust by acceleratingions usingelectricity.
• ISRO – TheIndian Space Research Organisation^[d] (ISRO/ˈɪsroʊ/) or (IAST :Bhāratīya Antrikṣ Anusandhān Saṅgaṭhan) is thenational space agency ofIndia, headquartered inBengaluru. It operates under theDepartment of Space (DOS) which is directly overseen by thePrime Minister of India, whileChairman of ISRO acts as executive of DOS as well. ISRO is the primary agency in India to perform tasks related to space based applications,space exploration and development of related technologies.^[108] It is one of six government space agencies in the world which possess full launch capabilities, deploycryogenic engines, launchextraterrestrial missions and operate large fleets of artificial satellites.^{[109][110][e]}

• Jet engine – is a type ofreaction engine discharging a fast-movingjet that generatesthrust byjet propulsion.

• Keel effect – Inaeronautics, the keel effect (also known as thependulum effect orpendulum stability^[111]) is the result of the sideforce-generating surfaces being above (or below) thecenter of mass (which coincides with thecenter of gravity) in an aircraft. Along withdihedral,sweepback, andweight distribution, keel effect is one of the four main design considerations in aircraftlateral stability.^[112]
• Kepler's laws of planetary motion – Inastronomy,Kepler's laws of planetary motion, published byJohannes Kepler between 1609 and 1619, describe the orbits ofplanets around theSun. The laws modified theheliocentric theory ofNicolaus Copernicus, replacing its circularorbits andepicycles with elliptical trajectories, and explaining how planetary velocities vary. The three laws state that:

1. The orbit of a planet is anellipse with the Sun at one of the two foci.
2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3. The square of a planet'sorbital period is proportional to the cube of the length of thesemi-major axis of its orbit.

The elliptical orbits of planets were indicated by calculations of the orbit ofMars. From this, Kepler inferred that other bodies in theSolar System, including those farther away from the Sun, also have elliptical orbits. The second law helps to establish that when a planet is closer to the Sun, it travels faster. The third law expresses that the farther a planet is from the Sun, the slower its orbital speed, and vice versa.

Isaac Newton showed in 1687 that relationships like Kepler's would apply in the Solar System as a consequence of his ownlaws of motion andlaw of universal gravitation.

• Kessler syndrome – (also called theKessler effect,^[113][114]collisional cascading, orablation cascade), proposed byNASA scientistDonald J. Kessler in 1978, is a theoretical scenario in which the density of objects inlow Earth orbit (LEO) due tospace pollution is high enough that collisions between objects could cause a cascade in which each collision generatesspace debris that increases the likelihood of further collisions.^[115] One implication is that the distribution of debris in orbit could render space activities and the use ofsatellites in specific orbital ranges difficult for many generations.^[115]
• Kinetic energy – Inphysics, thekinetic energy of an object is theenergy that it possesses due to itsmotion.^[116] It is defined as thework needed to accelerate a body of a given mass from rest to its statedvelocity. Having gained this energy during itsacceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body when decelerating from its current speed to a state of rest. Inclassical mechanics, the kinetic energy of a non-rotating object ofmassm traveling at aspeedv is$\frac{1}{2}m{v}^2$. Inrelativistic mechanics, this is a good approximation only whenv is much less than thespeed of light.
• Kite – is atetheredheavier-than-air or lighter-than-air craft withwing surfaces that react against the air to createlift anddrag forces.^[117] A kite consists of wings, tethers and anchors. Kites often have a bridle and tail to guide the face of the kite so the wind can lift it.^[118] Some kite designs don't need a bridle;box kites can have a single attachment point. A kite may have fixed or moving anchors that can balance the kite. One technical definition is that a kite is "a collection of tether-coupled wing sets".^[119] The name derives from its resemblance to a hoveringbird.^[120]
• Kutta condition – is a principle in steady-flowfluid dynamics, especiallyaerodynamics, that is applicable to solid bodies with sharp corners, such as thetrailing edges ofairfoils. It is named forGermanmathematician and aerodynamicistMartin Kutta.

Kuethe and Schetzer state the Kutta condition as follows:^{[121]: § 4.11}

A body with a sharp trailing edge which is moving through a fluid will create about itself acirculation of sufficient strength to hold the rearstagnation point at the trailing edge.

In fluid flow around a body with a sharp corner, the Kutta condition refers to the flow pattern in which fluid approaches the corner from above and below, meets at the corner, and then flows away from the body. None of the fluid flows around the sharp corner.

The Kutta condition is significant when using theKutta–Joukowski theorem to calculate the lift created by an airfoil with a sharp trailing edge. The value ofcirculation of the flow around the airfoil must be that value that would cause the Kutta condition to exist.

• Kutta–Joukowski theorem – is a fundamental theorem inaerodynamics used for the calculation of lift of anairfoil and any two-dimensional bodies including circular cylinders translating into a uniform fluid at a constant speed large enough so that the flow seen in the body-fixed frame is steady and unseparated. The theorem relates thelift generated by an airfoil to the speed of the airfoil through the fluid, the density of the fluid and thecirculation around the airfoil. The circulation is defined as the line integral around a closed-loop enclosing the airfoil of the component of the velocity of the fluidtangent to the loop.^[122] It is named afterMartin Kutta andNikolai Zhukovsky (or Joukowski) who first developed its key ideas in the early 20th century. Kutta–Joukowski theorem is aninviscid theory, but it is a good approximation for real viscous flow in typical aerodynamic applications.^[123]

• Lander –spacecraft designed to soft-land intact or almost undamaged on the surface of acelestial body and eventually take-off from it
• Landing – is the last part of aflight, where anaircraft, orspacecraft returns to the ground. When the flying object returns to water, the process is calledalighting, although it is commonly called "landing", "touchdown"^a or "splashdown" as well. A normal aircraft flight would include several parts of flight includingtaxi,takeoff,climb,cruise,descent and landing.
• Landing gear – is the undercarriage of anaircraft orspacecraft and may be used for eithertakeoff orlanding. For aircraft it is generally needed for both. Also, for aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land, and taxi without damage. Wheeled landing gear is the most common, withskis orfloats needed to operate from snow/ice/water and skids for vertical operation on land. Faster aircraft have retractable undercarriages, which fold away during flight to reducedrag.
• Lagrangian mechanics – Introduced by the Italian-French mathematician and astronomerJoseph-Louis Lagrange in 1788,Lagrangian mechanics is a formulation ofclassical mechanics and is founded on thestationary action principle.

Lagrangian mechanics defines a mechanical system to be a pair${\displaystyle (M,L)}$ of aconfiguration space${\displaystyle M}$ and a smooth function${\displaystyle L=L(q,v,t)}$ calledLagrangian. By convention,${\displaystyle L=T-V,}$ where${\displaystyle T}$ and${\displaystyle V}$ are thekinetic andpotential energy of the system, respectively. Here${\displaystyle q\in M,}$ and${\displaystyle v}$ is the velocity vector at${\displaystyle q}$${\displaystyle (v}$ is tangential to${\displaystyle M).}$ (For those familiar withtangent bundles,${\displaystyle L:TM\times {\mathbb{R}}_t\to \mathbb{R},}$ and${\displaystyle v\in T_{q}M).}$

Given the time instants${\displaystyle t_1}$ and${\displaystyle t_2,}$ Lagrangian mechanics postulates that a smooth path${\displaystyle x_0:[t_1,t_2]\to M}$ describes the time evolution of the given system if and only if${\displaystyle x_0}$ is astationary point of theaction functional

${\displaystyle \mathcal{S}[x]\stackrel{\text{def}}{=}{\int }_{t_1}^{t_2}L(x(t),\dot{x}(t),t)dt.}$

If${\displaystyle M}$ is an open subset of${\displaystyle {\mathbb{R}}^n}$ and${\displaystyle t_1,}$${\displaystyle t_2}$ are finite, then the smooth path${\displaystyle x_0}$ is a stationary point of${\displaystyle \mathcal{S}}$ if all its directional derivatives at${\displaystyle x_0}$ vanish, i.e., for every smooth${\displaystyle \delta :[t_1,t_2]\to {\mathbb{R}}^n,}$

${\displaystyle \delta \mathcal{S}\stackrel{\text{def}}{=}\frac{d}{d\epsilon }{|}_{\epsilon =0}\mathcal{S}\left[x_0+\epsilon \delta \right]=0.}$

The function${\displaystyle \delta (t)}$ on the right-hand side is calledperturbation orvirtual displacement. The directional derivative${\displaystyle \delta \mathcal{S}}$ on the left is known asvariation in physics andGateaux derivative in mathematics.

Lagrangian mechanics has been extended to allow for non-conservative forces.

• Lagrangian point – Incelestial mechanics, the Lagrange points/ləˈɡrɑːndʒ/ (alsoLagrangian points,L-points, orlibration points) are points near two largeorbiting bodies. Normally, the two objects exert an unbalanced gravitational force at a point, altering the orbit of whatever is at that point. At the Lagrange points, thegravitational forces of the two large bodies and thecentrifugal force balance each other.^[124] This can make Lagrange points an excellent location for satellites, as feworbit corrections are needed to maintain the desired orbit. Small objects placed in orbit at Lagrange points are in equilibrium in at least two directions relative to thecenter of mass of the large bodies.
• Laser broom – is a proposed ground-basedlaserbeam-powered propulsion system whose purpose is to sweepspace debris out of the path of otherartificial satellites such as theInternational Space Station. It would heat one side of an object enough to change its orbit and make it hit the atmosphere sooner. Space researchers have proposed that a laser broom may help mitigateKessler syndrome, a theoretical runaway cascade of collision events between orbiting objects.^[125] Space-based laser broom systems using a laser mounted on a satellite or space station have also been proposed.^{[126][127][128][129]}
• Laser Camera System – (LCS), is short-range, high precision autosynchronous triangulation scanner. The camera uses a laser to measure the distance between itself and points on a target and is able to create a three-dimensional representation of the area it has scanned.
• Latus rectum – is thechord parallel to the directrix and passing through a focus; its half-length is the semi-latus rectum (ℓ).
• Launch window – In the context ofspaceflight, launch period is the collection of days andlaunch window is the time period on a given day during which a particularrocket must be launched in order to reach its intended target.^[130][131] If the rocket is not launched within a given window, it has to wait for the window on the next day of the period.^[132] Launch periods and launch windows are very dependent on both the rocket's capability and the orbit to which it is going.^[133][134]
• Leading edge – The leading edge of anairfoil surface such as awing is its foremost edge and is therefore the part which first meets the oncoming air.^[135][136]
• Lift –
• Lift coefficient – is adimensionlesscoefficient that relates thelift generated by alifting body to thefluid density around the body, thefluid velocity and an associated reference area. A lifting body is afoil or a complete foil-bearing body such as afixed-wing aircraft.C_L is a function of the angle of the body to the flow, itsReynolds number and itsMach number. The lift coefficientc_l refers to the dynamic lift characteristics of atwo-dimensional foil section, with the reference area replaced by the foilchord.^[137][138]
• Lightcraft – The Lightcraft is aspace- orair-vehicle driven bybeam-powered propulsion, the energy source powering the craft being external. It was conceptualized by aerospace engineering professorLeik Myrabo atRensselaer Polytechnic Institute in 1976,^[139] who developed the concept further with working prototypes,^[140] funded in the 1980s by theStrategic Defense Initiative organization, and the decade after by theAdvanced Concept Division of theUS Air ForceAFRL,NASA'sMFSC and theLawrence Livermore National Laboratory.^{[141][142][143]}
• Lighter than air – Alifting gas orlighter than air gas is a gas that has a lower density than normal atmospheric gases and rises above them as a result. It is required foraerostats to createbuoyancy, particularly inlighter-than-air aircraft, which includefree balloons,moored balloons, andairships. Only certain lighter than air gases are suitable as lifting gases. Dry air has a density of about 1.29 g/L (gram per liter) atstandard conditions for temperature and pressure (STP) and an average molecular mass of 28.97 g/mol,^[144] and so lighter than air gases have a density lower than this.
• Liquid air cycle engine – (LACE), is a type ofspacecraft propulsion engine that attempts to increase its efficiency by gathering part of itsoxidizer from theatmosphere. A liquid air cycle engine usesliquid hydrogen (LH2) fuel to liquefy the air.
• Liquid fuel – Liquid fuels are combustible or energy-generating molecules that can be harnessed to createmechanical energy, usually producingkinetic energy; they also must take the shape of their container. It is the fumes of liquid fuels that are flammable instead of the fluid. Most liquid fuels in widespread use are derived fromfossil fuels; however, there are several types, such ashydrogen fuel (forautomotive uses), ethanol, andbiodiesel, which are also categorized as a liquid fuel. Many liquid fuels play a primary role in transportation and the economy. Liquid fuels are contrasted withsolid fuels andgaseous fuels.
• Liquid-propellant rocket – orliquid rocket, utilizes arocket engine that usesliquid propellants. Liquids are desirable because they have a reasonably high density and highspecific impulse (I_sp). This allows the volume of the propellant tanks to be relatively low. It is also possible to use lightweight centrifugalturbopumps to pump therocket propellant from the tanks into the combustion chamber, which means that the propellants can be kept under low pressure. This permits the use of low-mass propellant tanks that do not need to resist the high pressures needed to store significant amounts of gases, resulting in a lowmass ratio for the rocket.^{[citation needed]}
• Liquid rocket propellant – The highestspecific impulse chemicalrockets useliquid propellants (liquid-propellant rockets). They can consist of a single chemical (amonopropellant) or a mix of two chemicals, calledbipropellants. Bipropellants can further be divided into two categories;hypergolic propellants, which ignite when the fuel andoxidizer make contact, and non-hypergolic propellants which require an ignition source.^[145]
• Lithobraking – is a landing technique used by uncrewed space vehicles to safely reach the surface of a celestial body while reducing landing speed by impact with the body's surface.
• Loiter – Inaeronautics andaviation, loiter is the phase of flight consisting of flying over some small region.
• Low Earth orbit – (LEO), is anEarth-centered orbit close to the planet, often specified as anorbital period of 128 minutes or less (making at least 11.25 orbits per day) and aneccentricity less than 0.25.^[146] Most of the artificial objects inouter space are in LEO, with an altitude never more than about one-third of theradius of the Earth.^[147]
• Lunar Module – TheApollo Lunar Module, or simplyLunar Module (LM/ˈlɛm/), originally designated theLunar Excursion Module (LEM), was theLunar landerspacecraft that was flown betweenlunar orbit and the Moon's surface during theUnited States'Apollo program. It was the first crewed spacecraft to operate exclusively in the airless vacuum of space, and remains the only crewed vehicle to land anywhere beyond Earth.
• Lunar space elevator – orlunar spacelift, is a proposed transportation system for moving a mechanical climbing vehicle up and down a ribbon-shaped tethered cable that is set between the surface of theMoon "at the bottom" and a docking port suspended tens of thousands of kilometers above in space at the top.

• Mach number – Influid dynamics, the Mach number is adimensionless quantity representing the ratio offlow velocity past aboundary to the localspeed of sound.^[148][149]
• Magnetic sail – ormagsail, is a proposed method ofspacecraft propulsion which would use a static magnetic field to deflectcharged particles radiated by theSun as aplasma wind, and thus impart momentum to accelerate the spacecraft.^[150][151] A magnetic sail could also thrust directly against planetary and solarmagnetospheres.
• Magnetoplasmadynamic thruster – A magnetoplasmadynamic (MPD) thruster (MPDT) is a form ofelectrically powered spacecraft propulsion which uses theLorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet.
• Mass – is both aproperty of aphysical body and ameasure of itsresistance toacceleration (rate of change ofvelocity with respect to time) when anet force is applied.^[152] An object's mass also determines thestrength of itsgravitational attraction to other bodies. TheSI base unit of mass is thekilogram (kg). Inphysics, mass is not the same asweight, even though mass is often determined by measuring the object's weight using aspring scale, rather thanbalance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
• Mass driver – orelectromagnetic catapult, is a proposed method ofnon-rocket spacelaunch which would use alinear motor toaccelerate and catapultpayloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to makeelectromagnets. Sequential firing of a row of electromagnets accelerates the payload along a path. After leaving the path, the payload continues to move due tomomentum.
• Mechanics of fluids –
• Membrane mirror –
• Metre per second –
• Mini-magnetospheric plasma propulsion –
• Moment of inertia – otherwise known as themass moment of inertia,angular mass,second moment of mass, or most accurately,rotational inertia, of arigid body is a quantity that determines thetorque needed for a desiredangular acceleration about a rotational axis, akin to howmass determines theforce needed for a desiredacceleration. It depends on the body's mass distribution and the axis chosen, with larger moments requiring more torque to change the body's rate of rotation.
• Momentum – InNewtonian mechanics,linear momentum,translational momentum, or simplymomentum is the product of themass andvelocity of an object. It is avector quantity, possessing a magnitude and a direction. Ifm is an object's mass andv is its velocity (also a vector quantity), then the object's momentump is

${\displaystyle \mathbf{p}=m\mathbf{v}.}$

In theInternational System of Units (SI), theunit of measurement of momentum is thekilogrammetre per second (kg⋅m/s), which is equivalent to thenewton-second.

• Momentum wheel –
• Monopropellant rocket – ormonochemical rocket, is arocket that uses a singlechemical as itspropellant.
• Motion – Inphysics, motion is the phenomenon in which an object changes itsposition. Motion is mathematically described in terms ofdisplacement,distance,velocity,acceleration,speed, andtime. The motion of a body is observed by attaching aframe of reference to an observer and measuring the change in position of the body relative to that frame with change in time. The branch of physics describing the motion of objects without reference to its cause iskinematics; the branch studying forces and their effect on motion isdynamics.
• Multistage rocket – orstep rocket^[153] is alaunch vehicle that uses two or morerocketstages, each of which contains its ownengines andpropellant. Atandem orserial stage is mounted on top of another stage; aparallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Two-stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched.

• NACA –United States National Advisory Committee for Aeronautics, replaced byNASA in 1958.
• NASA –United States National Aeronautics and Space Administration.
• Navier–Stokes equations – Inphysics, the Navier–Stokes equations(/nævˈjeɪstoʊks/) are certainpartial differential equations which describe the motion ofviscous fluid substances, named after French engineer and physicistClaude-Louis Navier and Anglo-Irish physicist and mathematicianGeorge Gabriel Stokes. They were developed over several decades of progressively building the theories, from 1822 (Navier) to 1842–1850 (Stokes).

The Navier–Stokes equations mathematically expressconservation of momentum andconservation of mass forNewtonian fluids. They are sometimes accompanied by anequation of state relatingpressure,temperature anddensity.^[154] They arise from applyingIsaac Newton's second law tofluid motion, together with the assumption that thestress in the fluid is the sum of adiffusingviscous term (proportional to thegradient of velocity) and apressure term—hence describingviscous flow. The difference between them and the closely relatedEuler equations is that Navier–Stokes equations take viscosity into account while the Euler equations model onlyinviscid flow. As a result, the Navier–Stokes are aparabolic equation and therefore have better analytic properties, at the expense of having less mathematical structure (e.g. they are nevercompletely integrable).

• Newton (unit) – The newton (symbol:N) is theInternational System of Units (SI)derived unit offorce. It is named afterIsaac Newton in recognition of his work onclassical mechanics, specificallyNewton's second law of motion.

A newton is defined as 1 kg⋅m/s², which is the force which gives a mass of 1 kilogram an acceleration of 1 metre per second, per second.

• Newton's law of universal gravitation – is usually stated as that everyparticle attracts every other particle in the universe with aforce that isdirectly proportional to the product of their masses andinversely proportional to the square of the distance between their centers.^{[note 1]} The publication of the theory has become known as the "first great unification", as it marked the unification of the previously described phenomena of gravity on Earth with known astronomical behaviors.^{[155][156][157]}

This is a generalphysical law derived fromempirical observations by whatIsaac Newton calledinductive reasoning.^[158] It is a part ofclassical mechanics and was formulated in Newton's workPhilosophiæ Naturalis Principia Mathematica ("thePrincipia"), first published on 5 July 1687. When Newton presented Book 1 of the unpublished text in April 1686 to theRoyal Society,Robert Hooke made a claim that Newton had obtained the inverse square law from him.

In today's language, the law states that everypointmass attracts every other point mass by aforce acting along theline intersecting the two points. The force isproportional to theproduct of the two masses, and inversely proportional to thesquare of the distance between them.^[159]

The equation for universal gravitation thus takes the form:

${\displaystyle F=G\frac{m_1{m}_2}{r^2},}$

whereF is the gravitational force acting between two objects,m₁ andm₂ are the masses of the objects,r is the distance between thecenters of their masses, andG is thegravitational constant.

• Newton's laws of motion – are threelaws ofclassical mechanics that describe the relationship between themotion of an object and theforces acting on it. These laws can be paraphrased as follows:^[160]

Law 1. A body continues in its state of rest, or in uniform motion in a straight line, unless acted upon by a force.

Law 2. A body acted upon by a force moves in such a manner that the time rate of change ofmomentum equals the force.

Law 3. If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction.

The three laws of motion were first stated byIsaac Newton in hisPhilosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687.^[161] Newton used them to explain and investigate the motion of many physical objects and systems, which laid the foundation for Newtonian mechanics.^[162]

• Nose cone design – Given the problem of theaerodynamicdesign of thenose cone section of any vehicle or body meant to travel through acompressible fluid medium (such as arocket oraircraft,missile orbullet), an important problem is the determination of thenose cone geometrical shape for optimum performance. For many applications, such a task requires the definition of asolid of revolution shape that experiences minimal resistance to rapid motion through such a fluid medium.
• Nozzle – is a device designed to control the direction or characteristics of afluid flow (especially to increase velocity) as it exits (or enters) an enclosed chamber orpipe. A nozzle is often a pipe or tube of varying cross-sectional area, and it can be used to direct or modify the flow of a fluid (liquid orgas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them. In a nozzle, the velocity of fluid increases at the expense of its pressure energy.

• Orbit – Inphysics, an orbit is thegravitationally curvedtrajectory of anobject,^[163] such as the trajectory of aplanet around a star or anatural satellite around a planet. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites followelliptic orbits, with thecenter of mass being orbited at a focal point of the ellipse,^[164] as described byKepler's laws of planetary motion. For most situations, orbital motion is adequately approximated byNewtonian mechanics, which explainsgravity as a force obeying aninverse-square law.^[165] However,Albert Einstein'sgeneral theory of relativity, which accounts for gravity as due to curvature ofspacetime, with orbits followinggeodesics, provides a more accurate calculation and understanding of the exact mechanics of orbital motion.
• Orbit phasing – In astrodynamics, orbit phasing is the adjustment of the time-position of spacecraft along its orbit, usually described as adjusting the orbiting spacecraft's true anomaly.^[166] Orbital phasing is primarily used in scenarios where a spacecraft in a given orbit must be moved to a different location within the same orbit. The change in position within the orbit is usually defined as the phase angle, ϕ, and is the change in true anomaly required between the spacecraft's current position to the final position.
• Orbital eccentricity – Inastrodynamics, the orbital eccentricity of anastronomical object is adimensionless parameter that determines the amount by which itsorbit around another body deviates from a perfectcircle. A value of 0 is acircular orbit, values between 0 and 1 form anelliptic orbit, 1 is aparabolicescape orbit, and greater than 1 is ahyperbola. The term derives its name from the parameters ofconic sections, as everyKepler orbit is a conic section. It is normally used for the isolatedtwo-body problem, but extensions exist for objects following a rosette orbit through the galaxy.
• Orbital elements – are theparameters required to uniquely identify a specificorbit. Incelestial mechanics these elements are considered intwo-body systems using aKepler orbit. There are many different ways to mathematically describe the same orbit, but certain schemes, each consisting of a set of six parameters, are commonly used inastronomy andorbital mechanics. A real orbit and its elements change over time due to gravitationalperturbations by other objects and the effects ofgeneral relativity. A Kepler orbit is an idealized, mathematical approximation of the orbit at a particular time.
• Orbital inclination – measures the tilt of an object'sorbit around a celestial body. It is expressed as theangle between areference plane and theorbital plane oraxis of direction of the orbiting object.
• Orbital inclination change – is anorbital maneuver aimed at changing theinclination of an orbiting body'sorbit. This maneuver is also known as anorbital plane change as the plane of the orbit is tipped. This maneuver requires a change in the orbital velocity vector (delta v) at theorbital nodes (i.e. the point where the initial and desired orbits intersect, the line of orbital nodes is defined by the intersection of the two orbital planes).
• Orbital maneuver – Inspaceflight, an orbital maneuver (otherwise known as aburn) is the use ofpropulsion systems to change theorbit of aspacecraft.
• Orbital mechanics – orastrodynamics, is the application ofballistics andcelestial mechanics to the practical problems concerning the motion ofrockets and otherspacecraft. The motion of these objects is usually calculated fromNewton's laws of motion andlaw of universal gravitation. Orbital mechanics is a core discipline withinspace-mission design and control.
• Orbital node – is either of the two points where anorbitintersects aplane of reference to which it is inclined.^[167] Anon-inclined orbit, which iscontained in the reference plane, has no nodes.
• Orbital period – (alsorevolution period), is the time a givenastronomical object takes to complete oneorbit around another object, and applies inastronomy usually toplanets orasteroids orbiting theSun,moons orbiting planets,exoplanets orbiting otherstars, orbinary stars.
• Orbital station-keeping – Inastrodynamics, orbital station-keeping is keeping aspacecraft at a fixed distance from another spacecraft. It requires a series oforbital maneuvers made withthruster burns to keep the active craft in the same orbit as its target. For manylow Earth orbit satellites, the effects ofnon-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of ahomogeneous sphere, gravitational forces from Sun/Moon,solar radiation pressure and airdrag, must be counteracted.
• Orbiter Boom Sensor System – (OBSS), was a 50-foot (15.24 m) boom carried on board NASA's Space Shuttles. The boom was grappled by the Canadarm and served as an extension of the arm, doubling its length to a combined total of 100 feet (30 m).[1] At the far end of the boom was an instrumentation package of cameras and lasers used to scan the leading edges of the wings, the nose cap, and the crew compartment after each lift-off and before each landing. If flight engineers suspected potential damage to other areas, as evidenced in imagery captured during lift-off or the rendezvous pitch maneuver, then additional regions could be scanned.
• Osculating orbit – Inastronomy, and in particular inastrodynamics, the osculating orbit of an object in space at a given moment in time is the gravitationalKepler orbit (i.e. anelliptic or other conic one) that it would have around itscentral body ifperturbations were absent.^[168] That is, it is the orbit that coincides with the currentorbital state vectors (position andvelocity).

• Parallel axis theorem – also known asHuygens–Steiner theorem, or just asSteiner's theorem,^[169] named afterChristiaan Huygens andJakob Steiner, can be used to determine themoment of inertia or thesecond moment of area of arigid body about any axis, given the body's moment of inertia about aparallel axis through the object'scenter of gravity and theperpendiculardistance between the axes.
• Parasitic drag – also known asprofile drag,^{[170]: 254 [171]: 256} is a type ofaerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is a combination of form drag andskin friction drag.^{[172][170]: 641–642} It affects all objects regardless of whether they are capable of generatinglift. Total drag on an aircraft is made up of parasitic drag andlift-induced drag. Parasitic drag is so named because it is not useful, whereas lift-induced drag is the result of anairfoil generating lift. Parasitic drag comprises all types of drag except lift-induced drag.^[173]
• Perpendicular axes theorem – states that themoment of inertia of aplanar lamina (i.e. 2-D body) about an axis perpendicular to theplane of the lamina is equal to the sum of the moments of inertia of the lamina about the two axes at right angles to each other, in its own plane intersecting each other at the point where the perpendicular axis passes through it.

Define perpendicular axes${\displaystyle x}$,${\displaystyle y}$, and${\displaystyle z}$ (which meet at origin${\displaystyle O}$) so that the body lies in the${\displaystyle xy}$ plane, and the${\displaystyle z}$ axis is perpendicular to the plane of the body. LetI_x,I_y andI_z be moments of inertia about axisx,y,z respectively. Then the perpendicular axis theorem states that^[174]

${\displaystyle I_z=I_x+I_y}$

This rule can be applied with theparallel axis theorem and thestretch rule to find polar moments of inertia for a variety of shapes.

If a planar object (or prism, by thestretch rule) has rotational symmetry such that${\displaystyle I_x}$ and${\displaystyle I_y}$ are equal,^[175]

then the perpendicular axes theorem provides the useful relationship:

${\displaystyle I_z=2I_x=2I_y}$

• Pitch Angle –
• Plasma (physics) – (from Ancient Greek πλάσμα 'moldable substance')^[176] is one of thefour fundamental states of matter. It consists of a gas ofions – atoms or molecules which have at least one orbitalelectron stripped (or an extra electron attached) and, thus, an electric charge. It is the most abundant form ofordinary matter in theuniverse,^[177] being mostly associated withstars,^[178] including theSun.^[179][180] It extends to the rarefiedintracluster medium and possibly tointergalactic regions.^[181]
• Plug nozzle – is a type ofnozzle which includes a centerbody or plug around which the working fluid flows. Plug nozzles have applications in aircraft, rockets, and numerous other fluid flow devices.
• Pogo oscillation –
• Prandtl–Glauert singularity –
• Precession –
• Pressure –
• Pressure altitude –
• Pressure-fed engine –
• Propeller –
• Proper orbital elements –
• Pulsed inductive thruster –
• Pulsed plasma thruster –
• Propulsion –

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• Radar – system using the reflection from transmittedelectromagnetic waves to detect the distance and rough shape of an object, working even inouter space, unlikesonar
• Radio direction finder –
• Railgun –
• Ram accelerator –
• Ramjet –
• Rate of climb –
• RCS (Reaction control system) – set ofrocket thrusters used forspacecraft maneuvers over the craft's threerotation axes inouter space
• Reentry –
• Reflection –
• Relativistic rocket –
• Remote Manipulator System –
• Resistojet rocket –
• Reusable launch system –
• Reynolds number –
• RL-10 (rocket engine) –
• Rocket –
• Rocket engine –
• Rocket engine nozzle –
• Rocket fuel –
• Rocket launch –
• Rogallo wing – is a flexible type ofwing. In 1948,Francis Rogallo, aNASA engineer, and his wifeGertrude Rogallo, invented a self-inflating flexible wing they called theParawing, also known after them as the "Rogallo Wing" andflexible wing.^[182] NASA considered Rogallo's flexible wing as an alternative recovery system for theMercury andGeminispace capsules, and for possible use in other spacecraft landings, but the idea was dropped from Gemini in 1964 in favor ofconventional parachutes.
• Rudder –

• SABRE –
• Satellite –
• Saturn (rocket family) –
• Scalar (physics) – A quantity with magnitude but no direction.
• Schlieren –
• Schlieren photography –
• Scramjet –
• Second moment of area –
• Shock wave –
• SI –
• Single point of failure –
• Single-stage-to-orbit –spacecraft able to fly from acelestial body (usually theEarth or theMoon)'s surface to its orbit without using external boosters
• Skyhook (structure) –
• Slew –
• Stream function –
• Streamline –
• Solar panel –
• Solar sail –
• Solar thermal rocket –
• Solid of revolution –
• Solid rocket –
• Sound barrier –
• Space activity suit –
• Space elevator –
• Space fountain –
• Space Shuttle – crewedNASAspacecraft used between 1981 and 2011, consisting of a reusablespaceplane (theSpace Shuttle orbiter, capable ofairplane-like landing) attached to an expendableexternal tank (which disintegrated duringre-entry) and two recoverablesolid rocket boosters (which re-entered theEarth's atmosphere and splash-landed)
• Space Shuttle external tank – external tank attached to theorbiter and thesolid rocket boosters in theNASASpace Shuttle program
• Space Shuttle main engine –
• Space Shuttle orbiter – reusableNASAVTHLspaceplane used during theSpace Shuttle program (1981–2011)
• Space station – habitableartificial satellite
• Space suit –
• Space technology –
• Space transport –
• Spacecraft –
• Spacecraft design –
• Spacecraft propulsion –
• Spaceplane – vehicle capable of both atmospheric flight according to the laws ofaerodynamics (like anaircraft) andspaceflight inouter space (like aspacecraft)
• Special relativity –
• Specific impulse –
• Speed of sound –
• SRB –solid rocket booster
• SSTO –single-stage-to-orbit
• Staged combustion cycle (rocket) –
• Subsonic – inferior to thespeed of sound
• Supersonic – superior to thespeed of sound
• Surface of revolution –
• Sweep theory –

• Tait–Bryan rotations –
• Temperature –
• Terminal velocity – is the maximum velocity (speed) attainable by an object as it falls through afluid (air is the most common example). It occurs when the sum of thedrag force (F_d) and thebuoyancy is equal to the downward force ofgravity (F_G) acting on the object. Since thenet force on the object is zero, the object has zeroacceleration.^[183]
• Test target –
• Tether propulsion –
• Thermal protection system –
• Thermodynamics –
• Thrust –
• Thruster –
• Torricelli's equation – In physics, Torricelli's equation, or Torricelli's formula, is an equation created byEvangelista Torricelli to find the finalvelocity of an object moving with aconstant acceleration along an axis (for example, the x axis) without having a known time interval.

The equation itself is:^[184]

${\displaystyle v_f^2=v_i^2+2a\mathrm{\Delta }x}$

where

• ${\displaystyle v_f}$ is the object's finalvelocity along the x axis on which the acceleration is constant.
• ${\displaystyle v_i}$ is the object's initial velocity along the x axis.
• ${\displaystyle a}$ is the object'sacceleration along the x axis, which is given as a constant.
• ${\displaystyle \mathrm{\Delta }x}$ is the object's change in position along the x axis, also calleddisplacement.

This equation is valid along any axis on which the acceleration is constant.

• Total air temperature – In aviation,stagnation temperature is known as total air temperature and is measured by atemperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest,kinetic energy is converted tointernal energy. The air is compressed and experiences anadiabatic increase in temperature. Therefore, totalair temperature is higher than the static (or ambient) air temperature. Total air temperature is an essential input to anair data computer in order to enable the computation of static air temperature and hencetrue airspeed.
• Trajectory – or flight path, is the path that anobject withmass inmotion follows throughspace as a function of time. Inclassical mechanics, a trajectory is defined byHamiltonian mechanics viacanonical coordinates; hence, a complete trajectory is defined by position andmomentum, simultaneously. The mass might be aprojectile or asatellite.^[185] For example, it can be anorbit — the path of aplanet,asteroid, orcomet as it travels around acentral mass.
• Trailing edge –
• Trans Lunar Injection –
• Transonic –
• Transverse wave –
• Tripropellant rocket –
• Tsiolkovsky rocket equation –
• Turbomachinery –
• Two-stage-to-orbit –

• UFO – An unidentified flying object is any perceived aerial phenomenon that cannot be immediately identified or explained. On investigation, most UFOs areidentified as known objects or atmospheric phenomena, while a small number remain unexplained.

• V-2 rocket – TheV-2 (German:Vergeltungswaffe 2, "Retribution Weapon 2"), with the technical nameAggregat 4 (A4), was the world's first long-range^[186]guidedballistic missile. The missile, powered by aliquid-propellant rocket engine, was developed during theSecond World War inGermany as a "vengeance weapon" and assigned to attack Allied cities as retaliation for theAllied bombings against German cities. TheV-2 rocket also became the first artificial object to travel into space by crossing theKármán line with the vertical launch ofMW 18014 on 20 June 1944.^[187]
• Variable specific impulse magnetoplasma rocket – (VASIMR), is an electrothermal thruster under development for possible use inspacecraft propulsion. It usesradio waves toionize and heat aninertpropellant, forming a plasma, then amagnetic field to confine and accelerate the expandingplasma, generatingthrust. It is aplasma propulsion engine, one of several types of spacecraftelectric propulsion systems.^[188]
• Velocity – The velocity of an object is therate of change of itsposition with respect to aframe of reference, and is a function of time. Velocity is equivalent to a specification of an object'sspeed and direction ofmotion (e.g.60 km/h to the north). Velocity is a fundamental concept inkinematics, the branch ofclassical mechanics that describes the motion of bodies.

Velocity is a physicalvectorquantity; both magnitude and direction are needed to define it. Thescalarabsolute value (magnitude) of velocity is calledspeed, being a coherent derived unit whose quantity is measured in theSI (metric system) asmetres per second (m/s or m⋅s⁻¹). For example, "5 metres per second" is a scalar, whereas "5 metres per second east" is a vector. If there is a change in speed, direction or both, then the object is said to be undergoing anacceleration.

• Viscometer – (also calledviscosimeter) is an instrument used to measure theviscosity of afluid. For liquids with viscosities which vary withflow conditions, an instrument called arheometer is used. Thus, a rheometer can be considered as a special type of viscometer.^[189] Viscometers only measure under one flow condition.
• Viscosity – The viscosity of afluid is a measure of itsresistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example,syrup has a higher viscosity thanwater.^[190]
• Vortex generator – (VG), is anaerodynamic device, consisting of a smallvane usually attached to a lifting surface (orairfoil, such as anaircraft wing)^[191] or a rotor blade of awind turbine.^[192] VGs may also be attached to some part of an aerodynamic vehicle such as an aircraft fuselage or a car. When the airfoil or the body is in motion relative to the air, the VG creates avortex,^[191][193] which, by removing some part of the slow-moving boundary layer in contact with the airfoil surface, delays localflow separation andaerodynamic stalling, thereby improving the effectiveness of wings andcontrol surfaces, such asflaps,elevators,ailerons, andrudders.^[193]

• Wave drag – Inaeronautics, wave drag is a component of theaerodynamic drag on aircraft wings and fuselage, propeller blade tips andprojectiles moving attransonic andsupersonic speeds, due to the presence ofshock waves.^[194] Wave drag is independent ofviscous effects,^[195] and tends to present itself as a sudden and dramatic increase in drag as the vehicle increases speed to theCritical Mach number. It is the sudden and dramatic rise of wave drag that leads to the concept of asound barrier.
• Weight – Inscience andengineering, theweight of an object is theforce acting on the object due togravity.^{[196][197][198]}
• Weight function – is a mathematical device used when performing a sum, integral, or average to give some elements more "weight" or influence on the result than other elements in the same set. The result of this application of a weight function is aweighted sum orweighted average. Weight functions occur frequently instatistics andanalysis, and are closely related to the concept of ameasure. Weight functions can be employed in both discrete and continuous settings. They can be used to construct systems of calculus called "weighted calculus"^[199] and "meta-calculus".^[200]
• Wind tunnels – are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft will fly.NASA uses wind tunnels to testscale models of aircraft and spacecraft. Some wind tunnels are large enough to contain full-size versions of vehicles. The wind tunnel moves air around an object, making it seem as if the object is flying.
• Wing – is a type offin that produceslift while moving through air or some otherfluid. Accordingly, wings havestreamlinedcross-sections that are subject toaerodynamic forces and act asairfoils. A wing'saerodynamic efficiency is expressed as itslift-to-drag ratio. The lift a wing generates at a given speed andangle of attack can be one to twoorders of magnitude greater than the totaldrag on the wing. A high lift-to-drag ratio requires a significantly smallerthrust to propel the wings through the air at sufficient lift.
• Wright Flyer – TheWright Flyer (theKitty Hawk,^[201][202] also known asFlyer I or1903Flyer) made the first sustained flight by a mannedheavier-than-air powered and controlled aircraft—anairplane—on 17 December 1903.^[203] Invented and flown byOrville and Wilbur Wright, it marked the beginning of the"pioneer era" of aviation.
• Wright Glider – TheWright brothers designed, built and flew a series of three mannedgliders in 1900–1902 as they worked towards achieving poweredflight. They also made preliminary tests with akite in 1899. In 1911 Orville conducted tests with a much more sophisticated glider. Neither the kite nor any of the gliders were preserved, but replicas of all have been built.

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• Aerospace engineering
• List of aviation, aerospace and aeronautical abbreviations
• Engineering
• Glossary of engineering
• National Council of Examiners for Engineering and Surveying (NCEES)
• Fundamentals of Engineering Examination
• Principles and Practice of Engineering Examination (PE exam)
• Graduate Aptitude Test in Engineering (GATE)
• Glossary of areas of mathematics
• Glossary of artificial intelligence
• Glossary of astronomy
• Glossary of biology
• Glossary of chemistry
• Glossary of civil engineering
• Glossary of economics
• Glossary of mechanical engineering
• Glossary of physics
• Glossary of probability and statistics
• Glossary of structural engineering