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Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites.In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.
Several methods of pragmatic spacecraft propulsion have been developed, each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (oftenmonopropellant rockets) orresistojet rockets for orbital station-keeping, while a few usemomentum wheels for attitude control. Russian and antecedent Soviet bloc satellites have usedelectric propulsion for decades,[1] and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used electric propulsion such asion thrusters andHall-effect thrusters. Various technologies need to support everything from small satellites and robotic deep space exploration to space stations and human missions to Mars.
Hypothetical in-space propulsion technologies describe propulsion technologies that could meet future space science and exploration needs. These propulsion technologies are intended to provide effective exploration of the Solar System and may permit mission designers to plan missions to "fly anytime, anywhere, and complete a host of science objectives at the destinations" and with greater reliability and safety. With a wide range of possible missions and candidate propulsion technologies, the question of which technologies are "best" for future missions is a difficult one; expert opinion now holds that a portfolio of propulsion technologies should be developed to provide optimum solutions for a diverse set of missions and destinations.[2][3][4]
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The primary goals ofSpace exploration are to reach the destination safely, quickly, with a large quantity ofpayload mass, and relatively inexpensively. The act of reaching the destination requires an in-space propulsion system, and the other metrics are modifiers to this fundamental action.[5][4] Propulsion technologies can significantly improve a number of critical aspects of the mission.
When launching a spacecraft from Earth, a propulsion method must overcome a highergravitational pull to provide a positive net acceleration.[6] When in space, the purpose of apropulsion system is to change the velocity, orv, of a spacecraft, in order to establish the desired trajectory.[7]
In-space propulsion begins where theupper stage of thelaunch vehicle leaves off, performing the functions ofprimary propulsion,reaction control,station keeping,precision pointing, andorbital maneuvering. Themain engines used inspace provide the primary propulsive force fororbit transfer,planetary trajectories, and extraplanetary landing andascent. The reaction control and orbital maneuvering systems provide the propulsive force for orbit maintenance, position control, station keeping, and spacecraft attitude control.[5][3][4]
In orbit, any additionalimpulse, even tiny, will result in a change in the orbit path, in two ways:[8]
Earth's surface is situated fairly deep in agravity well; theescape velocity required to leave its orbit is 11.2 kilometers/second.[11] Thus for destinations beyond, propulsion systems need enough propellant and to be of high enough efficiency. The same is true for other planets and moons, albeit some have lower gravity wells.
As human beings evolved in a gravitational field of "oneg" (9.81m/s²), it would be most comfortable for a human spaceflight propulsion system to provide that acceleration continuously,[according to whom?] (though human bodies can tolerate much larger accelerations over short periods).[12] The occupants of a rocket or spaceship having such a propulsion system would be free from the ill effects offree fall, such as nausea, muscular weakness, reduced sense of taste, orleaching of calcium from their bones.[13][14]
TheTsiolkovsky rocket equation shows, using the law ofconservation of momentum, that for arocket engine propulsion method to change the momentum of a spacecraft, it must change the momentum of something else in the opposite direction. In other words, the rocket must exhaust mass opposite the spacecraft's acceleration direction, with such exhausted mass calledpropellant orreaction mass.[15]: Sec 1.2.1 [16] For this to happen, both reaction mass and energy are needed. The impulse provided by launching a particle of reaction mass with massm at velocityv ismv. But this particle has kinetic energymv²/2, which must come from somewhere. In a conventionalsolid,liquid, orhybrid rocket, fuel is burned, providing the energy, and the reaction products are allowed to flow out of theengine nozzle, providing the reaction mass. In anion thruster, electricity is used to accelerate ions behind the spacecraft. Here other sources must provide the electrical energy (e.g. asolar panel or anuclear reactor), whereas the ions provide the reaction mass.[6]
The rate of change ofvelocity is calledacceleration and the rate of change ofmomentum is calledforce.[17] To reach a given velocity, one can apply a small acceleration over a long period of time, or a large acceleration over a short time; similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations for a long time can often produce the same impulse as another which produces large accelerations for a short time.[18] However, when launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.[19]
Some designs however, operatewithout internal reaction mass by taking advantage of magnetic fields or light pressure to change the spacecraft's momentum.
When discussing the efficiency of a propulsion system, designers often focus on the effective use of the reaction mass, which must be carried along with the rocket and is irretrievably consumed when used.[20] Spacecraft performance can be quantified inamount of change in momentum per unit of propellant consumed, also calledspecific impulse. This is a measure of the amount ofimpulse that can be obtained from a fixed amount of reaction mass. The higher the specific impulse, the better the efficiency.Ion propulsion engines have high specific impulse (~3000 s) and low thrust[21] whereas chemical rockets likemonopropellant orbipropellant rocket engines have a low specific impulse (~300 s) but high thrust.[22]
The impulse per unit weight-on-Earth (typically designated by) has units of seconds.[18] Because the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass, with the same units as velocity (e.g., meters per second).[23] This measure is equivalent to theeffective exhaust velocity of the engine, and is typically designated.[24] Either the change in momentum per unit of propellant used by a spacecraft, or the velocity of the propellant exiting the spacecraft, can be used to measure its "specific impulse." The two values differ by a factor of thestandard acceleration due to gravity,gn, 9.80665 m/s² ().[25]
In contrast to chemical rockets, electrodynamic rockets useelectric or magnetic fields to accelerate a charged propellant. The benefit of this method is that it can achieve exhaust velocities, and therefore, more than 10 times greater than those of a chemical engine, producing steady thrust with far less fuel. With a conventional chemical propulsion system, 2% of a rocket's total mass might make it to the destination, with the other 98% having been consumed as fuel. With an electric propulsion system, 70% of what's aboard in low Earth orbit can make it to a deep-space destination.[26]
However, there is a trade-off. Chemical rockets transform propellants into most of the energy needed to propel them, but their electromagnetic equivalents must carry or produce the power required to create and accelerate propellants. Because there are currently practical limits on the amount of power available on a spacecraft, these engines are not suitable for launch vehicles or when a spacecraft needs a quick, largeimpulse, such as when it brakes to enter a capture orbit. Even so, becauseelectrodynamic rockets offer very high, mission planners areincreasingly willing to sacrifice power and thrust (and the extra time it willtake to get a spacecraft where it needs to go) in order to save large amountsof propellant mass.[25]
Spacecraft operate in many areas of space. These include orbital maneuvering, interplanetary travel, and interstellar travel.
Artificial satellites are firstlaunched into the desired altitude by conventional liquid/solid propelled rockets, after which the satellite may use onboard propulsion systems for orbital stationkeeping. Once in the desired orbit, they often need some form ofattitude control so that they are correctly pointed with respect to theEarth, theSun, and possibly someastronomical object of interest.[27] They are also subject todrag from the thinatmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital station-keeping).[28] Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.[29] A satellite's useful life is usually over once it has exhausted its ability to adjust its orbit.[30]
Forinterplanetary travel, a spacecraft can use its engines to leave Earth's orbit. It is not explicitly necessary as the initial boost given by the rocket, gravity slingshot, monopropellant/bipropellent attitude control propulsion system are enough for the exploration of the solar system (seeNew Horizons). Once it has done so, it must make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.[31] In between these adjustments, the spacecraft typically moves along its trajectory without accelerating. The most fuel-efficient means to move from one circular orbit to another is with aHohmann transfer orbit: the spacecraft begins in a roughly circular orbit around the Sun. A short period ofthrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.[32] Special methods such asaerobraking or aerocapture are sometimes used for this final orbital adjustment.[33]
Some spacecraft propulsion methods such assolar sails provide very low but inexhaustible thrust;[34] an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun, or constantly thrusting along its direction of motion to increase its distance from the Sun.[citation needed] The concept has been successfully tested by the JapaneseIKAROS solar sail spacecraft.[35]
Because interstellar distances are great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival remains a formidable challenge for spacecraft designers.[36] No spacecraft capable of short duration (compared to human lifetime)interstellar travel has yet been built, but many hypothetical designs have been discussed.
Spacecraft propulsion technology can be of several types, such as chemical, electric or nuclear. They are distinguished based on the physics of the propulsion system and how thrust is generated. Other experimental and more theoretical types are also included, depending on their technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications.[37]
Almost all types arereaction engines, which producethrust by expellingreaction mass, in accordance withNewton's third law of motion.[38][39][40] Examples includejet engines,rocket engines,pump-jet, and more uncommon variations such asHall–effect thrusters,ion drives,mass drivers, andnuclear pulse propulsion.[41]
A large fraction ofrocket engines in use today arechemical rockets; that is, they obtain the energy needed to generate thrust bychemical reactions to create a hot gas that is expanded to producethrust.[42] Many different propellant combinations are used to obtain these chemical reactions, including, for example,hydrazine,liquid oxygen,liquid hydrogen,nitrous oxide, andhydrogen peroxide.[43] They can be used as amonopropellant or inbi-propellant configurations.[44]
Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.[25] Most rocket engines areinternal combustionheat engines (although non-combusting forms exist).[45] Rocket engines generally produce a high-temperature reaction mass, as a hot gas, which is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber.[46] The extremely hot gas is then allowed to escape through a high-expansion ratio bell-shapednozzle, a feature that gives a rocket engine its characteristic shape.[45] The effect of the nozzle is to accelerate the mass, converting most of the thermal energy into kinetic energy,[45] where exhaust speeds reaching as high as 10 times the speed of sound at sea level are common.[citation needed]
The dominant form of chemical propulsion forsatellites has historically beenhydrazine, however, this fuel is highly toxic and at risk of being banned across Europe.[47] Non-toxic 'green' alternatives are now being developed to replace hydrazine.Nitrous oxide-based alternatives are garnering traction and government support,[48][49] with development being led by commercial companies Dawn Aerospace, Impulse Space,[50] and Launcher.[51] The first nitrous oxide-based system flown in space was by D-Orbit onboard their ION Satellite Carrier (space tug) in 2021, using sixDawn Aerospace B20 thrusters, launched upon aSpaceXFalcon 9 rocket.[52][53]
Rather than relying on high temperature andfluid dynamics to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic orelectromagnetic forces to accelerate the reaction mass directly, where the reaction mass is usually a stream ofions.[citation needed]
Ion propulsion rockets typically heat a plasma or charged gas inside amagnetic bottle and release it via amagnetic nozzle so that no solid matter needs to come in contact with the plasma.[54] Such an engine uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities.[55] For these drives, at the highest exhaust speeds, energetic efficiency and thrust are all inversely proportional to exhaust velocity.[citation needed] Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel.[citation needed]
Electric propulsion is commonly used for station keeping on commercialcommunications satellites and for prime propulsion on somescientific space missions because of their high specific impulse.[56] However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.[5][57][58][59]
The idea of electric propulsion dates to 1906, whenRobert Goddard considered the possibility in his personal notebook.[60]Konstantin Tsiolkovsky published the idea in 1911.[61]
Electric propulsion methods include:[62]
For some missions, particularly reasonably close to the Sun,solar energy may be sufficient, and has often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are callednuclear electric rockets.[64]
Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun.[citation needed] Chemical power generators are not used due to the far lower total available energy.[65] Beamed power to the spacecraft is considered to have potential, according to NASA and theUniversity of Colorado Boulder.[66][67]
With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits the amount of thrust that can be produced to a small value.[citation needed] Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle.[68]
Nuclear fuels typically have very highspecific energy, much higher than chemical fuels, which means that they can generate large amounts of energy per unit mass. This makes them valuable in spaceflight, as it can enable highspecific impulses, sometimes even at high thrusts. The machinery to do this is complex, but research has developed methods for their use in propulsion systems, and some have been tested in a laboratory.[69]
Here, nuclear propulsion moreso refers to the source of propulsion being nuclear, instead of anuclear electric rocket where anuclear reactor would provide power (instead of solar panels) for other types of electrical propulsion.
Nuclear propulsion methods include:
There are several different space drives that need little or no reaction mass to function.Field propulsion refers to propulsion systems in which thrust arises from interactions with externalfields or ambientmedia, rather than from the sustained expulsion of onboardreaction mass or reliance on solidchemical fuels.
Thelaw of conservation ofmomentum is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible).[citation needed] But space is not empty, especially space inside the Solar System; there are gravitation fields,magnetic fields,electromagnetic waves,solar wind and solar radiation.[70] Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux densityP of an EM wave is quantitatively 1/c2 times thePoynting vectorS, i.e.P =S/c2, where c is the velocity of light.[citation needed]
The concept ofsolar sails rely onradiation pressure from electromagnetic energy, but they require a large collection surface to function effectively.[71]E-sails propose to use very thin and lightweight wires holding an electric charge to deflect particles, which may have more controllable directionality.[citation needed]
Magnetic sails deflect charged particles from thesolar wind with a magnetic field, thereby imparting momentum to the spacecraft.[72] For instance, the so-calledMagsail is a large superconducting loop proposed for acceleration/deceleration in thesolar wind and deceleration in theInterstellar medium.[73] A variant is themini-magnetospheric plasma propulsion system[74] and its successor, themagnetoplasma sail,[75] which inject plasma at a low rate to enhance the magnetic field to more effectively deflect charged particles in a plasma wind.
Japan launched a solar sail-powered spacecraft,IKAROS in May 2010, which successfully demonstrated propulsion and guidance (and is still active as of this date).[when?][citation needed] As further proof of thesolar sail concept,NanoSail-D became the first such powered satellite to orbitEarth.[76] As of August 2017, NASA confirmed the Sunjammer solar sail project was concluded in 2014 with lessons learned for future space sail projects.[77] The U.K.Cubesail programme will be the first mission to demonstrate solar sailing in low Earth orbit, and the first mission to demonstrate full three-axis attitude control of a solar sail.[78]
The concept of agravitational slingshot is a form of propulsion to carry aspace probe onward to other destinations without the expense of reaction mass; harnessing the gravitational energy of other celestial objects allows the spacecraft to gain kinetic energy.[79] However, more energy can be obtained from the gravity assist if rockets are used via theOberth effect.
Atether propulsion system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object.[80]
Beam-powered propulsion is another method of propulsion without reaction mass, and includes sails pushed bylaser, microwave, or particle beams.[81]
Many spacecraft usereaction wheels orcontrol moment gyroscopes to control orientation in space.[82] A satellite or other space vehicle is subject to thelaw of conservation of angular momentum, which constrains a body from anet change inangular velocity. Thus, for a vehicle to change itsrelative orientation without expending reaction mass, another part of the vehicle may rotate in the opposite direction. Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum,[83] so such systems are designed to "bleed off" undesired rotational energies built up over time.
Advanced, and in some cases theoretical, propulsion technologies may use chemical or nonchemical physics to produce thrust but are generally considered to be of lower technical maturity with challenges that have not been overcome.[84] For both human and robotic exploration, traversing the solar system is a struggle against time and distance. The most distant planets are 4.5–6 billion kilometers from the Sun and to reach them in any reasonable time requires much more capable propulsion systems than conventional chemical rockets. Rapid inner solar system missions with flexible launch dates are difficult, requiring propulsion systems that are beyond today's current state of the art. The logistics, and therefore the total system mass required to support sustained human exploration beyond Earth to destinations such as the Moon, Mars, ornear-Earth objects, are daunting unless more efficient in-space propulsion technologies are developed and fielded.[85][86]
A variety of hypothetical propulsion techniques have been considered that require a deeper understanding of the properties of space, particularlyinertial frames and thevacuum state. Such methods are highly speculative and include:[citation needed]
A NASA assessment of itsBreakthrough Propulsion Physics Program divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are not impossible according to current theories.[87]
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Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods. Four numbers are shown. The first is theeffective exhaust velocity: the equivalent speed which the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However,
The second and third are the typical amounts of thrust and the typical burn times of the method; outside a gravitational potential, small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period, if the object is not significantly influenced by gravity.[citation needed] The fourth is the maximum delta-v the technique can give without staging. For rocket-like propulsion systems, this is a function of mass fraction and exhaust velocity; mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight.[citation needed] For a system to achieve this limit, the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower.[citation needed]
| Method | Effective exhaust velocity (km/s) | Thrust (N) | Firing duration | Maximum delta-v (km/s) | Technology readiness level |
|---|---|---|---|---|---|
| Solid-fuel rocket | <2.5 | <107 | Minutes | 7 | 9: Flight proven |
| Hybrid rocket | <4 | Minutes | >3 | 9: Flight proven | |
| Monopropellant rocket | 1–3[88] | 0.1–400[88] | Milliseconds–minutes | 3 | 9: Flight proven |
| Liquid-fuel rocket | <4.4 | <107 | Minutes | 9 | 9: Flight proven |
| Electrostatic ion thruster | 15–210[89] | Months–years | >100 | 9: Flight proven | |
| Hall-effect thruster (HET) | up to 50[90] | Months–years | >100 | 9: Flight proven[91] | |
| Resistojet rocket | 2–6 | 10−2–10 | Minutes | ? | 8: Flight qualified[92] |
| Arcjet rocket | 4–16 | 10−2–10 | Minutes | ? | 8: Flight qualified[citation needed] |
| Field-emission electric propulsion (FEEP) | 100[93]–130 | 10−6–10−3[93] | Months–years | ? | 8: Flight qualified[93] |
| Pulsed plasma thruster (PPT) | 20 | 0.1 | 80–400 days | ? | 7: Prototype demonstrated in space |
| Dual-mode propulsion rocket | 1–4.7 | 0.1–107 | Milliseconds–minutes | 3–9 | 7: Prototype demonstrated in space |
| Solar sails | 299,792.458,Speed of light | 9.08/km2 at 1 AU 908/km2 at 0.1 AU 10−10/km2 at 4ly | Indefinite | >40 |
|
| Tripropellant rocket | 2.5–5.3[citation needed] | 0.1–107[citation needed] | Minutes | 9 | 6: Prototype demonstrated on ground[95] |
| Magnetoplasmadynamic thruster (MPD) | 20–100 | 100 | Weeks | ? | 6: Model, 1 kW demonstrated in space[96] |
| Nuclear–thermal rocket | 9[97] | 107[97] | Minutes[97] | >20 | 6: Prototype demonstrated on ground |
| Propulsivemass drivers | 0–30 | 104–108 | Months | ? | 6: Model, 32 MJ demonstrated on ground |
| Tether propulsion | — | 1–1012 | Minutes | 7 | 6: Model, 31.7 km demonstrated in space[98] |
| Air-augmented rocket | 5–6 | 0.1–107 | Seconds–minutes | >7? | 6: Prototype demonstrated on ground[99][100] |
| Liquid-air-cycle engine | 4.5 | 103–107 | Seconds–minutes | ? | 6: Prototype demonstrated on ground |
| Pulsed-inductive thruster (PIT) | 10–80[101] | 20 | Months | ? | 5: Component validated in vacuum[101] |
| Variable-specific-impulse magnetoplasma rocket (VASIMR) | 10–300[citation needed] | 40–1,200[citation needed] | Days–months | >100 | 5: Component, 200 kW validated in vacuum |
| Magnetic-field oscillating amplified thruster (MOA) | 10–390[102] | 0.1–1 | Days–months | >100 | 5: Component validated in vacuum |
| Solar–thermal rocket | 7–12 | 1–100 | Weeks | >20 | 4: Component validated in lab[103] |
| Radioisotope rocket/Steam thruster | 7–8[citation needed] | 1.3–1.5 | Months | ? | 4: Component validated in lab |
| Nuclear–electric rocket | As electric propulsion method used | 4: Component,400 kW validated in lab | |||
| Orion Project (near-term nuclear pulse propulsion) | 20–100 | 109–1012 | Days | 30–60 | 3: Validated, 900 kg proof-of-concept[104][105] |
| Space elevator | — | — | Indefinite | >12 | 3: Validated proof-of-concept |
| Reaction Engines SABRE[106] | 30/4.5 | 0.1 – 107 | Minutes | 9.4 | 3: Validated proof-of-concept |
| Electric sails | 145–750, solar wind | ? | Indefinite | >40 | 3: Validated proof-of-concept |
| Magsail inSolar wind | — | 644[107][a] | Indefinite | 250–750 | 3: Validated proof-of-concept |
| Magnetoplasma sail inSolar wind[109] | 278 | 700 | Months–Years | 250–750 | 4: Component validated in lab[110] |
| Magsail inInterstellar medium[108] | — | 88,000 initially | Decades | 15,000 | 3: Validated proof-of-concept |
| Beam-powered/laser | As propulsion method powered by beam | 3: Validated, 71 m proof-of-concept | |||
| Launch loop/orbital ring | — | 104 | Minutes | 11–30 | 2:Technology concept formulated |
| Nuclear pulse propulsion (Project Daedalus' drive) | 20–1,000 | 109–1012 | Years | 15,000 | 2: Technology concept formulated |
| Gas-core reactor rocket | 10 – 20 | 103–106 | ? | ? | 2: Technology concept formulated |
| Nuclear salt-water rocket | 100 | 103–107 | Half-hour | ? | 2: Technology concept formulated |
| Fission sail | ? | ? | ? | ? | 2: Technology concept formulated |
| Fission-fragment rocket | 15,000 | ? | ? | ? | 2: Technology concept formulated |
| Nuclear–photonic rocket/Photon rocket | 299,792.458,Speed of light | 10−5–1 | Years–decades | ? | 2: Technology concept formulated |
| Fusion rocket | 100–1,000[citation needed] | ? | ? | ? | 2: Technology concept formulated |
| Antimatter-catalyzed nuclear pulse propulsion | 200–4,000 | ? | Days–weeks | ? | 2: Technology concept formulated |
| Antimatter rocket | 10,000–100,000[citation needed] | ? | ? | ? | 2: Technology concept formulated |
| Bussard ramjet | 2.2–20,000 | ? | Indefinite | 30,000 | 2: Technology concept formulated |
| Method | Effective exhaust velocity (km/s) | Thrust (N) | Firing duration | Maximum delta-v (km/s) | Technology readiness level |
Table Notes
There have been many ideas proposed for launch-assist mechanisms that have the potential of substantially reducing the cost of getting to orbit. Proposednon-rocket spacelaunch launch-assist mechanisms include:[111][112]
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Studies generally show that conventional air-breathing engines, such asramjets orturbojets are basically too heavy (have too low a thrust/weight ratio) to give significant performance improvement when installed on a launch vehicle.[citation needed] However, launch vehicles can beair launched from separate lift vehicles (e.g.B-29,Pegasus Rocket andWhite Knight) which do use such propulsion systems. Jet engines mounted on a launch rail could also be so used.[citation needed]
On the other hand, very lightweight or very high-speed engines have been proposed that take advantage of the air during ascent:
Normal rocket launch vehicles fly almost vertically before rolling over at an altitude of some tens of kilometers before burning sideways for orbit; this initial vertical climb wastes propellant but is optimal as it greatly reduces airdrag. Airbreathing engines burn propellant much more efficiently and this would permit a far flatter launch trajectory. The vehicles would typically fly approximately tangentially to Earth's surface until leaving the atmosphere then perform a rocket burn to bridge the finaldelta-v to orbital velocity.
For spacecraft already in very low-orbit,air-breathing electric propulsion could use residual gases in the upper atmosphere as a propellant. Air-breathing electric propulsion could make a new class of long-lived, low-orbiting missions feasible on Earth,Mars orVenus.[114][115]
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When a vehicle is to enter orbit around its destination planet, or when it is to land, it must adjust its velocity.[116] This can be done using any of the methods listed above (provided they can generate a high enough thrust), but there are methods that can take advantage of planetary atmospheres and/or surfaces.
Development of technologies will result in technical solutions that improve thrust levels,specific impulse, power,specific mass, (orspecific power), volume, system mass, system complexity, operational complexity, commonality with other spacecraft systems, manufacturability, durability, and cost. These types of improvements will yield decreased transit times, increased payload mass, safer spacecraft, and decreased costs. In some instances, the development of technologies within this technology area will result in mission-enabling breakthroughs that will revolutionize space exploration. There is no single propulsion technology that will benefit all missions or mission types; the requirements for in-space propulsion vary widely according to their intended application.[5][4]
One institution focused on developing primary propulsion technologies aimed at benefitting near- and mid-term science missions by reducing cost, mass, and/or travel times is theGlenn Research Center (GRC).[citation needed]Electric propulsion architectures are of particular interest to the GRC, includingion andHall thrusters.[citation needed] One system combinessolar sails, a form of propellantless propulsion which relies on naturally occurring starlight for propulsion energy, and Hall thrusters. Other propulsion technologies being developed include advanced chemical propulsion and aerocapture.[4][119][120]
Sustainable Propulsion technologies, such as solar cells and electric propulsion systems powered by renewable energy, are gaining attention for their potential to provide solutions for space travel, whilst aiming for more efficient energy sources and lesser harmful emissions. However, those technologies may be limited in terms of thrust and scalability.[121]
In 2023,Boeing andAirbus are the leading research companies in Sustainable Propulsion for space applications in terms ofpatent family publications, but mostly focus on hydrogen/fuel cells and sustainable fuels. The most important area in terms of patent family publications is electric propulsion. The number of patent family publications in electric propulsion systems has increased from only 70 in 2000 to 293 in 2023, with the top inventors being fromChina.[121]
The term "mission pull" defines a technology or a performance characteristic necessary to meet a planned NASA mission requirement. Any other relationship between a technology and a mission (an alternate propulsion system, for example) is categorized as "technology push." Also, a space demonstration refers to the spaceflight of a scaled version of a particular technology or of a critical technology subsystem. On the other hand, a space validation would serve as a qualification flight for future mission implementation. A successful validation flight would not require any additional space testing of a particular technology before it can be adopted for a science or exploration mission.[5]
Spacecraft propulsion systems are often first statically tested on Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully.[122] Rockets are usually tested at arocket engine test facility well away from habitation and other buildings for safety reasons.Ion drives are far less dangerous and require much less stringent safety, usually only a moderately large vacuum chamber is needed.[citation needed] Static firing of engines are done atground test facilities, and systems which cannot be adequately tested on the ground and require launches may be employed at alaunch site.
In science fiction, space ships use various means to travel, some of them scientifically plausible (like solar sails or ramjets), others, mostly or entirely fictitious (likeanti-gravity,warp drive,spindizzy orhyperspace travel).[123]: 8, 69–77 [124]: 142
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The shape and length of the combustion chamber and exit nozzle are essential design parameters for a rocket engine. The combustion chamber must be long enough for complete propellant combustion before the hot gases enter the nozzle, ensuring efficient combustion and maximizing thrust production.
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| Destinations | |||||||
| Space launch | |||||||
| Ground segment | |||||||
| Concepts | |||||||||
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| Physical propulsion | |||||||||
| Chemical propulsion |
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| Electrical propulsion |
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| Nuclear propulsion |
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| External power | |||||||||
| Related concepts | |||||||||
