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Apropellant (orpropellent) is amass that is expelled or expanded in such a way as to create athrust or anothermotive force in accordance withNewton's third law of motion, and "propel" a vehicle,projectile, orfluid payload. In vehicles, the engine that expels the propellant is called areaction engine. Although technically a propellant is the reaction mass used to create thrust, the term "propellant" is often used to describe a substance which contains both the reaction mass and the fuel that holds the energy used to accelerate the reaction mass. For example, the term "propellant" is often used inchemical rocket design to describe a combined fuel/propellant, although the propellants should not be confused with thefuel that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.
Vehicles can use propellants to move by ejecting a propellant backwards which creates an opposite force that moves the vehicle forward. Projectiles can use propellants that are expanding gases which provide the motive force to set the projectile in motion. Aerosol cans use propellants which are fluids that are compressed so that when the propellant is allowed to escape by releasing a valve, the energy stored by the compression moves the propellant out of the can and that propellant forces the aerosol payload out along with the propellant. Compressed fluid may also be used as a simple vehicle propellant, with the potential energy that is stored in the compressed fluid used to expel the fluid as the propellant. The energy stored in the fluid was added to the system when the fluid was compressed, such ascompressed air. The energy applied to the pump or thermal system that is used to compress the air is stored until it is released by allowing the propellant to escape. Compressed fluid may also be used only as energy storage along with some other substance as the propellant, such as with awater rocket, where the energy stored in the compressed air is the fuel and the water is the propellant.
Inelectrically powered spacecraft, electricity is used to accelerate the propellant. Anelectrostatic force may be used to expel positive ions, or theLorentz force may be used to expel negative ions and electrons as the propellant.Electrothermal engines use theelectromagnetic force to heat low molecular weight gases (e.g. hydrogen, helium, ammonia) into a plasma and expel the plasma as propellant. In the case of aresistojet rocket engine, the compressed propellant is simply heated usingresistive heating as it is expelled to create more thrust.
In chemical rockets and aircraft, fuels are used to produce an energetic gas that can be directed through anozzle, thereby producing thrust. In rockets, the burning ofrocket fuel produces an exhaust, and the exhausted material is usually expelled as a propellant under pressure through anozzle. The exhaust material may be agas,liquid,plasma, or asolid. In powered aircraft without propellers such asjets, the propellant is usually the product of the burning of fuel with atmospheric oxygen so that the resulting propellant product has more mass than the fuel carried on the vehicle.
Proposedphoton rockets would use therelativistic momentum of photons to create thrust. Even though photons do not have mass, they can still act as a propellant because they move at relativistic speed, i.e., the speed of light. In this caseNewton's third Law of Motion is inadequate to model the physics involved andrelativistic physics must be used.
In chemical rockets, chemical reactions are used toproduce energy which createsmovement of a fluid which is used to expel the products of that chemical reaction (and sometimes other substances) as propellants. For example, in a simple hydrogen/oxygen engine, hydrogen is burned (oxidized) to createH2O and the energy from the chemical reaction is used to expel the water (steam) to provide thrust. Often in chemical rocket engines, a higher molecular mass substance is included in the fuel to provide more reaction mass.
Rocket propellant may be expelled through an expansion nozzle as a cold gas, that is, without energetic mixing and combustion, to provide smallchanges in velocity to spacecraft by the use ofcold gas thrusters, usually as maneuvering thrusters.
To attain a useful density for storage, most propellants are stored as either a solid or a liquid.
A rocket propellant is amass that is expelled from a vehicle, such as a rocket, in such a way as to create athrust in accordance withNewton's third law of motion, and "propel" the vehicle forward. The engine that expels the propellant is called areaction engine. Although the term "propellant" is often used inchemical rocket design to describe a combined fuel/propellant, propellants should not be confused with thefuel that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.
Inelectrically powered spacecraft, electricity is used to accelerate the propellant. Anelectrostatic force may be used to expel positive ions, or theLorentz force may be used to expel negative ions and electrons as the propellant.Electrothermal engines use theelectromagnetic force to heat low molecular weight gases (e.g. hydrogen, helium, ammonia) into a plasma and expel the plasma as propellant. In the case of aresistojet rocket engine, the compressed propellant is simply heated usingresistive heating as it is expelled to create more thrust.
In chemical rockets and aircraft, fuels are used to produce an energetic gas that can be directed through anozzle, thereby producing thrust. In rockets, the burning ofrocket fuel produces an exhaust, and the exhausted material is usually expelled as a propellant under pressure through anozzle. The exhaust material may be agas,liquid,plasma, or asolid. In powered aircraft without propellers such asjets, the propellant is usually the product of the burning of fuel with atmospheric oxygen so that the resulting propellant product has more mass than the fuel carried on the vehicle.
The propellant or fuel may also simply be a compressed fluid, with the potential energy that is stored in the compressed fluid used to expel the fluid as the propellant. The energy stored in the fluid was added to the system when the fluid was compressed, such ascompressed air. The energy applied to the pump or thermal system that is used to compress the air is stored until it is released by allowing the propellant to escape. Compressed fluid may also be used only as energy storage along with some other substance as the propellant, such as with awater rocket, where the energy stored in the compressed air is the fuel and the water is the propellant.
Proposedphoton rockets would use therelativistic momentum of photons to create thrust. Even though photons do not have mass, they can still act as a propellant because they move at relativistic speed, i.e., the speed of light. In this case Newton's third Law of Motion is inadequate to model the physics involved andrelativistic physics must be used.
In chemical rockets, chemical reactions are used toproduce energy which createsmovement of a fluid which is used to expel the products of that chemical reaction (and sometimes other substances) as propellants. For example, in a simple hydrogen/oxygen engine, hydrogen is burned (oxidized) to createH2O and the energy from the chemical reaction is used to expel the water (steam) to provide thrust. Often in chemical rocket engines, a higher molecular mass substance is included in the fuel to provide more reaction mass.
Rocket propellant may be expelled through an expansion nozzle as a cold gas, that is, without energetic mixing and combustion, to provide smallchanges in velocity to spacecraft by the use ofcold gas thrusters, usually as maneuvering thrusters.
To attain a useful density for storage, most propellants are stored as either a solid or a liquid.
Propellants may be energized by chemical reactions to expel solid, liquid or gas. Electrical energy may be used to expel gases, plasmas, ions, solids or liquids. Photons may be used to provide thrust via relativistic momentum.
Propellants that explode in operation are of little practical use currently, although there have been experiments withPulse Detonation Engines. Also the newly synthesized bishomocubane based compounds are under consideration in the research stage as both solid and liquid propellants of the future.[1][2]
Solid fuel/propellants are used in forms calledgrains. A grain is any individual particle of fuel/propellant regardless of the size or shape. The shape and size of a grain determines the burn time, amount of gas, and rate of produced energy from the burning of the fuel and, as a consequence, thrust vs time profile.
There are three types of burns that can be achieved with different grains.
There are four different types of solid fuel/propellant compositions:
In rockets, three main liquid bipropellant combinations are used: cryogenic oxygen and hydrogen, cryogenic oxygen and a hydrocarbon, and storable propellants.[3]
Propellant combinations used forliquid propellant rockets include:
Common monopropellant used forliquid rocket engines include:
Electrically powered reactive engines use a variety of usually ionized propellants, including atomic ions, plasma, electrons, or small droplets or solid particles as propellant.
If the acceleration is caused mainly by theCoulomb force (i.e. application of a staticelectric field in the direction of the acceleration) the device is considered electrostatic. The types of electrostatic drives and their propellants:
These are engines that use electromagnetic fields to generate aplasma which is used as the propellant. They use a nozzle to direct the energized propellant. The nozzle itself may be composed simply of a magnetic field. Low molecular weight gases (e.g. hydrogen, helium, ammonia) are preferred propellants for this kind of system.[6]
Electromagnetic thrusters use ions as the propellant, which are accelerated by theLorentz force or by magnetic fields, either of which is generated by electricity:
Nuclear reactions may be used to produce the energy for the expulsion of the propellants. Many types of nuclear reactors have been used/proposed to produce electricity for electrical propulsion as outlined above.Nuclear pulse propulsion uses a series of nuclear explosions to create large amounts of energy to expel the products of the nuclear reaction as the propellant.Nuclear thermal rockets use the heat of a nuclear reaction to heat a propellant. Usually the propellant is hydrogen because the force is a function of the energy irrespective of the mass of the propellant, so the lightest propellant (hydrogen) produces the greatestspecific impulse.
A photonic reactive engine usesphotons as the propellant and their discrete relativistic energy to produce thrust.
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Compressed fluid orcompressed gas propellants are pressurized physically, by a compressor, rather than by a chemical reaction. The pressures and energy densities that can be achieved, while insufficient for high-performance rocketry and firearms, are adequate for most applications, in which case compressed fluids offer a simpler, safer, and more practical source of propellant pressure.
A compressed fluid propellant may simply be a pressurized gas, or a substance which is a gas at atmospheric pressure, but stored under pressure as a liquid.
In applications in which a large quantity of propellant is used, such aspressure washing andairbrushing,air may be pressurized by acompressor and used immediately. Additionally, a hand pump to compress air can be used for its simplicity in low-tech applications such asatomizers, plant misters andwater rockets. The simplest examples of such a system aresqueeze bottles for such liquids as ketchup and shampoo.
However, compressed gases are impractical as stored propellants if they do not liquify inside the storage container, because very high pressures are required in order to store any significant quantity of gas, and high-pressuregas cylinders andpressure regulators are expensive and heavy.
Liquefied gas propellants are gases at atmospheric pressure, but become liquid at a modest pressure. This pressure is high enough to provide useful propulsion of the payload (e.g. aerosol paint, deodorant, lubricant), but is low enough to be stored in an inexpensive metal can, and to not pose a safety hazard in case the can is ruptured.
The mixture of liquid and gaseous propellant inside the can maintains a constant pressure, called the liquid'svapor pressure. As the payload is depleted, the propellant vaporizes to fill the internal volume of the can. Liquids are typically 500-1000x denser than their corresponding gases at atmospheric pressure; even at the higher pressure inside the can, only a small fraction of its volume needs to be propellant in order to eject the payload and replace it with vapor.
Vaporizing the liquid propellant to gas requires some energy, theenthalpy of vaporization, which cools the system. This is usually insignificant, although it can sometimes be an unwanted effect of heavy usage (as the system cools, the vapor pressure of the propellant drops). However, in the case of afreeze spray, this cooling contributes to the desired effect (although freeze sprays may also contain other components, such aschloroethane, with a lower vapor pressure but higher enthalpy of vaporization than the propellant).
Chlorofluorocarbons (CFCs) were once often used as propellants,[7] but since theMontreal Protocol came into force in 1989, they have been replaced in nearly every country due to the negative effects CFCs have on Earth'sozone layer. The most common replacements of CFCs are mixtures of volatilehydrocarbons, typicallypropane, n-butane andisobutane.[8]Dimethyl ether (DME) andmethyl ethyl ether are also used. All these have the disadvantage of beingflammable.Nitrous oxide andcarbon dioxide are also used as propellants to deliver foodstuffs (for example,whipped cream andcooking spray). Medicinal aerosols such asasthma inhalers usehydrofluoroalkanes (HFA): eitherHFA 134a (1,1,1,2,-tetrafluoroethane) orHFA 227 (1,1,1,2,3,3,3-heptafluoropropane) or combinations of the two. More recently, liquidhydrofluoroolefin (HFO) propellants have become more widely adopted in aerosol systems due to their relatively low vapor pressure, lowglobal warming potential (GWP), and nonflammability.[9]
The practicality of liquified gas propellants allows for a broad variety of payloads.Aerosol sprays, in which a liquid is ejected as a spray, include paints, lubricants, degreasers, and protective coatings; deodorants and other personal care products; cooking oils. Some liquid payloads are not sprayed due to lower propellant pressure and/or viscous payload, as withwhipped cream andshaving cream or shaving gel. Low-power guns, such asBB guns,paintball guns, andairsoft guns, have solid projectile payloads. Uniquely, in the case of agas duster ("canned air"), the only payload is the velocity of the propellant vapor itself.
The most efficient fuel and oxidizer combination commonly used today for chemical liquid rockets is hydrogen (fuel) and oxygen (oxidizer)," continued Coates. The two elements are relatively simple and they burn easily when combined—and even better, the result of their reaction is simple water.
Refined petroleum is not the most efficient thrust-producing fuel for rockets, but what it lacks in thrust production it makes up for in density. It takes less volume of RP-1 to impart the same thrust force on a vehicle, and less volume equates to reduced stage size. ... A smaller booster stage means much less aerodynamic drag as the vehicle lifts off from near sea-level and accelerates up through the more dense (thicker) part of the atmosphere near the earth. The result of a smaller booster stage is it allows a more efficient ascent through the thickest part of the atmosphere, which helps improve the net mass lifted to orbit.
Freon chemical compounds in household refrigerators, air-cooling systems and asaDDT carrier in aerosolinsect bombs have been found to be more effective in extinguishing fires than carbon dioxide.
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