Propellant

Apropellant(orpropellent) is amassthat is expelled or expanded in such a way as to create athrustor anothermotive forcein accordance withNewton's third law of motion,and "propel" a vehicle,projectile,orfluidpayload. 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 designto describe a combined fuel/propellant, although the propellants should not be confused with thefuelthat 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 forcemay be used to expel positive ions, or theLorentz forcemay be used to expel negative ions and electrons as the propellant.Electrothermalengines use theelectromagnetic forceto heat low molecular weight gases (e.g. hydrogen, helium, ammonia) into a plasma and expel the plasma as propellant. In the case of aresistojet rocketengine, the compressed propellant is simply heated usingresistive heatingas 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 fuelproduces 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 rocketswould use therelativistic momentumof 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 Motionis inadequate to model the physics involved andrelativistic physicsmust be used.

In chemical rockets, chemical reactions are used toproduce energywhich createsmovement of a fluidwhich 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 createH₂Oand 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 mi xing and combustion, to provide smallchanges in velocityto 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.

Vehicle propellants

Main articles:Rocket propellantandJet propulsion

A rocket propellant is amassthat is expelled from a vehicle, such as a rocket, in such a way as to create athrustin 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 designto describe a combined fuel/propellant, propellants should not be confused with thefuelthat 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 forcemay be used to expel positive ions, or theLorentz forcemay be used to expel negative ions and electrons as the propellant.Electrothermalengines use theelectromagnetic forceto heat low molecular weight gases (e.g. hydrogen, helium, ammonia) into a plasma and expel the plasma as propellant. In the case of aresistojet rocketengine, the compressed propellant is simply heated usingresistive heatingas 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 fuelproduces 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 rocketswould use therelativistic momentumof 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 physicsmust be used.

In chemical rockets, chemical reactions are used toproduce energywhich createsmovement of a fluidwhich 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 createH₂Oand 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 mi xing and combustion, to provide smallchanges in velocityto 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.

Chemically powered

Solid propellant

Further information:Solid fuelandSolid-fuel rocket

• Composite propellantsmade from a solidoxidizersuch asammonium perchlorateorammonium nitrate,asynthetic rubbersuch asHTPB,PBAN,orPolyurethane(or energetic polymers such aspolyglycidyl nitrateorpolyvinyl nitratefor extra energy), optional high-explosive fuels (again, for extra energy) such asRDXornitroglycerin,and usually a powderedmetalfuelsuch asaluminum.
• Someamateurpropellants usepotassium nitrate,combined withsugar,epoxy,or other fuels and binder compounds.
• Potassium perchloratehas been used as an oxidizer, paired withasphalt,epoxy,and other binders.

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.

Grain

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.

Progressive burn

Usually a grain with multiple perforations or a star cut in the center providing a lot of surface area.

Degressive burn

Usually a solid grain in the shape of a cylinder or sphere.

Neutral burn

Usually a single perforation; as outside surface decreases the inside surface increases at the same rate.

Composition

There are four different types of solid fuel/propellant compositions:

Single-based fuel/propellant

A single based fuel/propellant has nitrocellulose as its chief explosives ingredient. Stabilizers and other additives are used to control the chemical stability and enhance its properties.

Double-based fuel/propellant

Double-based fuel/propellants consist of nitrocellulose with nitroglycerin or other liquid organic nitrate explosives added. Stabilizers and other additives are also used. Nitroglycerin reduces smoke and increases the energy output. Double-based fuel/propellants are used in small arms, cannons, mortars and rockets.

Triple-based fuel/propellant

Triple-based fuel/propellants consist of nitrocellulose, nitroguanidine, nitroglycerin or other liquid organic nitrate explosives. Triple-based fuel/propellants are used incannons.

Composite

Composites do not utilize nitrocellulose, nitroglycerin, nitroguanidine or any other organic nitrate as the primary constituent. Composites usually consist of a fuel such as metallic aluminum, a combustible binder such as synthetic rubber orHTPB,and an oxidizer such as ammonium perchlorate. Composite fuel/propellants are used in large rocket motors. In some applications, such as the US SLBM Trident II missile, nitroglycerin is added to the aluminum and ammonium perchlorate composite as an energetic plasticizer.

Liquid propellant

Further information:liquid fuelandliquid-propellant rocket

In rockets, three main liquid bipropellant combinations are used: cryogenic oxygen and hydrogen, cryogenic oxygen and a hydrocarbon, and storable propellants.

Cryogenicoxygen-hydrogencombination system

Used in upper stages and sometimes in booster stages of space launch systems. This is a nontoxic combination. This gives highspecific impulseand is ideal for high-velocity missions.

Cryogenic oxygen-hydrocarbon propellant system

Used for many booster stages of spacelaunch vehiclesas well as a smaller number ofsecond stages.This combination of fuel/oxidizer has high density and hence allows for a more compact booster design.

Storable propellant combinations

Used in almost all bipropellant low-thrust, auxiliary orreaction controlrocket engines, as well as in some in large rocket engines for first and second stages of ballistic missiles. They are instant-starting and suitable for long-term storage.

Propellant combinations used forliquid propellant rocketsinclude:

• Liquid oxygenandliquid hydrogen
• Liquid oxygenandkeroseneorRP-1
• Liquid oxygenandethanol
• Liquid oxygen andmethane
• Hydrogen peroxideand mentioned above alcohol orRP-1
• Red fuming nitric acid(RFNA) andkeroseneorRP-1
• RFNA andUnsymmetrical dimethylhydrazine(UDMH)
• Dinitrogen tetroxideand UDMH,MMH,and/orhydrazine

Common monopropellant used forliquid rocket enginesinclude:

• Hydrogen peroxide
• Hydrazine
• Red fuming nitric acid (RFNA)

Electrically powered

Main article:Electrically powered spacecraft propulsion

Electrically powered reactive engines use a variety of usually ionized propellants, including atomic ions, plasma, electrons, or small droplets or solid particles as propellant.

Electrostatic

Main article:Ion thruster

If the acceleration is caused mainly by theCoulomb force(i.e. application of a staticelectric fieldin the direction of the acceleration) the device is considered electrostatic. The types of electrostatic drives and their propellants:

• Gridded ion thruster– using positive ions as the propellant, accelerated by an electrically charged grid
  • NASA Solar Technology Application Readiness(NSTAR) – positive ions accelerated using high-voltage electrodes
  • HiPEP– using positive ions as the propellant, created using microwaves
  • Radiofrequency ion thruster– generalization of HiPEP
• Hall-effect thruster,including its subtypes Stationary Plasma Thruster (SPT) and Thruster with Anode Layer (TAL) – use the Hall effect to orient electrons to create positive ions for propellant
• Colloid ion thruster– electrostatic acceleration of droplets ofliquid saltas the propellant
• Field-emission electric propulsion– using electrodes to accelerate ionized liquid metal as a propellant
• Nano-particle field extraction thruster– using charged cylindricalcarbon nanotubesas propellant

Electrothermal

These are engines that use electromagnetic fields to generate aplasmawhich 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.

• Resistojet– using a usually inert compressed propellant that is energized by simpleresistive heating
• Arcjet– uses (usually) hydrazine or ammonia as a propellant which is energized with anelectrical arc
• Microwave– a type ofRadiofrequency ion thruster
• Variable specific impulse magnetoplasma rocket(VASIMR) – using microwave-generated plasma as the propellant and magnetic field to direct its expulsion

Electromagnetic

Main article:Plasma propulsion engine

Electromagnetic thrusters use ions as the propellant, which are accelerated by theLorentz forceor by magnetic fields, either of which is generated by electricity:

• Electrodeless plasma thruster– a complex system that usescold plasmaas a propellant that is accelerated byponderomotive force
• Magnetoplasmadynamic thruster– propellants include xenon, neon, argon, hydrogen, hydrazine, or lithium; expelled using the Lorentz force
• Pulsed inductive thruster– because this reactive engine uses a radial magnetic field, it acts on both positive and negative particles and so it may use a wide range of gases as a propellant including water, hydrazine, ammonia, argon, xenon and many others
• Pulsed plasma thruster– uses a Teflon plasma as a propellant, which is created by an electrical arc and expelled using the Lorentz force
• Helicon Double Layer Thruster– a plasma propellant is generated and excited from a gas using a helicon induced byhigh frequencyband radiowaves which form a magnetic nozzle in a cylinder

Nuclear

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 propulsionuses a series of nuclear explosions to create large amounts of energy to expel the products of the nuclear reaction as the propellant.Nuclear thermal rocketsuse 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.

Photonic

See also:Laser propulsionandPhoton rocket

A photonic reactive engine usesphotonsas the propellant and their discrete relativistic energy to produce thrust.

Projectile propellants

See also:Gunpowder,Smokeless powder,andFirearm propellant

Compressed fluid propellants

Compressed fluidorcompressed gaspropellants 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.

Compressed gas propellants

In applications in which a large quantity of propellant is used, such aspressure washingandairbrushing,airmay be pressurized by acompressorand 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 bottlesfor 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 cylindersandpressure regulatorsare expensive and heavy.

Liquified gas propellants

See also:Aerosol_spray § Aerosol_propellants

Principle

Liquified 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).

Propellant compounds

Chlorofluorocarbons(CFCs) were once often used as propellants,but since theMontreal Protocolcame 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-butaneandisobutane.Dimethyl ether(DME) andmethyl ethyl etherare also used. All these have the disadvantage of beingflammable.Nitrous oxideandcarbon dioxideare also used as propellants to deliver foodstuffs (for example,whipped creamandcooking spray). Medicinal aerosols such asasthma inhalersusehydrofluoroalkanes(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.

Payloads

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 creamandshaving creamor shaving gel. Low-power guns, such asBB guns,paintballguns, andairsoftguns, have solid projectile payloads. Uniquely, in the case of agas duster( "canned air" ), the only payload is the velocity of the propellant vapor itself.

See also

• Cartridge (firearms)
• Explosive material
• Fuel
• Propellant depot
• Spacecraft propulsion
• Specific impulse

Bibliography

• Clark, John D.(1972).Ignition! An Informal History of Liquid Rocket Propellants.Rutgers University Press.ISBN0-8135-0725-1.

External links

• Rocket Propellants
• Rocket propulsion elements, Sutton, George.P, Biblarz, Oscar 7th Ed
• Understanding and Predicting Gun Barrel Erosion – Weapons Systems Division Defence Science and Technology Organisation by Ian A. JohnstonArchived2013-07-10 at theWayback Machine