Editing Propellant (section)

==Vehicle propellants==
{{Main|Rocket propellant|Jet propulsion}}

A rocket propellant is a [[reaction mass|mass]] that is expelled from a vehicle, such as a rocket, in such a way as to create a [[thrust]] in accordance with [[Newton's third law of motion]], and "propel" the vehicle forward. The engine that expels the propellant is called a [[reaction engine]]. Although the term "propellant" is often used in [[Rocket engine|chemical rocket design]] to describe a combined fuel/propellant, propellants should not be confused with the [[fuel]] 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.

In [[electrically powered spacecraft propulsion|electrically powered spacecraft]], electricity is used to accelerate the propellant. An [[electrostatic force]] may be used to expel positive ions, or the [[Lorentz force]] may be used to expel negative ions and electrons as the propellant. [[Electrically powered spacecraft propulsion#Electrothermal|Electrothermal]] engines use the [[electromagnetic 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 a [[resistojet rocket]] engine, the compressed propellant is simply heated using [[Joule heating|resistive 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 a [[nozzle]], thereby producing thrust. In rockets, the burning of [[rocket fuel]] produces an exhaust, and the exhausted material is usually expelled as a propellant under pressure through a [[nozzle]]. The exhaust material may be a [[gas]], [[liquid]], [[Plasma (physics)|plasma]], or a [[solid]]. In powered aircraft without propellers such as [[Jet aircraft|jets]], 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 as [[Compressed-air energy storage|compressed 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 a [[water rocket]], where the energy stored in the compressed air is the fuel and the water is the propellant.

Proposed [[photon rocket]]s would use the [[Momentum#Four-vector formulation|relativistic 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 and [[relativistic physics]] must be used.

In chemical rockets, chemical reactions are used to [[Chemical energy|produce energy]] which creates [[fluid dynamics|movement 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 create {{H2O-nl}} 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 small [[delta-v|changes in velocity]] to spacecraft by the use of [[cold gas thruster]]s, 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|Solid fuel|Solid-fuel rocket}}
* [[Composite propellant]]s made from a solid [[oxidizer]] such as [[ammonium perchlorate]] or [[ammonium nitrate]], a [[synthetic rubber]] such as [[HTPB]], [[Polybutadiene acrylonitrile|PBAN]], or [[Polyurethane]] (or energetic polymers such as [[polyglycidyl nitrate]] or [[polyvinyl nitrate]] for extra energy), optional high-explosive fuels (again, for extra energy) such as [[RDX]] or [[nitroglycerin]], and usually a powdered [[metal]] [[fuel]] such as [[aluminum]].
* Some [[amateur rocketry|amateur]] propellants use [[potassium nitrate]], combined with [[sugar]], [[epoxy]], or other fuels and binder compounds.
* [[Potassium perchlorate]] has been used as an oxidizer, paired with [[Bitumen|asphalt]], [[epoxy]], and other binders.

Propellants that explode in operation are of little practical use currently, although there have been experiments with [[Pulse Detonation Engine]]s. Also the newly synthesized bishomocubane based compounds are under consideration in the research stage as both solid and liquid propellants of the future.<ref>{{cite journal|last1=Lal|first1=Sohan|last2=Rajkumar|first2=Sundaram|last3=Tare|first3=Amit|last4=Reshmi|first4=Sasidharakurup|last5=Chowdhury|first5=Arindrajit|last6=Namboothiri|first6=Irishi N. N.|title=Nitro-Substituted Bishomocubanes: Synthesis, Characterization, and Application as Energetic Materials|journal=Chemistry: An Asian Journal|date=December 2014|volume=9|issue=12|pages=3533–3541|doi=10.1002/asia.201402607|pmid=25314237|doi-access=free}}</ref><ref>{{cite journal|last1=Lal|first1=Sohan|last2=Mallick|first2=Lovely|last3=Rajkumar|first3=Sundaram|last4=Oommen|first4=Oommen P.|last5=Reshmi|first5=Sasidharakurup|last6=Kumbhakarna|first6=Neeraj|last7=Chowdhury|first7=Arindrajit|last8=Namboothiri|first8=Irishi|title=Synthesis and energetic properties of high-nitrogen substituted bishomocubanes|journal=J. Mater. Chem. A|volume=3|issue=44|pages=22118–22128|date=2015|doi=10.1039/C5TA05380C|url=http://uu.diva-portal.org/smash/get/diva2:878740/FULLTEXT01|doi-access=free}}</ref>

=====Grain=====
Solid fuel/propellants are used in forms called [[Solid-fuel rocket#Grain geometry|grains]]. 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 in [[cannon]]s.
; 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 or [[HTPB]], 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|liquid fuel|liquid-propellant rocket}}
In rockets, three main liquid bipropellant combinations are used:  cryogenic oxygen and hydrogen, cryogenic oxygen and a hydrocarbon, and storable propellants.<ref name=sutton2001>
{{cite book |last=Sutton|first=George |author2=Biblarz, Oscar  |title=Rocket Propulsion Elements |year=2001 |publisher=Willey |isbn=9781601190604 |oclc=75193234 }}</ref>
; [[Cryogenic]] [[liquid oxygen|oxygen]]-[[liquid hydrogen|hydrogen]] combination system: Used in upper stages and sometimes in booster stages of space launch systems. This is a nontoxic combination. This gives high [[specific impulse]] and is ideal for high-velocity missions.
; Cryogenic oxygen-hydrocarbon propellant system: Used for many booster stages of space [[launch vehicle]]s as well as a smaller number of [[second stage]]s. 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 or [[reaction control system|reaction control]] rocket 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 for [[bipropellant rocket|liquid propellant rocket]]s include:
* [[Liquid oxygen]] and [[liquid hydrogen]]<ref name=ars20130414a>
{{cite news|last=Hutchinson|first=Lee |title=New F-1B rocket engine upgrades Apollo-era design with 1.8 M lbs of thrust |url=https://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/ |access-date=2013-04-15 |newspaper=ARS technica |date=2013-04-14 |quote=''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.''}}</ref>
* [[Liquid oxygen]] and [[kerosene]] or [[RP-1]]<ref name=ars20130414b>
{{cite news|last=Hutchinson|first=Lee |title=New F-1B rocket engine upgrades Apollo-era design with 1.8 M lbs of thrust |url=https://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/2/ |page=2 |access-date=2013-04-15 |newspaper=ARS technica |date=2013-04-14 |quote=''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.''}}</ref>
* [[Liquid oxygen]] and [[ethanol]]
* Liquid oxygen and [[methane]]
* [[High-test peroxide|Hydrogen peroxide]] and mentioned above alcohol or [[RP-1]]
* [[Red fuming nitric acid]] (RFNA) and [[kerosene]] or [[RP-1]]
* RFNA and [[Unsymmetrical dimethylhydrazine]] (UDMH)
* [[Dinitrogen tetroxide]] and UDMH, [[monomethylhydrazine|MMH]], and/or [[hydrazine]]

Common monopropellant used for [[liquid-propellant rocket|liquid rocket engines]] include:
* Hydrogen peroxide
* [[Hydrazine]]
* Red fuming nitric acid (RFNA)

===Electrically powered===
{{Main|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|Ion thruster}}
If the acceleration is caused mainly by the [[Coulomb force]] (i.e. application of a static [[electric field]] in 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
** [[High Power Electric Propulsion|HiPEP]] – using positive ions as the propellant, created using microwaves
** [[Gridded ion thruster|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 thruster|Colloid ion thruster]] – electrostatic acceleration of droplets of [[Ionic liquid|liquid salt]] as the propellant
* [[Field-emission electric propulsion]] – using electrodes to accelerate ionized liquid metal as a propellant
* [[Nano-particle field extraction thruster]] – using charged cylindrical [[carbon nanotubes]] as propellant

====Electrothermal====
These are engines that use electromagnetic fields to generate a [[plasma (physics)|plasma]] 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.<ref>{{cite web|url=http://novosti-kosmonavtiki.ru/content/numbers/198/35.shtml|title=Native Electric Propulsion Engines Today|publisher=Novosti Kosmonavtiki|year=1999|issue=7|archive-url=https://web.archive.org/web/20110606033558/http://www.novosti-kosmonavtiki.ru/content/numbers/198/35.shtml|archive-date=6 June 2011|language=ru}}</ref>
* [[Resistojet rocket|Resistojet]] – using a usually inert compressed propellant that is energized by simple [[Joule heating|resistive heating]]
* [[Arcjet rocket|Arcjet]] – uses (usually) hydrazine or ammonia as a propellant which is energized with an [[electrical arc]]
* [[Ion thruster#Microwave electrothermal thrusters|Microwave]] – a type of [[Radiofrequency ion thruster]]
* [[Variable specific impulse magnetoplasma rocket]] (VASIMR) – using microwave-generated plasma as the propellant and magnetic field to direct its expulsion

====Electromagnetic====
{{Main|Plasma propulsion engine}}
Electromagnetic thrusters use ions as the propellant, which are accelerated by the [[Lorentz force]] or by magnetic fields, either of which is generated by electricity:
* [[Electrodeless plasma thruster]] – a complex system that uses [[cold plasma]] as a propellant that is accelerated by [[ponderomotive 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 by [[high frequency]] band 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 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 rocket]]s 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 greatest [[specific impulse]].

===Photonic===
{{See also|Laser propulsion|Photon rocket}}
A photonic reactive engine uses [[photon]]s as the propellant and their discrete relativistic energy to produce thrust.