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A propellant (or propellent) is a 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 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, 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. Electothermal 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 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, or a solid. In powered aircraft without propellers such as 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. 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 rockets would use the 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 produce energy which creates 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 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 changes in velocity to spacecraft by the use of cold 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.
- Composite propellants made from a solid oxidizer such as ammonium perchlorate or ammonium nitrate, a synthetic rubber such as HTPB, 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 propellants use potassium nitrate, combined with sugar, epoxy, or other fuels and binder compounds.
- Potassium perchlorate has been used as an oxidizer, paired with asphalt, epoxy, and other binders.
Propellants that explode in operation are of little practical use currently, although there have been experiments with Pulse 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.
Solid fuel/propellants are used in forms called 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.
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 cannons.
- 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.
In rockets, three main liquid bipropellant combinations are used: cryogenic oxygen and hydrogen, cryogenic oxygen and a hydrocarbon, and storable propellants.
- Cryogenic oxygen-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 vehicles as well as a smaller number of second 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 or 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 liquid propellant rockets include:
- Liquid oxygen and liquid hydrogen
- Liquid oxygen and kerosene or RP-1
- Liquid oxygen and ethanol
- Liquid oxygen and methane
- 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, MMH, and/or hydrazine
Common monopropellant used for liquid rocket engines include:
- Hydrogen peroxide
- Red fuming nitric acid (RFNA)
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 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
- 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 of 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
These are engines that use electromagnetic fields to generate a 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.
- Resistojet - using a usually inert compressed propellant that is energized by simple resistive heating
- Arcjet - uses (usually) hydrazine or ammonia as a propellant which is energized with an electrical arc
- 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 thrusters use ions as the propellant, which are accelerted by the Lorentz force or by magnetic fields, either of which is generated by electicity:
- 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 reacitve 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 Lorantz 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 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 greatest specific impulse.
A photonic reactive engine uses photons as the propellant and their discrete relativistic energy to produce thrust.
- Cartridge (firearms)
- Explosive material
- Propellant depot
- Spacecraft propulsion
- Specific impulse
- Lal, Sohan; Rajkumar, Sundaram; Tare, Amit; Reshmi, Sasidharakurup; Chowdhury, Arindrajit; Namboothiri, Irishi N. N. (December 2014). "Nitro-Substituted Bishomocubanes: Synthesis, Characterization, and Application as Energetic Materials". Chemistry: An Asian Journal. 9 (12): 3533–3541. doi:10.1002/asia.201402607. PMID 25314237.
- Lal, Sohan; Mallick, Lovely; Rajkumar, Sundaram; Oommen, Oommen P.; Reshmi, Sasidharakurup; Kumbhakarna, Neeraj; Chowdhury, Arindrajit; Namboothiri, Irishi (2015). "Synthesis and energetic properties of high-nitrogen substituted bishomocubanes". J. Mater. Chem. A. 3 (44): 22118–22128. doi:10.1039/C5TA05380C.
- Sutton, George; Biblarz, Oscar (2001). Rocket Propulsion Elements. Willey. ISBN 9781601190604. OCLC 75193234.
Hutchinson, Lee (2013-04-14). "New F-1B rocket engine upgrades Apollo-era design with 1.8 M lbs of thrust". ARS technica. Retrieved 2013-04-15.
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.
Hutchinson, Lee (2013-04-14). "New F-1B rocket engine upgrades Apollo-era design with 1.8 M lbs of thrust". ARS technica. p. 2. Retrieved 2013-04-15.
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.
- "Native Electric Propulsion Engines Today" (in Russian). Novosti Kosmonavtiki. 1999. Archived from the original on 6 June 2011.
- Clark, John D. (1972). Ignition! An Informal History of Liquid Rocket Propellants. Rutgers University Press. ISBN 0-8135-0725-1.
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