Propellant

• 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.[1][2]

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.

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.

In rockets, three main liquid bipropellant combinations are used: cryogenic oxygen and hydrogen, cryogenic oxygen and a hydrocarbon, and storable propellants.[3]

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[4]
• Liquid oxygen and kerosene or RP-1[5]
• 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
• Hydrazine
• 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
  • 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 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.[6]

• 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 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 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.

In applications in which a large quantity of propellant is used, such as pressure washing and airbrushing, air may be pressurized by a compressor and used immediately. Additionally, a hand pump to compress air can be used for its simplicity in low-tech applications such as atomizers, plant misters and water rockets. The simplest examples of such a system are squeeze 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-pressure gas cylinders and pressure regulators are expensive and heavy.

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's vapor 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, the enthalpy 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 a freeze spray, this cooling contributes to the desired effect (although freeze sprays may also contain other components, such as chloroethane, with a lower vapor pressure but higher enthalpy of vaporization than the propellant).

Chlorofluorocarbons (CFCs) were once often used as propellants,[7] but since the Montreal Protocol came into force in 1989, they have been replaced in nearly every country due to the negative effects CFCs have on Earth's ozone layer. The most common replacements of CFCs are mixtures of volatile hydrocarbons, typically propane, n-butane and isobutane.[8] Dimethyl ether (DME) and methyl ethyl ether are also used. All these have the disadvantage of being flammable. Nitrous oxide and carbon dioxide are also used as propellants to deliver foodstuffs (for example, whipped cream and cooking spray). Medicinal aerosols such as asthma inhalers use hydrofluoroalkanes (HFA): either HFA 134a (1,1,1,2,-tetrafluoroethane) or HFA 227 (1,1,1,2,3,3,3-heptafluoropropane) or combinations of the two. More recently, liquid hydrofluoroolefin (HFO) propellants have become more widely adopted in aerosol systems due to their relatively low vapor pressure, low global 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 with whipped cream and shaving cream or shaving gel. Low-power guns, such as BB guns, paintball guns, and airsoft guns, have solid projectile payloads. Uniquely, in the case of a gas duster ("canned air"), the only payload is the velocity of the propellant vapor itself.