Rocket propellant is used as a reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines or nuclear engines.
Overview Rockets create thrust by expelling mass rearward, at high velocity. The thrust produced can be calculated by multiplying the mass flow rate of the propellants by their exhaust velocity relative to the rocket (specific impulse). A rocket can be thought of as being accelerated by the pressure of the combusting gases against the combustion chamber and nozzle, not by "pushing" against the air behind or below it. Rocket engines perform best in outer space because of the lack of air pressure on the outside of the engine. In space it is also possible to fit a longer nozzle without suffering from flow separation.
Most chemical propellants release energy through redox chemistry, more specifically combustion. As such, both an oxidizing agent and a reducing agent (fuel) must be present in the mixture. Decomposition, such as that of highly unstable peroxide bonds in monopropellant rockets, can also be the source of energy.
In the case of bipropellant liquid rockets, a mixture of reducing fuel and oxidizing oxidizer is introduced into a combustion chamber, typically using a turbopump to overcome the pressure. As combustion takes place, the liquid propellant mass is converted into a huge volume of gas at high temperature and pressure. This exhaust stream is ejected from the engine nozzle at high velocity, creating an opposing force that propels the rocket forward in accordance with Newton's laws of motion.
Chemical rockets can be grouped by phase. Solid rockets use propellant in the solid phase, liquid fuel rockets use propellant in the liquid phase, gas fuel rockets use propellant in the gas phase, and hybrid rockets use a combination of solid and liquid or gaseous propellants.
In the case of solid rocket motors, the fuel and oxidizer are combined when the motor is cast. Propellant combustion occurs inside the motor casing, which must contain the pressures developed. Solid rockets typically have higher thrust, less specific impulse, shorter burn times, and a higher mass than liquid rockets, and additionally cannot be stopped once lit.
Rocket stages In space, the maximum change in velocity that a rocket stage can impart on its payload is primarily a function of its mass ratio and its exhaust velocity. This relationship is described by the rocket equation. Exhaust velocity is dependent on the propellant and engine used and closely related to specific impulse, the total energy delivered to the rocket vehicle per unit of propellant mass consumed. The mass ratio can also be affected by the choice of a given propellant.
Rocket stages that fly through the atmosphere usually use lower-performing, high-molecular-mass, high-density propellants due to the smaller and lighter tankage required. Upper stages, which mostly or only operate in the vacuum of space, tend to use the high-energy, high-performance, low-density liquid hydrogen fuel. For future planetary missions the use of local resources and solar energy for in situ propellant production is considered.
Solid chemical propellants Solid propellants come in two main types. "Composites" are mostly a mixture of granules of solid oxidizer, such as ammonium nitrate, ammonium dinitramide, ammonium perchlorate, or potassium nitrate in a polymer binding agent, with flakes or powders of energetic fuel compounds such as RDX, HMX, aluminium or beryllium. Plasticizers, stabilizers, and burn rate modifiers (e.g. iron oxide or copper oxide) can be added.
Single-, double-, or triple-bases are homogeneous mixtures of one to three primary ingredients, which must include fuel and oxidizer, and often include binders and plasticizers. All components are macroscopically indistinguishable and often blended as liquids and cured in a single batch. Ingredients can have multiple roles: RDX is both fuel and oxidizer, while nitrocellulose is fuel, oxidizer, and structural polymer.
Many propellants contain elements of double-base and composite propellants, and often contain energetic additives homogeneously mixed into the binder. In the case of gunpowder (a pressed composite without a polymeric binder), the fuel is charcoal, the oxidizer is potassium nitrate, and sulfur serves as a reaction catalyst while also being consumed to form reaction products such as potassium sulfide.
The newest nitramine solid propellants based on CL-20 (HNIW) can match the performance of NTO/UDMH storable liquid propellants, but cannot be throttled or restarted.
Extraterrestrial on-site production is being explored by combining aluminium and ice (ALICE).
Advantages Solid-propellant rockets are much easier to store and handle than liquid-propellant rockets. High propellant density makes for compact size as well. These features plus simplicity and low cost make solid-propellant rockets ideal for military applications.
Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid-fueled rockets in their boost stages (solid rocket boosters) for this reason.
Disadvantages
Solid-fuel rockets have lower specific impulse, a measure of propellant efficiency, than liquid-fuel rockets. As a result, the overall performance of solid upper stages is less than liquid stages even though the solid mass ratios are usually in the .91 to .93 range, as good as or better than most liquid-propellant upper stages. The high mass ratios possible with these unsegmented solid upper stages is a result of high propellant density and very high strength-to-weight ratio casings.
A drawback to solid rockets is that they cannot be throttled in real time, although a programmed thrust schedule can be created by adjusting the interior propellant geometry. Solid rockets can be vented to extinguish combustion or reverse thrust as a means of controlling range or accommodating stage separation. Casting large amounts of propellant requires consistency and repeatability to avoid cracks and voids in the completed motor. The blending and casting take place under computer control in a vacuum, and the propellant blend is spread thin and scanned to ensure that no large gas bubbles are introduced into the motor.
Solid-fuel rockets are intolerant to cracks and voids and require post-processing such as X-ray scans to identify faults. The combustion process is dependent on the surface area of the fuel. Voids and cracks represent local increases in burning surface area, increasing the local temperature, which increases the local rate of combustion. This positive feedback loop can easily lead to catastrophic failure of the case or nozzle.
History During the 1950s and 60s, researchers in the United States developed ammonium perchlorate composite propellant (APCP). This mixture is typically 69-70% finely ground ammonium perchlorate (an oxidizer), combined with 16-20% fine aluminium powder (a fuel), held together in a base of 11-14% polybutadiene acrylonitrile (PBAN) or hydroxyl-terminated polybutadiene (polybutadiene rubber fuel). The mixture is formed as a thickened liquid and then cast into the correct shape and cured into a firm but flexible load-bearing solid. Historically, the tally of APCP solid propellants is relatively small. The military, however, uses a wide variety of different types of solid propellants, some of which exceed the performance of APCP. A comparison of the highest specific impulses achieved with the various solid and liquid propellant combinations used in current launch vehicles is given in the article on solid-fuel rockets.M. D. Black, The Evolution of ROCKET TECHNOLOGY, 3rd Ed., 2012, payloadz.com ebook/History pp. 109-112 and pp. 114-119
In the 1970s and 1980s, the U.S. switched entirely to solid-fueled ICBMs: the LGM-30 Minuteman and LG-118A Peacekeeper (MX). In the 1980s and 1990s, the USSR/Russia also deployed solid-fueled ICBMs (RT-23, RT-2PM, and RT-2UTTH), but retains two liquid-fueled ICBMs (R-36 and UR-100N). All solid-fueled ICBMs on both sides had three initial solid stages, and those with multiple independently targeted warheads had a precision maneuverable bus used to fine-tune the trajectories of the re-entry vehicles.
Liquid chemical propellants
Advantages Liquid-fueled rockets have higher specific impulse than solid rockets and are capable of being throttled, shut down, and restarted. Only the combustion chamber of a liquid-fueled rocket needs to withstand high combustion pressures and temperatures. Cooling can be done regeneratively with the liquid propellant. On vehicles employing turbopumps or electric pumps, the propellant tanks are at a lower pressure than the combustion chamber, decreasing tank mass. For these reasons, most orbital launch vehicles use liquid propellants.
The primary specific impulse advantage of liquid propellants is due to the availability of high-performance oxidizers. Several practical liquid oxidizers (liquid oxygen, dinitrogen tetroxide, and hydrogen peroxide) have better specific impulse than the ammonium perchlorate used in most solid rockets when paired with suitable fuels.
Some gases, particularly oxygen and nitrogen, may be able to be collected from the upper atmosphere, and transferred up to low Earth orbit for use in propellant depots at substantially reduced cost.Jones, C., Masse, D., Glass, C., Wilhite, A., and Walker, M. (2010), "PHARO: Propellant harvesting of atmospheric resources in orbit," IEEE Aerospace Conference.
Disadvantages The main difficulties with liquid propellants are also with the oxidizers. Storable oxidizers, such as nitric acid and nitrogen tetroxide, tend to be extremely toxic and highly reactive, while cryogenic propellants by definition must be stored at low temperature and can also have reactivity/toxicity issues. Liquid oxygen (LOX) is the only flown cryogenic oxidizer. Others such as FLOX, a fluorine/LOX mix, have never been flown due to instability, toxicity, and explosivity. Several other unstable, energetic, and toxic oxidizers have been proposed: liquid ozone (O3), ClF3, and ClF5.
Liquid-fueled rockets require potentially troublesome valves, seals, and turbopumps, which increase the cost of the launch vehicle. Turbopumps are particularly troublesome due to high performance requirements.
Current cryogenic types Liquid oxygen (LOX) and liquid hydrogen. Used on the Centaur upper stage, the Delta IV rocket, the H-IIA rocket, most stages of the European Ariane 6, and the Space Launch System core and upper stages. LOX and liquid methane (from liquefied natural gas). Used on Zhuque-2, Vulcan, New Glenn, SpaceX Starship, and also planned for use on several rockets in development, including Soyuz-7, Rocket Lab Neutron and Long March 9. LOX and liquid propane. Used on the Aquila engines of Spectrum.
Current semi-cryogenic types LOX and highly refined kerosene (RP-1). Used for the first stages of the Atlas V, Falcon 9, Falcon Heavy, Soyuz, Zenit, Angara, and Long March 6, among others. This combination is widely regarded as the most practical for boosters that lift off at ground level and therefore must operate at full atmospheric pressure.
Current storable types Dinitrogen tetroxide (N2O4) and hydrazine (N2H4), MMH, or UDMH. Used in military, orbital, and deep-space rockets because both liquids are storable for long periods at reasonable temperatures and pressures. N2O4/UDMH is the main fuel for the Proton rocket, older Long March rockets (LM 1-4), PSLV, Fregat, and Briz-M upper stages. This combination is hypergolic, making for attractively simple ignition sequences. The major inconvenience is that these propellants are highly toxic and require careful handling.