Rocket engine propellants

A rocket engine uses stored rocket propellants as the reaction mass. Reaction mass forms a high speed propulsive jet of fluid, usually high temperature gas. Rocket engines are type of engines using reaction, producing thrust by ejecting mass rearward, according to the Newton's third law of action and reaction. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist.

RD-180 test firing at Marshall Space Flight Center, source wiki

Rocket engines produce thrust by the ejection of an exhaust fluid that has been accelerated to high speed through a propelling nozzle. The fluid is usually a gas created by high pressure combustion of solid or liquid propellants, consisting of fuel and oxidizer, inside a combustion chamber. As the gases expand through the nozzle, they are accelerated to very high speed, and the reaction to this pushes the engine in the opposite direction, as you can see on the Figure below. Rocket propellant is the reaction mass or working mass of a rocket, which is ejected at the highest achievable velocity from a rocket engine to produce thrust.

Thrust works on Newton's 3rd law - every action has an equal and opposite reaction

As a note to remind: The difference between the jet engine and rocket engine is that jet engines get their oxygen to burn fuel from the air they work in. Rockets need to carry their own oxygen with them, but that allows them to operate in space. That leads to another thing. Jet engines have two openings, an intake and an exhaust nozzle. Rocket engines on other hand have one opening only, an exhaust nozzle.

As, I wrote in previous article Jet and rocket engines, an internal combustion heat engines work on the principle of the ideal gas law. An internal combustion engine has a chamber, which has fuel added to it which ignites in order to raise the temperature of the gas. When the heat is added, the gas expands. Piston raises in case of piston engines. In case of turbine, the hot air is forced through the chamber and turns the turbine.

Rocket propellant is material used by the rocket as the reaction mass that is ejected from rocket engine to produce thrust. 

Thermal rocket engines use an inert propellant, heated by electricity (electrothermal propulsion) or a nuclear reactor (nuclear thermal rocket). Chemical rocket engines are powered by exothermic reduction-oxidation chemical reactions of the propellant. 

Chemical rockets can be grouped by phase. The main types of chemical rocket propellants are 4: solid, liquid, gas, and hybrid. 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.

Solid propellants

In case of solid rocket engines, propellant combustion occurs inside the motor casing. 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. 

Solid propellant rockets are either composites or single, double or triple bases. 

Composites are composed mostly of a mixture of granules of solid oxidized, such as ammonium nitrate, ammonium dinitramide, ammonium perchlorate, or potassium nitrate in a polymer binding agent, with flakes or powders of energetic fuel compounds (examples: RDX, HMX, aluminium, beryllium), plasticizers, stabilizers, and/or burn rate modifiers (iron oxide, copper oxide).

Single, double or triple based are homogeneous mixtures of one to three primary ingredients. These primary ingredients must include fuel and oxidizer and often include binders and plasticizers. All components are usually blended as liquids and cured in a single batch. They can play different roles. For example, RDX can be both a fuel and oxidizer while nitrocellulose can be a fuel, oxidizer and structural polymer. 

Liquid propellants

A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid propellants. Liquids are good because they have a reasonably high density and high specific impulse. The main types of liquid propellants are storable propellants, which tend to be hypergolic, and cryogenic propellants.

Liquid rockets can be monopropellant rockets using a single type of propellant, or bipropellant rockets using two types of propellant. Tripropellant rockets using three types of propellant are rare. Liquid propellants are also used in hybrid rockets, with some of the advantages of a solid rocket.

Bipropellant liquid rockets, a mixture of reducing fuel and oxidizing oxidizer is fed into a combustion chamber, typically using a turbopump. 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.

Current cryogenic types:

1. Liquid oxygen (LOX) and highly refined kerosene (RP-1): Used for the first stages of the Atlas V, Falcon 9, Falcon Heavy, Soyuz, Zenit, and developmental rockets like Angara and Long March 6. This combination is generally considered as the most practical for boosters that lift off at ground level and therefore must operate at full atmospheric pressure.

2. Liquid oxygen (LOX) and liquid hydrogen: Used on the Centaur upper stage, the Delta IV rocket, the H-IIA rocket, most stages of the Ariane 5, and the Space Launch System (SLS) core and upper stages.

3. Liquid oxygen (LOX) and liquid methane are planned for use on several rockets in development, including Vulcan, New Glenn, and SpaceX Starship.

Gas propellants

A gas propellant usually involves to use some sort of compressed gas. But because of low density and high mass of pressure vessel, gas propellants are not used very often. 

GOX (gaseous oxygen) was used as the oxidizer for the Buran program's orbital maneuvering system,  see my previous article Buran program.

Hybrid propellants

Hybrid propellants: a storable oxidizer used with a solid fuel, which retains most virtues of both liquids (high specific impulse ISP) and solids (simplicity).

Hybrid rocket propulsion system
source: wiki

In basic concept, a hybrid rocket consists of a pressure vessel (tank) containing the liquid oxidizer, the combustion chamber containing the solid propellant, and a mechanical device separating the two. When thrust is desired, a suitable ignition source is introduced in the combustion chamber and the valve is opened. The liquid oxidizer flows into the combustion chamber where it is vaporized and then reacted with the solid propellant. Combustion occurs in a boundary layer diffusion flame adjacent to the surface of the solid propellant.

Generally, the liquid propellant is the oxidizer and the solid propellant is the fuel because solid oxidizers are extremely dangerous and lower performing than liquid oxidizers. Furthermore, using a solid fuel such as Hydroxyl-terminated polybutadiene or paraffin wax allows for the incorporation of high-energy fuel additives such as aluminium, lithium, or metal hydrides.

Inert propellant

Thermal rockets use inert propellants of low molecular weight that are chemically compatible with the heating mechanism at high temperatures. 

Solar thermal rockets and nuclear thermal rockets typically propose to use liquid hydrogen for a specific impulse of around 600–900 seconds, or in some cases water that is exhausted as steam for a specific impulse of about 190 seconds. 

Nuclear thermal rockets use the heat of nuclear fission, replacing the chemical energy of the propellants in case of chemical rockets.  A nuclear engine was considered as a replacement for the J-2 used on the S-II and S-IVB stages on the Saturn V and Saturn I rockets.  Meanwhile, Soviets studied nuclear engines for their own moon rockets, especially upper stages of the N-1. However they never entered an extensive testing program like the one the U.S. conducted throughout the 1960s at the Nevada Test Site. American nuclear rockets did not fly before the space race ended.

Sketch of a solid core fission nuclear thermal rocket with tap-off turbine pump, source wiki

Solar thermal rockets use concentrated sunlight to heat a propellant. A solar thermal rocket is a theoretical spacecraft propulsion that makes use of solar power to directly heat reaction mass.

There are 2 solar thermal propulsion concepts, which are differing in the method by which they use solar power to heat up the propellant: indirect solar heating and direct solar heating. Most proposed designs for solar thermal rockets use hydrogen as the propellant due to its low molecular weight which gives specific impulse of up to 1000 seconds (10 kN·s/kg) using heat exchangers made of rhenium. Ammonia has been also proposed as a propellant. 

Solar electric propulsion refers to the combination of solar cells and electric thrusters to propel a spacecraft through outer space. Solar electric propulsion has a higher specific impulse than normal chemical rockets, thus requiring less propellant mass to be launched with a spacecraft.

There are few launched missions based on this system: BepiColombo mission to Mercury, Dawn asteroids mission, Deep Space 1 asteroid and comet mission, and Hayabusa asteroid mission.