Liquid rocket propulsion is a method of propulsion that uses liquid propellants to generate the thrust needed to launch and propel a rocket. It is commonly used in space launch vehicles, missiles, and other high-speed vehicles that require a lot of energy to overcome Earth's gravity and travel at high speeds.
Liquid rockets source their energy from one or more liquid propellants. The propellant is mixed and ignited inside the rocket engine to create a high-velocity exhaust gas. The force of the gas escaping the engine through the nozzle creates the thrust that propels the rocket forward.
Liquid rocket engines offer several advantages over other types of rocket propulsion systems, such as solid rocket engines. These advantages include greater control over the amount of thrust produced, the ability to stop and restart the engine, and the ability to adjust the engine's performance during flight. However, liquid rocket engines also tend to be more complex and require more maintenance than solid rocket engines.
A liquid rocket propulsion system utilizes several major components for its functionality. Here is a description of some of the most important components of the rocket:
A liquid rocket generates thrust by combusting one or more propellants. To store these compounds, the propulsion system must include one or more tanks. The tanks themselves must be designed to be lightweight, while also being able to potentially withstand high internal pressures and cryogenic compounds.
A bipropellant rocket is a rocket which uses two distinct propellants for its propulsion. Inside the engine, the two propellants are mixed together with an injector before combustion. One propellant will act as the fuel, and the other propellant acts as the oxidizer. A commonly-used propellant combination is RP-1 (a refined kerosene) and LOX (a shorthand for Liquid Oxygen).
A monopropellant rocket, on the other hand, uses only a single fuel source. Inside the engine, a catalyst chemically breaks down the propellant into multiple chemical compounds. This process releases the thermal energy necessary to propel the rocket.
The feed system is responsible for delivering propellants from the tanks to the engine.
A pressure-fed feed system uses a pressurized gas to force the pressurant out of the tanks and into the engine. This gas is generally inert and is stored inside an additional pressurant tank.
In order for this process to function correctly, the pressure of the gas and propellants must be greater than the pressure inside the combustion chamber of the engine. Additionally, there must be an array of flow control valves or pressure regulators to ensure that fuel/oxidizer flows into the engine at the correct rate.
A pump-fed system typically uses a small portion of chemical energy already stored in the propellant to drive turbine(s) that power propellant pumps. Pump-fed engines are much more complicated than pressure-fed feed systems and beyond the scope of most amateur teams. However, they do not require thick-walled tanks to contain high pressures, and the overall weight savings make pump-fed systems a clear choice for larger vehicles. This video by Everyday Astronaut introduces a variety of pump types and rocket engine "cycles."
The engine is responsible for combusting/decomposing the propellants and ensuring the most efficient transfer of energy from the explosion to thrust.
An injector ensures that the propellants are mixed properly before combustion. The reaction takes place in the combustion chamber, and the remaining hot gas is then forced through a converging-diverging nozzle and out of the engine assembly.