A rocket engine is a type of heat engine that is used for spacecraft propulsion. Rocket engines take their reaction mass from one or more tanks or solid fuel and form it into a hypersonic jet, obtaining thrust in accordance with Newton's third law.
Principle of operation
Chemical rocket engines produce a high temperature gaseous exhaust. This is achieved by the combustion of solid, liquid or gaseous propellant, containing oxidiser and a fuel, within a combustion chamber at high pressure. The hot gas produced is then allowed to escape through a narrow hole (the throat), into a large bell or cone shaped expansion nozzle giving a rocket engine its characteristic shape. The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy. Exhaust speeds as high as 10 times the speed of sound at sea level are not uncommon.
Part of the rocket engine's thrust comes from the gas pressure inside the combustion chamber but the majority comes from the pressure against the inside of the expansion nozzle. Inside the combustion chamber the gas produces a similar force against all the sides of the combustion chamber but the throat gives no force producing an unopposed resultant force from the diametrically opposite end of the chamber. As the gases expand inside the nozzle they press against the bell's walls forcing the rocket engine in one direction, and accelerating the gases in the opposite direction.
The reaction mass's combustion temperature is typically far higher than the melting point of the nozzle and combustion chamber materials. It is critical that these materials be prevented from combusting, melting or vapourising to the point of failure. To avoid this problem rockets can use ablative cooling where materials erode in a controlled fashion, or very high temperature materials, such as graphite, ceramics or certain exotic metals.
Alternatively, rockets may use more common construction materials such as aluminum, steel or copper alloys and employ cooling systems to prevent the construction material itself becoming too hot. Many engines use regenerative cooling, where the propellant is passed through tubes around the combustion chamber or nozzle; other techniques such as curtain cooling or film cooling may be employed to give essentially unlimited nozzle and chamber life.
Types of rocket engines
By fuel type:
- Solid rocket motor
- Hybrid rocket motor
- Monopropellant rocket motor
- Bipropellant rocket
- Tripropellant rocket
- Dual mode propulsion rocket
- Nuclear-thermal rocket
- Electric rocket engine
By engine cycle:
- Solid rocket motor
- Pressure-fed cycle rocket engine
- Gas-generator cycle rocket engine
- Tap-off cycle rocket engine
- Expander cycle rocket engine
- Staged combustion cycle rocket engine
- Electrothermal rocket engine (arc-jets, resisto-jets)
- Electrostatic rocket engine (Kaufman thruster, radiofrequency ion thruster)
- Electromagnetic rocket engines (JxB thruster, eigenfield accelerator, stationary plasma thruster (SPT), plasma pulse thruster)