Editing Liquid-propellant rocket (section)

== Principle of operation ==
Liquid rocket engines have tankage and pipes to store and transfer propellant, an injector system and one or more combustion chambers with associated [[rocket engine nozzle|nozzles]].

Typical liquid propellants have densities roughly similar to water, approximately {{cvt|0.7|to|1.4|g/cm3}}. An exception is [[liquid hydrogen]] which has a much lower density, while requiring only relatively modest [[Saturated vapor pressure|pressure to prevent vaporization]]. The density and low pressure of liquid propellants permit lightweight tankage: approximately 1% of the contents for dense propellants and around 10% for liquid hydrogen. The increased tank mass is due to liquid hydrogen's low density and the mass of the required insulation.

For injection into the combustion chamber, the propellant pressure at the injectors needs to be greater than the chamber pressure. This is often achieved with a pump. Suitable pumps usually use centrifugal [[turbopumps]] due to their high power and light weight, although [[Reciprocating pump|reciprocating pumps]] have been employed in the past. Turbopumps are usually lightweight and can give excellent performance; with an on-Earth weight well under 1% of the thrust. Indeed, overall [[Thrust-to-weight ratio|thrust to weight ratios]] including a turbopump have been as high as 155:1 with the SpaceX [[Merlin 1D]] rocket engine and up to 180:1 with the vacuum version.<ref>{{cite web |title=Thomas Mueller's answer to Is SpaceX's Merlin 1D's thrust-to-weight ratio of 150+ believable? - Quora |url=https://www.quora.com/Is-SpaceXs-Merlin-1Ds-thrust-to-weight-ratio-of-150+-believable/answer/thomas-mueller-11 |website=www.quora.com}}</ref> Instead of a pump, some designs use a tank of a high-pressure inert gas such as helium to pressurize the propellants. These rockets often provide lower [[delta-v]] because the mass of the pressurant tankage reduces performance. In some designs for high altitude or vacuum use the tankage mass can be acceptable.

The major components of a rocket engine are therefore the [[combustion chamber]] (thrust chamber), [[pyrotechnic igniter]], [[Rocket propellant|propellant]] feed system, valves, regulators, propellant tanks and the [[rocket engine nozzle]]. For feeding propellants to the combustion chamber, liquid-propellant engines are either [[Pressure-fed engine (rocket)|pressure-fed]] or [[Pump-fed engine|pump-fed]], with pump-fed engines working in a variety of [[engine cycle]]s.