Editing Pressure-fed engine (section)

{{Short description|Rocket engine operation method}}
[[Image:Pressure fed rocket cycle.svg|thumb|right|270px|Pressure-fed rocket cycle.  Propellant tanks are pressurized to directly supply fuel and oxidizer to the engine, eliminating the need for [[turbopump]]s.]]
The '''pressure-fed engine''' is a class of [[rocket engine]] designs. A separate gas supply, usually [[helium]], pressurizes the propellant tanks to force fuel and oxidizer to the combustion chamber. To maintain adequate flow, the tank pressures must exceed the combustion chamber pressure.

Pressure fed engines have simple plumbing and have no need for complex and occasionally unreliable [[turbopump]]s. A typical startup procedure begins with opening a valve, often a one-shot pyrotechnic device, to allow the pressurizing gas to flow through check valves into the propellant tanks. Then the propellant valves in the engine itself are opened. If the fuel and oxidizer are [[hypergolic]], they burn on contact; non-hypergolic fuels require an igniter. Multiple burns can be conducted by merely opening and closing the propellant valves as needed. If the pressurization system also has activating valves, they can be operated electrically, or by gas pressure controlled by smaller electrically operated valves.

Care must be taken, especially during long burns, to avoid excessive cooling of the pressurizing gas due to [[Adiabatic_process#Adiabatic_free_expansion_of_a_gas|adiabatic expansion]]. Cold helium won't liquify, but it could freeze a propellant, decrease tank pressures, or damage components not designed for low temperatures. The [[Apollo Lunar Module]] [[Descent Propulsion System]] was unusual in storing its helium in a [[Supercritical fluid|supercritical]] but very cold state. It was warmed as it was withdrawn through a [[heat exchanger]] from the ambient temperature fuel.<ref name=LMDE />

[[File:OMS Pod schematic.png|thumb|This is a diagram of the pressure fed, reusable [[Orbital Maneuvering System|Orbital Manouevering System]] pod, of which there were two on either side of the shuttle’s [[Stabilizer (aeronautics)|stabiliser]]. It was used on the [[Space Shuttle orbiter|Space Shuttle orbiter (or simply Space Shuttle)]] for [[Orbit insertion|orbital insertion]], manoeuvring the orbiter in space, and the deorbit burn. The [[AJ10|AJ10-190]] engines could be reused for up to 100 missions. ]]

[[File:Ssme_schematic_(updated).svg|thumb|Diagram of an [[RS-25|RS-25 (or Space Shuttle Main Engine)]], that used a [[Staged combustion cycle|twin shaft staged combustion cycle]]. There were three of these on the back of the orbiter. Comparing the diagram of the RS-25 to that of the Orbital Manoeuvring System (OMS), it is clear that the RS-25 engine is far more complex. The record for the most space shuttle missions an individual RS-25 engine has been used on is 19. ]]

Spacecraft [[Spacecraft attitude control|attitude control]] and [[orbital maneuver]]ing thrusters are almost universally pressure-fed designs.<ref name=London>{{cite book |author=JOHN R. LONDON III | title = LEO on the Cheap | publisher = Air University Press  | date = October 1994 | pages = 68–69 | url = http://www.quarkweb.com/foyle/leocheap_book.pdf | isbn = 0-89499-134-5}}</ref>
Examples include the Reaction Control (RCS) and the [[Space Shuttle Orbital Maneuvering System|Orbital Maneuvering (OMS)]] engines of the [[Space Shuttle]] orbiter; the RCS and Service Propulsion System (SPS) engines on the [[Apollo Command/Service Module]]; the [[SuperDraco]] (in-flight abort) and [[Draco_(rocket_engine_family)|Draco]] (RCS) engines on the [[SpaceX Dragon 2]]; and the RCS, ascent and descent engines on the [[Apollo Lunar Module]].<ref name=LMDE>{{cite web |title=LM Descent Propulsion Development Diary |access-date=5 June 2012 |url=http://www.astronautix.com/craft/lmdlsion.htm |publisher=Encyclopedia Astronautica |url-status=dead |archive-url=https://web.archive.org/web/20120606201132/http://www.astronautix.com/craft/lmdlsion.htm |archive-date=6 June 2012 }}</ref>

Some launcher [[upper stage]]s also use pressure-fed engines. These include the Aerojet [[AJ10]] and TRW [[TR-201]] used in the second stage of [[Delta II]] launch vehicle, 
and the [[Kestrel (rocket engine)|Kestrel]] engine of the [[Falcon 1]] by SpaceX.<ref name="spacex_fug">{{cite news |url=http://www.spacex.com/Falcon1UsersGuide.pdf |title=Falcon 1 Users Guide |date=2008-09-28 |access-date=5 June 2012 |publisher=SpaceX |url-status=dead |archive-url=https://web.archive.org/web/20121002181416/http://www.spacex.com/Falcon1UsersGuide.pdf |archive-date=2 October 2012 }}</ref>

The 1960s [[Sea Dragon (rocket)|Sea Dragon]] concept by [[Robert Truax]] for a [[big dumb booster]] would have used pressure-fed engines.

Pressure-fed engines have practical limits on propellant pressure, which in turn limits combustion chamber pressure.  High pressure propellant tanks require thicker walls and stronger materials which make the vehicle tanks heavier, thereby reducing performance and payload capacity.  The lower stages of [[launch vehicle]]s often use either [[Solid-fuel rocket|solid fuel]] or [[Pump-fed engine|pump-fed]] liquid fuel engines instead, where high pressure ratio nozzles are considered desirable.<ref name=London />

Other vehicles or companies using pressure-fed engine:
*[[OTRAG (rocket)]]
*[[Quad (rocket)]] of [[Armadillo Aerospace]]
*[[XCOR EZ-Rocket]] of [[XCOR Aerospace]]
*[[Masten Space Systems]]
*[[Aquarius Launch Vehicle]]
*NASA's [[Project Morpheus]] prototype lander
*[[NASA Mighty Eagle]] mini lunar lander
*[[Comisión Nacional de Actividades Espaciales|CONAE]]'s [[Tronador (rocket)|Tronador II]] upper stage{{cn|date=September 2014}}
*[[Copenhagen Suborbitals]]' Spica