View source for Liquid-propellant rocket

{{Short description|Rocket engine that uses liquid fuels and oxidizers}}
{{Distinguish|Rocket-powered aircraft}}
[[File:Liquid-Fuel Rocket Diagram.svg|thumb|300px|A simplified diagram of a liquid-propellant rocket.{{ordered list
 | list_style=margin-left:0;
 | item_style=list-style-position:inside;
 | [[Liquid rocket propellant|Liquid rocket fuel]].
 | [[Oxidizing agent|Oxidizer]].
 | Pumps carry the fuel and oxidizer.
 | The [[combustion chamber]] mixes and burns the two liquids.
 | Combustion product gasses enter the [[rocket engine nozzle |nozzle]] through a throat.
 | Exhaust exits the rocket.
}}]]
A '''liquid-propellant rocket''' or '''liquid rocket''' uses a [[rocket engine]] burning [[liquid rocket propellant|liquid propellants]]. (Alternate approaches use gaseous or [[Solid-propellant rocket |solid propellants]].) Liquids are desirable propellants because they have reasonably high density and their combustion products have high [[Specific impulse|specific impulse (''I''<sub>sp</sub>)]]. This allows the volume of the propellant tanks to be relatively low.

== Types ==
Liquid rockets can be [[monopropellant rocket]]s using a single type of propellant, or [[bipropellant rocket]]s using two types of propellant.  [[Tripropellant rocket]]s using three types of propellant are rare. Liquid oxidizer propellants are also used in [[hybrid rocket]]s, with some of the advantages of a [[solid rocket]]. '''Bipropellant liquid rockets''' use a liquid [[fuel]] such as [[liquid hydrogen]] or [[RP-1]], and a liquid [[oxidizer]] such as [[liquid oxygen]]. The engine may be a [[cryogenic rocket engine]], where the fuel and oxidizer, such as hydrogen and oxygen, are gases which have been liquefied at very low temperatures.

Most designs of liquid rocket engines are [[Rocket engine throttling|throttleable]] for variable thrust operation. Some allow control of the propellant mixture ratio (ratio at which oxidizer and fuel are mixed). Some can be shut down and, with a suitable ignition system or self-igniting propellant, restarted.

[[Hybrid rocket]]s apply a liquid or gaseous oxidizer to a solid fuel.<ref name="Sutton"/>{{rp|354–356}}

== Advantages and disadvantages ==
The use of liquid propellants has a number of advantages:
* A liquid rocket engine can be tested prior to use, whereas for a solid rocket motor a rigorous [[quality management]] must be applied during manufacturing to ensure high reliability.<ref name=":02">[http://cobweb.ecn.purdue.edu/~propulsi/propulsion/rockets/liquids.html NASA:Liquid rocket engines], 1998, Purdue University</ref>
* Liquid systems enable higher [[specific impulse]] than solids and hybrid rocket motors and can provide very high tankage efficiency.
* A liquid rocket engine can also usually be reused for several flights, as in the [[Space Shuttle]] and [[Falcon 9]] series rockets, although reuse of solid rocket motors was also effectively demonstrated during the Shuttle program.
* The flow of propellant into the combustion chamber can be throttled, which allows for control over the magnitude of the thrust throughout the flight. This enables real-time error correction during the flight along with efficiency gains.<ref name=":1">{{Cite book |last1=Heister |first1=Stephen D. |url=http://dx.doi.org/10.1017/9781108381376 |title=Rocket Propulsion |last2=Anderson |first2=William E. |last3=Pourpoint |first3=Timothée L. |last4=Cassady |first4=R. Joseph |date=2019-02-07 |publisher=Cambridge University Press |doi=10.1017/9781108381376 |isbn=978-1-108-38137-6|s2cid=203039055 }}</ref>
* Shutdown and restart capabilities allow for multiple burn cycles throughout a flight.<ref name=":22">{{Citation |chapter=History and principles of rocket propulsion |chapter-url=http://dx.doi.org/10.1007/3-540-27041-8_1 |series=Springer Praxis Books |date=2005 |pages=1–34 |access-date=2023-11-29 |publisher=Springer Berlin Heidelberg |doi=10.1007/3-540-27041-8_1 |isbn=978-3-540-22190-6 |title=Rocket and Spacecraft Propulsion }}</ref>
* In the case of an emergency, liquid propelled rockets can be shutdown in a controlled manner, which provides an extra level of safety and mission abort capability.<ref name=":22" />

[[File:Space Shuttle Main Engine Maintenance - GPN-2000-000548.jpg|right|thumb|Bipropellant liquid rockets are simple in concept but due to high temperatures and high speed moving parts, very complex in practice.]]
Use of liquid propellants can also be associated with a number of issues:

* Because the propellant is a very large proportion of the mass of the vehicle, the [[Center of gravity of an aircraft|center of mass]] shifts significantly rearward as the propellant is used; one will typically lose control of the vehicle if its center mass gets too close to the center of drag/pressure.
* When operated within an atmosphere, pressurization of the typically very thin-walled propellant tanks must guarantee positive [[gauge pressure]] at all times to avoid catastrophic collapse of the tank.
* Liquid propellants are subject to ''[[slosh]]'', which has frequently led to loss of control of the vehicle. This can be controlled with slosh baffles in the tanks as well as judicious control laws in the [[guidance system]].
* They can suffer from [[pogo oscillation]] where the rocket suffers from uncommanded cycles of acceleration.
* Liquid propellants often need [[ullage motor]]s in zero-gravity or during staging to avoid sucking gas into engines at start up. They are also subject to vortexing within the tank, particularly towards the end of the burn, which can also result in gas being sucked into the engine or pump.
* Liquid propellants can leak, especially [[hydrogen]], possibly leading to the formation of an explosive mixture.
* [[Turbopumps]] to pump liquid propellants are complex to design, and can suffer serious failure modes, such as overspeeding if they run dry or shedding fragments at high speed if metal particles from the manufacturing process enter the pump.
* [[Cryogenic propellant]]s, such as liquid oxygen, freeze atmospheric water vapor into ice. This can damage or block seals and valves and can cause leaks and other failures. Avoiding this problem often requires lengthy ''chilldown'' procedures which attempt to remove as much of the vapor from the system as possible. Ice can also form on the outside of the tank, and later fall and damage the vehicle. External foam insulation can cause issues as shown by the [[Space Shuttle Columbia disaster]]. Non-cryogenic propellants do not cause such problems.
* Non-storable liquid rockets require considerable preparation immediately before launch. This makes them less practical than [[solid rocket]]s for most weapon systems.

== 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.

== Pressurization ==
Liquid propellants are often pumped into the combustion chamber with a lightweight centrifugal [[turbopump]]. Recently, some aerospace companies have used electric pumps with batteries. In simpler, small engines, an inert gas stored in a tank at a high pressure is sometimes used instead of pumps to force propellants into the combustion chamber. These engines may have a higher mass ratio, but are usually more reliable, and are therefore used widely in satellites for orbit maintenance.<ref name="Sutton">{{cite book|last=Sutton|first=George P.|title=Rocket Propulsion Elements, 3rd edition|year=1963|publisher=John Wiley & Sons|location=New York|pages=25, 186, 187}}</ref>

==Propellants==
{{main article|Liquid rocket propellant}}
Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:

===Cryogenic===
* Liquid oxygen ([[LOX]], O<sub>2</sub>) and liquid [[hydrogen]] ([[LH2|LH{{sub|2}}]], H<sub>2</sub>) – [[Space Shuttle]] main engines, [[Space Launch System]] [[Space Launch System core stage|core stage]], [[Ariane 5]] main stage and the Ariane 5 ECA second stage, the [[BE-3]] of Blue Origin's New Shepard, the first and second stage of the [[Delta IV rocket|Delta IV]], the upper stages of the [[Ares I]], [[Saturn V (rocket)|Saturn V]]'s [[S-II|second]] and [[S-IVB|third stages]], [[Saturn IB (rocket)|Saturn IB]], and [[Saturn I (rocket)|Saturn I]] as well as [[Centaur (rocket stage)|Centaur]] rocket stage, the upper stages of the [[Long March 3]], [[Long March 5]], [[Long March 8]], the first stage and second stage of the [[H-II]], [[H-IIA]], [[H-IIB]], and the upper stage of the [[Geosynchronous Satellite Launch Vehicle|GSLV Mk-II]] and [[Geosynchronous Satellite Launch Vehicle Mark III|GSLV Mk-III]]. The main advantages of this mixture are a clean burn (water vapor is the only combustion product) and high performance.<ref name="jaxa-lng">{{cite web |url=https://global.jaxa.jp/projects/engineering/components/lng/index.html |title=About LNG Propulsion System |work=[[JAXA]] |access-date=2020-08-25}}</ref>
* Liquid oxygen (LOX) and [[liquid methane rocket fuel|liquid methane]] (CH<sub>4</sub>, [[liquefied natural gas]], LNG) – the [[Raptor (rocket engine family)|Raptor]] (SpaceX) and [[BE-4]] (Blue Origin) engines. (See also [[Propulsion Cryogenics & Advanced Development]] project of NASA, and [[Project Morpheus]].)

One of the most efficient mixtures, [[oxygen]] and [[hydrogen]], suffers from the extremely low temperatures required for storing liquid hydrogen (around {{cvt|20|K|disp=or}}) and very low fuel density ({{cvt|70|kg/m3|lb/ft3|disp=or}}, compared to RP-1 at {{cvt|820|kg/m3|lb/ft3|disp=or}}), necessitating large tanks that must also be lightweight and insulating. Lightweight foam insulation on the [[Space Shuttle external tank]] led to the {{OV|102}}'s [[Space Shuttle Columbia disaster|destruction]], as a piece broke loose, damaged its wing and caused it to break up on [[atmospheric reentry]].

Liquid methane/LNG has several advantages over LH{{sub|2}}. Its performance (max. [[specific impulse]]) is lower than that of LH{{sub|2}} but higher than that of RP1 (kerosene) and solid propellants, and its higher density, similarly to other hydrocarbon fuels, provides higher thrust to volume ratios than LH{{sub|2}}, although its density is not as high as that of RP1.<ref name="airbus"/>  This makes it specially attractive for [[reusable launch system]]s because higher density allows for smaller motors, propellant tanks and associated systems.<ref name="jaxa-lng" /> LNG also burns with less or no soot (less or no coking) than RP1, which eases reusability when compared with it, and LNG and RP1 burn cooler than LH{{sub|2}} so LNG and RP1 do not deform the interior structures of the engine as much. This means that engines that burn LNG can be reused more than those that burn RP1 or LH{{sub|2}}. Unlike engines that burn LH{{sub|2}}, both RP1 and LNG engines can be designed with a shared shaft with a single turbine and two turbopumps, one each for LOX and LNG/RP1.<ref name="airbus">{{cite web|title=LOX/Methane The Future is Green |first=Dr. Gerald |last=Hagemann |date=November 4, 2015|url=http://www.academie-air-espace.com/upload/doc/ressources/Launchers/slides/hagemann.pdf|access-date=November 29, 2022}}</ref> In space, LNG does not need heaters to keep it liquid, unlike RP1.<ref>{{cite web|title=Methane Engine Just for Future Space Transportation |url=https://www.ihi.co.jp/var/ezwebin_site/storage/original/application/c947f865f960ed20f82895dcaa4bbbb1.pdf|publisher=IHI Corporation|access-date=November 29, 2022}}</ref><!--less soot eases fuel injector unclogging and refurbishment for reusability. and methane can also be used to autogenously pressurize rocket fuel tanks, eliminating the need for Helium, which is normally used to provide additional structural stability to the rocket. --> LNG is less expensive, being readily available in large quantities. It can be stored for more prolonged periods of time, and is less explosive than LH{{sub|2}}.<ref name="jaxa-lng"/>

===Semi-cryogenic===
* Liquid oxygen (LOX) and [[RP-1]] (kerosene) – [[Saturn V (rocket)|Saturn V]]'s [[S-IC|first stage]], [[Zenit rocket]], [[R-7 Semyorka|R-7]]-derived vehicles including [[Soyuz (rocket family)|Soyuz]], [[Delta rocket|Delta]], [[Saturn I (rocket)|Saturn I]], and [[Saturn IB (rocket)|Saturn IB]] first stages, [[Titan (rocket family)|Titan I]] and [[Atlas rocket]]s, [[Falcon 1]] and [[Falcon 9]], [[Long March 5]], [[Long March 6]], [[Long March 7]] and [[Long March 8]] first stages. 
* Liquid oxygen (LOX) and alcohol ([[ethanol]], C<sub>2</sub>H<sub>5</sub>OH) – early liquid rockets, like [[Germany|German]] ([[World War II]]) A4, aka [[V-2]], and [[Redstone (rocket)|Redstone]]
* Liquid oxygen (LOX) and [[gasoline]] – [[Robert Goddard (scientist)|Robert Goddard]]'s first liquid rocket
* Liquid oxygen (LOX) and [[carbon monoxide]] (CO) – proposed for a Mars ''hopper'' vehicle (with a specific impulse of approximately 250{{nbsp}}s), principally because carbon monoxide and oxygen can be straightforwardly produced by [[Zirconia]] electrolysis from the Martian atmosphere without requiring use of any of the Martian water resources to obtain Hydrogen.<ref name="landis2001">{{cite journal |last=Landis |title=Mars Rocket Vehicle Using In Situ Propellants |journal=Journal of Spacecraft and Rockets |date=2001 |volume=38 |issue=5 |pages=730–735 |doi=10.2514/2.3739 |url=http://arc.aiaa.org/doi/abs/10.2514/2.3739?journalCode=jsr |bibcode=2001JSpRo..38..730L |url-access=subscription }}</ref>

=== Non-cryogenic/storable/hypergolic ===
[[File:Messerschmitt Me 163B USAF.jpg|thumb|left| The [[NMUSAF]]'s Me 163B ''Komet'' rocket plane]]

Many non-cryogenic bipropellants are [[hypergolic propellant|hypergolic]] (self igniting).
* [[T-Stoff]] (80% hydrogen peroxide, H<sub>2</sub>O<sub>2</sub> as the oxidizer) and [[C-Stoff]] (methanol, {{chem2|CH3OH}}, and hydrazine hydrate, {{chem2|N2H4*''n''(H2O)}} as the fuel) – used for the Hellmuth-Walter-Werke [[HWK 109-509]]A, -B and -C engine family used on the [[Messerschmitt Me 163]]B Komet, an operational rocket fighter plane of [[World War II]], and [[Bachem Ba 349|Ba 349 ''Natter'']] crewed [[Takeoff#Vertical takeoff|VTO]] interceptor prototypes.
* [[Nitric acid]] (HNO<sub>3</sub>) and kerosene – [[Soviet Union|Soviet]] [[Bereznyak-Isayev BI-1|BI-1]] and [[Mikoyan-Gurevich I-270|MiG I-270]] rocket fighter prototypes, [[Scud]]-A, aka [[SS-1]] [[SRBM]]
* Inhibited red fuming nitric acid (I[[RFNA]], HNO<sub>3</sub> + N<sub>2</sub>O<sub>4</sub>) and unsymmetric dimethyl hydrazine ([[UDMH]], (CH<sub>3</sub>)<sub>2</sub>N<sub>2</sub>H<sub>2</sub>) – Soviet [[Scud]]-C, aka [[SS-1]]-c,-d,-e
* Nitric acid 73% with [[dinitrogen tetroxide]] 27% (AK27) and kerosene/gasoline mixture (TM-185) – various Russian (USSR) cold-war ballistic missiles ([[R-12 Dvina|R-12]], [[Scud]]-B,-D), [[Iran]]: [[Shahab-5]], [[North Korea]]: [[Taepodong-2]]
* [[High-test peroxide]] (H<sub>2</sub>O<sub>2</sub>) and kerosene – [[United Kingdom|UK]] (1970s) [[Black Arrow]], [[United States|USA]] Development (or study): BA-3200
* [[Hydrazine]] (N<sub>2</sub>H<sub>4</sub>) and [[red fuming nitric acid]] – [[MIM-3 Nike Ajax]] Anti-aircraft missile
* Unsymmetric dimethylhydrazine ([[UDMH]]) and [[dinitrogen tetroxide]] (N<sub>2</sub>O<sub>4</sub>) – [[Proton rocket|Proton]], [[Rokot]], [[Long March 2 (rocket family)|Long March 2]] (used to launch [[Shenzhou (spacecraft)|Shenzhou]] crew vehicles.)
* [[File:LGM-25C Titan II Test Launch.jpg|thumb|Titan II]][[Aerozine 50]] (50% UDMH, 50% hydrazine) and [[dinitrogen tetroxide]] (N<sub>2</sub>O<sub>4</sub>) – [[Titan (rocket family)|Titans 2–4]], Apollo [[lunar module]], Apollo [[service module]], interplanetary probes (Such as [[Voyager 1]] and [[Voyager 2]])
* [[Monomethylhydrazine]] (MMH, (CH<sub>3</sub>)HN<sub>2</sub>H<sub>2</sub>) and dinitrogen tetroxide (N<sub>2</sub>O<sub>4</sub>) – [[Space Shuttle orbiter]]'s [[orbital maneuvering system]] (OMS) engines and [[Reaction control system]] (RCS) thrusters. [[SpaceX]]'s [[Draco (rocket engine)|Draco]] and SuperDraco engines for the [[Dragon spacecraft]].

For [[storable propellant|storable]] [[ICBM]]s and most spacecraft, including crewed vehicles, planetary probes, and satellites, storing cryogenic propellants over extended periods is unfeasible. Because of this, mixtures of [[hydrazine]] or its derivatives in combination with nitrogen oxides are generally used for such applications, but are toxic and [[carcinogenic]]. Consequently, to improve handling, some crew vehicles such as [[Dream Chaser]] and [[Space Ship Two]] plan to use [[hybrid rocket]]s with non-toxic fuel and oxidizer combinations.

==Injectors==
The injector implementation in liquid rockets determines the percentage of the theoretical performance of the [[Rocket engine nozzle|nozzle]] that can be achieved. A poor injector performance causes unburnt propellant to leave the engine, giving poor efficiency.

Additionally, injectors are also usually key in reducing thermal loads on the nozzle; by increasing the proportion of fuel around the edge of the chamber, this gives much lower temperatures on the walls of the nozzle.

===Types of injectors===
Injectors can be as simple as a number of small diameter holes arranged in carefully constructed patterns through which the fuel and oxidizer travel. The speed of the flow is determined by the square root of the pressure drop across the injectors, the shape of the hole and other details such as the density of the propellant.

The first injectors used on the V-2 created parallel jets of fuel and oxidizer which then combusted in the chamber. This gave quite poor efficiency.

Injectors today classically consist of a number of small holes which aim jets of fuel and oxidizer so that they collide at a point in space a short distance away from the injector plate. This helps to break the flow up into small droplets that burn more easily.

The main types of injectors are
* Shower head
* Self-impinging doublet
* Cross-impinging triplet
* Centripetal or swirling
* [[pintle injector|Pintle]]

The pintle injector permits good mixture control of fuel and oxidizer over a wide range of flow rates. The pintle injector was used in the [[Apollo Lunar Module]] engines ([[Descent Propulsion System]]) and the [[Kestrel (rocket engine)|Kestrel]] engine, it is currently used in the [[Merlin (rocket engine)|Merlin]] engine on [[Falcon 9]] and [[Falcon Heavy]] rockets.

The [[RS-25]] engine designed for the [[Space Shuttle]] uses a system of fluted posts, which use heated hydrogen from the preburner to vaporize the liquid oxygen flowing through the center of the posts<ref>Sutton, George P. and Biblarz, Oscar, ''Rocket Propulsion Elements'', 7th ed., John Wiley & Sons, Inc., New York, 2001.</ref> and this improves the rate and stability of the combustion process; previous engines such as the F-1 used for the [[Apollo program]] had significant issues with oscillations that led to destruction of the engines, but this was not a problem in the RS-25 due to this design detail.

[[Valentin Glushko]] invented the centripetal injector in the early 1930s, and it has been almost universally used in Russian engines. Rotational motion is applied to the liquid (and sometimes the two propellants are mixed), then it is expelled through a small hole, where it forms a cone-shaped sheet that rapidly atomizes.  Goddard's first liquid engine used a single impinging injector.  German scientists in WWII experimented with impinging injectors on flat plates, used successfully in the [[Wasserfall]] missile.

===Combustion stability===

To avoid instabilities such as ''chugging,'' which is a relatively low speed oscillation, the engine must be designed with enough pressure drop across the injectors to render the flow largely independent of the chamber pressure. This pressure drop is normally achieved by using at least 20% of the chamber pressure across the injectors.

Nevertheless, particularly in larger engines, a high speed combustion oscillation is  easily triggered, and these are not well understood. These high speed oscillations tend to disrupt the gas side boundary layer of the engine, and this can cause the cooling system to rapidly fail, destroying the engine. These kinds of oscillations are much more common on large engines, and plagued the development of the [[Saturn V]], but were finally overcome.

Some combustion chambers, such as those of the [[RS-25]] engine, use [[Helmholtz resonator]]s as damping mechanisms to stop particular resonant frequencies from growing.

To prevent these issues the RS-25 injector design instead went to a lot of effort to vaporize the propellant prior to injection into the combustion chamber. Although many other features were used to ensure that instabilities could not occur, later research showed that these other features were unnecessary, and the gas phase combustion worked reliably.

Testing for stability often involves the use of small explosives. These are detonated within the chamber during operation, and causes an impulsive excitation. By examining the pressure trace of the chamber to determine how quickly the effects of the disturbance die away, it is possible to estimate the stability and redesign features of the chamber if required.

==Engine cycles==
For liquid-propellant rockets, four different ways of powering the injection of the propellant into the chamber are in common use.<ref>{{cite web|url= http://www.aero.org/publications/crosslink/winter2004/03_sidebar3.html|title= Sometimes, Smaller is Better|access-date= 2010-06-01|archive-url= https://web.archive.org/web/20120414212704/http://www.aero.org/publications/crosslink/winter2004/03_sidebar3.html|archive-date= 2012-04-14|url-status= dead}}</ref>

Fuel and oxidizer must be pumped into the combustion chamber against the pressure of the hot gasses being burned, and engine power is limited by the rate at which propellant can be pumped into the combustion chamber. For atmospheric or launcher use, high pressure, and thus high power, engine cycles are desirable to minimize [[gravity drag]]. For orbital use, lower power cycles are usually fine.

;[[Pressure-fed cycle (rocket)|Pressure-fed cycle]]: The propellants are forced in from pressurised (relatively heavy) tanks. The heavy tanks mean that a relatively low pressure is optimal, limiting engine power, but all the fuel is burned, allowing high efficiency. The pressurant used is frequently helium due to its lack of reactivity and low density. Examples: [[AJ-10]], used in the Space Shuttle [[Orbital Maneuvering System|OMS]], Apollo [[Service Propulsion System|SPS]], and the second stage of the [[Delta II]].
;[[Electric pump-fed engine|Electric pump-fed]]: An [[electric motor]], generally a [[brushless DC electric motor]], drives the [[pump]]s. The electric motor is powered by a battery pack. It is relatively simple to implement and reduces the complexity of the [[turbomachinery]] design, but at the expense of the extra dry mass of the battery pack. Example engine is the [[Rutherford (rocket engine)|Rutherford]] designed and used by [[Rocket Lab]].
;[[Gas-generator cycle (rocket)|Gas-generator cycle]]: A small percentage of the propellants are burnt in a preburner to power a turbopump and then exhausted through a separate nozzle, or low down on the main one. This results in a reduction in efficiency since the exhaust contributes little or no thrust, but the pump turbines can be very large, allowing for high power engines. Examples: [[Saturn V]]'s [[F-1 engine|F-1]] and [[J-2 engine|J-2]], [[Delta IV]]'s [[RS-68]], [[Ariane 5]]'s [[HM7B]], [[Falcon 9 v1.1|Falcon 9]]'s [[Merlin 1D|Merlin]].
;[[Tap-off cycle]]: Takes hot gases from the main [[combustion chamber]] of the rocket engine and routes them through engine [[turbopump]] turbines to pump propellant, then is exhausted. Since not all propellant flows through the main combustion chamber, the tap-off cycle is considered an open-cycle engine. Examples include the [[J-2S]] and [[BE-3]].
;[[Expander cycle]]: Cryogenic fuel (hydrogen, or methane) is used to cool the walls of the combustion chamber and nozzle. Absorbed heat vaporizes and expands the fuel which is then used to drive the turbopumps before it enters the combustion chamber, allowing for high efficiency, or is bled overboard, allowing for higher power turbopumps. The limited heat available to vaporize the fuel constrains engine power. Examples: [[RL10]] for [[Atlas V]] and Delta IV second stages (closed cycle), [[H-II]]'s [[LE-5]] (bleed cycle).
;[[Staged combustion cycle (rocket)|Staged combustion cycle]]: A fuel- or oxidizer-rich mixture is burned in a preburner and then drives turbopumps, and this high-pressure exhaust is fed directly into the main chamber where the remainder of the fuel or oxidizer undergoes combustion, permitting very high pressures and efficiency. Examples: [[SSME]], [[RD-191]], [[LE-7]].
;[[Staged combustion cycle (rocket)#Full-flow staged combustion cycle|Full-flow staged combustion cycle]]: Fuel- and oxidizer-rich mixtures are burned in separate preburners and driving the turbopumps, then both high-pressure exhausts, one oxygen rich and the other fuel rich, are fed directly into the main chamber where they combine and combust, permitting very high pressures and high efficiency. Example: [[SpaceX Raptor]].

===Engine cycle tradeoffs===
Selecting an engine cycle is one of the earlier steps to rocket engine design.  A number of tradeoffs arise from this selection, some of which include:
{| class="wikitable"
|-
|+ Tradeoff comparison among popular engine cycles
|-
! rowspan=2 |
! colspan=4 | Cycle type
|-
! Gas generator
! Expander cycle
! Staged-combustion
! Pressure-fed
|-
| {{Yes|'''Advantages'''}}
| Simple; low dry mass; allows for high power turbopumps for high thrust
| High specific impulse; fairly low complexity
| High specific impulse; high combustion chamber pressures allowing for high thrust
| Simple; no turbopumps; low dry mass; high specific impulse
|-
| {{No|'''Disadvantages'''}}
| Lower specific impulse
| Must use cryogenic fuel; heat transfer to the fuel limits available power to the turbine and thus engine thrust
| Greatly increased complexity &, therefore, mass (more-so for full-flow)
| Tank pressure limits combustion chamber pressure and thrust; heavy tanks and associated pressurization hardware
|}

==Cooling==
{{main article|Rocket engine cooling}}
Injectors are commonly laid out so that a fuel-rich layer is created at the combustion chamber wall. This reduces the temperature there, and downstream to the throat and even into the nozzle and permits the combustion chamber to be run at higher pressure, which permits a higher expansion ratio nozzle to be used which gives a higher ''I{{sub|SP}}'' and better system performance.<ref>Rocket Propulsion elements - Sutton Biblarz, section 8.1</ref> A liquid rocket engine often employs [[regenerative cooling (rocketry)|regenerative cooling]], which uses the fuel or less commonly the oxidizer to cool the chamber and nozzle.

==Ignition==
Ignition can be performed in many ways, but perhaps more so with liquid propellants than other rockets a consistent and significant ignitions source is required; a delay of ignition (in some cases as small as a few tens of milliseconds) can cause overpressure of the chamber due to excess propellant. A [[hard start]] can even cause an engine to explode.

Generally, ignition systems try to apply flames across the injector surface, with a mass flow of approximately 1% of the full mass flow of the chamber.

Safety interlocks are sometimes used to ensure the presence of an ignition source before the main valves open; however reliability of the interlocks can in some cases be lower than the ignition system. Thus it depends on whether the system must fail safe, or whether overall mission success is more important. Interlocks are rarely used for upper, uncrewed stages where failure of the interlock would cause loss of mission, but are present on the RS-25 engine, to shut the engines down prior to liftoff of the Space Shuttle. In addition, detection of successful ignition of the igniter is surprisingly difficult, some systems use thin wires that are cut by the flames, pressure sensors have also seen some use.

Methods of ignition include [[Pyrotechnic initiator|pyrotechnic]], electrical (spark or hot wire), and chemical.  [[Hypergolic]] propellants have the advantage of self igniting, reliably and with less chance of hard starts.  In the 1940s, the Russians began to start engines with hypergols, to then switch over to the primary propellants after ignition. This was also used on the American [[F-1 rocket engine]] on the [[Apollo program]].

Ignition with a [[pyrophoric]] agent: [[Triethylaluminium]] ignites on contact with air and will ignite and/or decompose on contact with water, and with any other oxidizer—it is one of the few substances sufficiently pyrophoric to ignite on contact with cryogenic [[liquid oxygen]]. The [[enthalpy of combustion]], Δ<sub>c</sub>H°, is {{cvt|-5105.70|±|2.90|kJ/mol}}. Its easy ignition makes it particularly desirable as a [[rocket engine]] [[Pyrotechnic initiator|ignitor]].  May be used in conjunction with [[triethylborane]] to create triethylaluminum-triethylborane, better known as TEA-TEB.

==History==
{{main|History of rockets}}
===Russia–Soviet Union===

[[File:Ракета 09 и 10.jpg|thumb|Rocket 09 (left) and 10 (GIRD-09 and GIRD-X). Museum of Cosmonautics and Rocket Technology; St. Petersburg.|352x352px]]
The idea of a liquid-fueled rocket as understood in the modern context first appeared in 1903 in the book ''Exploration of the Universe with Rocket-Propelled Vehicles''<ref>Russian title ''Issledovaniye mirovykh prostranstv reaktivnymi priborami'' (''Исследование мировых пространств реактивными приборами'')</ref> by the Russian rocket scientist [[Konstantin Eduardovitch Tsiolkovsky|Konstantin Tsiolkovsky]]. The magnitude of his contribution to [[astronautics]] is astounding, including the [[Tsiolkovsky rocket equation]], multi-staged rockets, and using liquid oxygen and liquid hydrogen in liquid propellant rockets.{{sfn|Siddiqi|2000|p=1}} Tsiolkovsky influenced later rocket scientists throughout Europe, like [[Wernher von Braun]]. Soviet search teams at [[Peenemünde]] found a German translation of a book by Tsiolkovsky of which "almost every page...was embellished by von Braun's comments and notes."{{sfn|Siddiqi|2000|p=27}} Leading Soviet rocket-engine designer [[Valentin Glushko]] and rocket designer [[Sergey Korolev]] studied Tsiolkovsky's works as youths{{sfn|Siddiqi|2000|p=6–7,333}} and both sought to turn Tsiolkovsky's theories into reality.{{sfn|Siddiqi|2000|p=3,166,182,187,205–206,208}} 

From 1929 to 1930 in [[Leningrad]] Glushko pursued rocket research at the [[Gas Dynamics Laboratory]] (GDL), where a new research section was set up for the study of liquid-propellant and [[Spacecraft electric propulsion|electric rocket engines]]. This resulted in the creation of ORM (from "Experimental Rocket Motor" in Russian) engines {{ill|ORM-1|ru|ОРМ-1}} to {{ill|ORM-52|ru|ОРМ-52}}.<ref name="Glushko">{{cite book |last1=Glushko |first1=Valentin |title=Developments of Rocketry and Space Technology in the USSR |date=1 January 1973 |publisher=Novosti Press Pub. House |pages=12–14,19|oclc=699561269}}</ref>  A total of 100 bench tests of liquid-propellant rockets were conducted using various types of fuel, both low and high-boiling and thrust up to 300&nbsp;kg was achieved.<ref name="RSB_GDL">{{cite web |last1=Zak |first1=Anatoly |title=Gas Dynamics Laboratory |url=http://www.russianspaceweb.com/gdl.html |website=Russian Space Web |access-date=20 July 2022}}</ref><ref name="Glushko" />

During this period in [[Moscow]], [[Friedrich Zander|Fredrich Tsander]] – a scientist and inventor – was designing and building liquid rocket engines which ran on compressed air and gasoline. Tsander investigated high-energy fuels including powdered metals mixed with gasoline. In September 1931 Tsander formed the Moscow based '[[Group for the Study of Reactive Motion]]',{{sfn|Chertok|2005|p=165 Vol 1}} better known by its Russian acronym "GIRD".{{sfn|Siddiqi|2000|p=4}} In May 1932, Sergey Korolev replaced Tsander as the head of GIRD. On 17 August 1933, [[Mikhail Tikhonravov]] launched the first Soviet liquid-propelled rocket (the GIRD-9), fueled by liquid oxygen and jellied gasoline. It reached an altitude of {{convert|400|m|ft}}.<ref>{{Cite web|url=https://www.airspacemag.com/space/the-man-behind-the-curtain-22131111/|title=The Man Behind the Curtain|date=November 2007|author=[[Asif Azam Siddiqi|Asif Siddiqi]]|archiveurl=https://web.archive.org/web/20210403054225/https://www.airspacemag.com/space/the-man-behind-the-curtain-22131111/|archivedate=2021-04-03|url-status=live}}</ref> In January 1933 Tsander began development of the GIRD-X rocket. This design burned liquid oxygen and gasoline and was one of the first engines to be regeneratively cooled by the liquid oxygen, which flowed around the inner wall of the combustion chamber before entering it. Problems with burn-through during testing prompted a switch from gasoline to less energetic alcohol. The final missile, {{convert|2.2|m|ft}} long by {{convert|140|mm|in}} in diameter, had a mass of {{convert|30|kg|lb}}, and it was anticipated that it could carry a {{convert|2|kg|lb}} payload to an altitude of {{convert|5.5|km|mi}}.<ref name=Albrecht>{{cite book | last = Albrecht | first = Ulrich | title = The Soviet Armaments Industry | publisher = Routledge | date = 1993 | pages = 74–75 |isbn=3-7186-5313-3}}</ref> The GIRD X rocket was launched on 25 November 1933 and flew to a height of 80 meters.<ref name='Tsander'>{{cite book |last1=Tsander |first1=F. A. |title=Problems of Flight by Jet Propulsion-Interplanetary Flights (Translated from Russian) |date=1964 |publisher=Israel Program for Scientific Translations |pages=32, 38-39, 58-59 |url=https://epizodyspace.ru/bibl/inostr-yazyki/nasa/tsander_problems.pdf |access-date=13 June 2022}}</ref>

In 1933 GDL and GIRD merged and became the [[Reactive Scientific Research Institute]] (RNII).  At RNII Gushko continued the development of liquid propellant rocket engines ОРМ-53 to ОРМ-102, with {{ill|ORM-65|ru|ОРМ-65}} powering the [[Korolyov RP-318|RP-318 rocket-powered aircraft]].<ref name="Glushko" /> In 1938 [[Leonid Dushkin]] replaced Glushko and continued development of the ORM engines, including the engine for the rocket powered interceptor, the [[Bereznyak-Isayev BI-1]].<ref>{{Cite book |last1=Gordon |first1=E. |title=Soviet X-planes |last2=Sweetman |first2=Bill |publisher=Motorbooks International |others=Bill Sweetman |year=1992 |isbn=978-0-87938-498-2 |location=Osceola, WI |pages=47 |oclc=22704082}}</ref> At RNII Tikhonravov worked on developing oxygen/alcohol liquid-propellant rocket engines.{{sfn|Chertok|2005|p=167 Vol 1}} Ultimately liquid propellant rocket engines were given a low priority during the late 1930s at RNII, however the research was productive and very important for later achievements of the Soviet rocket program.{{sfn|Siddiqi|2000|p=8-9}}

===Peru===
[[File:Paulet_Avion-Torpedo_System.png|thumb|[[Pedro Paulet]]'s ''[[Avión Torpedo|Avion Torpedo]]'' of 1902, featuring a [[Aircraft canopy|canopy]] fixed to a [[Delta wing|delta]] [[tiltwing]] for horizontal or vertical flight.]]
Peruvian [[Pedro Paulet]], who had experimented with rockets throughout his life in [[Peru]], wrote a letter to [[El Comercio (Peru)|''El Comercio'']] in [[Lima]] in 1927, claiming he had experimented with a liquid rocket engine while he was a student in Paris three decades earlier.<ref name=":6">{{Cite journal |last=Paulet de Vásquez |first=Sara |date=2002 |title=Pedro Paulet: pionero peruano del espacio |url=https://docplayer.es/10525831-Pedro-paulet-pionero-peruano-del-espacio.html |journal=Ciencia y tecnología |location=[[Lima]] |pages=5–12}}</ref><ref>{{cite journal|url=https://ntrs.nasa.gov/search.jsp?R=19770026106 | title = The alleged contributions of Pedro E. Paulet to liquid-propellant rocketry | journal = Nasa, Washington Essays on the History of Rocketry and Astronautics, Vol. 2 | date = September 1977 | publisher = NASA| last1 = Ordway | first1 = F. I. }}</ref> Historians of early rocketry experiments, among them [[Max Valier]], [[Willy Ley]], and [[John Drury Clark|John D. Clark]], have given differing amounts of credence to Paulet's report. Valier applauded Paulet's liquid-propelled rocket design in the Verein für Raumschiffahrt publication ''Die Rakete'', saying the engine had "amazing power" and that his plans were necessary for future rocket development.{{sfn|Mejía|2017|pp=115-116}} [[Hermann Oberth]] would name Paulet as a pioneer in rocketry in 1965.<ref name=":42">{{Cite book |last=Fitzgerald |first=Michael |title=Hitler's Secret Weapons of Mass Destruction: The Nazi Plan for Final Victory |year=2018 |pages=Chapter 3 |quote=Paulet was clearly a pioneer in the field of rocketry and it is unsurprising that the Nazis were keen to recruit him to assist their efforts. The German Astronautical Society invited him to Germany to become part of a team of researchers into rocket propulsion and he was initially interested, but when he discovered that the intention was to construct a weapon that would be used for military purposes he declined the invitation. As late as 1965, Oberth described him as one of the true pioneers of rocket science.}}</ref> [[Wernher von Braun]] would also describe Paulet as "the pioneer of the liquid fuel propulsion motor" and stated that "Paulet helped [[Moon landing|man reach the Moon]]".<ref name=":6" /><ref name=":0">{{Cite news |title=El peruano que se convirtió en el padre de la astronáutica inspirado por Julio Verne y que aparece en los nuevos billetes de 100 soles |language=es |work=[[BBC News]] |url=https://www.bbc.com/mundo/noticias-america-latina-38197437 |access-date=2022-03-11}}</ref><ref name="vb2">{{cite book |last1=Von Braun |first1=Wernher |url=https://books.google.com/books?id=Z8huNAAACAAJ&q=Histoire+mondiale+de+lastronautique |title=Histoire Mondiale de L'Astronautique |last2=Ordway III |first2=Frederick I. |publisher=Larousse / Paris -Match |year=1968 |location=París |pages=51–52}}</ref><ref name=":2">{{Cite book |last=Fitzgerald |first=Michael |title=Hitler's Secret Weapons of Mass Destruction: The Nazi Plan for Final Victory |year=2018 |pages=Chapter 3 |quote=Even Wernher von Braun described Paulet as 'one of the fathers of aeronautics' and 'the pioneer of the liquid fuel propulsion motor'. He declared that 'by his efforts, Paulet helped man reach the Moon'.}}</ref><ref name=":5">{{Cite book |last=Harding |first=Robert C. |title=Space Policy in Developing Countries: The Search for Security and Development on the Final Frontier |publisher=[[Routledge]] |year=2012 |isbn=9781136257902 |pages=156 |quote=Peru holds a special place among Latin America's EMSAs because the country was home to Pedro Paulet, who invented the world's first liquid-propelled rocket engine in 1895 and the first modern rocket propulsion system in 1900. ... According to Wernher von Braun, 'Paulet should be considered the pioneer of the liquid fuel propulsion motor ... by his efforts, Paulet helped man reach the moon.' Paulet went on to found Peru's National Pro-Aviation League, a precursor of the Peruvian Air Force.}}</ref> Paulet was later approached by [[Nazi Germany]], being invited to join the [[Astronomische Gesellschaft]] to help develop rocket technology, though he refused to assist after discovering that the project was destined for weaponization and never shared the formula for his propellant.<ref name=":03">{{Cite news |title=El peruano que se convirtió en el padre de la astronáutica inspirado por Julio Verne y que aparece en los nuevos billetes de 100 soles |language=es |work=[[BBC News]] |url=https://www.bbc.com/mundo/noticias-america-latina-38197437 |access-date=2022-03-11}}</ref><ref name=":4">{{Cite book |last=Fitzgerald |first=Michael |title=Hitler's Secret Weapons of Mass Destruction: The Nazi Plan for Final Victory |year=2018 |pages=Chapter 3 |quote=Paulet was clearly a pioneer in the field of rocketry and it is unsurprising that the Nazis were keen to recruit him to assist their efforts. The German Astronautical Society invited him to Germany to become part of a team of researchers into rocket propulsion and he was initially interested, but when he discovered that the intention was to construct a weapon that would be used for military purposes he declined the invitation. As late as 1965, Oberth described him as one of the true pioneers of rocket science.}}</ref> According to filmmaker and researcher Álvaro Mejía, [[Frederick I. Ordway III]] would later attempt to discredit Paulet's discoveries in the context of the [[Cold War]] and in an effort to shift the public image of von Braun away from his history with Nazi Germany.<ref name=":7">{{Cite web |date=2012-04-05 |title=Un documental reivindicará al peruano Paulet como pionero de la astronáutica |url=https://www.lainformacion.com/tecnologia/un-documental-reivindicara-al-peruano-paulet-como-pionero-de-la-astronautica_QtfLj5fEZHsXhIPh1BEkV3/ |access-date=2022-03-11 |website=[[EFE]] |language=es |last1=Com |first1=Lainformacion }}</ref>

===United States===
[[Image:Goddard and Rocket.jpg|thumb|[[Robert H. Goddard]], bundled against the cold [[New England]] weather of March 16, 1926, holds the launching frame of his most notable invention &mdash; the first liquid rocket.]]The first ''flight'' of a liquid-propellant rocket took place on March 16, 1926 at [[Auburn, Massachusetts]], when American professor Dr. [[Robert H. Goddard]] launched a vehicle using [[liquid oxygen]] and gasoline as propellants.<ref>
{{cite web|url=http://liftoff.msfc.nasa.gov/news/2003/news-goddard.asp |title=Re-Creating History |publisher=NASA |url-status=dead |archive-url=https://web.archive.org/web/20071201210444/http://liftoff.msfc.nasa.gov/news/2003/news-goddard.asp |archive-date=2007-12-01 }}</ref> The rocket, which was dubbed "Nell", rose just 41 feet during a 2.5-second flight that ended in a cabbage field, but it was an important demonstration that rockets using liquid propulsion were possible. Goddard proposed liquid propellants about fifteen years earlier and began to seriously experiment with them in 1921. The German-Romanian [[Hermann Oberth]] published a book in 1923 suggesting the use of liquid propellants.<ref>{{Cite web |title=Hermann Julius Oberth |url=https://pioneersofflight.si.edu/content/hermann-julius-oberth |access-date=2024-11-16 |website=pioneersofflight.si.edu |language=en}}</ref>

===Germany===
In Germany, engineers and scientists became enthralled with liquid propulsion, building and testing them in the late 1920s within [[Opel RAK]], the world's first rocket program, in Rüsselsheim. According to [[Max Valier]]'s account,<ref>Max, Valier, Raketenfahrt: Eine technische Möglichkeit Gebundene Ausgabe – Großdruck, 1. Januar 1930, De Gruyter Oldenbourg, Reprint 2019 ({{ISBN|978-3-486-76182-5}})</ref> Opel RAK rocket designer, [[Friedrich Wilhelm Sander]] launched two liquid-fuel rockets at Opel Rennbahn in [[Rüsselsheim]] on April 10 and April 12, 1929. These Opel RAK rockets have been the first European, and after Goddard the world's second, liquid-fuel rockets in history. In his book "Raketenfahrt" Valier describes the size of the rockets as of 21&nbsp;cm in diameter and with a length of 74&nbsp;cm, weighing 7&nbsp;kg empty and 16&nbsp;kg with fuel. The maximum thrust was 45 to 50 kp, with a total burning time of 132 seconds. These properties indicate a gas pressure pumping. The main purpose of these tests was to develop the liquid rocket-propulsion system for a Gebrüder-Müller-Griessheim aircraft<ref>{{cite web|title=Fritz von Opel, Speech at Deutsches Museum, April 3, 1968, re-print in "Opel Post" |date=May 1968|page=4ff |url=https://opelpost.com/wp-content/uploads/2018/04/Opel_Post_1968_3_Mai.pdf}}</ref> under construction for a planned flight across the English channel. Also spaceflight historian [[Frank H. Winter]], curator at National Air and Space Museum in Washington, DC, confirms the Opel group was working, in addition to their solid-fuel rockets used for land-speed records and the world's first crewed rocket-plane flights with the [[Opel RAK.1]], on liquid-fuel rockets.<ref>Frank H. Winter, "1928-1929 Forerunners of the Shuttle: the 'Von Opel Flights'", SPACEFLIGHT, Vol. 21,2, Feb. 1979</ref> By May 1929, the engine produced a thrust of 200&nbsp;kg (440&nbsp;lb.) "for longer than fifteen minutes and in July 1929, the Opel RAK collaborators were able to attain powered phases of more than thirty minutes for thrusts of 300 kg (660-lb.) at Opel's works in Rüsselsheim," again according to Max Valier's account. The Great Depression brought an end to the Opel RAK activities. After working for the German military in the early 1930s, Sander was arrested by Gestapo in 1935, when private rocket-engineering became forbidden in Germany. He was convicted of treason to 5 years in prison and forced to sell his company, he died in 1938.<ref>{{cite magazine|first=Walter J.|last=Boyne|title=Rocket Men |magazine=Air Force Magazine|date=September 2004|url=https://www.airandspaceforces.com/PDF/MagazineArchive/Documents/2004/September%202004/0904rocket.pdf}}</ref> Max Valier's (via [[Arthur Rudolph]] and Heylandt), who died while experimenting in 1930, and Friedrich Sander's work on liquid-fuel rockets was confiscated by the German military, the [[Heereswaffenamt]] and integrated into the activities under General [[Walter Dornberger]] in the early and mid-1930s in [[Kummersdorf|a field]] near Berlin.<ref>{{cite book|url=https://archive.org/details/bub_gb_n-MDAAAAMBAJ|page=[https://archive.org/details/bub_gb_n-MDAAAAMBAJ/page/n77 716]|quote=Popular Mechanics 1931 curtiss.|title=Popular Mechanics|first=Hearst|last=Magazines|date=1 May 1931|publisher=Hearst Magazines|via=Internet Archive}}</ref> Max Valier was a co-founder of an amateur research group, the [[Verein für Raumschiffahrt|VfR]], working on liquid rockets in the early 1930s, and many of whose members eventually became important rocket technology pioneers, including [[Wernher von Braun]]. Von Braun served as head of the army research station that designed the [[V-2 rocket]] weapon for the Nazis. 
	
[[File:Heinkel He 176 3-view line drawing.svg|thumb|Drawing of the He 176 V1 prototype rocket aircraft]]
By the late 1930s, use of rocket propulsion for crewed flight began to be seriously experimented with, as Germany's [[Heinkel He 176]] made the first crewed rocket-powered flight using a liquid rocket engine, designed by German aeronautics engineer [[Hellmuth Walter]] on June 20, 1939.<ref>Volker Koos, ''Heinkel He 176 – Dichtung und Wahrheit,'' Jet&Prop 1/94 p. 17–21</ref>  The only production rocket-powered combat aircraft ever to see military service, the [[Messerschmitt Me 163|Me 163]] ''Komet'' in 1944-45, also used a Walter-designed liquid rocket engine, the [[Walter HWK 109-509]], which produced up to 1,700 kgf (16.7&nbsp;kN) thrust at full power.

===Post World War II===
After World War II the American government and military finally seriously considered liquid-propellant rockets as weapons and began to fund work on them. The Soviet Union did likewise, and thus began the [[Space Race]].

In 2010s [[3D printing|3D printed]] engines started being used for spaceflight. Examples of such engines include [[SuperDraco]] used in [[launch escape system]] of the [[SpaceX Dragon 2]] and also engines used for first or second stages in [[launch vehicle]]s from [[Astra (American spaceflight company)|Astra]],<ref>{{cite web | url=https://www.flickr.com/photos/jurvetson/49960952648/ | title=Astra Rocket Engine — Delphin 3.0 | date=June 2020 }}</ref> [[Orbex]],<ref>{{cite web | url=https://www.aero-mag.com/orbex-single-piece-rocket-engine-3d-printing-slm-800/ | title=Orbex builds single-piece rocket engine 3D printed on SLM 800 - Aerospace Manufacturing | date=13 February 2019 }}</ref><ref>{{cite web | url=https://www.3dnatives.com/en/orbex-3d-printed-engine-130220195/ | title=Orbex unveiled largest 3D printed rocket engine in the world | date=13 February 2019 }}</ref> [[Relativity Space]],<ref>{{cite web | url=https://www.space.com/relativity-space-autonomous-rocket-factory.html | title=Relativity Space will 3D-print rockets at new autonomous factory in Long Beach, California | website=[[Space.com]] | date=28 February 2020 }}</ref> [[Skyrora]],<ref>{{cite web | url=https://techcrunch.com/2020/02/03/launch-startup-skyrora-successfully-tests-3d-printed-rocket-engines-powered-by-plastic-waste/ | title=Launch startup Skyrora successfully tests 3D-printed rocket engines powered by plastic waste | date=3 February 2020 }}</ref> or Launcher.<ref>{{cite web | url=https://www.cnbc.com/2019/02/20/brooklyn-rocket-start-up-launcher-gets-largest-single-piece-3d-printed-engine.html | title=A tiny start-up based in Brooklyn has a 3D-printed rocket engine it says is the largest in the world | website=[[CNBC]] | date=20 February 2019 }}</ref><ref>{{cite web | url=https://spacenews.com/launcher-af-pitch-award/ | title=Air Force funding keeps Launcher development on track | date=14 November 2019 }}</ref><ref>{{cite web | url=https://arstechnica.com/science/2020/11/meet-launcher-a-company-building-a-rocket-engine-with-eight-employees/ | title=Meet Launcher, the rocket engine builder with just eight employees | date=9 November 2020 }}</ref>

==See also==
* [[Comparison of orbital launch systems]]
* [[Comparison of orbital launchers families]]
* [[Comparison of orbital rocket engines]]
* [[Comparison of solid-fuelled orbital launch systems]]
* [[List of space launch system designs]]
* [[List of missiles]]
* [[List of orbital launch systems]]
* [[List of sounding rockets]]
* [[List of military rockets]]

==References==
{{Reflist}}

== Sources cited ==
* {{cite book |last1=Baker |first1=David |last2=Zak |first2=Anatoly |title=Race for Space 1: Dawn of the Space Age |date=9 September 2013 |publisher=RHK |url=https://books.apple.com/au/book/race-for-space-1-dawn-of-the-space-age/id634833085 |access-date=21 July 2022}}
* {{cite book |last1=Chertok |first1=Boris |title=Rockets and People Volumes 1-4 |date=2005 |publisher=National Aeronautics and Space Administration |url=https://www.nasa.gov/connect/ebooks/rockets_people_vol1_detail.html |access-date=21 July 2022}}
* {{cite journal |last1=Mejía |first1=Álvaro |url=https://revistas.ucsp.edu.pe/index.php/persona/article/view/209/230 |title=Pedro Paulet, sabio multidisciplinario |journal=Persona & Cultura |publisher=Universidad Católica San Pablo |year=2017 |volume=14 |issue=14 |edition= |pages=95–122 |doi=10.36901/persona.v14i14.209 |s2cid=258143557 |language=es |access-date=|doi-access=free }}
* {{cite book |last1=Siddiqi |first1=Asif |title=Challenge to Apollo : the Soviet Union and the space race, 1945-1974 |date=2000 |publisher=National Aeronautics and Space Administration, NASA History Div. |location=Washington, D.C. |url=https://history.nasa.gov/SP-4408pt1.pdf |access-date=21 July 2022}}

==External links==
* [http://www.risacher.org/rocket/ An online book entitled ''"How to Design, Build, and Test Small Liquid-Fuel Rocket Engines"'']
* [http://www.erichwarsitz.com The Heinkel He 176, worlds's first liquid-fuel rocket aircraft]

{{Spacecraft propulsion}}
{{Rocket engines}}
{{Authority control}}

{{DEFAULTSORT:Liquid-Propellant Rocket}}
[[Category:American inventions]]
[[Category:Rocket propulsion]]
[[Category:Rocket engines by propellant]]