Editing Spacecraft propulsion (section)

===Chemical propulsion===
{{Main|Rocket engine}}

[[File:SpaceX engine test fire.jpg|thumb|right|[[SpaceX]]'s [[Kestrel (rocket engine)|Kestrel engine]] is tested.]]

A large fraction of [[rocket engine]]s in use today are [[Rocket engine#Chemically powered|chemical rockets]]; that is, they obtain the energy needed to generate thrust by [[chemical reactions]] to create a hot gas that is expanded to produce [[thrust]].<ref>{{Cite web |title=Chapter 14: Launch – NASA Science |url=https://science.nasa.gov/learn/basics-of-space-flight/chapter14-1/ |access-date=2024-04-19 |website=science.nasa.gov |date=20 July 2023 |language=en-US}}</ref> Many different propellant combinations are used to obtain these chemical reactions, including, for example, [[hydrazine]], [[liquid oxygen]], [[liquid hydrogen]], [[nitrous oxide]], and [[hydrogen peroxide]].<ref>{{Cite web |title=4.0 In-Space Propulsion – NASA |url=https://www.nasa.gov/smallsat-institute/sst-soa/in-space_propulsion/ |access-date=2024-04-25 |language=en-US}}</ref> They can be used as a [[Monopropellant rocket|monopropellant]] or in [[Bipropellant rocket|bi-propellant]] configurations.<ref>{{Cite web |title=4.0 In-Space Propulsion – NASA |url=https://www.nasa.gov/smallsat-institute/sst-soa/in-space_propulsion/ |access-date=2024-04-20 |language=en-US}}</ref>

Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.<ref name=":4" /> Most rocket engines are [[internal combustion engine|internal combustion]] [[heat engines]] (although non-combusting forms exist).<ref name="Leishman">{{Cite book |last=Leishman |first=J. Gordon |date=2023-01-01 |title=Introduction to Aerospace Flight Vehicles - Rocket Engines |url=https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/rocket-engines/ |language=en|publisher=Embry Riddle Aeronautical University}}</ref> Rocket engines generally produce a high-temperature reaction mass, as a hot gas, which is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber.<ref>{{Cite web |title=Rocket Propulsion |url=https://www.grc.nasa.gov/www/k-12/airplane/rocket.html |access-date=April 21, 2024 |website=[[NASA]]}}</ref> The extremely hot gas is then allowed to escape through a high-expansion ratio bell-shaped [[de Laval nozzle|nozzle]], a feature that gives a rocket engine its characteristic shape.<ref name="Leishman"/> The effect of the nozzle is to accelerate the mass, converting most of the thermal energy into kinetic energy,<ref name="Leishman"/> where exhaust speeds reaching as high as 10 times the speed of sound at sea level are common.{{citation needed|date = July 2023}}

====Green chemical propulsion====
The dominant form of chemical propulsion for [[satellite]]s has historically been [[hydrazine]], however, this fuel is highly toxic and at risk of being banned across Europe.<ref>{{Cite web |date=2017-10-25 |title=Hydrazine ban could cost Europe's space industry billions |url=https://spacenews.com/hydrazine-ban-could-cost-europes-space-industry-billions/ |access-date=2022-08-19 |website=SpaceNews |language=en-US}}</ref> Non-toxic 'green' alternatives are now being developed to replace hydrazine. [[Nitrous oxide]]-based alternatives are garnering traction and government support,<ref>{{Cite web |last=Urban |first=Viktoria |date=2022-07-15 |title=Dawn Aerospace granted €1.4 million by EU for green propulsion technology |url=https://spacewatch.global/2022/07/dawn-aerospace-granted-e1-4-million-by-eu-for-green-propulsion-technology/ |access-date=2022-08-19 |website=SpaceWatch.Global |language=en-US}}</ref><ref>{{Cite web |title=International research projects {{!}} Ministry of Business, Innovation & Employment |url=https://www.mbie.govt.nz/science-and-technology/space/nzspacetalk/international-research-projects/ |access-date=2022-08-19 |website=www.mbie.govt.nz}}</ref> with development being led by commercial companies Dawn Aerospace, Impulse Space,<ref>{{Cite web |last=Berger |first=Eric |date=2022-07-19 |title=Two companies join SpaceX in the race to Mars, with a launch possible in 2024 |url=https://arstechnica.com/science/2022/07/relativity-and-impulse-space-say-theyre-flying-to-mars-in-late-2024/ |access-date=2022-08-19 |website=Ars Technica |language=en-us}}</ref> and Launcher.<ref>{{Cite web |date=2021-06-15 |title=Launcher to develop orbital transfer vehicle |url=https://spacenews.com/launcher-to-develop-orbital-transfer-vehicle/ |access-date=2022-08-19 |website=SpaceNews |language=en-US}}</ref> The first nitrous oxide-based system flown in space was by D-Orbit onboard their ION Satellite Carrier ([[space tug]]) in 2021, using six [[Dawn Aerospace]] B20 thrusters, launched upon a [[SpaceX]] [[Falcon 9]] rocket.<ref>{{Cite web |title=Dawn Aerospace validates B20 Thrusters in space – Bits&Chips |date=6 May 2021 |url=https://bits-chips.nl/artikel/dawn-aerospace-validates-b20-thrusters-in-space/ |access-date=2022-08-19 |language=en-US}}</ref><ref>{{Cite web |title=Dawn B20 Thrusters Proven In Space |url=https://www.dawnaerospace.com/latest-news/b20-thrusters-proven-in-space |access-date=2022-08-19 |website=Dawn Aerospace |language=en-US}}</ref>