Editing Spacecraft propulsion (section)

===Air-breathing engines===
{{Main|Jet engine|Air-breathing electric propulsion}}

{{more citations needed section | date = July 2023}}
Studies generally show that conventional air-breathing engines, such as [[ramjets]] or [[turbojets]] are basically too heavy (have too low a thrust/weight ratio) to give significant performance improvement when installed on a launch vehicle.{{Citation needed|date=April 2024}} However, launch vehicles can be [[air launch]]ed from separate lift vehicles (e.g. [[B-29 Superfortress|B-29]], [[Pegasus rocket|Pegasus Rocket]] and [[Scaled Composites White Knight|White Knight]]) which do use such propulsion systems. Jet engines mounted on a launch rail could also be so used.{{Citation needed|date=April 2024}}

On the other hand, very lightweight or very high-speed engines have been proposed that take advantage of the air during ascent:
* [[Reaction Engines SABRE|SABRE]] – a lightweight hydrogen fuelled turbojet with precooler<ref name="SABRE">{{cite web
 |author=Anonymous 
 |year=2006 
 |url=http://www.reactionengines.co.uk/sabre.html 
 |archive-url=https://web.archive.org/web/20070222125903/http://www.reactionengines.co.uk/sabre.html 
 |archive-date=2007-02-22 
 |title=The Sabre Engine 
 |publisher=Reaction Engines Ltd. 
 |access-date=2007-07-26 
}}</ref>
* [[ATREX]] – a lightweight hydrogen fuelled turbojet with precooler<ref>{{cite journal
 |author1=Harada, K. |author2=Tanatsugu, N. |author3=Sato, T. | title=Development Study on ATREX Engine
 | journal=Acta Astronautica
 | year=1997 | volume=41 | issue=12 | pages=851–862
 | doi=10.1016/S0094-5765(97)00176-8 |bibcode=1997AcAau..41..851T}}</ref>
* [[Liquid air cycle engine]] – a hydrogen-fuelled jet engine that liquifies the air before burning it in a rocket engine
* [[Scramjet]] – jet engines that use supersonic combustion
* [[Shcramjet]] – similar to a scramjet engine, however it takes advantage of shockwaves produced from the aircraft in the combustion chamber to assist in increasing overall efficiency.

Normal rocket launch vehicles fly almost vertically before rolling over at an altitude of some tens of kilometers before burning sideways for orbit; this initial vertical climb wastes propellant but is optimal as it greatly reduces airdrag. Airbreathing engines burn propellant much more efficiently and this would permit a far flatter launch trajectory. The vehicles would typically fly approximately tangentially to Earth's surface until leaving the atmosphere then perform a rocket burn to bridge the final [[delta-v]] to orbital velocity.

For spacecraft already in very low-orbit, [[air-breathing electric propulsion]] could use residual gases in the upper atmosphere as a propellant. Air-breathing electric propulsion could make a new class of long-lived, low-orbiting missions feasible on Earth, [[Mars]] or [[Venus]].<ref>{{cite news|title=World-first firing of air-breathing electric thruster|url=http://www.esa.int/Our_Activities/Space_Engineering_Technology/World-first_firing_of_air-breathing_electric_thruster|access-date=7 March 2018|work=Space Engineering & Technology|publisher=[[European Space Agency]]|date=5 March 2018}}</ref><ref>[http://erps.spacegrant.org/uploads/images/2015Presentations/IEPC-2015-271_ISTS-2015-b-271.pdf Conceptual design of an air-breathing electric propulsion system] {{Webarchive|url=https://web.archive.org/web/20170404043702/http://erps.spacegrant.org/uploads/images/2015Presentations/IEPC-2015-271_ISTS-2015-b-271.pdf |date=2017-04-04 }}. (PDF). 30th International Symposium on Space Technology and Science. 34th International Electric Propulsion Conference and 6th Nano-satellite Symposium. Hyogo-Kobe, Japan July 4, 2015.</ref>