Editing Spacecraft propulsion (section)

==Planetary and atmospheric propulsion==
[[File:Lightcraft.jpg|thumb|A successful proof of concept [[Lightcraft]] test, a subset of [[beam-powered propulsion]]]]

===Launch-assist mechanisms===
{{Main|Space launch}}

There have been many ideas proposed for launch-assist mechanisms that have the potential of substantially reducing the cost of getting to orbit. Proposed [[non-rocket spacelaunch]] launch-assist mechanisms include:<ref>{{Cite web |date=1970-01-01 |title=Can We Get Into Space Without Big Rockets? |url=https://science.howstuffworks.com/can-get-into-space-without-big-rocket.htm |access-date=2024-04-28 |website=HowStuffWorks |language=en-us}}</ref><ref>{{Cite web |last=Bolonkin |first=Alexander |date=January 2011 |title=Review of new ideas, innovations of non- rocket propulsion systems for Space Launch and Flight (Part 2) |url=https://www.researchgate.net/publication/268426650 |access-date=April 28, 2024 |website=www.researchgate.net}}</ref>

*[[Skyhook (structure)|Skyhook]] (requires reusable suborbital launch vehicle, not feasible using presently available materials)
*[[Space elevator]] (tether from Earth's surface to geostationary orbit, cannot be built with existing materials)
*[[Launch loop]] (a very fast enclosed rotating loop about 80&nbsp;km tall)
*[[Space fountain]] (a very tall building held up by a stream of masses fired from its base)
*[[Orbital ring]] (a ring around Earth with spokes hanging down off bearings)
*[[Mass driver|Electromagnetic catapult]] ([[railgun]], [[coilgun]]) (an electric gun)
*[[Rocket sled launch]]
*[[Space gun]] ([[Project HARP]], [[ram accelerator]]) (a chemically powered gun)
*[[Beam-powered propulsion]] rockets and jets powered from the ground via a beam
*[[High-altitude platform]]s to assist initial stage

===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>

===Planetary arrival and landing===
{{Main|Atmospheric entry}}
{{More sources needed|section|date=April 2024}}
[[File:Pathfinder Air Bags - GPN-2000-000484.jpg|thumb|right|A test version of the [[Mars Pathfinder]] airbag system]]
When a vehicle is to enter orbit around its destination planet, or when it is to land, it must adjust its velocity.<ref>{{Cite web |title=Chapter 4: Trajectories – NASA Science |url=https://science.nasa.gov/learn/basics-of-space-flight/chapter4-1/ |access-date=2024-04-24 |website=science.nasa.gov |date=20 July 2023 |language=en-US}}</ref> This can be done using any of the methods listed above (provided they can generate a high enough thrust), but there are methods that can take advantage of planetary atmospheres and/or surfaces.

* [[Aerobraking]] allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere<ref>{{Cite web |title=Definition of AEROBRAKE |url=https://www.merriam-webster.com/dictionary/aerobrake |access-date=2024-04-24 |website=www.merriam-webster.com |language=en}}</ref> at the low point of the orbit. This can save a considerable amount of fuel because it takes much less delta-V to enter an elliptical orbit compared to a low circular orbit. Because the braking is done over the course of many orbits, heating is comparatively minor, and a heat shield is not required. This has been done on several Mars missions such as ''[[Mars Global Surveyor]]'', ''[[2001 Mars Odyssey]]'', and ''[[Mars Reconnaissance Orbiter]]'', and at least one Venus mission, ''[[Magellan (spacecraft)|Magellan]]''.
* [[Aerocapture]] is a much more aggressive manoeuver, converting an incoming hyperbolic orbit to an elliptical orbit in one pass. This requires a heat shield and more controlled navigation because it must be completed in one pass through the atmosphere, and unlike aerobraking no preview of the atmosphere is possible. If the intent is to remain in orbit, then at least one more propulsive maneuver is required after aerocapture—otherwise the low point of the resulting orbit will remain in the atmosphere, resulting in eventual re-entry. Aerocapture has not yet been tried on a planetary mission, but the [[Skip reentry|re-entry skip]] by [[Zond 6]] and [[Zond 7]] upon lunar return were aerocapture maneuvers, because they turned a hyperbolic orbit into an elliptical orbit. On these missions, because there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.
* A [[ballute]] is an inflatable drag device.<ref>{{Cite web |title=Definition of BALLUTE |url=https://www.merriam-webster.com/dictionary/ballute |access-date=2024-04-26 |website=www.merriam-webster.com |language=en}}</ref>
* [[Parachute]]s can land a probe on a planet or moon with an atmosphere, usually after the atmosphere has scrubbed off most of the velocity, using a [[Atmospheric reentry|heat shield]].
* [[Airbag]]s can soften the final landing.
* [[Lithobraking]], or stopping by impacting the surface, is usually done by accident. However, it may be done deliberately with the probe expected to survive (see, for example, the [[Deep Impact (spacecraft)|Deep Impact spacecraft]]), in which case very sturdy probes are required.