Editing Spacecraft propulsion (section)

==Purpose and function==
{{more citations needed section | date = July 2023}}
[[Space exploration]] is about reaching the destination safely (mission enabling), quickly (reduced transit times), with a large quantity of [[Payload (air and space craft)|payload]] mass, and relatively inexpensively (lower cost). The act of reaching the destination requires an in-space propulsion system, and the other metrics are modifiers to this fundamental action.{{Sfn|Meyer|2012|p=5}}<ref name="leone" /> Propulsion technologies can significantly improve a number of critical aspects of the mission.

When launching a spacecraft from Earth, a propulsion method must overcome a higher [[gravity drag|gravitational]] pull to provide a positive net acceleration.<ref name="beginners_guide">{{cite web |last=Benson |first=Tom |title=Guided Tours: Beginner's Guide to Rockets |url=http://exploration.grc.nasa.gov/education/rocket/guided.htm |archive-url=https://web.archive.org/web/20130814022045/http://exploration.grc.nasa.gov/education/rocket/guided.htm |archive-date=2013-08-14 |access-date=2007-08-02 |publisher=NASA}}</ref> When in space, the purpose of a [[propulsion system]] is to change the velocity, or ''v'', of a spacecraft.<ref>{{cite web |last=Zobel |first=Edward A. |year=2006 |title=Summary of Introductory Momentum Equations |url=http://id.mind.net/~zona/mstm/physics/mechanics/momentum/introductoryProblems/momentumSummary2.html |archive-url=https://web.archive.org/web/20070927025532/http://id.mind.net/~zona/mstm/physics/mechanics/momentum/introductoryProblems/momentumSummary2.html |archive-date=September 27, 2007 |access-date=2007-08-02 |publisher=Zona Land}}</ref>

In-space propulsion begins where the [[upper stage]] of the [[launch vehicle]] leaves off, performing the functions of [[rocket propulsion|primary propulsion]], [[reaction engine|reaction control]], [[orbital station-keeping|station keeping]], [[Spacecraft attitude control|precision pointing]], and [[orbital maneuver]]ing. The [[rocket engine|main engine]]s used in [[space]] provide the primary propulsive force for [[Geostationary orbit|orbit transfer]], [[Hohmann transfer orbit|planetary trajectories]], and extra [[List of landings on extraterrestrial bodies|planetary landing]] and [[wikt:ascent|ascent]]. The reaction control and orbital maneuvering systems provide the propulsive force for orbit maintenance, position control, station keeping, and spacecraft attitude control.{{Sfn|Meyer|2012|p=5}}<ref name="mason" /><ref name="leone" />

In orbit, any additional [[Impulse (physics)|impulse]], even tiny, will result in a change in the orbit path, in two ways:<ref name="z677">{{cite web | title=In-Space Propulsion Technology Products for NASA's Future Science and Exploration Missions | url=https://ntrs.nasa.gov/api/citations/20110016163/downloads/20110016163.pdf | access-date=2024-08-03}}</ref>
* Prograde/retrograde (i.e. acceleration in the tangential/opposite in tangential direction), which increases/decreases altitude of orbit.
* Perpendicular to orbital plane, which changes [[orbital inclination change|orbital inclination]].{{citation needed|date = July 2023}}
Earth's surface is situated fairly deep in a [[gravity well]]; the [[escape velocity]] required to leave its orbit is 11.2 kilometers/second.<ref>{{Cite web |date=2024-02-23 |title=Escape velocity {{!}} Definition, Formula, Earth, Moon, & Facts {{!}} Britannica |url=https://www.britannica.com/science/escape-velocity |access-date=2024-04-20 |website=www.britannica.com |language=en}}</ref> Thus for destinations beyond, propulsion systems need enough propellant and to be of high enough efficiency. The same is true for other planets and moons, albeit some have lower gravity wells.

As human beings evolved in a gravitational field of "one ''g''" (9.81m/s²), it would be most comfortable for a human spaceflight propulsion system to provide that acceleration continuously,{{according to whom|date = July 2023}} (though human bodies can tolerate much larger accelerations over short periods).<ref>{{Cite web |title=The Jump to Light Speed Is a Real Killer |url=https://www.scientificamerican.com/article/star-wars-science-light-speed/ |access-date=2024-04-20 |website=Scientific American |language=en}}</ref> The occupants of a rocket or spaceship having such a propulsion system would be free from the ill effects of [[free fall]], such as nausea, muscular weakness, reduced sense of taste, or [[leaching (chemistry)|leaching]] of calcium from their bones.<ref>{{Cite journal |last1=Wolfe |first1=J. W. |last2=Rummel |first2=J. D. |date=1992 |title=Long-term effects of microgravity and possible countermeasures |url=https://pubmed.ncbi.nlm.nih.gov/11536970/ |journal=Advances in Space Research |volume=12 |issue=1 |pages=281–284 |doi=10.1016/0273-1177(92)90296-a |issn=0273-1177 |pmid=11536970|bibcode=1992AdSpR..12a.281W }}</ref><ref>{{Cite journal |date=December 22, 2022 |title=Human Health during Space Travel: State-of-the-Art Review |pmc=9818606 |last1=Krittanawong |first1=C. |last2=Singh |first2=N. K. |last3=Scheuring |first3=R. A. |last4=Urquieta |first4=E. |last5=Bershad |first5=E. M. |last6=MacAulay |first6=T. R. |last7=Kaplin |first7=S. |last8=Dunn |first8=C. |last9=Kry |first9=S. F. |last10=Russomano |first10=T. |last11=Shepanek |first11=M. |last12=Stowe |first12=R. P. |last13=Kirkpatrick |first13=A. W. |last14=Broderick |first14=T. J. |last15=Sibonga |first15=J. D. |last16=Lee |first16=A. G. |last17=Crucian |first17=B. E. |journal=Cells |volume=12 |issue=1 |page=40 |doi=10.3390/cells12010040 |doi-access=free |pmid=36611835 }}</ref>