View source for Astronomy:Spacecraft propulsion

{{short description|Method used to accelerate spacecraft}}
[[File:Shuttle Main Engine Test Firing.jpg|thumb|A remote camera captures a close-up view of an [[Engineering:RS-25|RS-25]] during a test firing at the John C. Stennis Space Center in Hancock County, Mississippi.]]
[[File:Engine of APOLLO Eagle.png|thumb|Bipropellant rocket engines of the Apollo Lunar Module reaction control system (RCS)]]

'''Spacecraft propulsion''' is any method used to accelerate [[Engineering:Spacecraft|spacecraft]] and artificial [[Astronomy:Satellite|satellite]]s. '''In-space propulsion''' exclusively deals with propulsion systems used in the vacuum of space and should not be confused with [[Engineering:Space launch|space launch]] or [[Astronomy:Atmospheric entry|atmospheric entry]].

Several methods of pragmatic spacecraft propulsion have been developed, each having its own drawbacks and advantages.  Most satellites have simple reliable chemical thrusters (often [[Engineering:Monopropellant rocket|monopropellant rocket]]s) or [[Engineering:Resistojet rocket|resistojet rocket]]s for [[Astronomy:Orbital station-keeping|orbital station-keeping]], while a few use [[Engineering:Reaction wheel|momentum wheels]] for [[Astronomy:Spacecraft attitude control|attitude control]]. Russian and antecedent Soviet bloc satellites have used [[Engineering:Spacecraft electric propulsion|electric propulsion]] for decades,{{not verified in body|date = July 2023}} and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used [[Engineering:Ion thruster|ion thruster]]s and [[Physics:Hall-effect thruster|Hall-effect thruster]]s (two different types of [[Engineering:Electrically powered spacecraft propulsion|electric propulsion]]) to great success.

Hypothetical in-space propulsion technologies describe the propulsion technologies that could meet future [[Astronomy:Space research|space science and exploration]] needs. These propulsion technologies are intended to provide effective [[Astronomy:Space exploration|exploration]] of the [[Astronomy:Solar System|Solar System]] and will permit mission designers to plan missions to "fly anytime, anywhere, and complete a host of science objectives at the destinations" and with greater reliability and safety. With a wide range of possible missions and candidate propulsion technologies, the question of which technologies are "best" for future missions is a difficult one; expert opinion now holds that a portfolio of propulsion technologies should be developed to provide optimum solutions for a diverse set of missions and destinations.<ref name="meyer">

==Purpose and function==
In-space propulsion begins where the upper stage of the [[Engineering:Launch vehicle|launch vehicle]] leaves off, performing the functions of [[Engineering:Rocket propulsion|primary propulsion]], [[Engineering:Reaction engine|reaction control]], [[Astronomy:Orbital station-keeping|station keeping]], [[Astronomy:Spacecraft attitude control|precision pointing]], and [[Astronomy:Orbital maneuver|orbital maneuver]]ing. The [[Engineering:Rocket engine|main engine]]s used in [[Space|space]] provide the primary propulsive force for [[Astronomy:Geostationary orbit|orbit transfer]], [[Astronomy:Hohmann transfer orbit|planetary trajectories]], and extra [[Engineering:List of landings on extraterrestrial bodies|planetary landing]] and 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/>

When in space, the purpose of a propulsion system is to change the velocity, or ''v'', of a spacecraft. Because this is more difficult for more massive spacecraft, designers generally discuss spacecraft performance in ''amount of change in momentum per unit of propellant consumed'' also called [[Physics:Specific impulse|specific impulse]].<ref>{{cite web|last=Zobel |first=Edward A. |year=2006 |url=http://id.mind.net/~zona/mstm/physics/mechanics/momentum/introductoryProblems/momentumSummary2.html |title=Summary of Introductory Momentum Equations |publisher=Zona Land |access-date=2007-08-02 |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 }}</ref> The higher the specific impulse, the better the efficiency. [[Engineering:Ion thruster|Ion propulsion engines]] have high specific impulse (~3000 s) and low thrust<ref>{{Cite web|url=https://www2.l3t.com/edd/pdfs/datasheets/EP_Thrusters-XIPS_PPU%20Overview%20datasheet.pdf|title=Xenon Ion Propulsion System (XIPS) Thrusters|website=L3 Technologies|archive-url=https://web.archive.org/web/20180417143504/http://www2.l3t.com/edd/pdfs/datasheets/EP_Thrusters-XIPS_PPU%20Overview%20datasheet.pdf|archive-date=17 April 2018|access-date=16 March 2019}}</ref>  whereas chemical rockets like [[Engineering:Monopropellant rocket|monopropellant]] or bipropellant rocket engines have a low specific impulse (~300 s) but high thrust.<ref>{{Cite web|url=http://www.space-propulsion.com/brochures/bipropellant-thrusters/bipropellant-thrusters.pdf|title=Chemical Bipropellant thruster family|website=Ariane Group|access-date=16 March 2019}}</ref>

When launching a spacecraft from Earth, a propulsion method must overcome a higher [[Astronomy:Gravity drag|gravitational]] pull to provide a positive net acceleration.<ref name="beginners_guide">{{cite web | last = Benson | first = Tom | url = http://exploration.grc.nasa.gov/education/rocket/guided.htm | title = Guided Tours: Beginner's Guide to Rockets | publisher = NASA | access-date = 2007-08-02 | archive-date = 2013-08-14 | archive-url = https://web.archive.org/web/20130814022045/http://exploration.grc.nasa.gov/education/rocket/guided.htm }}</ref>
In orbit, any additional impulse, even very tiny, will result in a change in the orbit path, in two ways:{{citation needed|date = July 2023}}
* prograde/retrograde (i.e. acceleration in the tangential/opposite in tangential direction), which increases/decreases altitude of orbit; and
* perpendicular to orbital plane, which changes [[Astronomy:Orbital inclination change|orbital inclination]].{{citation needed|date = July 2023}}

The rate of change of [[Physics:Velocity|velocity]] is called [[Physics:Acceleration|acceleration]], and the rate of change of [[Physics:Momentum|momentum]] is called [[Physics:Force|force]].{{citation needed|date = July 2023}} To reach a given velocity, one can apply a small acceleration over a long period of time, or one can apply a large acceleration over a short time; similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time.{{citation needed|date = July 2023}} This means that for maneuvering in space, a propulsion method that produces tiny accelerations but runs for a long time can produce the same impulse as a propulsion method that produces large accelerations for a short time.{{citation needed|date = July 2023}} When launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.{{citation needed|date = July 2023}}

Earth's surface is situated fairly deep in a [[Physics:Gravity well|gravity well]]; the [[Astronomy:Escape velocity|escape velocity]] required to get out of it is 11.2 kilometers/second.{{citation needed|date = July 2023}}{{relevance inline|date=July 2023|reason=The statement of Earth's escape velocity seems misplaced in this paragraph.}} As human beings evolved in a gravitational field of "one g" (9.8&nbsp;m/s²), an ideal propulsion system for human spaceflight would be one that provides that acceleration continuously,{{according to whom|date = July 2023}} (though human bodies can tolerate much larger accelerations over short periods).{{citation needed|date = July 2023}} The occupants of a rocket or spaceship having such a propulsion system would be free from all the ill effects of [[Physics:Free fall|free fall]], such as nausea, muscular weakness, reduced sense of taste, or [[Engineering:Leaching|leaching]] of calcium from their bones.{{citation needed|date = July 2023}}

The [[Tsiolkovsky rocket equation]] proves using the law of [[Physics:Conservation of momentum|conservation of momentum]] that for a [[Engineering:Rocket engine|Rocket engine]] propulsion method to change the momentum of a spacecraft it must change the momentum of something else in the opposite direction. In other words, the rocket must exhaust mass opposite the spacecraft's acceleration direction, with such exhausted mass called [[Physics:Propellant|propellant]] or reaction mass.<ref>{{Cite book |last=Turner |first=Martin J. L. |title=Rocket and spacecraft propulsion: principles, practice and new developments |date=2009 |publisher=Praxis Publ |isbn=978-3-540-69202-7 |edition=3rd |series=Springer-Praxis books in astronautical engineering |location=Chichester, UK}}</ref>{{Rp|location=Sec 1.2.1}}<ref name="ReactiveFlyingMachines">{{Cite web|last=Tsiolkovsky|first=K.|title=Reactive Flying Machines|url=http://epizodsspace.airbase.ru/bibl/inostr-yazyki/tsiolkovskii/tsiolkovskii-nhedy-t2-1954.pdf}}</ref>  Some designs operate [[Astronomy:Spacecraft propulsion#Without internal reaction mass|without internal reaction mass]] by taking advantage of things like magnetic fields or light pressure to change the spacecraft's momentum.

In order for a rocket to work, it needs two things: reaction mass and energy. The impulse provided by launching a particle of reaction mass having mass ''m'' at velocity ''v'' is ''mv''. But this particle has kinetic energy ''mv''²/2, which must come from somewhere. In a conventional solid, [[Engineering:Liquid-propellant rocket|liquid]], or hybrid rocket, the fuel is burned, providing the energy, and the reaction products are allowed to flow out the back, providing the reaction mass.  In an [[Engineering:Ion thruster|ion thruster]], electricity is used to accelerate ions out the back. Here some other source must provide the electrical energy (perhaps a solar panel or a [[Physics:Nuclear reactor|nuclear reactor]]), whereas the ions provide the reaction mass.<ref name="beginners_guide" />

When discussing the efficiency of a propulsion system, designers often focus on the effective use of the reaction mass, which must be carried along with the rocket and is irretrievably consumed when used.{{citation needed|date = July 2023}} One measure of the amount of impulse that can be obtained from a fixed amount of reaction mass is the [[Physics:Specific impulse|specific impulse]], the impulse per unit weight-on-Earth (typically designated by <math>I_\text{sp}</math>), with units of seconds.{{citation needed|date = July 2023}} Because the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of the measure, impulse per unit mass, with the same units as velocity (e.g., meters per second).{{citation needed|date = July 2023}} This measure is equivalent to the effective exhaust velocity of the engine, and is typically designated <math>v_{e}</math>.{{citation needed|date = July 2023}}  (Confusingly, both values{{clarify|date = July 2023}}<!--THREE MEASURES LISTED, CANNOT USE "BOTH". BREAK PARAGRAPH AT "Because the weight..." IF "Isp" IS INTENDED AS EXCLUDED FROM DISCUSSION.--> are sometimes called specific impulse.{{citation needed|date = July 2023}}) The two values differ by a factor of the [[Physics:Standard gravity|standard acceleration due to gravity, ''g''<sub>n</sub>]], 9.80665&nbsp;m/s² (<math>I_\text{sp} g_\mathrm{n} = v_{e}</math>).{{citation needed|date = July 2023}}

A rocket with a high exhaust velocity can achieve the same impulse with less reaction mass; however, the energy required for that impulse is proportional to the exhaust velocity, so that more mass-efficient engines require much more energy, and are typically less energy-efficient.{{citation needed|date = July 2023}} This is a problem if the engine is to provide a large amount of thrust: to generate a large amount of impulse per second, it must use a large amount of energy per second.{{citation needed|date = July 2023}} As such, high-mass-efficient engine designs require enormous amounts of energy per second to produce high thrusts, but generally also tend to provide lower thrust (due to the unavailability of high amounts of energy).{{citation needed|date = July 2023}}

In-space propulsion represents technologies that can significantly improve a number of critical aspects of the mission. [[Astronomy:Space exploration|Space exploration]] is about getting somewhere safely (mission enabling), getting there quickly (reduced transit times), getting a lot of mass there (increased payload mass), and getting there cheaply (lower cost). The simple act of "getting" there requires the employment of an in-space propulsion system, and the other metrics are modifiers to this fundamental action.{{Sfn|Meyer|2012|p=5}}<ref name=leone/>

Development of technologies will result in technical solutions that improve thrust levels, Isp, power, specific mass, (or [[Engineering:Power-to-weight ratio|specific power]]), volume, system mass, system complexity, operational complexity, commonality with other spacecraft systems, manufacturability, durability, and cost. These types of improvements will yield decreased transit times, increased payload mass, safer spacecraft, and decreased costs. In some instances, development of technologies within this technology area (TA) will result in mission-enabling breakthroughs that will revolutionize space exploration. There is no single propulsion technology that will benefit all missions or mission types. The requirements for in-space propulsion vary widely according to their intended application. The described technologies should support everything from small [[Astronomy:Satellite|satellite]]s and [[Astronomy:Deep space exploration|robotic deep space exploration]] to space stations and [[Astronomy:Human mission to Mars|human missions to Mars applications]].{{Sfn|Meyer|2012|p=5}}<ref name=leone/>

===Defining technologies===
Furthermore, the term "mission pull" defines a technology or a performance characteristic necessary to meet a planned NASA mission requirement. Any other relationship between a technology and a mission (an alternate propulsion system, for example) is categorized as "technology push." Also, a space demonstration refers to the spaceflight of a scaled version of a particular technology or of a critical technology subsystem. On the other hand, a space validation would serve as a qualification flight for future mission implementation. A successful validation flight would not require any additional space testing of a particular technology before it can be adopted for a science or exploration mission.{{Sfn|Meyer|2012|p=5}}

==Operating domains==
Spacecraft operate in many areas of space. These include orbital maneuvering, interplanetary travel, and interstellar travel.

===Orbital===
{{Main|Astronomy:Orbital mechanics}}
Artificial satellites are first [[Engineering:Rocket launch|launched]] into the desired altitude by conventional liquid/solid propelled rockets after which the satellite may use onboard propulsion systems for orbital stationkeeping. Once in the desired orbit, they often need some form of [[Astronomy:Spacecraft attitude control|attitude control]] so that they are correctly pointed with respect to the [[Astronomy:Earth|Earth]], the [[Astronomy:Sun|Sun]], and possibly some [[Astronomy:Astronomy|astronomical]] object of interest.<ref>{{cite news |author1=Hess, M. |author2=Martin, K. K. |author3=Rachul, L. J. | title=Thrusters Precisely Guide EO-1 Satellite in Space First | publisher=NASA | date=February 7, 2002 | url=http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm | access-date=2007-07-30 |archive-url = https://web.archive.org/web/20071206154134/http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm <!-- Bot retrieved archive --> |archive-date = 2007-12-06}}</ref> They are also subject to drag from the thin atmosphere,  so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital station-keeping).<ref>{{cite web  |last=Phillips  |first=Tony  |date=May 30, 2000  |url=https://science.nasa.gov/headlines/y2000/ast30may_1m.htm  |archive-url=https://web.archive.org/web/20000619105529/https://science.nasa.gov/headlines/y2000/ast30may_1m.htm  |archive-date=June 19, 2000  |title=Solar S'Mores  |publisher=NASA  |access-date=2007-07-30 
}}</ref> Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.<ref>{{cite web | last=Olsen | first=Carrie | date=September 21, 1995 | url=http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html | title=Hohmann Transfer & Plane Changes | publisher=NASA | access-date=2007-07-30 |archive-url = https://web.archive.org/web/20070715042552/http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html <!-- Bot retrieved archive --> |archive-date = 2007-07-15}}</ref> A satellite's useful life is usually over once it has exhausted its ability to adjust its orbit.{{citation needed|date = July 2023}}

===Interplanetary===
{{Main|Astronomy:Interplanetary spaceflight}}
For interplanetary travel, a spacecraft can use its engines to leave Earth's orbit. It is not explicitly necessary as the initial boost given by the rocket, gravity slingshot, monopropellant/bipropellent attitude control propulsion system are enough for the exploration of the solar system (see [[Astronomy:New Horizons|New Horizons]]). Once it has done so, it must somehow make its way to its destination.  Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.<ref>{{cite web|author=Staff |date=April 24, 2007 |url=http://mars.jpl.nasa.gov/odyssey/mission/cruise.html |title=Interplanetary Cruise |publisher=NASA |work=2001 Mars Odyssey |access-date=2007-07-30 |archive-url=https://web.archive.org/web/20070802071234/http://mars.jpl.nasa.gov/odyssey/mission/cruise.html |archive-date=August 2, 2007 }}</ref> In between these adjustments, the spacecraft simply moves along its trajectory without accelerating. The most fuel-efficient means to move from one circular orbit to another is with a [[Astronomy:Hohmann transfer orbit|Hohmann transfer orbit]]: the spacecraft begins in a roughly circular orbit around the Sun.  A short period of [[Thrust|thrust]] in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination.  The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.<ref>{{cite news | first=Dave | last=Doody | title=Chapter 4. Interplanetary Trajectories | work=Basics of Space Flight | publisher=NASA JPL | date=February 7, 2002 | url=http://www2.jpl.nasa.gov/basics/bsf4-1.html | access-date=2007-07-30 | archive-url=https://web.archive.org/web/20070717143018/http://www2.jpl.nasa.gov/basics/bsf4-1.html | archive-date=July 17, 2007 }}</ref>  Special methods such as [[Astronomy:Aerobraking|aerobraking]] or aerocapture are sometimes used for this final orbital adjustment.<ref>{{cite conference|first=S. |last=Hoffman |title=A comparison of aerobraking and aerocapture vehicles for interplanetary missions |book-title=AIAA and AAS, Astrodynamics Conference |pages=25 p |publisher=American Institute of Aeronautics and Astronautics |date=August 20–22, 1984 |location=Seattle, Washington |url=http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 |access-date=2007-07-31 |archive-url=https://web.archive.org/web/20070927230504/http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 |archive-date=September 27, 2007 }}</ref>

[[File:Ssunsail.jpg|thumb|right|Artist's concept of a solar sail]]
Some spacecraft propulsion methods such as [[Physics:Solar sail|solar sail]]s provide very low but inexhaustible thrust;<ref>{{cite web|author=Anonymous |year=2007 |url=http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html |title=Basic Facts on Cosmos 1 and Solar Sailing |publisher=The Planetary Society |access-date=2007-07-26 |archive-url=https://web.archive.org/web/20070703052531/http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html |archive-date=July 3, 2007 }}</ref> an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun, or constantly thrusting along its direction of motion to increase its distance from the Sun.{{citation needed|date = July 2023}} The concept has been successfully tested by the Japanese [[Engineering:IKAROS|IKAROS]] solar sail spacecraft.{{citation needed|date = July 2023}}

===Interstellar===
{{Main|Engineering:Interstellar travel}}
No spacecraft capable of short duration (compared to human lifetime) [[Engineering:Interstellar travel|interstellar travel]] has yet been built, but many hypothetical designs have been discussed. Because interstellar distances are very great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time.  Acquiring such a velocity on launch and getting rid of it on arrival remains a formidable challenge for spacecraft designers.<ref>{{cite web | last=Rahls | first=Chuck | date=December 7, 2005 | url=http://www.physorg.com/news8817.html | title=Interstellar Spaceflight: Is It Possible? | publisher=Physorg.com | access-date=2007-07-31 }}</ref>

==Propulsion technology==
The technology areas are divided into four basic groups: (1) Chemical propulsion, (2) Electric propulsion, (3) Advanced propulsion technologies, and (4) Supporting technologies; based on the physics of the propulsion system and how it derives thrust as well as its technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications.{{Sfn|Meyer|2012|p=10}}

===Chemical propulsion===
A large fraction of the [[Engineering:Rocket engine|rocket engine]]s in use today are [[Engineering: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|thrust]].{{citation needed|date = July 2023}} Many different propellant combinations are used to obtain these chemical reactions, including, for example, [[Chemistry:Hydrazine|hydrazine]], [[Physics:Liquid oxygen|liquid oxygen]], [[Chemistry:Liquid hydrogen|liquid hydrogen]], [[Chemistry:Nitrous oxide|nitrous oxide]], and [[Chemistry:Hydrogen peroxide|hydrogen peroxide]].{{citation needed|date = July 2023}} They can be used as a [[Engineering:Monopropellant rocket|monopropellant]] or in bi-propellant configurations.{{citation needed|date = July 2023}}

====Green chemical propulsion====
The dominant form of chemical propulsion for [[Astronomy:Satellite|satellite]]s has historically been [[Chemistry:Hydrazine|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. [[Chemistry:Nitrous oxide|Nitrous oxide]]-based alternatives are garnering a lot of 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 ever flown in space was by D-Orbit onboard their ION Satellite Carrier ([[Engineering:Space tug|space tug]]) in 2021, using six Dawn Aerospace B20 thrusters, launched upon a [[Company:SpaceX|SpaceX]] crafted [[Engineering:Falcon 9|Falcon 9]] rocket.<ref>{{Cite web |title=Dawn Aerospace validates B20 Thrusters in space – Bits&Chips |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>

====Reaction engines====
{{Main|Engineering:Reaction engine}}

Reaction engines produce [[Thrust|thrust]] by expelling reaction mass, in accordance with Newton's third law of motion.{{citation needed|date = July 2023}}<ref><!--THIS IS UNNECESSARY, IN THIS DITOR'S VIEW:-->This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."{{citation needed|date = July 2023}}</ref> Examples include [[Engineering:Jet engine|jet engine]]s, [[Engineering:Rocket engine|rocket engine]]s, [[Engineering:Pump-jet|pump-jet]], and more uncommon variations such as [[Physics:Hall-effect thruster|Hall–effect thrusters]], ion drives, mass drivers, and [[Physics:Nuclear pulse propulsion|nuclear pulse propulsion]].{{citation needed|date = July 2023}}

====Rocket engines====
{{Main|Engineering:Rocket engine}}
[[File:SpaceX engine test fire.jpg|thumb|right|SpaceX's Kestrel engine is tested.]]
Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.{{citation needed|date = July 2023}} Most rocket engines are [[Earth:Internal combustion engine|internal combustion]] heat engines (although non-combusting forms exist).{{citation needed|date = July 2023}} 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.{{citation needed|date = July 2023}} The extremely hot gas is then allowed to escape through a high-expansion ratio bell-shaped  [[Astronomy:De Laval nozzle|nozzle]], a feature that gives a rocket engine its characteristic shape.{{citation needed|date = July 2023}} The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy, where exhaust speeds reaching as high as 10 times the speed of sound at sea level are common.{{citation needed|date = July 2023}}

Ion propulsion rockets can heat a plasma or charged gas inside a magnetic bottle and release it via a [[Physics:Magnetic nozzle|magnetic nozzle]] so that no solid matter needs to come in contact with the plasma.{{citation needed|date = July 2023}} The machinery to do this is complex, but research into [[Physics:Nuclear fusion|nuclear fusion]] has developed methods, some of which have been proposed to be used in propulsion systems, and some have been tested in a lab.{{citation needed|date = July 2023}} (See the Wikipedia article on [[Engineering:Rocket engine|rocket engine]]s for a listing of various kinds of rocket engines using different heating methods, including chemical, electrical, solar, and nuclear.)

===Electric propulsion===
[[File:Ion Engine Test Firing - GPN-2000-000482.jpg|thumb|right|225px|NASA's 2.3 kW NSTAR ion thruster for the Deep Space&nbsp;1 spacecraft during a hot fire test at the Jet Propulsion Laboratory]]{{Main|Spacecraft electric propulsion}}
[[Engineering:Electrically powered spacecraft propulsion|Electric propulsion]] is commonly used for station keeping{{clarify|date = July 2023}} on commercial communications satellites and for prime propulsion on some [[Astronomy:Space exploration|scientific space missions]] because of their high specific impulse. However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.{{Sfn|Meyer|2012|p=5}}<ref name="tomsik">Tomsik, Thomas M. "[http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%20Propulsion/NASA-TM-2000-209941,%20Advances%20in%20Cryo%20Propellant%20Densification%20Technology.pdf Recent advances and applications in cryogenic propellant densification technology] {{Webarchive|url=https://web.archive.org/web/20141129035753/http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%20Propulsion/NASA-TM-2000-209941,%20Advances%20in%20Cryo%20Propellant%20Densification%20Technology.pdf|date=2014-11-29}}." NASA TM 209941 (2000).</ref><ref name="oleson">Oleson, S., and J. Sankovic. "[http://adsabs.harvard.edu/full/2000ESASP.465..717O Advanced Hall electric propulsion for future in-space transportation]." Spacecraft Propulsion. Vol. 465. 2000.</ref><ref>Dunning, John W., Scott Benson, and Steven Oleson. "NASA's electric propulsion program." 27th International Electric Propulsion Conference, Pasadena, CA, IEPC-01-002. 2001.</ref>

Rather than relying on high temperature and [[Physics:Fluid dynamics|fluid dynamics]] to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or [[Physics:Electromagnetism|electromagnetic]] forces to accelerate the reaction mass directly, where the reaction mass is usually a stream of [[Physics:Ion|ion]]s.{{citation needed|date = July 2023}} Such an engine typically uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities.{{citation needed|date = July 2023}} For these drives, at the highest exhaust speeds, energetic efficiency and thrust are all inversely proportional to exhaust velocity.{{citation needed|date = July 2023}} Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel.{{citation needed|date = July 2023}}

The idea of electric propulsion dates back to 1906, when Robert Goddard considered the possibility in his personal notebook.<ref name="choueiri">{{cite journal | last = Choueiri | first = Edgar Y. | year = 2004 | title = A Critical History of Electric Propulsion: The First 50 Years (1906–1956) | journal = Journal of Propulsion and Power | volume = 20 | issue = 2 | pages = 193–203 | url = http://alfven.princeton.edu/publications/choueiri-jpp-2004 | doi = 10.2514/1.9245 | citeseerx = 10.1.1.573.8519 | access-date = 2016-10-18 | archive-date = 2019-04-28 | archive-url = https://web.archive.org/web/20190428155604/https://alfven.princeton.edu/publications/choueiri-jpp-2004 }}</ref> [[Biography:Konstantin Tsiolkovsky|Konstantin Tsiolkovsky]] published the idea in 1911.{{citation needed|date = July 2023}}

One institution focused on developing primary propulsion technologies, including electric, aimed at benefitting near and mid-term science missions by reducing cost, mass, and/or travel times is the [[Organization:Glenn Research Center|Glenn Research Center]] (GRC).{{citation needed|date = July 2023}} Electric propulsion architectures of particular interest to the GRC are [[Engineering:Ion thruster|Ion]] and Hall thrusters.{{citation needed|date = July 2023}} One system combines [[Physics:Solar sail|solar sail]]s, a form of propellantless propulsion which relies on naturally-occurring starlight for propulsion energy, and Hall thrusters. Other propulsion technologies being developed include advanced chemical propulsion and aerocapture.<ref name="leone" /><ref name="grcspace">[https://www1.grc.nasa.gov/space/sep/ Solar Electric Propulsion (SEP)]. Glenn Research Center. NASA. 2019</ref><ref name="glenion">[http://www.grc.nasa.gov/WWW/ion/ Ion propulsion system research] {{Webarchive|url=https://web.archive.org/web/20060901214224/http://www.grc.nasa.gov/WWW/ion/|date=2006-09-01}}. Glenn Research Center. NASA. 2013</ref>

For some missions, particularly reasonably close to the Sun, [[Physics:Solar energy|solar energy]] may be sufficient, and has very often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are called [[Engineering:Nuclear electric rocket|nuclear electric rocket]]s.{{citation needed|date = July 2023}}

With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits the amount of thrust that can be produced to a small value.{{citation needed|date = July 2023}} Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle.{{citation needed|date = July 2023}}

Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun.{{citation needed|date = July 2023}} Chemical power generators are not used due to the far lower total available energy.{{citation needed|date = July 2023}} Beamed power to the spacecraft is considered to have potential.{{according to whom|date = July 2023}}{{citation needed|date = July 2023}}
[[File:Xenon hall thruster.jpg|thumb|6 kW Hall thruster in operation at the [[Organization:NASA|NASA]] Jet Propulsion Laboratory ]]
Electromagnetic methods include:{{citation needed|date = July 2023}}
* ion thrusters, which accelerate ions first and later neutralize the ion beam with an electron stream emitted from a cathode called a neutralizer;{{citation needed|date = July 2023}}
**electrostatic ion thruster
**[[Physics:Gridded ion thruster|gridded ion thruster]]
**[[Engineering:Field-emission electric propulsion|field-emission electric propulsion]]
**[[Engineering:MagBeam|MagBeam]] thrusters
**[[Physics:Hall-effect thruster|Hall-effect thruster]]
**[[Engineering:Colloid thruster|colloid thruster]]
* electrothermal thrusters, wherein electromagnetic fields are used to generate a plasma to increase the [[Physics:Heat|heat]] of the bulk propellant, the thermal energy imparted to the propellant gas is then converted into kinetic energy by a [[Physics:Nozzle|nozzle]] of either physical material construction or by magnetic means;{{citation needed|date = July 2023}}
**[[Engineering:Arcjet rocket|DC arcjets]]
**[[Engineering:Arcjet rocket|microwave arcjets]]
**helicon double-layer thrusters
* electromagnetic thrusters, wherein ions are accelerated either by the Lorentz Force or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration;{{citation needed|date = July 2023}}
**[[Physics:Plasma propulsion engine|plasma propulsion engine]]s
**[[Engineering:Magnetoplasmadynamic thruster|magnetoplasmadynamic thruster]]s
**[[Engineering:Electrodeless plasma thruster|electrodeless plasma thruster]]s
**[[Engineering:Pulsed inductive thruster|pulsed inductive thruster]]s
**[[Engineering:Pulsed plasma thruster|pulsed plasma thruster]]s
**variable specific impulse magnetoplasma rockets (VASIMR)
**[[Engineering:Vacuum arc thruster|vacuum arc thruster]]s, and
*[[Physics:Mass driver|mass driver]]s designed for propulsion.{{citation needed|date = July 2023}}

=== Without internal reaction mass ===
[[File:Solarsail msfc.jpg|right|thumb|NASA study of a solar sail. The sail would be half a kilometer wide.]]

====EM wave-based propulsion====
The [[Physics:Conservation law|law of conservation]] of [[Physics:Momentum|momentum]] is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible).{{citation needed|date = July 2023}} But space is not empty, especially space inside the Solar System; there are gravitation fields, [[Physics:Magnetic field|magnetic field]]s, [[Physics:Electromagnetic waves|electromagnetic waves]], [[Astronomy:Solar wind|solar wind]] and solar radiation.{{citation needed|date = July 2023}} Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux density '''P''' of an EM wave is quantitatively 1/c<sup>2</sup> times the [[Poynting vector]] '''S''', i.e. '''P''' = '''S'''/c<sup>2</sup>, where c is the velocity of light.{{citation needed|date = July 2023}} [[Engineering:Field propulsion|Field propulsion]] methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft; however, because many of these phenomena are diffuse in nature, corresponding propulsion structures must be proportionately large.{{citation needed|date = July 2023}}

====Tether propulsion====
There are several different space drives that need little or no reaction mass to function. A tether propulsion system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object.<ref>{{cite news|first=Dave |last=Drachlis |title=NASA calls on industry, academia for in-space propulsion innovations |publisher=NASA |date=October 24, 2002 |url=http://www.msfc.nasa.gov/news/news/releases/2002/02-269.html |access-date=2007-07-26 |archive-url=https://web.archive.org/web/20071206095134/http://www.msfc.nasa.gov/news/news/releases/2002/02-269.html |archive-date=December 6, 2007 }}</ref>

====Solar and magnetic sails====
A satellite or other space vehicle is subject to the law of conservation of angular momentum, which constrains a body from a [[Physics:Net force|net change]] in [[Physics:Angular velocity|angular velocity]]. Thus, for a vehicle to change its relative orientation without expending reaction mass, another part of the vehicle may rotate in the opposite direction.  Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum,<ref>{{cite book| first=Desmond | last=King-Hele | title=Satellite orbits in an atmosphere: Theory and application | publisher=Springer| year=1987 | url=https://books.google.com/books?id=HSffDG356TkC&pg=PA6|page=6 | isbn=978-0-216-92252-5 }}</ref> so secondary systems are designed to "bleed off" undesired rotational energies built up over time. Accordingly, many spacecraft utilize reaction wheels or control moment gyroscopes to control orientation in space.<ref>{{cite journal  | last = Tsiotras  | first = P. |author2=Shen, H. |author3=Hall, C. D. | title = Satellite attitude control and power tracking with energy/momentum wheels  | journal = Journal of Guidance, Control, and Dynamics  | volume = 43  | issue = 1  | pages = 23–34  | year = 2001  | issn = 0731-5090  | doi = 10.2514/2.4705 |bibcode = 2001JGCD...24...23T | url = http://www.ae.gatech.edu/people/tsiotras/Papers/jgcd99.pdf| citeseerx = 10.1.1.486.3386 }}
</ref>

The concept of [[Physics:Solar sail|Solar sail]]s rely on [[Astronomy:Radiation pressure|radiation pressure]] from electromagnetic energy, but they require a large collection surface to function effectively.{{citation needed|date = July 2023}} E-sails propose to use very thin and lightweight wires holding an electric charge to deflect particles, which may have more controllable directionality.{{citation needed|date = July 2023}}

[[Physics:Magnetic sail|Magnetic sail]]s deflect charged particles from the [[Astronomy:Solar wind|solar wind]] with a magnetic field, thereby imparting momentum to the spacecraft.<ref>{{Cite journal |last=Djojodihardjo |first=Harijono |date=November 2018 |title=Review of Solar Magnetic Sailing Configurations for Space Travel |url=http://link.springer.com/10.1007/s42423-018-0022-4 |journal=Advances in Astronautics Science and Technology |language=en |volume=1 |issue=2 |pages=207–219 |doi=10.1007/s42423-018-0022-4 |issn=2524-5252}}</ref> For instance, the so-called [[Physics:Magnetic sail#Magsail (MS)|Magsail]] is a large superconducting loop proposed for acceleration/deceleration in the [[Astronomy:Solar wind|solar wind]] and deceleration in the [[Astronomy:Interstellar medium|Interstellar medium]].<ref>{{Cite journal |last=Zubrin |first=Robert M. |last2=Andrews |first2=Dana G. |date=March 1991 |title=Magnetic sails and interplanetary travel |url=https://arc.aiaa.org/doi/10.2514/3.26230 |journal=Journal of Spacecraft and Rockets |language=en |volume=28 |issue=2 |pages=197–203 |doi=10.2514/3.26230 |issn=0022-4650}}</ref> A variant is the mini-magnetospheric plasma propulsion system<ref>{{Cite journal |last=Winglee |first=R. M. |last2=Slough |first2=J. |last3=Ziemba |first3=T. |last4=Goodson |first4=A. |date=September 2000 |title=Mini‐Magnetospheric Plasma Propulsion: Tapping the energy of the solar wind for spacecraft propulsion |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999JA000334 |journal=Journal of Geophysical Research: Space Physics |language=en |volume=105 |issue=A9 |pages=21067–21077 |doi=10.1029/1999JA000334 |issn=0148-0227}}</ref> and its successor, the [[Physics:Magnetic sail#Magnetoplasma sail (MPS)|magnetoplasma sail]],<ref>{{Cite journal |last=Funaki |first=Ikkoh |last2=Asahi |first2=Ryusuke |last3=Fujita |first3=Kazuhisa |last4=Yamakawa |first4=Hiroshi |last5=Ogawa |first5=Hiroyuki |last6=Otsu |first6=Hirotaka |last7=Nonaka |first7=Satoshi |last8=Sawai |first8=Shujiro |last9=Kuninaka |first9=Hitoshi |date=2003-06-23 |title=Thrust Production Mechanism of a Magnetoplasma Sail |url=https://arc.aiaa.org/doi/10.2514/6.2003-4292 |journal=34th AIAA Plasmadynamics and Laser Conference |language=en |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2003-4292 |isbn=978-1-62410-096-3}}</ref> which inject plasma at a low rate to enhance the magnetic field to more effectively deflect charged particles in a plasma wind.

Japan launched a solar sail-powered spacecraft, [[Engineering:IKAROS|IKAROS]], in May 2010, which successfully demonstrated propulsion and guidance (and is still active as of this date).{{when|date = July 2023}}{{citation needed|date = July 2023}} As a further proof of the [[Physics:Solar sail|solar sail]] concept, [[Engineering:NanoSail-D|NanoSail-D]] became the first such powered satellite to orbit [[Astronomy:Earth|Earth]].<ref>{{Cite web |url= http://www.nasa.gov/mission_pages/tdm/solarsail|title=Solar Sail Demonstrator|date=19 September 2016}}</ref> As of August 2017, NASA confirmed the Sunjammer solar sail project was concluded in 2014 with lessons learned for future space sail projects.<ref>{{Cite web|url=https://www.nasa.gov/mission_pages/tdm/solarsail/index.html|title=Solar Sail Demonstrator|date=19 September 2016}}</ref> The U.K. Cubesail programme will be the first mission to demonstrate solar sailing in low Earth orbit, and the first mission to demonstrate full three-axis attitude control of a solar sail.<ref>{{cite web|title=Space Vehicle Control|url=http://www.surrey.ac.uk/ssc/research/space_vehicle_control/cubesail/news/index.htm|website=University of Surrey|access-date=8 August 2015|archive-date=7 May 2016|archive-url=https://web.archive.org/web/20160507025010/http://www.surrey.ac.uk/ssc/research/space_vehicle_control/cubesail/news/index.htm}}</ref>

====Other propulsion types====
The concept of a gravitational slingshot is a form of propulsion to carry a [[Engineering:Space probe|space probe]] onward to other destinations without the expense of reaction mass; harnessing the gravitational energy of other celestial objects allows the spacecraft can pick up kinetic energy.<ref>{{Cite journal | year = 2004 | pages = 619–000 | doi = 10.1119/1.1621032| last2 =   Cacioppo| last3 =   Gangopadhyaya| first1 = J. J. | volume = 72| last1 = Dykla | first2 = R.| first3 = A. | journal = American Journal of Physics | title = Gravitational slingshot| issue = 5|bibcode = 2004AmJPh..72..619D | url = http://ecommons.luc.edu/cgi/viewcontent.cgi?article=1007&context=physics_facpubs}}</ref> However, even more energy can be obtained from the gravity assist if rockets are used.{{citation needed|date = July 2023}}

[[Astronomy:Beam-powered propulsion|Beam-powered propulsion]] is another method of propulsion without reaction mass, and includes sails pushed by [[Engineering:Laser propulsion|laser]], microwave, or particle beams.{{citation needed|date = July 2023}}

===Advanced propulsion technology===
Advanced, and in some cases theoretical, propulsion technologies may use chemical or nonchemical physics to produce thrust, but are generally considered to be of lower technical maturity with challenges that have not been overcome.{{Sfn|Meyer|2012|p=20}}  For both human and robotic exploration, traversing the solar system is a struggle against time and distance. The most distant planets are 4.5–6 billion kilometers from the Sun and to reach them in any reasonable time requires much more capable propulsion systems than conventional chemical rockets. Rapid inner solar system missions with flexible launch dates are difficult, requiring propulsion systems that are beyond today's current state of the art. The logistics, and therefore the total system mass required to support sustained human exploration beyond Earth to destinations such as the Moon, Mars or Near Earth Objects,
are daunting unless more efficient in-space propulsion technologies are developed and fielded.{{Sfn|Meyer|2012|p=6}}<ref name=huntsb>
{{cite journal
|doi=10.1061/40476(299)45
|citeseerx = 10.1.1.83.3242
|title=Robotics Challenges for Robotic and Human Mars Exploration
|journal=Robotics 2000
|year=2000
|last1=Huntsberger
|first1=Terry
|last2=Rodriguez
|first2=Guillermo
|last3=Schenker
|first3=Paul S.
|isbn=978-0-7844-0476-8
|pages=340–346}}</ref>

[[File:Wormhole travel as envisioned by Les Bossinas for NASA.jpg|thumb|right|Artist's conception of a warp drive design]]
A variety of hypothetical propulsion techniques have been considered that require a deeper understanding of the properties of space, particularly [[Physics:Inertial frame of reference|inertial frames]] and the [[Physics:Vacuum state|vacuum state]].  To date, such methods are highly speculative and include:{{citation needed|date = July 2023}}
{{colbegin}}
*[[Engineering:Black hole starship|Black hole starship]]
*[[Engineering:Breakthrough Propulsion Physics Program#Quantum vacuum energy experiments|Differential sail]]
*[[Physics:Gravitational shielding|Gravitational shielding]]
*[[Engineering:Field propulsion|Field propulsion]]
**Diametric drive
**Disjunction drive
**Pitch drive
**Bias drive
*[[Engineering:Photon rocket|Photon rocket]]
*[[Physics:Quantum vacuum thruster|Quantum vacuum thruster]]
*[[Engineering:Nano electrokinetic thruster|Nano electrokinetic thruster]]
*[[Engineering:Reactionless drive|Reactionless drive]]
**[[Physics:Abraham–Minkowski controversy|Abraham—Minkowski drive]]
**[[Alcubierre drive]]
**[[Engineering:Dean drive|Dean drive]]
**[[Engineering:EmDrive|EmDrive]]
**[[Physics:Heim theory|Heim theory]]
**[[Physics:Woodward effect|Woodward effect]]

{{colend}}

A NASA assessment of its [[Engineering:Breakthrough Propulsion Physics Program|Breakthrough Propulsion Physics Program]] divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are not impossible according to current theories.<ref>{{cite conference
 | first =Marc
 | last =Millis
 | title =Assessing Potential Propulsion Breakthroughs
 | book-title =New Trends in Astrodynamics and Applications II
 | date =June 3–5, 2005
 | location =Princeton, NJ
 | url =https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060000022.pdf
 }}</ref>

===Table of methods===
Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods. Four numbers are shown.  The first is the [[Physics:Specific impulse|effective exhaust velocity]]: the equivalent speed that the propellant leaves the vehicle.  This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However,
* if the delta-v is much more than the exhaust velocity, then exorbitant amounts of fuel are necessary (see the section on calculations, above),{{according to whom|date = July 2023}} and
* if it is much more than the delta-v, then, proportionally more energy is needed; if the power is limited, as with solar energy, this means that the journey takes a proportionally longer time.{{according to whom|date = July 2023}}
The second and third are the typical amounts of thrust and the typical burn times of the method; outside a gravitational potential small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period, if the object is not significantly influenced by gravity.{{citation needed|date = July 2023}} The fourth is the maximum delta-v the technique can give without staging. For rocket-like propulsion systems this is a function of mass fraction and exhaust velocity; mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight.{{citation needed|date = July 2023}} For a system to achieve this limit, the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower.{{citation needed|date = July 2023}}

{| class="wikitable sortable" style="text-align:center;"
|+ Propulsion methods
! style="text-align:left;"  | Method
! [[Physics:Specific impulse|Effective exhaust <br/>velocity]] (km/s)
! [[Thrust]] (N)
! Firing <br/>duration
! Maximum <br/>[[Astronomy:Delta-v|delta-v]] (km/s)
! style="text-align:left;"  | [[Engineering:Technology readiness level#NASA definitions|Technology <br/>readiness level]]
|-
| style="text-align:left;"  | Solid-fuel rocket || {{ntsh|2.5}}<2.5 || <10<sup>7</sup> || {{Ntsh|60}}Minutes || {{Nts|7}}
| style="text-align:left;"  | {{Nts|9}}: Flight proven
|-
| style="text-align:left;"  | Hybrid rocket || <4 ||  || {{Ntsh|60}}Minutes || {{Ntsh|3}}>3
| style="text-align:left;"  | {{Nts|9}}: Flight proven
|-
| style="text-align:left;"  | [[Engineering:Monopropellant rocket|Monopropellant rocket]] || {{Ntsh|2}}1 – 3<ref name=":0">{{Cite web|url=http://www.space-propulsion.com/brochures/hydrazine-thrusters/hydrazine-thrusters.pdf|title=Chemical monopropellant thruster family|website=Ariane Group|access-date=16 March 2019}}</ref>||{{Ntsh|{{#expr:10^.5}}}}0.1 – 400<ref name=":0" />||{{Ntsh|1}}Milliseconds – minutes || {{Nts|3}}
| style="text-align:left;"  | {{Nts|9}}: Flight proven
|-
| style="text-align:left;"  | Liquid-fuel rocket || {{ntsh|4.4}}<4.4 || <10<sup>7</sup> || {{Ntsh|60}}Minutes || {{Nts|9}}
| style="text-align:left;"  | {{Nts|9}}: Flight proven
|-
| style="text-align:left;"  | Electrostatic ion thruster || {{Ntsh|112.5}}15 – 210<ref>{{cite web| title = ESA Portal – ESA and ANU make space propulsion breakthrough|url=https://cordis.europa.eu/article/id/25070-esa-and-australian-team-develop-breakthrough-in-space-propulsion|date=18 January 2006|publisher=European Union}}</ref> ||  || {{Ntsh|{{#expr:3600*24*365.25*12^-.5}}}}Months – years || {{Ntsh|100}}>100
| style="text-align:left;"  | {{Nts|9}}: Flight proven
|-
| style="text-align:left;"  | [[Physics:Hall-effect thruster|Hall-effect thruster]] (HET) || {{Ntsh|29}}up to 50<ref>{{Cite web |url=https://www.grc.nasa.gov/WWW/hall/overview/overview.htm |title=Overview of Hall thrusters |access-date=2020-05-29 |archive-date=2020-05-23 |archive-url=https://web.archive.org/web/20200523031148/https://www.grc.nasa.gov/WWW/hall/overview/overview.htm }}</ref> || || {{Ntsh|{{#expr:3600*24*365.25*12^-.5}}}}Months – years || {{Ntsh|100}}>100
| style="text-align:left;"  | {{Nts|9}}: Flight proven<ref>Hall-effect thrusters have been used on Russian and antecedant Soviet bloc satellites for decades.{{Original research inline|date = July 2023}}{{citation needed|date = July 2022}}</ref>
|-
| style="text-align:left;"  | [[Engineering:Resistojet rocket|Resistojet rocket]] || {{Ntsh|4}}2 – 6 || {{Ntsh|{{#expr:10^-.5}}}}10<sup>−2</sup> – 10 || {{Ntsh|60}}Minutes || {{dunno}}
| style="text-align:left;"  | {{Nts|8}}: Flight qualified<ref>[https://www.webcitation.org/5sq1FWURx?url=http://pdf.aiaa.org/preview/CDReadyMJPC2005_1177/PV2005_4260.pdf ''A Xenon Resistojet Propulsion System for Microsatellites''] (Surrey Space Centre, University of Surrey, Guildford, Surrey)</ref>
|-
| style="text-align:left;"  | [[Engineering:Arcjet rocket|Arcjet rocket]] || {{Ntsh|10}}4 – 16 || {{Ntsh|{{#expr:10^-.5}}}}10<sup>−2</sup> – 10 || {{Ntsh|60}}Minutes || {{dunno}}
| style="text-align:left;"  | {{Nts|8}}: Flight qualified{{Citation needed|date=February 2010}}
|-
| style="text-align:left;"  | [[Engineering:Field-emission electric propulsion|Field-emission<br />electric propulsion]] (FEEP) || {{Ntsh|115}}100<ref name="feep">{{cite web| url = http://www.alta-space.com/index.php?page=feep| archive-url = https://web.archive.org/web/20110707120124/http://www.alta-space.com/index.php?page=feep| archive-date = 2011-07-07| title = Alta - Space Propulsion, Systems and Services - Field Emission Electric Propulsion<!-- Bot generated title -->}}</ref> – 130 || {{Ntsh|{{#expr:10^-4.5}}}}10<sup>−6</sup> – 10<sup>−3</sup><ref name="feep"/> || {{Ntsh|{{#expr:3600*24*365.25*12^-.5}}}}Months – years || {{dunno}}
| style="text-align:left;"  | {{Nts|8}}: Flight qualified<ref name="feep"/>
|-
| style="text-align:left;"  | [[Engineering:Pulsed plasma thruster|Pulsed plasma thruster]] (PPT) || {{Nts|20}} || {{Nts|0.1}} || {{Ntsh|{{#expr:3600000*10^((1+ln2/ln10)/2)}}}}80 – 400 days || {{dunno}}
| style="text-align:left;"  | {{Nts|7}}: Prototype demonstrated in space
|-
| style="text-align:left;"  | [[Engineering:Dual mode propulsion rocket|Dual-mode propulsion rocket]] || {{Ntsh|2.85}}1 – 4.7 || {{Ntsh|1000}}0.1 – 10<sup>7</sup> || {{Ntsh|1}}Milliseconds – minutes || {{Ntsh|6}}3 – 9
| style="text-align:left;"  | {{Nts|7}}: Prototype demonstrated in space
|-
| style="text-align:left;"  | [[Physics:Solar sail|Solar sail]]s
| [[Astronomy:Radiation pressure|{{Nts|299792.458}}]], [[Physics:Speed of light|Speed of light]]
| {{Ntsh|9}} 9.08/km<sup>2</sup> at 1&nbsp;[[Astronomy:Astronomical unit|AU]]<br />908/km<sup>2</sup> at 0.1&nbsp;AU<br />10<sup>−10</sup>/km<sup>2</sup> at 4 [[Astronomy:Light-year|ly]]
| Indefinite
|| {{Ntsh|40}}>40
| {{Ntsh|6.9}}{{unbulleted list
 | 9: Light pressure attitude-control flight proven
 | 6: Model, 196 m<sup>2</sup> 1.12 mN 400 m/s delta-v demonstrated in interplanetary space<ref>{{cite web |url=http://www.isas.jaxa.jp/home/IKAROS-blog/?itemid=1017|title=今日の IKAROS(8/29) – Daily Report – Aug 29, 2013 | publisher=Japan Aerospace Exploration Agency (JAXA) |date=29 August 2013 | access-date=8 June 2014 |language=ja}}</ref>
 }}
|-
| style="text-align:left;"  | [[Engineering:Tripropellant rocket|Tripropellant rocket]] || {{Ntsh|3.9}}2.5 – 5.3{{Citation needed|date=February 2011}} || {{Ntsh|1000}}0.1 – 10<sup>7</sup>{{Citation needed|date=February 2011}} || {{Ntsh|60}}Minutes || {{Nts|9}}
| style="text-align:left;"  | {{Nts|6}}: Prototype demonstrated on ground<ref>[http://www.astronautix.com/engines/rd701.htm RD-701<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20100210203238/http://www.astronautix.com/engines/rd701.htm |date=2010-02-10 }}</ref>
|-
| style="text-align:left;"  | [[Engineering:Magnetoplasmadynamic thruster|Magnetoplasmadynamic<br />thruster]] (MPD) || {{Ntsh|60}}20 – 100 || {{Nts|100}} || {{Ntsh|{{#expr:3600*24*7}}}}Weeks || {{dunno}}
| style="text-align:left;"  | {{Nts|6}}: Model, 1&nbsp;kW demonstrated in space<ref>{{cite web| url = https://translate.google.com/translate?hl=en&sl=ja&u=http://www.isas.jaxa.jp/ISASnews/No.190/labo-5.html| title = Google Translate<!-- Bot generated title -->}}</ref>
|-
| style="text-align:left;"  | [[Engineering:Nuclear thermal rocket|Nuclear–thermal rocket]] || {{Nts|9}}<ref name="rd0410">[http://www.astronautix.com/engines/rd0410.htm RD-0410<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20090408122011/http://www.astronautix.com/engines/rd0410.htm |date=2009-04-08 }}</ref> || {{Ntsh|10000000}}10<sup>7</sup><ref name="rd0410"/> || {{Ntsh|60}}Minutes<ref name="rd0410"/> || {{Ntsh|20}}>20
| style="text-align:left;"  | {{Nts|6}}: Prototype demonstrated on ground
|-
| style="text-align:left;"  | Propulsive [[Physics:Mass driver|mass driver]]s || {{Ntsh|15}}0 – 30 || {{Ntsh|1000000}}10<sup>4</sup> – 10<sup>8</sup> || {{Ntsh|{{#expr:3600*24*31}}}}Months || {{dunno}}
| style="text-align:left;"  | {{Nts|6}}: Model, 32&nbsp;MJ demonstrated on ground
|-
| style="text-align:left;"  | Tether propulsion || {{n/a}} || {{Ntsh|1000000}}1 – 10<sup>12</sup> || {{Ntsh|60}}Minutes || {{Ntsh|7}}7
| style="text-align:left;"  | {{Nts|6}}: Model, 31.7&nbsp;km demonstrated in space<ref name="Tether">[http://www.yes2.info/ Young Engineers' Satellite 2<!-- Bot generated title -->]  {{webarchive|url=https://web.archive.org/web/20030210014335/http://www.yes2.info/ |date=2003-02-10 }}</ref>
|-
| style="text-align:left;"  | [[Engineering:Air-augmented rocket|Air-augmented rocket]] || {{Ntsh|5.5}}5 – 6 || {{Ntsh|1000}}0.1 – 10<sup>7</sup> || {{Ntsh|{{#expr:60^.5}}}}Seconds – minutes || {{Ntsh|7}}>7?
| style="text-align:left;"  | {{Nts|6}}: Prototype demonstrated on ground<ref>[http://astronautix.com/lvs/gnom.htm Gnom] {{webarchive|url=https://web.archive.org/web/20100102204131/http://astronautix.com/lvs/gnom.htm |date=2010-01-02 }}</ref><ref>[http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html NASA GTX] {{webarchive |url=https://web.archive.org/web/20081122140310/http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html |date=November 22, 2008 }}</ref>
|-
| style="text-align:left;"  | [[Engineering:Liquid air cycle engine|Liquid-air-cycle engine]] || {{Nts|4.5}} || {{Ntsh|100000}}10<sup>3</sup> – 10<sup>7</sup> || {{Ntsh|{{#expr:60^.5}}}}Seconds – minutes || {{dunno}}
| style="text-align:left;"  | {{Nts|6}}: Prototype demonstrated on ground
|-
| style="text-align:left;"  | [[Engineering:Pulsed inductive thruster|Pulsed-inductive thruster]] (PIT) || {{Ntsh|45}}10 – 80<ref name="PIT">{{cite web| url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930023164_1993023164.pdf| title = The PIT MkV pulsed inductive thruster}}</ref> || {{Nts|20}} || {{Ntsh|{{#expr:3600*24*31}}}}Months || {{dunno}}
| style="text-align:left;"  | {{Nts|5}}: Component validated in vacuum<ref name="PIT"/>
|-
| style="text-align:left;"  | Variable-specific-impulse<br />magnetoplasma rocket<br />(VASIMR) || {{Ntsh|155}}10 – 300{{Citation needed|date=February 2011}} || {{Ntsh|620}}40 – 1,200{{Citation needed|date=February 2011}} || {{Ntsh|{{#expr:3600*24*31^.5}}}}Days – months || {{Ntsh|100}}>100
| style="text-align:left;"  | {{Nts|5}}: Component, 200&nbsp;kW validated in vacuum
|-
| style="text-align:left;"  | [[Engineering:Magnetic field oscillating amplified thruster|Magnetic-field oscillating<br />amplified thruster]] (MOA) || {{Ntsh|70}}10 – 390<ref name="MOA">{{cite web| url = http://scidoc.org/articlepdfs/IJASAR/IJASAR-2470-4415-10-102.pdf| title = Thermal velocities in the plasma of a MOA Device, M.Hettmer, Int J Aeronautics Aerospace Res. 2023;10(1):297-300}}</ref> || {{Ntsh|{{#expr:10^-.5}}}}0.1 – 1 || {{Ntsh|{{#expr:3600*24*31^.5}}}}Days – months || {{Ntsh|100}}>100
| style="text-align:left;"  | {{Nts|5}}: Component validated in vacuum
|-
| style="text-align:left;"  | [[Engineering:Solar thermal rocket|Solar–thermal rocket]] || {{Ntsh|9.5}}7 – 12 || {{Ntsh|10}}1 – 100 || {{Ntsh|{{#expr:3600*24*7}}}}Weeks || {{Ntsh|20}}>20
| style="text-align:left;"  | {{Nts|4}}: Component validated in lab<ref>{{cite news|url=https://spaceref.com/press-release/pratt-whitney-rocketdyne-wins-22-million-contract-option-for-solar-thermal-propulsion-rocket-engine/|title= Pratt & Whitney Rocketdyne Wins $2.2 Million Contract Option for Solar Thermal Propulsion Rocket Engine|date= June 25, 2008|publisher=Pratt & Whitney [[Company:Rocketdyne|Rocketdyne]])}}</ref>
|-
| style="text-align:left;"  | [[Engineering:Radioisotope rocket|Radioisotope rocket]]/[[Astronomy:World Is Not Enough (spacecraft propulsion)|Steam thruster]] || {{Ntsh|7.5}}7 – 8{{citation needed|date=January 2011}} || {{Ntsh|1.4}}1.3 – 1.5 || {{Ntsh|{{#expr:3600*24*31}}}}Months || {{dunno}}
| style="text-align:left;"  | {{Nts|4}}: Component validated in lab
|-
| style="text-align:left;"  | [[Engineering:Nuclear electric rocket|Nuclear–electric rocket]]
| colspan=4 {{n/a|As electric propulsion method used}}
| style="text-align:left;"  | {{Nts|4}}: Component, 400&nbsp;kW validated in lab
|-
| style="text-align:left;"  | [[Astronomy:Project Orion (nuclear propulsion)|Orion Project]] (near-term<br />nuclear pulse propulsion) || {{Ntsh|60}}20 – 100 || {{Ntsh|{{#expr:10^10.5}}}}10<sup>9</sup> – 10<sup>12</sup><!--not a misprint--> || {{Ntsh|{{#expr:3600*24*7}}}}Days || {{Ntsh|45}}30 – 60
| style="text-align:left;"  | {{Nts|3}}: Validated, 900&nbsp;kg proof-of-concept<ref>{{cite web| url=http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html#PascalB| title=Operation Plumbbob|date=July 2003| access-date=2006-07-31}}</ref><ref>{{cite web| url=http://nuclearweaponarchive.org/Usa/Tests/Brownlee.html| title=Learning to Contain Underground Nuclear Explosions| first=Robert R.| last=Brownlee|date=June 2002| access-date=2006-07-31}}</ref>
|-
| style="text-align:left;"  | [[Astronomy:Space elevator|Space elevator]] || {{n/a}} || {{n/a}} || Indefinite || {{Ntsh|12}}>12
| style="text-align:left;"  | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;"  | Reaction Engines SABRE<ref name="SABRE"/> || {{Ntsh|17.25}}30/4.5 || {{Ntsh|1000}}0.1 – 10<sup>7</sup> || {{Ntsh|60}}Minutes || {{Nts|9.4}}
| style="text-align:left;"  | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;"  | [[Engineering:Electric sail|Electric sail]]s
| [[Astronomy:Solar wind#Emission|145 – 750, solar wind]]
| {{dunno}} || Indefinite || {{Ntsh|40}}>40
| style="text-align:left;"  | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;"  | [[Physics:Magnetic sail#Magsail (MS)|Magsail]] in [[Astronomy:Solar wind|Solar wind]]|| {{n/a}} || {{Ntsh|644}}{{Nts|644}}<ref name=":AndrewsZubrin90">{{Cite journal |last1=Andrews |first1=Dana |last2=Zubrin |first2=Robert |date=1990 |title=MAGNETIC SAILS AND INTERSTELLAR TRAVEL |url=https://www.academia.edu/78476000 |journal=Journal of the British Interplanetary Society |volume=43 |pages=265–272 |via=JBIS}}</ref>{{Efn|Divided by 3.1 correction factor.<ref name=":Freeland2015" />}} || Indefinite || {{Ntsh|250}}250-750
| style="text-align:left;"  | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;"  | [[Physics:Magnetic sail#Magnetoplasma sail (MPS)|Magnetoplasma sail]] in [[Astronomy:Solar wind|Solar wind]]<ref name=":222">{{Cite journal |last1=Funaki |first1=Ikkoh |last2=Kajimura |first2=Yoshihiro |last3=Ashida |first3=Yasumasa |last4=Yamakawa |first4=Hiroshi |last5=Nishida |first5=Hiroyuki |last6=Oshio |first6=Yuya |last7=Ueno |first7=Kazuma |last8=Shinohara |first8=Iku |last9=Yamamura |first9=Haruhito |last10=Yamagiwa |first10=Yoshiki |date=2013-07-14 |title=Magnetoplasma Sail with Equatorial Ring-current |url=https://arc.aiaa.org/doi/10.2514/6.2013-3878 |journal=49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference |series=Joint Propulsion Conferences |language=en |location=San Jose, CA |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2013-3878 |isbn=978-1-62410-222-6}}</ref>|| 278 || {{Ntsh|700}}700 || {{Ntsh|{{#expr:3600*24*31}}}}Months - Years || {{Ntsh|250}}250-750
| style="text-align:left;"  | {{Nts|4}}: Component validated in lab<ref name=":3">{{Citation |last1=Funaki |first1=Ikkoh |title=Solar Wind Sails |date=2012-03-21 |url=http://www.intechopen.com/books/exploring-the-solar-wind/solar-wind-sails |work=Exploring the Solar Wind |editor-last=Lazar |editor-first=Marian |publisher=InTech |language=en |bibcode=2012esw..book..439F |doi=10.5772/35673 |isbn=978-953-51-0339-4 |access-date=2022-06-13 |last2=Yamakaw |first2=Hiroshi|s2cid=55922338 |doi-access=free }}</ref>
|-
| style="text-align:left;" | [[Physics:Magnetic sail#Magsail (MS)|Magsail]] in [[Astronomy:Interstellar medium|Interstellar medium]]<ref name=":Freeland2015">{{Cite journal |last=Freeland |first=R.M. |date=2015 |title=Mathematics of Magsail |url=https://bis-space.com/shop/product/mathematics-of-magsails/ |journal=Journal of the British Interplanetary Society |volume=68 |pages=306–323 |via=bis-space.com}}</ref>
|{{n/a}}
|88,000 initially
|{{Ntsh|{{#expr:3600*24*365*10}}}}Decades
|{{Ntsh|15000}}15,000
| style="text-align:left;" |{{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;"  | [[Astronomy:Beam-powered propulsion|Beam-powered]]/[[Engineering:Laser propulsion|laser]]
| colspan=4 {{n/a|As propulsion method powered by beam}}
| style="text-align:left;"  | {{Nts|3}}: Validated, 71&nbsp;m proof-of-concept
|-
| style="text-align:left;"  | [[Physics:Launch loop|Launch loop]]/[[Astronomy:Orbital ring|orbital ring]] || {{n/a}} || {{Ntsh|10000}}10<sup>4</sup> || {{Ntsh|60}}Minutes || {{Ntsh|20.5}}11 – 30
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Physics:Nuclear pulse propulsion|Nuclear pulse propulsion]]<br />([[Engineering:Project Daedalus|Project Daedalus]]' drive) || {{Ntsh|510}}20 – 1,000 || {{Ntsh|{{#expr:10^10.5}}}}10<sup>9</sup> – 10<sup>12</sup><!--not a misprint--> || {{Ntsh|{{#expr:3600*24*365.25}}}}Years || {{Ntsh|15000}}15,000
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Engineering:Gas core reactor rocket|Gas-core reactor rocket]] || {{Ntsh|15}}10 – 20 || {{Ntsh|{{#expr:10^4.5}}}}10<sup>3</sup> – 10<sup>6</sup> || {{dunno}} || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Engineering:Nuclear salt-water rocket|Nuclear salt-water rocket]] || {{Nts|100}} || {{Ntsh|100000}}10<sup>3</sup> – 10<sup>7</sup> || {{Ntsh|1800}}Half-hour || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Physics:Fission sail|Fission sail]] || {{dunno}} || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Engineering:Fission-fragment rocket|Fission-fragment rocket]] || {{Nts|15000}} || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Engineering:Nuclear photonic rocket|Nuclear–photonic rocket]]/[[Engineering:Photon rocket|Photon rocket]] || [[Astronomy:Radiation pressure|{{Nts|299792.458}}]], [[Physics:Speed of light|Speed of light]] || {{Ntsh|{{#expr:10^-2.5}}}}10<sup>−5</sup> – 1 || {{Ntsh|{{#expr:3600*24*365.25*10^.5}}}}Years – decades || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Physics:Fusion rocket|Fusion rocket]] || {{Ntsh|550}}100 – 1,000{{citation needed|date=January 2011}} || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Physics:Antimatter-catalyzed nuclear pulse propulsion|Antimatter-catalyzed<br />nuclear pulse propulsion]] || {{Ntsh|2100}}200 – 4,000 || {{dunno}} || {{Ntsh|{{#expr:3600*24*7^.5}}}}Days – weeks || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Physics:Antimatter rocket|Antimatter rocket]] || {{Ntsh|55000}}10,000 – 100,000{{citation needed|date=January 2011}}  || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;"  | [[Physics:Bussard ramjet|Bussard ramjet]] || {{Ntsh|10001.1}}2.2 – 20,000 || {{dunno}} || Indefinite || {{Ntsh|30000}}30,000
| style="text-align:left;"  | {{Nts|2}}: Technology concept formulated
|-
! style="text-align:left;"  | Method
! [[Physics:Specific impulse|Effective exhaust <br/>velocity]] (km/s)
! [[Thrust]] (N)
! Firing <br/>duration
! Maximum <br/>[[Astronomy:Delta-v|delta-v]] (km/s)
! style="text-align:left;" | [[Engineering:Technology readiness level#NASA definitions|Technology <br/>readiness level]]
|}
'''Table Notes'''{{Notelist}}

===Testing===
Spacecraft propulsion systems are often first statically tested on Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully. Rockets are usually tested at a [[Engineering:Rocket engine test facility|rocket engine test facility]] well away from habitation and other buildings for safety reasons. Ion drives are far less dangerous and require much less stringent safety, usually only a large-ish vacuum chamber is needed. Famous static test locations can be found at [[Engineering:Rocket engine test facility#Rocket ground test facilities|Rocket Ground Test Facilities]]. Some systems cannot be adequately tested on the ground and test launches may be employed at a Rocket Launch Site.

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

===Launch-assist mechanisms===
{{Main|Engineering:Space launch}}
There have been many ideas proposed for launch-assist mechanisms that have the potential of drastically reducing the cost of getting into orbit. Proposed [[Astronomy:Non-rocket spacelaunch|non-rocket spacelaunch]] launch-assist mechanisms include:

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

===Air-breathing engines===
{{Main|Engineering:Jet engine|Engineering:Air-breathing electric propulsion}}
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 any significant performance improvement when installed on a launch vehicle itself. However, launch vehicles can be [[Engineering:Air launch|air launch]]ed from separate lift vehicles (e.g. B-29, Pegasus Rocket and [[Engineering:Scaled Composites White Knight|White Knight]]) which do use such propulsion systems.  Jet engines mounted on a launch rail could also be so used.

On the other hand, very lightweight or very high speed engines have been proposed that take advantage of the air during ascent:
* 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>
* [[Engineering:ATREX|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>
* [[Engineering:Liquid air cycle engine|Liquid air cycle engine]] – a hydrogen fuelled jet engine that liquifies the air before burning it in a rocket engine
* [[Astronomy:Scramjet|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 [[Astronomy:Delta-v|delta-v]] to orbital velocity.

For spacecraft already in very low-orbit, [[Engineering:Air-breathing electric propulsion|air-breathing electric propulsion]] would use residual gases in the upper atmosphere as propellant. Air-breathing electric propulsion could make a new class of long-lived, low-orbiting missions feasible on Earth, [[Astronomy:Mars|Mars]] or [[Astronomy:Venus|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=[[Organization:European Space Agency|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|Astronomy:Atmospheric entry}}
[[File:Pathfinder Air Bags - GPN-2000-000484.jpg|thumb|right|A test version of the [[Astronomy:Mars Pathfinder|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.  This can be done using all the methods listed above (provided they can generate a high enough thrust), but there are a few methods that can take advantage of planetary atmospheres and/or surfaces.

* [[Astronomy:Aerobraking|Aerobraking]] allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere 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 ''[[Astronomy:Mars Global Surveyor|Mars Global Surveyor]]'', ''[[Astronomy:2001 Mars Odyssey|2001 Mars Odyssey]]'', and ''[[Astronomy:Mars Reconnaissance Orbiter|Mars Reconnaissance Orbiter]]'', and at least one Venus mission, ''[[Engineering:Magellan (spacecraft)|Magellan]]''.
* [[Engineering:Aerocapture|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 much trickier 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 re-entry skip by [[Astronomy:Zond 6|Zond 6]] and [[Astronomy:Zond 7|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 [[Engineering:Ballute|ballute]] is an inflatable drag device.
* [[Engineering:Parachute|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 heat shield.
* [[Engineering:Airbag|Airbag]]s can soften the final landing.
* [[Engineering:Lithobraking|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, [[Engineering:Deep Impact (spacecraft)|Deep Impact (spacecraft)]]), in which case very sturdy probes are required.

==In fiction==
In science fiction, space ships use various means to travel, some of them scientifically plausible (like solar sails or ramjets), others, mostly or entirely fictitious (like [[Physics:Anti-gravity|anti-gravity]], [[Engineering:Warp drive|warp drive]], spindizzy or hyperspace travel).<ref name="visual">{{Cite book|last=Ash|first=Brian|url=https://books.google.com/books?id=-SUYAAAAIAAJ&q=Visual+Encyclopedia+of+Science+Fiction|title=The Visual Encyclopedia of Science Fiction|date=1977|publisher=Harmony Books|isbn=978-0-517-53174-7|language=en}}</ref>{{Rp|8, 69–77}}<ref name=":Prucher">{{Cite book|last=Prucher|first=Jeff|url=https://books.google.com/books?id=lJCS0reqmFUC&q=Earthling+%22science+fiction%22&pg=PP2|title=Brave New Words: The Oxford Dictionary of Science Fiction|date=2007-05-07|publisher=Oxford University Press|isbn=978-0-19-988552-7|pages=|language=en}}</ref>{{Rp|142}}

==Further reading==
{{more science citations needed|section|date=July 2023}}
* {{cite book |author=Heister, Stephen D. |author2=Anderson, William E. |author3=Pourpoint, Timothée L. |author4=Cassady, R. Joseph |date=2019 |title=Rocket Propulsion |edition= |volume=47 |series=Cambridge Aerospace Series |location=Cambridge England |publisher=Cambridge University Press |isbn=978-1-108-39506-9 |url=https://books.google.com/books?id=zZCFDwAAQBAJ |access-date=22 July 2023}}
* {{cite book |author=Sutton, George P. |author2=Biblarz, Oscar |date=2016 |title=Rocket Propulsion Elements |edition=9th |location=New York, NY |publisher=John Wiley & Sons |isbn=978-1-118-75365-1 |url=https://books.google.com/books?id=XwSRDQAAQBAJ |access-date=22 July 2023}}
* {{cite journal |author=Taploo, A |author2=Lin, Li |author3=Keidar, Michael | date = 1 September 2021 | title = Analysis of Ionization in Air-Breathing Plasma Thruster | journal = Physics of Plasmas  | volume = 28 | issue = 9 | page = 093505 | doi = 10.1063/5.0059896 | bibcode = 2021PhPl...28i3505T | s2cid = 240531647}}{{primary source inline|date = July 2023}} See also: {{cite journal |author=Taploo, A |author2=Lin, Li |author3=Keidar, Michael | date = 2022 | title = Air Ionization in Self-Neutralizing Air-Breathing Plasma Thruster | journal = J. Electr. Propuls.  | volume = 1 | issue =  1| page = 25 | doi = 10.1007/s44205-022-00022-x | bibcode = 2022JElP....1...25T | s2cid = 253556114| doi-access = free }}{{primary source inline|date = July 2023}}
{{cite journal |author=Taploo, A |author2=Lin, Li |author3=Keidar, Michael | date = 2022 | title = Taploo, A., Soni, V., Solomon, H. et al. Characterization of a circular arc electron source for a self-neutralizing air-breathing plasma thruster. J Electr Propuls 2, 21 (2023). https://doi.org/10.1007/s44205-023-00058-7
==See also==
{{Div col}}
* [[Alcubierre drive]]
* [[Physics:Anti-gravity|Anti-gravity]]
* [[Physics:Artificial gravity|Artificial gravity]]
* [[Astronomy:Atmospheric entry|Atmospheric entry]]
* [[Engineering:Breakthrough Propulsion Physics Program|Breakthrough Propulsion Physics Program]]
* [[Engineering:Flight dynamics (spacecraft)|Flight dynamics (spacecraft)]]
* [[Engineering:Index of aerospace engineering articles|Index of aerospace engineering articles]]
* [[Interplanetary Transport Network]]
* Interplanetary travel
* List of aerospace engineering topics
* [[Astronomy:Lists of rockets|Lists of rockets]]
* [[Physics:Magnetic sail|Magnetic sail]]
* [[Astronomy:Orbital maneuver|Orbital maneuver]]
* [[Astronomy:Orbital mechanics|Orbital mechanics]]
* [[Physics:Plasma propulsion engine|Plasma propulsion engine]]
* [[Engineering:Pulse detonation engine|Pulse detonation engine]]
* [[Engineering:Rocket|Rocket]]
* Rocket engine nozzles
* [[Astronomy:Satellite|Satellite]]
* [[Physics:Solar sail|Solar sail]]
* [[Engineering:Spaceflight|Spaceflight]]
* [[Engineering:Space launch|Space launch]]
* [[Physics:Space travel using constant acceleration|Space travel using constant acceleration]]
* [[Physics:Specific impulse|Specific impulse]]
* [[Tsiolkovsky rocket equation]]
{{Div col end}}

==References==
{{reflist|2}}

==External links==
* [https://web.archive.org/web/20040402092917/http://www.grc.nasa.gov/WWW/bpp/ NASA Breakthrough Propulsion Physics project]
* [http://www.projectrho.com/rocket/rocket3c2.html Different Rockets] {{Webarchive|url=https://web.archive.org/web/20100529014413/http://www.projectrho.com/rocket/rocket3c2.html |date=2010-05-29 }}
* [http://www.islandone.org/LEOBiblio/ Earth-to-Orbit Transportation Bibliography] {{Webarchive|url=https://web.archive.org/web/20160615200556/http://www.islandone.org/LEOBiblio/ |date=2016-06-15 }}
* [http://vc.airvectors.net/tarokt.html Spaceflight Propulsion] – a detailed survey by Greg Goebel, in the public domain
* [http://www.cpia.jhu.edu/ Johns Hopkins University, Chemical Propulsion Information Analysis Center]
* [http://software.lpre.de/ Tool for Liquid Rocket Engine Thermodynamic Analysis]
* [http://howthingsfly.si.edu/ Smithsonian National Air and Space Museum's How Things Fly website]
* Fullerton, Richard K. "[http://spacecraft.ssl.umd.edu/design_lib/ICES01-2200.EVA_roadmaps.pdf Advanced EVA Roadmaps and Requirements]." Proceedings of the 31st International Conference on Environmental Systems. 2001.
* [http://www.projectrho.com/public_html/rocket/engineintro.php ''Atomic Rocket'' – Engines]: A site listing and detailing real, theoretical and fantasy space engines.
{{Spaceflight}}
{{Spacecraft propulsion}}

{{DEFAULTSORT:Spacecraft Propulsion}}
[[Category:Spacecraft propulsion| ]]
[[Category:Spacecraft components]]
[[Category:NASA programs]]
[[Category:Discovery and exploration of the Solar System]]

{{Sourceattribution|Spacecraft propulsion}}