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{{short description|Type of rocket engine}}
A '''cold gas thruster''' (or a '''cold gas propulsion system''') is a type of [[rocket engine]] which uses the expansion of a (typically inert) pressurized [[gas]] to generate [[thrust]]. As opposed to traditional rocket engines, a cold gas thruster does not house any combustion and therefore has lower thrust and [[specific impulse|efficiency]] compared to conventional [[monopropellant]] and [[Liquid-propellant rocket|bipropellant]] rocket engines. Cold gas thrusters have been referred to as the "simplest manifestation of a rocket engine" because their design consists only of a fuel tank, a regulating valve, a propelling nozzle, and the little required plumbing. They are the cheapest, simplest, and most reliable propulsion systems available for orbital maintenance, maneuvering and [[Spacecraft attitude control|attitude control]].{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}} 

Cold gas thrusters are predominantly used to provide stabilization for smaller space missions which require contaminant-free operation.<ref name=":0" /> Specifically, [[CubeSat]] propulsion system development has been predominantly focused on cold gas systems because CubeSats have strict regulations against [[Pyrotechnic valves|pyrotechnics]] and hazardous materials.<ref>{{Cite web|url=https://www.researchgate.net/publication/266737929|title=Micropropulsion systems for cubesats|website=ResearchGate|language=en|access-date=2018-12-14}}</ref>

== Design ==
[[File:Cold gas thruster diagram.png|thumb|Schematic of a cold gas propulsion system|290x290px]]
The nozzle of a cold gas thruster is generally a [[De Laval nozzle|convergent-divergent nozzle]] that provides the required thrust in flight. The nozzle is shaped such that the high-pressure, low-velocity gas that enters the nozzle is accelerated as it approaches the throat (the narrowest part of the nozzle), where the gas velocity matches the speed of sound.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}

== Performance ==
Cold gas thrusters benefit from their simplicity; however, they do fall short in other respects. The advantages and disadvantages of a cold gas system can be summarized as:

=== Advantages ===
* A lack of combustion in the nozzle of a cold gas thruster allows its usage in situations where regular liquid rocket engines would be too hot. This eliminates the need to engineer heat management systems.
* The simple design allows the thrusters to be smaller than regular rocket engines, which makes them a suitable choice for missions with limited volume and weight requirements.
* The cold gas system and its fuel are inexpensive compared to regular rocket engines.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}
* The simple design is less prone to failures than a traditional rocket engine.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}
* The fuels used in a cold gas system are safe to handle both before and after firing the engine. If inert fuel is used the cold gas system is one of the safest possible rocket engines.<ref name=":0">{{Cite journal|last1=Nguyen|first1=Hugo|last2=Köhler|first2=Johan|last3=Stenmark|first3=Lars|date=2002-01-01|title=The merits of cold gas micropropulsion in state-of-the-art space missions|journal=Iaf Abstracts|pages=785|url=https://www.researchgate.net/publication/228785798|bibcode=2002iaf..confE.785N}}</ref>
* Cold gas thrusters do not build up a net charge on the spacecraft during operation.
* Cold gas thrusters require very little electrical energy to operate, which is useful, for example, when a spacecraft is in the shadow of the planet it is orbiting.

=== Disadvantages ===
* A cold gas system cannot produce the high thrust that combustive rocket engines can achieve.
* The maximum thrust of a cold gas thruster is dependent upon the pressure in the storage tank. As fuel is used up with simple compressed-gas systems, the pressure decreases and maximum thrust decreases.<ref name=":2">{{Cite journal|last1=Tummala|first1=Akshay|last2=Dutta|first2=Atri|last3=Tummala|first3=Akshay Reddy|last4=Dutta|first4=Atri|date=9 December 2017|title=An Overview of Cube-Satellite Propulsion Technologies and Trends|journal=Aerospace|language=en|volume=4|issue=4|pages=58|doi=10.3390/aerospace4040058|doi-access=free|bibcode=2017Aeros...4...58T |hdl=10057/15652|hdl-access=free}}</ref> With liquefied gases, pressure will remain relatively constant as the liquid gas volatilizes and is used up in a manner similar to aerosol cans.

=== Thrust ===
[[Thrust]] is generated by momentum exchange between the exhaust and the spacecraft, which is given by [[Newton's second law]] as <math>F=\dot{m}V_e</math> where <math>\dot{m}</math> is the mass flow rate, and <math>V_e</math> is the velocity of the exhaust.

For a cold gas thruster in space, where the thrusters are designed for infinite expansion (since the ambient pressure is zero), the thrust is given as

<math>F=A_tP_c\gamma \left [ \left (\frac{2}{\gamma - 1}\right ) \left( \frac{2}{\gamma + 1} \right) \left (1 - \frac{P_e}{P_c} \right) \right ] + P_eA_e </math>

Where <math>A_t</math> is the area of the throat, <math>P_c</math> is the chamber pressure in the nozzle, <math>\gamma</math> is the [[specific heat ratio]], <math>P_e</math> is the exit pressure of the propellant, and <math>A_e</math> is the exit area of the nozzle.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}

=== Specific Impulse ===
The [[specific impulse]] (I<sub>sp</sub>) of a rocket engine is the most important metric of efficiency; a high specific impulse is normally desired. Cold gas thrusters have a significantly lower specific impulse than most other rocket engines because they do not take advantage of chemical energy stored in the propellant. The theoretical specific impulse for cold gases is given by

<math>I_{sp} = \frac{C^*}{g_0} \gamma \sqrt{\left ( \frac{2}{\gamma - 1} \right) \left ( \frac{2}{\gamma +1} \right )^ \frac{\gamma + 1}{\gamma - 1} \left ( 1 - \frac{P_e}{P_c} \right ) ^ {\frac{\gamma - 1}{\gamma}} }</math>

where <math>g_0</math> is [[standard gravity]] and <math>C^*</math> is the [[characteristic velocity]] which is given by

<math>C^* = \frac{a_0}{\gamma \left( \frac{2}{\gamma + 1} \right) ^ \frac{\gamma +1}{2(\gamma - 1)}}</math>

where <math>a_0</math> is the [[sonic velocity]] of the propellant.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}

== Propellants ==
Cold gas systems can use either a solid, liquid or gaseous propellant storage system; but the propellant must exit the nozzle in gaseous form. Storing liquid propellant may pose attitude control issues due to the sloshing of fuel in its tank. 

When choosing a propellant, a high specific impulse, and a high specific impulse per unit volume of propellant should be considered.<ref name=":2" />

Overview of the specific impulses of propellants suitable for a cold gas propulsion system:
{| class="wikitable"
|+Propellants and Efficiencies <ref name=":0" />
! Cold Gas
! Molecular<br /> weight M<br /> (u)
! Theoretical<br /> ''I<sub>sp</sub>''<br /> (sec)
! Measured<br /> ''I<sub>sp</sub>''<br /> (sec)
! Density<br /> (g/cm<sup>3</sup>)
|-
| [[Hydrogen gas|H<sub>2</sub>]]
| 2.0
| 296
| 272
| 0.02
|-
| [[Helium|He]]
| 4.0
| 179
| 165
| 0.04
|-
| [[Neon|Ne]]
| 20.2
| 82
| 75
| 0.19
|-
| [[Nitrogen|N<sub>2</sub>]]
| 28.0
| 80
| 73
| 0.28
|-
| [[Oxygen|O<sub>2</sub>]]
| 32.0
| ?
| 
| 
|-
| [[Argon|Ar]]
| 40.0
| 57
| 52
| 0.44
|-
| [[Krypton|Kr]]
| 83.8
| 39
| 37
| 1.08
|-
| [[Xenon|Xe]]
| 131.3
| 31
| 28
| 2.74
|-
| [[Dichlorodifluoromethane|CCl<sub>2</sub>F<sub>2</sub>]] (Freon-12)
| 120.9
| 46
| 37
| Liquid
|-
| [[Tetrafluoromethane|CF<sub>4</sub>]]
| 88.0
| 55
| 45
| 0.96
|-
| [[Methane|CH<sub>4</sub>]]
| 16.0
| 114
| 105
| 0.19
|-
| [[Ammonia|NH<sub>3</sub>]]
| 17.0
| 105
| 96
| Liquid
|-
| [[Nitrous oxide|N<sub>2</sub>O]]
| 44.0
| 67
| 61
| Liquid
|-
| [[Carbon dioxide|CO<sub>2</sub>]]
| 44.0
| 67
| 61
| Liquid
|-
|}

Properties at 0°C and {{cvt|241|bar|psi}}.

== Applications ==
=== Human Propulsion ===
Cold gas thrusters are especially well suited for [[astronaut propulsion unit]]s due to the inert and non-toxic nature of their propellants.

==== Hand-Held Maneuvering Unit ====
Main article: [[Hand-Held Maneuvering Unit]]

The Hand-Held Maneuvering Unit (HHMU) used on the [[Gemini 4]] and [[Gemini 10|10]] missions used pressurized oxygen to facilitate the astronauts' [[Extravehicular activity|extravehicular activities]].<ref>{{Cite web |url=https://airandspace.si.edu/collection-objects/maneuvering-unit-hand-held-white-gemini-4 |title=Maneuvering Unit, Hand-Held, White, Gemini 4 |date=2016-03-20 |website=National Air and Space Museum |language=en |access-date=2018-12-12 |archive-date=2019-06-30 |archive-url=https://web.archive.org/web/20190630153214/https://airandspace.si.edu/collection-objects/maneuvering-unit-hand-held-white-gemini-4 |url-status=dead }}</ref> Although the patent of the HHMU does not categorize the device as a cold gas thruster, the HHMU is described as a "propulsion unit utilizing the thrust developed by a pressurized gas escaping various nozzle means."<ref>{{patent|US|3270986|Hand-Held Self-Maneuvering Unit}}</ref>

==== Manned Maneuvering Unit ====
{{main|Manned Maneuvering Unit}}

Twenty-four cold gas thrusters using pressurized gaseous nitrogen were used on the Manned Maneuvering Unit (MMU). The thrusters provided full 6-degree-of-freedom control to the astronaut wearing the MMU. Each thruster provided {{cvt|1.4|lbf|N}} of thrust. The two propellant tanks onboard provided a total of {{cvt|40|lb|kg}} of gaseous nitrogen at {{cvt|4500|psi|MPa}}, which provided sufficient propellant to generate a change in velocity of {{cvt|110|to|135|ft/sec|m/s}}. At a nominal mass, the MMU had a translational acceleration of 0.3&nbsp;±&nbsp;0.05&nbsp;ft/sec<sup>2</sup> (9.1&nbsp;±&nbsp;1.5&nbsp;cm/s<sup>2</sup>) and a rotational acceleration of 10.0&nbsp;±&nbsp;3.0&nbsp;deg/sec<sup>2</sup> (0.1745&nbsp;±&nbsp;0.052&nbsp;rad/sec<sup>2</sup>)<ref>Lenda, J. A. "[https://ntrs.nasa.gov/search.jsp?R=19790008382 Manned maneuvering unit: User's guide]." (1978).</ref>

=== Vernier Engines ===
{{See also|Vernier thruster}}

Larger cold gas thrusters are employed to help in the attitude control of the first stage of the [[SpaceX]] [[Falcon 9]] rocket as it returns to land.<ref>{{Cite web |url=https://www.spacex.com/news/2015/06/24/why-and-how-landing-rockets |title=The why and how of landing rockets |last=plarson |date=2015-06-25 |website=SpaceX|access-date=2018-12-16}}</ref>

=== Automotive ===
In a [[Twitter|tweet]] in June 2018, [[Elon Musk]] proposed the use of air-based cold gas thrusters to improve car performance.<ref>{{cite tweet |user=elonmusk |author-link=Elon Musk |number=1005577738332172289 |date= June 9, 2018 |title=SpaceX option package for new Tesla Roadster will include ~10 small rocket thrusters arranged seamlessly around car. These rocket engines dramatically improve acceleration, top speed, braking & cornering. Maybe they will even allow a Tesla to fly … }}</ref>

In September 2018, [[Robert Bosch GmbH|Bosch]] successfully tested its proof-of-concept safety system for righting a slipping motorcycle using cold gas thrusters. The system senses a sideways wheel slip and uses a lateral cold gas thruster to keep the motorcycle from slipping further.<ref>{{Cite web |url=https://www.bosch-presse.de/pressportal/de/en/greater-safety-on-two-wheels-bosch-innovations-for-the-motorcycles-of-the-future-157824.html |title=Greater safety on two wheels: Bosch innovations for the motorcycles of the future |website=Bosch Media Service |date=10 July 2018 |language=en |access-date=2018-12-14}}</ref>

== Research ==
The main focus of research {{asof|2014|lc=y}} is miniaturization of cold gas thrusters using [[microelectromechanical systems]].<ref>{{Cite journal |last1=Kvell |first1=U |last2=Puusepp |first2=M |last3=Kaminski |first3=F |last4=Past |first4=J-E |last5=Palmer |first5=K |last6=Grönland |first6=T-A |last7=Noorma |first7=M |date=2014 |title=Nanosatellite orbit control using MEMS cold gas thrusters |journal=Proceedings of the Estonian Academy of Sciences |volume=63|issue=2S |pages=279 |doi=10.3176/proc.2014.2s.09 |issn=1736-6046 |doi-access=free}}</ref>

== See also ==
* [[Resistojet rocket]]
* [[Monopropellant rocket|Monopropellant Rocket]]
* [[CubeSat]]

== References ==
{{Reflist}}

{{Spacecraft propulsion}}

[[Category:Spacecraft components]]
[[Category:Spacecraft propulsion]]