Editing Hypergolic propellant

{{Short description|Type of rocket engine fuel}}
{{wikt | hypergolic}}
[[File:Hypergolic Fuel for MESSENGER.jpg|thumb|The attendant wears a full [[hazmat suit]] due to the hazards of the hypergolic fuel [[hydrazine]], here being loaded onto the ''[[MESSENGER]]'' space probe]]
A '''hypergolic propellant''' is a [[rocket propellant]] combination used in a [[rocket engine]], whose components [[Spontaneous combustion|spontaneously ignite]] when they come into contact with each other.

The two propellant components usually consist of a [[fuel]] and an [[oxidizer]]. The main advantages of hypergolic propellants are that they can be stored as liquids at room temperature and that engines which are powered by them are easy to ignite reliably and repeatedly. Common hypergolic propellants are extremely [[toxicity|toxic]] or [[corrosiveness|corrosive]], making them difficult to handle.

In contemporary usage, the terms "hypergol" and "hypergolic propellant" usually mean the most common such propellant combination: [[dinitrogen tetroxide]] plus [[hydrazine]].<ref>{{Cite journal |last1=Melof |first1=Brian M. |last2=Grubelich |first2=Mark C. |date=2000-11-15 |title=Investigation of Hypergolic Fuels with Hydrogen Peroxide |url=https://www.osti.gov/biblio/767866 |language=English|journal= 3rd International Hydrogen Peroxide Propulsion Conference|osti=767866 }}</ref>

== History ==
The fact that [[turpentine]] may spontaneously combust when mixed with [[nitric acid]] was discovered as early as the late 17th century by [[Frederick Slare]],<ref>{{Cite journal |last=Slare |first=Frederick |date=1694 |title=An Account of Some Experiments Relating to the Production of Fire and Flame, Together with an Explosion; Made by the Mixture of Two Liquors Actually Cold |url=https://www.jstor.org/stable/102461 |journal=Philosophical Transactions  |volume=18 |pages=201–218 |jstor=102461 |issn=0260-7085}}</ref><ref>{{Cite book |last=Newman |first=William R. |url=https://books.google.com/books?id=NXGYDwAAQBAJ&pg=PA457 |title=Newton the Alchemist: Science, Enigma, and the Quest for Nature's "Secret Fire" |date=2018-12-11 |publisher=Princeton University Press |isbn=978-0-691-17487-7 |language=en}}</ref> but it remained a scientific curiosity for centuries until it was proposed to use it for [[rocket-assisted take off]] during WWII.<ref>{{US patent|2489051A}}</ref>

In 1935, [[Hellmuth Walter]] discovered that [[hydrazine hydrate]] was hypergolic with [[high-test peroxide]] of 80–83%. He was probably the first to discover this phenomenon, and set to work developing a fuel. Prof. Otto Lutz  assisted the [[Hellmuth Walter Kommanditgesellschaft|Walter Company]] with the development of ''[[C-Stoff]]'', which contained 30% hydrazine hydrate, 57% [[methanol]], and 13% water, and spontaneously ignited with high-strength [[hydrogen peroxide]].<ref name="Ignition" />{{rp|13}} BMW developed engines burning a hypergolic mix of nitric acid with various combinations of amines, xylidines, and [[aniline]]s.<ref name="Benecke">{{cite book |chapter=BMW Developments |last=Lutz |first=O. | editor1-last=Benecke | editor1-first=T. H. | editor2-last=Quick | editor2-first=A.W. | editor3-last=Schulz | editor3-first=W. | title=History of German Guided Missiles Development (Guided Missiles Seminar. 1956. Munich) | publisher=Appelhans | series=Advisory Group for Aerospace Research and Development-AG-20 | year=1957 | url=https://books.google.com/books?id=O5tNswEACAAJ | pages=420}}</ref>

Hypergolic propellants were discovered independently, for the second time, in the U.S. by [[GALCIT]] and Navy Annapolis researchers in 1940. They developed engines powered by aniline and [[red fuming nitric acid]].<ref name="Sutton">{{cite book | last=Sutton | first=G. P. | title=History of Liquid Propellant Rocket Engines | publisher=American Institute of Aeronautics and Astronautics | series=Library of flight | year=2006 | isbn=978-1-56347-649-5 | url=https://books.google.com/books?id=s1C9Oo2I4VYC}}</ref> [[Robert Goddard]], [[Reaction Motors]], and [[Curtiss-Wright]] worked on aniline/nitric acid engines in the early 1940s, for small missiles and jet assisted take-off ([[JATO]]). The project resulted in the successful JATO of several [[Martin PBM Mariner|Martin PBM]] and PBY bombers, but the project was disliked because of the toxic properties of both fuel and oxidizer, as well as the high [[freezing point]] of aniline. The second problem was eventually solved by the addition of small quantities of [[furfuryl alcohol]] to the aniline.<ref name="Ignition"/>{{rp|22-23}}

[[File:Walter Triebwerk HWK109-509 A Luftwaffenmuseum Berlin-Gatow Denis Apel.JPG|thumb|right|An early hypergolic-propellant rocket engine, the Walter 109-509A of 1942–45]]
In Germany from the mid-1930s through [[World War II]], rocket propellants were broadly classed as [[monergol]]s, hypergols, nonhypergols and [[Hybrid-propellant rocket|lithergols]]. The ending ''ergol'' is a combination of [[Greek language|Greek]] ''ergon'' or work, and Latin ''oleum'' or oil, later influenced by the chemical suffix ''-ol'' from [[Alcohol (chemistry)|alcohol]].<ref group="Note">"-ergol", ''Oxford English Dictionary''</ref>  Monergols were [[monopropellant]]s, while nonhypergols were [[Bipropellant rocket|bipropellants]] that required external ignition, and lithergols were solid/liquid hybrids. Hypergolic propellants (or at least hypergolic ignition) were far less prone to [[hard start]]s than electric or pyrotechnic ignition. The "hypergole" terminology was coined by Dr. Wolfgang Nöggerath, at the Technical University of [[Braunschweig]] (Brunswick), Germany.<ref>{{citation |title=Peenemünde West: Die Erprobungsstelle der Luftwaffe für geheime Fernlenkwaffen und deren Entwicklungsgeschichte |trans-title=Peenemünde West: The Luftwaffe's test center for secret guided missiles and the history of their development |last=Botho |first=Stüwe |location=Peene Münde West |publisher=Weltbildverlag |isbn=9783828902947 |year=1998 |page=220 |language=de}}</ref>

The only rocket-powered fighter ever deployed was the [[Messerschmitt Me 163]]B ''Komet'', which had an [[HWK 109-509]], a rocket motor which consumed methanol/hydrazine as fuel and high-test peroxide ''[[T-Stoff]]'' as oxidizer. The hypergolic rocket motor had the advantage of fast climb and quick-hitting tactics at the cost of being very volatile and capable of exploding with any degree of inattention. Other proposed combat rocket fighters such as the [[Heinkel P.1077|Heinkel ''Julia'']] and reconnaissance aircraft like the [[DFS 228]] were meant to use the Walter 509 series of rocket motors, but besides the Me 163, only the [[Bachem Ba 349]] ''Natter'' vertical launch expendable fighter was ever flight-tested with the Walter rocket propulsion system as its primary sustaining thrust system for military-purpose aircraft.

The earliest [[ballistic missiles]], such as the Soviet [[Sputnik (rocket)|R-7]] that launched [[Sputnik 1]] and the U.S. [[Atlas (rocket family)|Atlas]] and [[HGM-25A Titan I|Titan-1]], used [[kerosene]] and [[liquid oxygen]]. Although they are preferred in space launchers, the difficulties of storing a [[cryogenics|cryogen]] such as liquid oxygen in a missile that had to be kept launch ready for months or years at a time led to a switch to hypergolic propellants in the U.S. [[LGM-25C Titan II|Titan II]] and in most Soviet ICBMs such as the [[R-36 (missile)|R-36]], but the difficulties of such corrosive and toxic materials, including injury-causing leaks and the explosion of a Titan-II in its silo,<ref>{{Cite book |last=Schlosser |first=Eric |title=Command and control: nuclear weapons, the Damascus Accident, and the illusion of safety |date=2013 |publisher=The Penguin Press |isbn=978-1-59420-227-8 |location=New York, NY}}</ref> led to their near universal replacement with [[Solid-fuel rocket|solid-fuel]] boosters, first in Western [[SLBM|submarine-launched ballistic missiles]] and then in land-based U.S. and Soviet ICBMs.<ref name="Ignition"/>{{rp|47}}

In the 1960s, late variants of French [[Véronique (rocket)|Véronique]] [[sounding rocket]] and the [[Vesta (rocket)|Vesta]] rocket, as well as the first stage of the first orbital SLV [[Diamant]] used<ref>{{Cite web |title=Nitric acid/Turpentine |url=http://www.astronautix.com/n/nitricacidturpentine.html |access-date=2024-11-30 |website=www.astronautix.com}}</ref> the combination of nitric acid and turpentine discovered by Slare. It may also be used in [[amateur rocketry]].<ref>{{Cite web |title=Breaking Bad rocket style: Cooking fuel in a trailer lab {{!}} Pythom Space |url=https://www.pythomspace.com/updates/breaking-bad-rocket-style-cooking-fuel-in-a-trailer-lab |access-date=2024-11-30 |website=www.pythomspace.com |language=en}}</ref>

The [[Apollo Lunar Module]], used in the [[Apollo program|Moon landings]], employed hypergolic fuels in both the descent and ascent rocket engines. The [[Apollo command and service module|Apollo spacecraft]] used the same combination for the [[Service Propulsion System]]. Those spacecraft and the [[Space Shuttle]] (among others) used hypergolic propellants for their [[reaction control system]]s.

The trend among Western space-launch agencies is away from large hypergolic rocket engines and toward hydrogen/oxygen engines or methane/oxygen and [[RP-1]]/oxygen engines for various [[Liquid-propellant_rocket#Advantages_and_disadvantages|advantages and disadvantages]]. [[Ariane (rocket family)|Arianes]] 1 through 4, with their hypergolic [[Multistage rocket|first and second stages]] (and optional hypergolic boosters on the Ariane 3 and 4) have been retired and replaced with the Ariane 5, which uses a first stage fueled by liquid hydrogen and liquid oxygen. The Titan II, III, and IV, with their hypergolic first and second stages, have also been retired for the Atlas V (RP-1/oxygen) and Delta IV (hydrogen/oxygen). Hypergolic propellants are still used in upper stages, when multiple burn-coast periods are required, and in [[launch escape system]]s.

== Characteristics ==
[[File:OMS Pod removal.png|thumb|Hypergolic propellant tanks of the [[Orbital Maneuvering System]] of Space Shuttle ''Endeavour'']]

=== Advantages ===
Hypergolically fueled rocket engines are usually simple and reliable because they need no ignition system. Although larger hypergolic engines in some launch vehicles use [[turbopump]]s, most hypergolic engines are pressure fed. A gas, usually [[helium]], is fed to the propellant tanks under pressure through a series of [[check valve|check]] and [[safety valve]]s. The propellants, in turn, flow through control valves into the combustion chamber; there, their instant contact ignition prevents a mixture of unreacted propellants from accumulating and then igniting in a potentially catastrophic [[hard start]].

As hypergolic rockets do not need an ignition system, they can fire any number of times by simply opening and closing the propellant valves until the propellants are exhausted, so are uniquely suited for spacecraft maneuvering and well-suited, though not uniquely so, as upper stages of such space launchers as the [[Delta II]] and [[Ariane 5]], which must perform more than one burn. Restartable nonhypergolic rocket engines nevertheless exist, notably the cryogenic (oxygen/hydrogen) [[RL-10]] on the [[Centaur (rocket stage)|Centaur]] and the [[J-2 (rocket engine)|J-2]] on the [[Saturn V]]. The [[RP-1]]/LOX [[Merlin (rocket engine family)|Merlin]] on the [[Falcon 9]] can also be restarted.<ref>{{Cite web|title=SpaceX|url=http://www.spacex.com/|access-date=2021-12-29|website=SpaceX|language=en}}</ref>

The most common hypergolic fuels, [[hydrazine]], [[monomethylhydrazine]], and [[unsymmetrical dimethylhydrazine]] (UDMH), and oxidizer, [[nitrogen tetroxide]], are all liquid at ordinary temperatures and pressures.  They are therefore sometimes called "storable liquid propellants".  They are suitable for use in spacecraft missions lasting many years. The [[cryogenics|cryogenity]] of [[liquid hydrogen]] and [[liquid oxygen]] has so far limited their practical use to space launch vehicles where they need to be stored only briefly.<ref>{{cite web
|url=https://www.permanent.com/space-transportation-propellants.html
|title=Fuel Propellants - Storable, and Hypergolic vs. Ignitable by Mike Schooley
|url-status=live
|archive-url=https://web.archive.org/web/20210724084512/https://www.permanent.com/space-transportation-propellants.html
|archive-date=24 July 2021}}</ref><!-- could be, historically; but would need a source.  For more modern rocket designs, after c. 2016, there are certainly cryogenic-propellant rocket stages that are planning on months-long duration for use of cryopropellants on interplanetary flights  --> As the largest issue with the usage of cryogenic propellants in interplanetary space is boil-off, which is largely dependent on [[Square–cube_law|the scale]] of spacecraft, for larger craft such as [[SpaceX_Starship#Starship_spacecraft|Starship]] this is less of an issue.

Another advantage of hypergolic propellants is their high density compared to cryogenic propellants. [[Liquid oxygen|LO<sub>2</sub>]] has a density of 1.14 g/ml, while  hypergolic oxidizers such as [[nitric acid]] or [[nitrogen tetroxide]] have a density of 1.55 g/ml and 1.45 g/ml, respectively. [[Liquid_hydrogen|LH<sub>2</sub>]] fuel offers extremely high performance, yet its density only warrants its use in the largest of rocket stages, while mixtures of hydrazine and UDMH have a density at least 10 times greater.<ref>{{cite web
|url=http://www.braeunig.us/space/propel.htm#tables
|title=PROPERTIES OF ROCKET PROPELLANTS
|website=braeunig.us
|url-status=live
|archive-url=https://web.archive.org/web/20220526100431/http://www.braeunig.us/space/propel.htm#tables
|archive-date=26 May 2022}}</ref> This is of great importance in [[Space probe|space probes]], as the higher propellant density allows the size of their propellant tanks to be reduced significantly, which in turn allows the probes to fit within a smaller [[payload fairing]].

=== Disadvantages ===
Relative to their mass, traditional hypergolic propellants possess a lower [[Heat of combustion|calorific value]] than cryogenic propellant combinations like LH<sub>2</sub>/LO<sub>2</sub> or [[Methane#Fuel|LCH<sub>4</sub>]]/LO<sub>2</sub>.<ref name=NIST>{{cite book |title=NIST Chemistry WebBook |date=2021 |publisher=NIST Office of Data and Informatics |series=NIST Standard Reference Database Number 69 |doi=10.18434/T4D303 |last1=Linstrom |first1=Peter }}</ref> A launch vehicle that uses hypergolic propellant must therefore carry a greater mass of fuel than one that uses these cryogenic fuels.

The [[corrosivity]], [[toxicity]], and [[carcinogen]]icity of traditional hypergolics necessitate expensive safety precautions.<ref>{{Internet Archive|NASA_NTRS_Archive_20090029348|A Summary of NASA and USAF Hypergolic Propellant Related Spills and Fires }}</ref><ref>{{YouTube|id=Zha9DyS-PPA|title="Toxic Propellant Hazards"}}</ref> Failure to follow adequate safety procedures with an exceptionally dangerous UDMH-nitric acid propellant mixture nicknamed [[Devil's venom|"devil's venom"]], for example, resulted in the deadliest rocketry accident in history, the [[Nedelin catastrophe]].<ref>{{citation|url=http://www.spacesafetymagazine.com/space-disasters/nedelin-catastrophe/historys-launch-padfailures-nedelin-disaster-part-1/|title=The Nedelin Catastrophe, Part 1 |date=28 October 2014 |url-status=live |archive-date=15 November 2014 |archive-url=https://web.archive.org/web/20141115205657/http://www.spacesafetymagazine.com/space-disasters/nedelin-catastrophe/historys-launch-padfailures-nedelin-disaster-part-1/}}</ref>

== Hypergolic combinations ==

=== Common ===
Common hypergolic propellant combinations include:<ref>{{cite web |url=http://www.braeunig.us/space/propel.htm |title=ROCKET PROPELLANTS |website=braeunig.us}}</ref>
* [[Aerozine 50]] + [[nitrogen tetroxide]] (NTO) is widely used in historical American rockets, including the Titan II and all engines in the [[Apollo Lunar Module]]. Aerozine 50 is a mixture of 50% UDMH and 50% straight hydrazine (N<sub>2</sub>H<sub>4</sub>).<ref name="Ignition"/>{{rp|45}}
* Monomethylhydrazine (MMH) + NTO is used in smaller engines and reaction control thrusters: [[Apollo command and service module]] [[Reaction_control_system|RCS]],<ref>{{cite book
 |date=December 1971
 |title=Apollo 11 Mission Report - Performance of the Command and Service Module Reaction Control System
 |url=https://ntrs.nasa.gov/api/citations/19720017210/downloads/19720017210.pdf
 |archive-url=https://web.archive.org/web/20220712145711/https://ntrs.nasa.gov/api/citations/19720017210/downloads/19720017210.pdf
 |archive-date=12 July 2022
 |publisher=NASA - Lyndon B. Johnson Space Center
 |pages= 4,8
}}</ref> Space Shuttle [[Orbital Maneuvering System|OMS]] and [[Reaction_control_system|RCS]];<ref>{{cite book
 |last=T.A.
 |first=Heppenheimer
 |date=2002
 |title=''Development of the Shuttle, 1972–1981 - Volume 2.''
 |url=https://books.google.com/books?id=LqhqBgAAQBAJ
 |publisher=Smithsonian Institution Press
 |page= <!-- or pages= -->
 |isbn=1-58834-009-0
}}</ref> [[Ariane 5]] EPS;<ref>{{cite web | url =http://www.spacelaunchreport.com/ariane5.html#config | archive-url =https://archive.today/20130202175747/http://www.spacelaunchreport.com/ariane5.html#config | url-status =usurped | archive-date =February 2, 2013 | title=Space Launch Report: Ariane 5 Data Sheet}}</ref> [[Draco (rocket engine)|Draco]] thrusters used by the [[SpaceX Dragon]] spacecraft.<ref name=sxu20071210>{{cite web|url=http://www.spacex.com/updates_archive.php?page=121007 |title=SpaceX Updates|publisher=[[SpaceX]] |date=2007-12-10 |access-date=2010-02-03 |url-status=dead |archive-url=https://web.archive.org/web/20110104061453/http://www.spacex.com/updates_archive.php?page=121007 |archive-date=January 4, 2011 }}</ref>
* [[Triethylborane]]/[[triethylaluminium]] (TEA-TEB) + [[liquid oxygen]] is used during the ignition process of some rocket engines that use liquid oxygen, used by the [[Merlin (rocket engine family)|SpaceX Merlin Engine Family]] and [[Rocketdyne F-1]].
* UDMH + NTO is frequently used by [[Roscosmos]], such as in the [[Proton (rocket family)]], and supplied by them to France for the Ariane 1 first and second stages (replaced with [[UH 25]]) and [[Indian Space Research Organisation|ISRO]] rockets using [[Vikas (rocket engine)|Vikas engine]].<ref>{{Cite web|url=http://www.hindu.com/2001/12/03/stories/2001120300481300.htm|archive-url=https://web.archive.org/web/20140323164318/http://www.hindu.com/2001/12/03/stories/2001120300481300.htm|url-status=dead|archive-date=2014-03-23|title=ISRO tests Vikas engine|date=2014-03-23|work=[[The Hindu]]|access-date=2019-07-29}}</ref>

=== Less common or obsolete ===

Less-common or obsolete hypergolic propellants include:
* Aniline + nitric acid (unstable, explosive) is used in the [[WAC Corporal]].<ref>{{cite web
|url=https://airandspace.si.edu/collection-objects/rocket-liquid-fuel-sounding-wac-corporal/nasm_A19590009000
|title=WAC Corporal Sounding Rocket
|url-status=live
|archive-url=https://web.archive.org/web/20220107072025/https://airandspace.si.edu/collection-objects/rocket-liquid-fuel-sounding-wac-corporal/nasm_A19590009000
|archive-date=7 January 2022}}</ref> 
* Aniline + hydrogen peroxide (dust-sensitive, explosive)
* [[Furfuryl alcohol]] + [[red fuming nitric acid]] – [[Copenhagen Suborbitals]] SPECTRA Engine<ref>{{cite web |title=Project SPECTRA - Experimental evaluation of a Liquid storable propellant |url=http://copenhagensuborbitals.com/public/spectra.pdf |archive-url=https://web.archive.org/web/20131104140052/http://copenhagensuborbitals.com/public/spectra.pdf |archive-date=4 November 2013 |url-status=dead}}</ref><ref name="Ignition"/>{{rp|27}}
* Furfuryl alcohol + [[Nitric_acid#Anhydrous_nitric_acid|white fuming nitric acid]]<ref name="Ignition"/>{{rp|27}}
* Hydrazine + nitric acid (toxic but stable) was abandoned due to lack of reliable ignition. No engine with this combination ever went into mass production.<ref>{{cite web|title=Nitric acid/Hydrazine|url=http://www.astronautix.com/n/nitricacidhydrazine.html|publisher=Astronautix.com|access-date=January 13, 2023}}</ref>
* Kerosene + (high-test peroxide + catalyst) is used in [[Gamma (Rocket engine)|Gamma]], with the peroxide first decomposed by a catalyst. Cold hydrogen peroxide and kerosene are not hypergolic, but concentrated hydrogen peroxide (referred to as high-test peroxide or HTP) run over a catalyst produces free oxygen and steam at over {{Convert|700|C|F|-2}}, which is hypergolic with kerosene.<ref>{{Cite web|url = http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=31490.0;attach=519423|title = High Test Peroxide|access-date = July 11, 2014|format = pdf}}</ref>
*[[Tonka (fuel)|Tonka]] (TG-02, about 50% [[triethylamine]] and 50% [[xylidine]]) typically oxidized with nitric acid or its anhydrous nitric oxide derivatives (AK-2x group in the Soviet Union) e.g. [[red fuming nitric acid|AK-20F]] (80% HNO<sub>3</sub> and 20% N<sub>2</sub>O<sub>4</sub> with [[Reaction inhibitor|inhibitor]]).<ref name="Ignition"/>{{rp|14-15,116}}
* ''[[T-Stoff]]'' (stabilized >80% peroxide) + ''[[C-Stoff]]'' (methanol, hydrazine, water, catalyst) was used in [[Messerschmitt Me 163]] World War II German rocket fighter aircraft, for its [[Walter HWK 109-509|Walter 109-509A]] engine.<ref name="Ignition"/>{{rp|13}}
* Turpentine + red fuming nitric acid (flown in French Diamant A first-stage)<ref>{{cite web
|url=http://www.b14643.de/Spacerockets/Specials/European_Rocket_engines/engines.htm
|title=European space-rocket liquid-propellant engines
|url-status=live
|archive-url=https://web.archive.org/web/20210723223834/https://b14643.de/Spacerockets/Specials/European_Rocket_engines/engines.htm
|archive-date=23 July 2021}}</ref>
* UDMH + red fuming nitric acid is used in the [[MGM-52 Lance]] missile system,<ref>{{cite web
|url=http://www.astronautix.com/p/p8e-9.html
|title=P8E-9
|url-status=live
|archive-url=https://web.archive.org/web/20220512200958/http://www.astronautix.com/p/p8e-9.html
|archive-date=12 May 2022}}</ref> [[RM-81_Agena|Agena]] and [[Able_(rocket_stage)|Able]] Upper Stages, and Isayev-built maneuvering engines.<ref>{{cite web
|url=http://www.astronautix.com/n/nitricacidudmh.html
|title=Nitric Acid/UDMH
|url-status=live
|archive-url=https://web.archive.org/web/20220701200932/http://astronautix.com/n/nitricacidudmh.html
|archive-date=1 July 2022}}</ref>

=== Proposed, remain unflown ===

* [[Chlorine trifluoride]] (ClF<sub>3</sub>) + all known fuels – Briefly considered as an oxidizer given its high hypergolicity with all standard fuels, it was ultimately abandoned in the 1970s due to the difficulty of handling the substance safely. ClF<sub>3</sub> is known to burn concrete and gravel.<ref name="Ignition">{{Cite book | last = Clark | first = John D. | author-link = John Drury Clark | title = Ignition! An Informal History of Liquid Rocket Propellants | publisher = Rutgers University Press | year = 1972 | isbn = 978-0-8135-0725-5 |url = https://library.sciencemadness.org/library/books/ignition.pdf | url-status=live | archive-url=https://web.archive.org/web/20220710061023/https://library.sciencemadness.org/library/books/ignition.pdf |archive-date=10 July 2022}}</ref>{{rp|74}} [[Chlorine pentafluoride]] (ClF<sub>5</sub>) presents the same hazards, but offers higher [[specific impulse]] than ClF<sub>3</sub>.
* [[Pentaborane(9)]] and diborane + nitrogen tetroxide – Pentaborane(9), a so-called [[zip fuel]], was studied by Soviet rocket scientist [[Valentin_Glushko|V. P. Glushko]] for use in combination with nitrogen tetroxide in the [[RD-270|RD-270M]] rocket engine. This propellant combination would have yielded a significant increase in performance, but was ultimately given up due to toxicity concerns.<ref name=L1>[http://www.astronautix.com/engines/rd270.htm Astronautix: '''RD-270'''] {{webarchive|url=https://web.archive.org/web/20090430234903/http://astronautix.com/engines/rd270.htm |date=2009-04-30 }}.</ref>
* [[Tetramethylethylenediamine]] +red fuming nitric acid is a sightly less-toxic alternative to hydrazine and its derivatives.

== Related technology ==
[[Pyrophoric]] substances, which ignite spontaneously in the presence of air, are also sometimes used as rocket fuels themselves or to ignite other fuels. For example, a mixture of [[triethylborane]] and [[triethylaluminium]] (which are both separately and even more so together pyrophoric), was used for engine starts in the [[SR-71 Blackbird]] and in the [[F-1 (rocket engine)|F-1]] engines on the [[NASA]] [[Saturn V]] rocket, and is used in the [[Merlin (rocket engine family)|Merlin]] engines on the [[SpaceX]] [[Falcon 9]] rockets.

== Notes ==
{{reflist|group=Note|colwidth=60em}}

== References ==
{{reflist|colwidth=30em}}

==Further reading==
{{refbegin|colwidth=60em}}
* {{cite book |title=Ignition! An Informal History of Liquid Rocket Propellants |last=Clark |first=John |author-link=John Drury Clark |year=1972 |publisher=Rutgers University Press |location=New Brunswick, New Jersey |isbn=0-8135-0725-1 |url = https://library.sciencemadness.org/library/books/ignition.pdf | url-status=live | archive-url=https://web.archive.org/web/20220710061023/https://library.sciencemadness.org/library/books/ignition.pdf |archive-date=10 July 2022}}
* ''Modern Engineering for Design of Liquid-Propellant Rocket Engines'', Huzel & Huang, pub. AIAA, 1992. {{ISBN|1-56347-013-6}}.
* ''History of Liquid Propellant Rocket Engines'', G. Sutton, pub. AIAA 2005. {{ISBN|1-56347-649-5}}.
{{refend}}

== External links ==
* {{cite web|title=Hypergolic Reaction |url=http://www.periodicvideos.com/videos/feature_hypergolic.htm|work=[[The Periodic Table of Videos]]|publisher=[[University of Nottingham]]|year=2009}}

{{spacecraft propulsion}}

[[Category:Rocket propellants]]
[[Category:Soviet inventions]]