Editing Monopropellant (section)

==Research in brief ==
Much work was done in the US in the 1950s and 1960s to attempt to find better and more energetic monopropellants. For the most part, researchers came to the conclusion that any single substance that contained enough energy to compete with bipropellants would be too unstable to handle safely under practical conditions. With new materials, control systems and requirements for high-performance thrusters, engineers are currently{{When|date=March 2011}} re-examining this assumption.{{Citation needed|date=March 2011}}

Many partially nitrated alcohol esters are suitable for use as monopropellants. "Trimethylene glycol dinitrate" <!-- a glycol must have the hydroxyls on adjacent carbon atoms --> or [[1,3-propanediol]] dinitrate is [[isomer]]ic with [[PGDN]], and produced as a fractional byproduct in all but the most exacting laboratory conditions; the marginally lower [[specific gravity]] (and thus [[energy density]]) of this compound argues against its use, but the minor differences in chemistry may prove useful in the future.{{Citation needed|date=March 2011}}

The related "dinitrodiglycol", more properly termed [[diethylene glycol dinitrate]] in modern notation, was widely used in World War 2 Germany, both alone as a liquid monopropellant and [[colloidal]] with [[nitrocellulose]] as a solid propellant. The otherwise desirable characteristics of this compound; it is quite stable, easy to manufacture, and has a very high energy density; are marred by a high [[freeze point]] (-11.5 deg. C) and pronounced thermal expansion, both being problematic in spacecraft. "Dinitrochlorohydrin" and "tetranitrodiglycerin" are also likely candidates, though no current use is known. The polynitrates of long chain and [[aromatic hydrocarbons]] are invariably room temperature solids, but many are soluble in simple alcohols or ethers in high proportion, and may be useful in this state.{{Citation needed|date=March 2011}}

[[Hydrazine]],<ref name="Sutton, 230" /><ref>{{cite web|url=http://cs.astrium.eads.net/sp/SpacecraftPropulsion/MonopropellantThrusters.html|archive-url=https://web.archive.org/web/20100327105117/http://cs.astrium.eads.net/sp/SpacecraftPropulsion/MonopropellantThrusters.html|archive-date=March 27, 2010|title=Monopropellant Hydrazine Thrusters|publisher=EADS Astrium}}</ref> [[ethylene oxide]],<ref>{{cite web|url=http://www.rocketmotorparts.com/resources/ethylene_oxide.pdf|archive-url=https://web.archive.org/web/20120321015445/http://www.rocketmotorparts.com/resources/ethylene_oxide.pdf|archive-date=March 21, 2012|title=ethylene_oxide.pdf}}</ref> [[hydrogen peroxide]] (especially in its German World War II form as ''[[T-Stoff]]''),<ref>{{cite web|url=http://www.rocketmotorparts.com/resources/h2o2.pdf|archive-url=https://web.archive.org/web/20120321015449/http://www.rocketmotorparts.com/resources/h2o2.pdf|archive-date=March 21, 2012|title=h2o2.pdf}}</ref> and [[nitromethane]]<ref>{{cite web|url=http://www.rocketmotorparts.com/resources/nitromethane.pdf|archive-url=https://web.archive.org/web/20120321015452/http://www.rocketmotorparts.com/resources/nitromethane.pdf|archive-date=March 21, 2012|title=nitromethane.pdf}}</ref> are common rocket monopropellants. As noted the specific impulse of monopropellants is lower<ref name="spacehandbookprops"/><ref>{{harvnb|Sutton|1992}}, p.&nbsp;36</ref> than bipropellants and can be found with the Air Force Chemical Equilibrium Specific Impulse Code tool.<ref>{{cite web|url=http://www.dunnspace.com/isp.htm|archive-url=https://web.archive.org/web/20131020061623/http://www.dunnspace.com/isp.htm|archive-date=October 20, 2013|title=Rocket Engine Specific Impulse Program|last=Dunn|first=Bruce P.|date=2001|publisher=Dunn Engineering}}</ref>

One newer monopropellant under development is [[nitrous oxide]], both neat and in the form of [[nitrous oxide fuel blend]]s.  Nitrous oxide offers the advantages of being self-pressurizing and of being relatively non-toxic, with a specific impulse intermediate between hydrogen peroxide and hydrazine.<ref name="ZakirovSweeting2001">{{cite journal|last1=Zakirov|first1=Vadim|last2=Sweeting|first2=Martin|last3=Lawrence|first3=Timothy|last4=Sellers|first4=Jerry|title=Nitrous oxide as a rocket propellant|journal=Acta Astronautica|volume=48|issue=5–12|year=2001|pages=353–362|doi=10.1016/S0094-5765(01)00047-9|bibcode=2001AcAau..48..353Z}}</ref> [[Nitrous oxide]] generates oxygen upon decomposition, and it is possible to blend it with fuels to form a monopropellant mixture with a specific impulse up to 325&nbsp;s, comparable to [[Hypergolic propellant|hypergolic bipropellants]].<ref name=awst20120521>{{cite news |last=Morring |first=Frank Jr. |title=SpaceX To Deliver Green-Propulsion Testbed To ISS |url=http://aviationweek.com/awin/spacex-deliver-green-propulsion-testbed-iss |access-date=July 13, 2014 |newspaper=Aviation Week and Space Technology |date=May 21, 2012}}</ref> In 2018 a new precious metal catalyst was invented for use with nitrous oxide -  rhodium oxide on alumina spheres – which is more stable at higher temperatures than pure rhodium or iridium.<ref>{{Cite web |title=Catalysts |url=https://www.americanelements.com/catalysts |access-date=2024-01-05 |website=American Elements |language=en}}</ref> 

Direct comparison of physical properties, performance, cost, storability, toxicity, storage requirements and accidental release measures for hydrogen peroxide, [[hydroxylammonium nitrate]] (HAN), hydrazine and various cold gas monopropellants shows that hydrazine is the highest performing in terms of specific impulse. However, hydrazine is also the most expensive and toxic. In addition HAN and hydrogen peroxide have the highest density impulse (total impulse per given unit volume).<ref name=Wernimont2006>{{cite journal|last1=Wernimont|first1=Eric|title=System Trade Parameter Comparison of Monopropellants: Hydrogen Peroxide vs Hydrazine and Others|journal=42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit|year=2006|doi=10.2514/6.2006-5236|isbn=978-1-62410-038-3|url=http://www.hydrogen-peroxide.us/chemical-handling-toxicity/AIAA-2006-5236_hydrogen_peroxide_versus_hydrazine.pdf}}</ref>