Editing Monopropellant rocket (section)

==Chemical-reaction based monopropellant rockets==
The simplest monopropellant rockets depend on the [[chemical decomposition]] of a storable propellant after passing it over a catalyst bed.<ref>{{Cite book |last1=Price |first1=T |title=The Status of Monopropellant Hydrazine Technology |last2=Evans |first2=D |date=February 15, 1968 |publisher=National Aeronautics and Space Administration |series=TR 32-1227 |location=Pasadena, California |pages=1–2}}</ref> The power for the thruster comes from the high pressure gas created during the decomposition reaction that allows a [[Rocket engine nozzle|rocket nozzle]] to speed up the gas to create thrust.

The most commonly used monopropellant is [[hydrazine]] ({{chem2|N2H4, or H2N\sNH2}}), a compound unstable in the presence of a [[Catalysis|catalyst]] and which is also a strong [[reducing agent]]. The most common catalyst is granular [[alumina]] (aluminum oxide, {{chem2|Al2O3}}) coated with [[iridium]]. These coated granules are usually under the commercial labels Aerojet S-405 (previously made by [[Shell plc|Shell]])<ref>{{cite web |url=http://ir.aerojetrocketdyne.com/releasedetail.cfm?releaseid=708514 |title=Aerojet Announces Licensing and Manufacture of Spontaneous Monopropellant Catalyst S-405 |work=aerojetrocketdyne.com |author=Aerojet Rocketdyne |date=12 Jun 2003 |access-date=9 Jul 2015 |archive-date=5 December 2017 |archive-url=https://web.archive.org/web/20171205194721/http://ir.aerojetrocketdyne.com/releasedetail.cfm?releaseid=708514 |url-status=dead }}</ref> or [[Heraeus#Sold, restructured and disinvested businesses|W.C. Heraeus]] H-KC 12 GA (previously made by Kali Chemie).<ref>{{cite book|author1=Wilfried Ley|author2=Klaus Wittmann|author3=Willi Hallmann|title=Handbook of Space Technology|url=https://books.google.com/books?id=5LBx4EmBix8C&pg=PA317|year=2009|publisher=John Wiley & Sons|isbn=978-0-470-74241-9|page=317}}</ref> There is no [[igniter]] with hydrazine.  Aerojet S-405 is a spontaneous catalyst, that is, hydrazine decomposes on contact with the catalyst. The [[Chemical decomposition|decomposition]] is highly [[exothermic]] and produces a {{convert|1000|C|F}} gas that is a mixture of [[nitrogen]], [[hydrogen]] and [[ammonia]]. The main limiting factor of the monopropellant rocket is its life, which mainly depends on the life of the catalyst. The catalyst may be subject to catalytic poisoning and catalytic attrition which results in the catalyst failure. Another monopropellant is [[hydrogen peroxide]], which, when purified to 90% or higher concentration, is self-decomposing at high temperatures or when a catalyst is present.

Most chemical-reaction monopropellant rocket systems consist of a [[fuel tank]], usually a [[titanium]] or [[aluminium]] sphere, with an [[Ethylene propylene rubber|ethylene-propylene rubber]] container or a [[surface tension]] [[propellant management device]] filled with the fuel. The tank is then pressurized with [[helium]] or [[nitrogen]], which pushes the fuel out to the motors. A [[Pipe (fluid conveyance)|pipe]] leads from the tank to a [[poppet valve]], and then to the decomposition chamber of the rocket motor. Typically, a [[satellite]] will have not just one motor, but two to twelve, each with its own valve.

The [[Spacecraft attitude control|attitude control]] rocket motors for satellites and [[space probe]]s are often very small, {{convert|25|mm|in|abbr=on}} or so in [[diameter]], and mounted in groups that point in four directions (within a plane).

The rocket is fired when the [[computer]] sends [[direct current]] through a small [[electromagnet]] that opens the poppet valve. The firing is often very brief, a few [[millisecond]]s, and — if operated in air — would sound like a pebble thrown against a metal trash can; if on for long, it would make a piercing hiss.

Chemical-reaction monopropellants are not as efficient as some other propulsion technologies. Engineers choose monopropellant systems when the need for simplicity and reliability outweigh the need for high delivered impulse. If the propulsion system must produce large amounts of thrust, or have a high [[specific impulse]], as on the main motor of an interplanetary spacecraft, other technologies are used.