Editing Monopropellant rocket (section)

==Solar-thermal based monopropellant thrusters==
A concept to provide [[low Earth orbit]] (LEO) [[propellant depot]]s that could be used as way-stations for other spacecraft to stop and refuel on the way to beyond-LEO missions has proposed that waste gaseous [[hydrogen]]—an inevitable byproduct of long-term [[liquid hydrogen]] storage in the [[Radiative heat transfer|radiative heat]] environment of [[outer space|space]]—would be usable as a monopropellant in a [[Solar thermal rocket|solar-thermal]] propulsion system.  The waste hydrogen would be productively utilized for both [[orbital stationkeeping|orbital station-keeping]] and attitude control, as well as providing limited propellant and thrust to use for [[Orbital maneuver#Non-impulsive maneuvers|orbital maneuvers]] to better [[Space rendezvous|rendezvous]] with other spacecraft that would be inbound to receive fuel from the depot.<ref name=aiaa20100902>
{{cite web |last=Zegler |first=Frank |title=Evolving to a Depot-Based Space Transportation Architecture |url=http://www.ulalaunch.com/site/docs/publications/DepotBasedTransportationArchitecture2010.pdf |work=AIAA SPACE 2010 Conference & Exposition |publisher=AIAA |access-date=2011-01-25 |author2=Bernard Kutter |date=2010-09-02 |page=3 |quote=the waste hydrogen that has boiled off happens to be the best known propellant (as a monopropellant in a basic solar-thermal propulsion system) for this task. A practical depot must evolve hydrogen at a minimum rate that matches the station keeping demands. |url-status=dead |archive-url=https://web.archive.org/web/20111020010301/http://www.ulalaunch.com/site/docs/publications/DepotBasedTransportationArchitecture2010.pdf |archive-date=2011-10-20 }}</ref>

Solar-thermal monopropellant thrusters are also integral to the design of a next-generation cryogenic [[upper stage]] [[rocket]] proposed by U.S. company [[United Launch Alliance]] (ULA).  The [[Advanced Common Evolved Stage]] (ACES) is intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA [[Centaur (rocket stage)|Centaur]] and ULA [[Delta IV#Vehicle description|Delta Cryogenic Second Stage]] (DCSS) upper stage vehicles.  The ACES [[Integrated Vehicle Fluids]] option eliminates all [[hydrazine]] and [[helium]] from the space vehicle—normally used for attitude control and station keeping—and depends instead on solar-thermal monopropellant thrusters using waste hydrogen.<ref name=aiaa20100902_p5>Zegler and Kutter, 2010, p. 5.</ref>