Editing Rocket engine (section)

==Ignition==
{{Further|Combustion}}
With liquid and hybrid rockets, immediate ignition of the propellants as they first enter the combustion chamber is essential.

With liquid propellants (but not gaseous), failure to ignite within milliseconds usually causes too much liquid propellant to be inside the chamber, and if/when ignition occurs the amount of hot gas created can exceed the maximum design pressure of the chamber, causing a catastrophic failure of the pressure vessel. This is sometimes called a ''[[hard start]]'' or a ''rapid unscheduled disassembly'' (RUD).<ref name=aw20121126>
{{cite news |last=Svitak|first=Amy |title=Falcon 9 RUD? |url=http://www.aviationweek.com/Blogs.aspx?plckBlogId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385&plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:c973f72f-55d3-4374-b722-df31a8d333e6 |archive-url=https://web.archive.org/web/20140321053215/http://www.aviationweek.com/Blogs.aspx?plckBlogId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385&plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:c973f72f-55d3-4374-b722-df31a8d333e6 |url-status=dead |archive-date=2014-03-21 |access-date=2014-03-21 |newspaper=Aviation Week |date=2012-11-26 }}</ref>

Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, a plasma torch can be used,{{citation needed|date=October 2016}} or electric spark ignition<ref name=nsf20161003>
{{cite news |last=Belluscio|first=Alejandro G. |title=ITS Propulsion – The evolution of the SpaceX Raptor engine |work=[[NASASpaceFlight.com]] |date=2016-10-03 |url=https://www.nasaspaceflight.com/2016/10/its-propulsion-evolution-raptor-engine/ |access-date=2016-10-03 }}</ref> may be employed. Some fuel/oxidiser combinations ignite on contact ([[hypergolic]]), and non-hypergolic fuels can be "chemically ignited" by priming the fuel lines with hypergolic propellants (popular in Russian engines).

Gaseous propellants generally will not cause [[hard start]]s, with rockets the total injector area is less than the throat thus the chamber pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at ignition.

Solid propellants are usually ignited with one-shot pyrotechnic devices and combustion usually proceeds through total consumption of the propellants.<ref name=Sutton/>

Once ignited, rocket chambers are self-sustaining and igniters are not needed and combustion usually proceeds through total consumption of the propellants.  Indeed, chambers often spontaneously reignite if they are restarted after being shut down for a few seconds. Unless designed for re-ignition, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the pyrotechnic igniter or even refueling of the propellants.<ref name=Sutton/>