Editing Rocket engine (section)

===Throttling===

Rockets can be throttled by controlling the propellant combustion rate <math> \dot{m}</math> (usually measured in kg/s or lb/s). In liquid and hybrid rockets, the propellant flow entering the chamber is controlled using valves, in [[solid rocket]]s it is controlled by changing the area of propellant that is burning and this can be designed into the propellant grain (and hence cannot be controlled in real-time).

Rockets can usually be throttled down to an exit pressure of about one-third of ambient pressure<ref name=Sutton/> (often limited by flow separation in nozzles) and up to a maximum limit determined only by the mechanical strength of the engine.

In practice, the degree to which rockets can be throttled varies greatly, but most rockets can be throttled by a factor of 2 without great difficulty;<ref name=Sutton/> the typical limitation is combustion stability, as for example, injectors need a minimum pressure to avoid triggering damaging oscillations (chugging or combustion instabilities); but injectors can be optimised and tested for wider ranges.

For example, some more recent liquid-propellant engine designs that have been optimised for greater throttling capability ([[BE-3]], [[Raptor (rocket engine)|Raptor]]) can be throttled to as low as 18–20 per cent of rated thrust.<!-- eg, Blue Orgin BE-3 --><ref name="sn20150407">
{{cite news |last1=Foust|first=Jeff |title=Blue Origin Completes BE-3 Engine as BE-4 Work Continues |url=http://spacenews.com/blue-origin-completes-be-3-engine-as-be-4-work-continues/ |access-date=2016-10-20 |work=Space News |date=2015-04-07 }}</ref><!-- eg, SpaceX Raptor --><ref name="sfi20160927">{{cite news |url= http://www.spaceflightinsider.com/organizations/space-exploration-technologies/elon-musk-shows-off-interplanetary-transport-system/ |title= Elon Musk Shows Off Interplanetary Transport System |publisher= Spaceflight Insider |last= Richardson |first= Derek |date= 2016-09-27 |access-date= 2016-10-20 |archive-date= 2016-10-01 |archive-url= https://web.archive.org/web/20161001225649/http://www.spaceflightinsider.com/organizations/space-exploration-technologies/elon-musk-shows-off-interplanetary-transport-system/ |url-status= dead }}</ref>

Solid rockets can be throttled by using shaped grains that will vary their surface area over the course of the burn.<ref name="Sutton" />