Editing Rocket (section)

====Net thrust====
{{For|a more detailed model of the net thrust of a rocket engine that includes the effect of atmospheric pressure|Rocket engine#Net thrust}}
[[File:Rocket nozzle expansion.svg|thumb|upright|[[Rocket engine#Nozzle|A rocket jet shape]] varies based on external air pressure. From top to bottom:{{unbulleted list|Underexpanded|Ideally expanded|Overexpanded|Grossly overexpanded}}]]

A typical rocket engine can handle a significant fraction of its own mass in propellant each second, with the propellant leaving the nozzle at several kilometres per second. This means that the [[thrust-to-weight ratio]] of a rocket engine, and often the entire vehicle can be very high, in extreme cases over 100. This compares with other jet propulsion engines that can exceed 5 for some of the better<ref>{{cite web |url=http://www.geae.com/engines/military/j85/index.html |title=General Electric J85 |publisher=Geae.com |date=2012-09-07 |access-date=2012-12-10 |url-status=dead |archive-url=https://web.archive.org/web/20110722155949/http://www.geae.com/engines/military/j85/index.html |archive-date=2011-07-22 }}</ref> engines.<ref>{{cite web |url=http://www.thrustssc.com/thrustssc/Club/Secure/Arfons_Last_Stand.html |title=Mach 1 Club |publisher=Thrust SSC |access-date=2016-05-28 |url-status=dead |archive-url=https://web.archive.org/web/20160617103717/http://www.thrustssc.com/thrustssc/Club/Secure/Arfons_Last_Stand.html |archive-date=2016-06-17 }}</ref>

The net thrust of a rocket is

{{block indent|<math>F_n = \dot{m}\;v_{e},</math><ref name="RPE7"/>{{rp|2–14}}}}

where

{{block indent|<math> \dot{m} =\,</math>propellant flow (kg/s or lb/s)}}
{{block indent|<math>v_{e} =\,</math>the [[effective exhaust velocity]] (m/s or ft/s).}}

The effective exhaust velocity <math>v_{e}</math> is more or less the speed the exhaust leaves the vehicle, and in the vacuum of space, the effective exhaust velocity is often equal to the actual average exhaust speed along the thrust axis. However, the effective exhaust velocity allows for various losses, and notably, is reduced when operated within an atmosphere.

The rate of propellant flow through a rocket engine is often deliberately varied over a flight, to provide a way to control the thrust and thus the airspeed of the vehicle. This, for example, allows minimization of aerodynamic losses<ref name=maxq/> and can limit the increase of [[g-force|''g''-forces]] due to the reduction in propellant load.