Editing Rocket engine (section)

===Net thrust===
{{Main|Thrust}}
Below is an approximate equation for calculating the net thrust of a rocket engine:<ref>{{cite book|author=George P. Sutton|author2=Oscar Biblarz|name-list-style=amp|title=Rocket Propulsion Elements|edition=8th|publisher=Wiley Interscience|date=2010|isbn=9780470080245|url=https://archive.org/details/Rocket_Propulsion_Elements_8th_Edition_by_Oscar_Biblarz_George_P._Sutton/page/34/mode/2up}} See Equation 2-14.</ref>

{{block indent|<math>F_n = \dot{m}\;v_{e} = \dot{m}\;v_{e-opt} + A_{e}(p_{e} - p_{amb})</math>}}

{| border="0" cellpadding="2" style="margin-left:1em"
|-
|align=right|where:
|&nbsp;
|-
!align=right|<math>\dot{m}</math>
|align=left|=&nbsp; exhaust gas mass flow
|- 
!align=right|<math>v_{e}</math>
|align=left|=&nbsp; effective exhaust velocity (sometimes otherwise denoted as ''c'' in publications)
|-
!align=right|<math>v_{e-opt}</math>
|align=left|=&nbsp; effective jet velocity when Pamb = Pe
|-
!align=right|<math>A_{e}</math>
|align=left|=&nbsp; flow area at nozzle exit plane (or the plane where the jet leaves the nozzle if separated flow)
|-
!align=right|<math>p_{e}</math>
|align=left|=&nbsp; static pressure at nozzle exit plane
|-
!align=right|<math>p_{amb}</math>
|align=left|=&nbsp; ambient (or atmospheric) pressure
|}

Since, unlike a jet engine, a conventional rocket motor lacks an air intake, there is no 'ram drag' to deduct from the gross thrust. Consequently, the net thrust of a rocket motor is equal to the gross thrust (apart from static back pressure).

The <math>\dot{m}\;v_{e-opt}\,</math> term represents the momentum thrust, which remains constant at a given throttle setting, whereas the <math>A_{e}(p_{e} - p_{amb})\,</math> term represents the pressure thrust term. At full throttle, the net thrust of a rocket motor improves slightly with increasing altitude, because as atmospheric pressure decreases with altitude, the pressure thrust term increases. At the surface of the Earth the pressure thrust may be reduced by up to 30%, depending on the engine design. This reduction drops roughly exponentially to zero with increasing altitude.

Maximum efficiency for a rocket engine is achieved by maximising the momentum contribution of the equation without incurring penalties from over expanding the exhaust. This occurs when <math>p_{e} = p_{amb}</math>. Since ambient pressure changes with altitude, most rocket engines spend very little time operating at peak efficiency.

Since specific impulse is force divided by the rate of mass flow, this equation means that the specific impulse varies with altitude.