Editing Rocket engine (section)

===Vacuum specific impulse, I<sub>sp</sub>===

Due to the specific impulse varying with pressure, a quantity that is easy to compare and calculate with is useful. Because rockets [[choked flow|choke]] at the throat, and because the supersonic exhaust prevents external pressure influences travelling upstream, it turns out that the pressure at the exit is ideally exactly proportional to the propellant flow <math> \dot{m}</math>, provided the mixture ratios and combustion efficiencies are maintained. It is thus quite usual to rearrange the above equation slightly:<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 3-33.</ref>

{{block indent|<math> F_{vac} = C_f\, \dot{m}\, c^*</math>}}

and so define the ''vacuum Isp'' to be:

{{block indent|<math>v_{evac} = C_f\, c^* \,</math>}}

where:

{{block indent|1=<math>c^*</math> &thinsp;= &thinsp;the [[characteristic velocity]] of the combustion chamber (dependent on propellants and combustion efficiency)}}
{{block indent|1=<math>C_f</math> &thinsp;= &thinsp;the thrust coefficient constant of the nozzle (dependent on nozzle geometry, typically about 2)}}

And hence:
{{block indent|<math> F_n = \dot{m}\, v_{evac} - A_{e}\, p_{amb}</math>}}