Rocket engine
Vacuum specific impulse, I

Due to the specific impulse varying with pressure, a quantity that is easy to compare and calculate with is useful. Because rockets 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 m ˙ {\displaystyle {\dot {m}}}, provided the mixture ratios and combustion efficiencies are maintained. It is thus quite usual to rearrange the above equation slightly:

and so define the vacuum Isp to be:

where:

And hence:
keywords:forbidden_words