As can be seen from the thrust equation the effective speed of the exhaust, Ve, has a large impact on the amount of thrust produced from a particular quantity of fuel burnt per second. The thrust-seconds (impulse) per unit of propellant is called Specific Impulse (Isp) or effective exhaust velocity and this is one of the most important figures that describes a rocket's performance.

Vacuum Isp

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 formula_9, 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:

Specific impulse
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