Editing Rocket engine (section)

===Specific impulse===
{{Specific impulse|align=right}}
{{Main|Specific impulse}}

The most important metric for the efficiency of a rocket engine is [[impulse (physics)|impulse]] per unit of [[propellant]], this is called [[specific impulse]] (usually written <math>I_{sp}</math>). This is either measured as a speed (the ''effective exhaust velocity'' <math>v_{e}</math> in metres/second or ft/s) or as a time (seconds). For example, if an engine producing 100 pounds of thrust runs for 320 seconds and burns 100 pounds of propellant, then the specific impulse is 320 seconds. The higher the specific impulse, the less propellant is required to provide the desired impulse.

The specific impulse that can be achieved is primarily a function of the propellant mix (and ultimately would limit the specific impulse), but practical limits on chamber pressures and the nozzle expansion ratios reduce the performance that can be achieved.