Editing Specific impulse (section)

====Rocketry====
In rocketry, the only reaction mass is the propellant, so the specific impulse is calculated using an alternative method, giving results with units of seconds. Specific impulse is defined as the thrust integrated over time per unit [[weight]]-on-Earth of the propellant:<ref name="SINasa">{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html|title=Specific impulse|last=Benson|first=Tom|date=11 July 2008|publisher=[[NASA]]|access-date=22 December 2009}}</ref>

<math display="block">I_\text{sp} = \frac{v_\text{e}}{g_0},</math>

where

*<math>I_\text{sp}</math> is the specific impulse measured in seconds,
*<math>v_\text{e}</math> is the average exhaust speed along the axis of the engine (in m/s or ft/s),
*<math>g_0</math> is the [[standard gravity]] (in m/s<sup>2</sup> or ft/s<sup>2</sup>).

In rockets, due to atmospheric effects, the specific impulse varies with altitude, reaching a maximum in a vacuum. This is because the exhaust velocity is not simply a function of the chamber pressure, but is [[de Laval nozzle|a function of the difference between the interior and exterior of the combustion chamber]]. Values are usually given for operation at sea level ("sl") or in a vacuum ("vac").