# How do you measure specific impulse Isp of a rocket engine?

Specific impulse (Isp), is one of the two most important parameters used to characterise a rocket engine. The other being thrust (T).

For example, the RS-25 Space Shuttle Main Engine (SSME) has a specific impulse in a vacuum of 452.3 seconds and at sea level of 366 seconds. These are often multiplied by g, the acceleration due to gravity (9.81 m/s^2) to give the specific impulse as an "effective velocity" of the exhaust gases.

I'm not interested in the theory or application of the concept, but how it is measured in practice, such as by mounting a rocket in a rig and test firing it.

RS-25 test firing

TIA

• Or one can simply measure force in pounds (pounds-force, to be precise) and mass consumption in pounds per second (pounds-mass per second, to be precise). Voila! The ratio $F/\dot m$ yields ISP in seconds. Strictly speaking, one does need to divide by 1g, but that has a numerical value of 1 when one uses pounds-force and pounds-mass. There are times when US customary units come in handy; this is one of them. There are lots and lots of other times when US customary units do not come in handy. Apr 15 at 11:31
• @Hobbes Thanks. I get it now $Isp = F/(\dot m g_0)$ Apr 15 at 13:00