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



1 Answer 1


Going from first principles:

Specific impulse is inversely proportional to specific fuel consumption (SFC) by the relationship Isp = 1/(go·SFC) for SFC in kg/(N·s) and Isp = 3600/SFC for SFC in lb/(lbf·hr).

So you can calculate Isp by measuring thrust (force on the test rig) and fuel consumption.

  • 4
    $\begingroup$ 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. $\endgroup$ Apr 15 at 11:31
  • $\begingroup$ @DavidHammen This is clearer, although I disagree about the units. The metric equivalent of pounds-force is kg-force (N/g). And of course mass flow in in kg/sec, so the result is still in seconds. I was playing with this online app which uses metric. Honestly Isp should use velocity units in metric, but maybe it uses seconds to make it invariant under the metric or imperial system. omnicalculator.com/physics/specific-impulse $\endgroup$
    – Galerita
    Apr 15 at 12:47
  • $\begingroup$ @Hobbes Thanks. I get it now $Isp = F/(\dot m g_0)$ $\endgroup$
    – Galerita
    Apr 15 at 13:00
  • $\begingroup$ @DavidHammen It doesn't matter what units you use--to express it in /seconds you have to match your units or convert them as the case may be. It's just kg-force isn't commonly mentioned. $\endgroup$ Apr 16 at 4:26
  • $\begingroup$ kg-f is a made-up unit to allow people to buy bananas and to measure Isp in seconds. If kg-force is felt to be a real unit then bananas should be bought by the 1/32.174... Slug. Yes? [ :-) ] [ I suspect that one is more likely to get slugged than to get what one wants if ordering in Slugs - or even in Slugs-force.] $\endgroup$ Apr 17 at 13:16

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