I'm almost certainly missing something here so I'd be grateful if someone could explain but how do I convert thrust in newtons to specific impulse?
thanks.
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$\begingroup$ there is also some helpful information in answers to How do I get Specific impulse in (s) from (N * s/kg)? and What is total impulse? $\endgroup$ – uhoh Dec 31 '19 at 3:49
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You can't directly convert them; they're two different measures. As Organic Marble points out, thrust is loosely analogous to a car's horsepower; specific impulse is loosely analogous to a car's gas mileage.
Thrust in newtons is equal to the propellant mass exhaust flow rate in kg/s times the specific impulse in N·s/kg (which is dimensionally equivalent to m/s), however.
You more often see specific impulse given in seconds; multiply seconds by 9.81 to get N·s/kg. Note that the use of standard gravity as a conversion factor does not mean that specific impulse changes under different gravity. Rather, it's required because of the historical conflation of "pound" as both a unit of force and a unit of mass. Specific impulse in seconds is really "pounds-force seconds per pound-mass" which has the same dimensionality as N·s/kg.
answered Dec 31 '19 at 0:29
