Specific Impulse Math Question

[rdkelley8]

Hi everyone,

I've just embarked on trying to figure out some of the math behind launching rockets. Forgive me if this is a novice question, but for some reason I've been staring at this for the last hour and am unable to see what is probably obvious:

I'm trying to calculate specific impulse. In its simplest form, specific impulse is just I_sp = thrust in newtons / fuel consumption in kg/s, from Kerbal's wiki on the matter. Yet I can't seem to get that to work using the info on the parts wiki. For example, the max thrust of the Thumper is 275 kN and the consumption at 1 atm is 22.63 kg/s...yet 275/22.63 is 12.15, or an I_sp in seconds of 12.15 x 9.80 = 119.09s. The wiki says the Thumper has a 165s I_sp at 1 atm.

What am I missing here? Am I not suppose to use Max Thrust for F_T? Is there some conversion I was suppose to make? Hmmmm...

Thanks,

[diomedea]

Many engines have been re-balanced, in later KSP versions, but even more important, the ISP relationship is now correct in KSP 1.0. The FF of the Thumper is 0.16995 U/s, that multiplied by the gravity (9.81 m/s^2) gives a weight of 1.667 kg/s. Yes the thrust at 0 ASL is 275 kN. Thrust divided by FF (in weight) gives ISP, and that is 165 at 0 ASL for the Thumper. Beware the KSP wiki is not always kept up-to-date.

[EtherDragon]

Hi Rdkelly8 the fuel consumption for an engine is constant - it never changes (It used to, but that is now corrected in 1.0). The Thrust is actually what changes based on the atmosphere.