How to calculate combined Isp

[SDIR]

PI was wondering, if you have multiple engines with differnt isps, how would you go about finding out their combined isps? I'm asking because I want to weigh the pros and cons of having an aerospike and two 24-77s or just having a T45 instead.

Edited June 23, 2013 by SDIR

[Ben314]

You can either treat each separate engine as a separate stage (if you're using them at different times), or take an average of Isp, weighted by thrust. Orrrrr a far easier option: http://www.kerbalspacecalculator.net16.net/stageDVCalc.html

[purpletarget]

The Weighted Thrust averaging formula is incorrect. You need to figure out the full Thrust, and the full mDot of all your engines and run it through the ISP formula, or use an mDot weighted average.;

Check here.

[SDIR]

Thanks for the tips! Especially purpletarget's isp calculations. I also found out that there was a negligible difference in Isp of 1 between two 24-77s and one aerospike.

Edited June 23, 2013 by SDIR