question about calculating delta-v

[Roderik]

so i'm trying to go to moho and doing it all scientific and calculating everything but when calculating the delta-v of a stage and that has multiple engines (same kind) on it do you add the isp up or do you just use the isp from 1 engine?

[Bill Phil]

If it's the same engine then you use its ISP, no changes.

Now if you use different engines, you sum the forces and divide by the sum of the mass flow to get effective exhaust velocity. In fact, that formula works all the time with multiple engines. 2 of the same engines, you get two times the force and two times the mass flow, resulting in no change.

[Roderik]

so for multiple engines you do this Isp = ( F1 + F2 + ... ) / ( ( F1 / Isp1 ) + ( F2 / Isp2 ) + ... )

F = thrust of engine and the numbers indicate that it's a different engine.

[Bill Phil]

Not ISP1, but the mass flow. You can solve for the mass flow by dividing the force of the engibe by its ISP and 9.8.

Unless you use ISP in meters per second...

[Red Iron Crown]

Where F is thrust. This is a rearrangment of a mass flow weighted average:

Isp = (á¹Â1∙Isp1 + á¹Â2∙Isp2 + ...) / (á¹Â1 + á¹Â2 + ...)

Edited October 5, 2015 by Red Iron Crown

[K^2]

so for multiple engines you do this Isp = ( F1 + F2 + ... ) / ( ( F1 / Isp1 ) + ( F2 / Isp2 ) + ... )

F = thrust of engine and the numbers indicate that it's a different engine.

This is weighted by number of each kind of engine. So index runs for all engines, even if some values repeat. But yes, this is the right formula.