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How to calculate Delta-V of multiple engines and/or stages


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Hello!

When .24 came out I lost my Delta-V calculator (KER) so I want to do the Delta-V calculations by hand. I can do the basic single stage/single engine:

(ISP * 9.82 * ln(wet/dry)

I've googled and got nothing that I could understand(never been good with the symbols). All I want to know is how to calculate a stage with 2+ engines and ships with multiple engines and/or multiple stages. I need something simple (as I put above) so I can wrap my head around it.

Thanks for your time.

-Codey

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For identical parallel engines it stays the same (except mass goes up), for stages, the difference in wet and dry mass is only the fuel in the active stage.

for (very) different engines parallel.. eh. I don't know by heart. :) for reasonably-similar-isp engines, just add the ISPs and divide by number of engines for a close approximation?

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Hello!

When .24 came out I lost my Delta-V calculator (KER) so I want to do the Delta-V calculations by hand. I can do the basic single stage/single engine:

(ISP * 9.82 * ln(wet/dry)

I've googled and got nothing that I could understand(never been good with the symbols). All I want to know is how to calculate a stage with 2+ engines and ships with multiple engines and/or multiple stages. I need something simple (as I put above) so I can wrap my head around it.

Thanks for your time.

-Codey

Codey,

It's easy, you just sum the delta-vs for each stage to get the total delta-v. The only change is that the mass ratio for a lower stage includes the wet mass of all the stages above it as part of its dry mass (e.g. the stages above act as 'payload'). For big rockets it is a lot easier to use Kerbal Engineer, or if you're feeling nerdy, a spreadsheet with tables of the wet and dry masses of various components.

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Calculation by hand is done stage by stage.

Assuming you are completely stock and use no third party calculation tools, you can only ever manually calculate the bottom-most stage in the stack.

Therefore:

(1) For each engine in this stage, calculcate thrust * Isp

(2) Sum up all these numbers

(3) Divide by total thrust of this stage to get average stage Isp

(4) Put your vessel on the launchpad, go to map mode, click the little nfo box, note your rocket's wet mass

(5) In the VAB, remove all fuel from that stage, then repeat step 4 to get your rocket's dry mass

(6) You can now calculate this stage's dV

(7) In the VAB, remove that stage, and return to (1) to calculate the next stage

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