A question about the calculation of the delta-v

[goldenpeach]

First of all,I know how to calculate the specific impulse of an engine and the delta-v of a rocket.

However,I still have a question.

If on the same stage you have engine with a different specific impulse,how do you calculate the delta-v?

What value do you take for the specific impulse in the delta-v calculation?

Thank you,in advance,for the answer!

[tavert]

It used to be incorrect on the wiki, but looks like somebody fixed it: http://wiki.kerbalspaceprogram.com/wiki/Tutorial:Advanced_Rocket_Design

Specific impulse is proportional to thrust divided by propellant mass flow rate. So to combine different engines with different specific impulses, you want to find the total thrust and the total propellant mass flow rate.

The mass flow rate of each engine is proportional to its thrust divided by its specific impulse. So take the thrust divided by specific impulse for each engine and sum the results. Divide the total thrust by this number to get the combined effective specific impulse.

[goldenpeach]

Thank you!

I didn't think I could consider all the engines as one engine and doing the isp calculation on that "engine"

By the way,There is an error in the wiki:the exact equation for the delta-v is: Ve*ln(m0/m1)

where

Ve=ips*surface gravity(9.81 m/s)

m0=masse of the rocket before the burn

m1=masse of the rocket after the burn

Anyone know how I could make the modification?

[tavert]

If you read the note below the equation, "If the specific impulse is given in seconds it is necessary to multiply this value by 9.816 m·s⻲ (see also Terminology about Isp)." There's a recurring discussion about specific impulse and its units, you can register and have a go at editing though, also a good idea to look at the discussion page first.

Keep in mind that effective exhaust velocity is not "specific impulse in seconds" times current local surface gravity, as that would change with location. The g0 term relating specific impulse in seconds to effective exhaust velocity is a constant of proportionality that never changes, it's just a convention due to a historical bad choice of units.