Calculating dV

[Khrissetti]

I wondered if anyone could help. I'm familiar with the basic mathematics of calculating dV but I am preparing a Moho return craft and I suspect the calculation is a little trickier than full mass, dry mass and isp.

As you can see in the image below, the ship is being built as a lander stuck on the front of an interplanetary module with 4 nuclear engines, plus a train of drop-tanks strapped on the back:

so:

1) When inputting the isp, how do you calculate four identically-sized engines? Does the isp remain the same or multiplied by 4?

2) Would you work it out as one equation, starting with a full compliment of fueled drop tanks and ending with a drained interplanetary stage or would you work out the dV each droptank-worth of fuel gives you and add them together?

Cheers.

Edited July 22, 2013 by Khrissetti

[Mr Shifty]

You'd work out the dV for each tank separately then add them together. Remember to subtract the mass of each tank as you drop it. You can get a reasonable (conservative) approximation by doing it as one equation (since the masses of the drop tanks aren't that significant compared to the mass of fuel.)

[Khrissetti]

You'd work out the dV for each tank separately then add them together. Remember to subtract the mass of each tank as you drop it. You can get a reasonable (conservative) approximation by doing it as one equation (since the masses of the drop tanks aren't that significant compared to the mass of fuel.)

Thanks, I did suspect so. How about the isp issue?

[Mr Shifty]

Here's an example I worked out for someone else a few weeks ago:

469438

[Mr Shifty]

Thanks, I did suspect so. How about the isp issue?

For ISP, you actually have to work out a weighted average. So if the engines are all identical, you just use the ISP for one engine.

[Khrissetti]

Super, that's great. Time to see how many droptanks I'll need.

[inigma]

Its all in the http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet