[Spaceships] Combined Ships and Delta-V

[Adelus]

Asking this after doing a search of the forums and not finding a totally satisfactory answer.

I'm curious as to how the External Clamp rules work with delta-V.

A thread a few years back ( http://forums.sjgames.com/showthread.php?t=65783 ) had one solution, namely the (fuel tank mass) / (combined ship mass) * 20 = New # of Fuel Tanks one, but I'm not sure how this holds up. How does one account for ships with different quantities of fuel per tank and with both/all ships thrusting?

[RogerBW]

I would be inclined to use the proper formula instead of the gameable approximation you get by renumbering fuel tanks. That's fine as far as it goes, but if you push it beyond there it will simply break.

This is not going to be simple.

You know the mass of each tank, it's 1/20 the total spaceship mass for that SM. Assume the structural mass of the tank is zero. Convert from dV-per-1/20-mass to effective exhaust velocity: divide by ln(1/0.95).

Now the dV for a pair of starting and ending vehicle masses will be Vexh*ln(start/end). (This is what that multiplier table approximates.)

For different vehicles linked together, maybe with different drives… for a start you need to know the different burn rates (how long it takes to empty a tank). Divide dV per tank by acceleration (and rescale to fit units if you're not using a calculator that does that for you: 1 mile/s from 1 gravity takes 164.10742 seconds.) Then work out how much fuel is burned by the slow-burning ship while the fast-burning ship runs its tanks dry, and do a dV calculation on that; then start from there and do another one for the remainder of the slow-burning ship's fuel.

[Ulzgoroth]

Originally Posted by Adelus

A thread a few years back ( http://forums.sjgames.com/showthread.php?t=65783 ) had one solution, namely the (fuel tank mass) / (combined ship mass) * 20 = New # of Fuel Tanks one, but I'm not sure how this holds up. How does one account for ships with different quantities of fuel per tank and with both/all ships thrusting?

It holds up perfectly if all the thrusters used have the same delta-V per tank. (If not, it works but it's not obvious what to do with the results.)

How many ships are thrusting in that case only affects the total acceleration. Accel for a multi-ship unit is simply the sum of the thrust (acceleration * mass) for each of the ships, divided by the mass of the full collection.

If you wind up with a mix of thrusters with different delta-V per tank values...things are a bit more complicated and I'm not going to try to solve it now.