How to calculate how much fuel I need?

[bobbytheboy416]

I have decided to embark on the exciting endeavour to fly to EVE, land a space-plane, do science, and return with said space-planes.

However, instead of slapping on a load of engines, and tanks, and see how far it can take me (my normal method), I thought I would try to do some mathematics to calculate exactly how much fuel I will need (even though the farthest I have ever been is to Duna).

Either way, I cannot seam to get the calculations right.

So far, I have been experimenting with said maths by having a single LV-N motor, n number of fuel tanks, and 40 Tonnes of space-plane.

I know the maths is completely off because I've calculated that all the fuel I would need (for the return stage) is two MK1 jet-fuel containers - which cannot be right.

So can I get you to walk me through the calculative steps to find out how much fuel I would need, and the method to calculate all other parameters for future calculations.

Thanks

[Cairol]

Look here:

http://forum.kerbalspaceprogram.com/threads/96985-1-0-2-WAC-s-Delta-V-Map-(24-05-2015)

The mod "Kerbal Engineer Redux" will show you your stages delta/v. As will MechJeb.

Add some fuel for your maneuvers and your mess ups and you should be good.

Edited June 18, 2015 by Cairol

[FancyMouse]

Sounds you haven't even mentioned deltaV yourself, which is possibly a sign of a way-off calculation.

And I would be curious about the design of a space plane that can take off on Eve.

[bobbytheboy416]

I have come up with a method (again) form first principals which is:

Total_ISP = Total_Thrust / (Sum_for_each_engine(Engine_Thrust / Engine_ISP)

Acceleration = Total_Thrust / Total_Mass

Time_of_burn = Delta_V / Acceleration

Fuel_used = Total_Thrust * Time_of_burn / Total_ISP

IS THIS METHOD CORRECT?????

[Red Iron Crown]

The first two look good. The first is simpler if all engines are the same, just use the Isp for that engine.

Time_of_burn = Delta_V / Acceleration

This one is tricky, because your acceleration changes over the duration of the burn as mass decreases. It needs to be the average acceleration for the burn in the calculation.

Fuel_used = Total_Thrust * Time_of_burn / Total_ISP

Should be Fuel_used = Total_Thrust * Time_of_burn / (Total_ISP * 9.81) if Isp is in seconds.

[Jason Patterson]

No, it's not. It doesn't account for things like changing mass as a function of time. You really need to use the Tsiolkovsky rocket equation for this sort of thing.

[Arkalius]

First, figure out how much delta-v your mission requires. This can be tricky, but once you have it, you're in a good place.

The Tsiolkovsky rocket equation tells you the delta-v of your rocket, which will be Isp * 9.81 * ln( m0/m1 ) Where Isp is specific impulse in seconds, m0 is full mass of the craft, and m1 is the dry mass (ie all fuel used up). To figure out how much mass of fuel you'd need, you would have:

m1 * ( e ^ (dV / (Isp * 9.81) ) - 1 )

However, this isn't super useful when designing a spacecraft. For one, you can't just simply add more fuel mass on its own. If you need more fuel, you need more tankage, which adds dry mass. Second, you're likely to build a rocket in stages, dropping dry mass over time as you go. This is why a mod like Kerbal Engineer Redux is useful. It will tell you how much delta-v you have in your rocket in each stage, helping you build what you need and plan it out. You can see how the numbers change as you move things around and add more or less fuel etc.

[Lynch]

Take a look at my Thread for further instructions

[h=2]Kerbal Rocketeering 101 - By Professor Lynch [1.0.2][/h]