Fuel to Transport VS Fuel Used Ratio

[ErevanGaming]

Hi everybody =)

Was wondering if there's a general ratio when transporting fuel with fuel used to transport them relative to the gravity of the planet where the fuel comes from?

Like for example (not accurate) =D
If Kerbin has 1G and launching 100 units of LFO uses 4 times amount of LFO (400 units), for Mun which has 0.166G, it would only use like 265.6 LFOs?

 

Regards and thanks in advance =)

[bewing]

It depends heavily on your engine's Isp during launch. Also the launch profile, drag, and many other factors. However, you can boil it all down to one number -- the payload fraction. But it varies between each ship, so it's not a fixed number.

In general, however, it doesn't really matter. Once you are making fuel on the Mun, you have an infinite amount of it available (it just takes a little time). If your transport ship only burns 25% of its fuel load getting the other 75% to a tanker ship in orbit -- then 75% of infinity is still infinity. It just becomes a question of logistics in moving those infinite amounts of fuel around at that point.

 

 

 

 

[Oiff]

It is even cheaper than you think to reach munar orbit compared to LKO.

Low gravity, no drag, full rocket ISP immediately, low orbital velocity and low orbital altitude all add up to very little fuel consumption to get into orbit.  Of course landing can be expensive though as there is no drag.

[Plusck]

There is no direct ratio possible, firstly because the cost of getting from the surface to orbit will vary a lot depending on the radius of your planet or moon and your intended orbit.

Compare Moho and Vall: Moho has higher gravity (2.7 compared to 2.3 m/s² at the surface) but Moho is significantly smaller. Therefore it takes much the same dv to get to low orbit (860 m/s to 15km for Vall, 870 m/s to 20km for Moho according to the delta-v map). Despite its higher gravity, therefore, Moho is cheaper to refuel from.

The second factor is your TWR when full of fuel. You want high TWR to avoid gavity losses, but you need to maximise their ISP and minimise their weight. Terriers trump Aerospikes on both counts, but only if the local gravity is low enough not to rack up gravity losses.

The third reason no direct ratio is possible is because the rocket equation is logarithmic with respect to the mass fraction.  For example, a single orange tank powered by twin Aerospikes and loaded with landing gear + wheels + solar panels + bits and bobs (say 8 tonnes dry mass, 40 tonnes wet mass) will get a dv of about 5350 m/s. If you use that as fuel lifter on Moho, spending about 1000m/s going up to orbit then 1000 m/s going back down again afterwards, you will use 11 tonnes LF+Ox getting to orbit, and need to keep 3 tonnes on board to go back down. That difference of 3300 m/s - what you can store in the fuel depot - is actually only 56% of the total fuel load, 18 tonnes. However, add a tonne of dead weight to your lifter and you only lose a bit more than half a tonne of fuel, because most of the fuel is spent when the lifter is nearly all fuel. When mass fraction is high, weight savings give diminishing returns...

So basically you're left with:

1. doing the orbital equations (radius, gravity goes in, required dv to get to orbit comes out) then converting that to real dv to orbit depending on TWR (basically a function of time to accelerate to orbital speed multiplied by local acceleration due to gravity) and plugging that into the rocket equation (mass fraction and ISP in, percentage fuel available out)... OR....
2. a rule of thumb: you need to spend nearly 50% of your fuel to lift fuel from Moho or Vall. Virtually none for Gilly. Virtually all from Eve. And the rest in between.

[Spricigo]

For me, the real question is: How much of my time (in real life) it will take to have the fuel I need where and when I need?

Some people will bring fuel from the surface of [celestial body] with an efficient nuclear tanker. I'd rather use the effective TwinBoar powered supertanker.

 

[ErevanGaming]

Thanks everybody for your comments! Should have known that my question was a broad one =">