Fuel vs Oxidiser

[Virtualgenius]

I am shocking at maths how do you work out the LF to oxidiser ratio so if you had 1000 LF what should its oxidser be

[Captain Sierra]

It's a 9:11 LF:0 ratio. Given that, you have the porportion 9/11 = 1000/x

Now it's basic algebra. Get X out of the bottom, then do multiplcation and division to isolate x.

9x/11=1000

9x=11000

x=1222.22; 2 repeating.

Is there an easier way? Probably, but this is what came to my head first and it works.

[Kasuha]

Almost all tanks provide the right ratio of LF and oxidizer (if full). Exceptions are plane fuel tanks with no oxidizer, and ROUND-8 tank which has just a tiny bit of LF surplus.

[capi3101]

Also the Oscar-B. The Oscar-B is 45.03% Fuel / 54.97% Oxidizer, and the Round-8 is 45.05%/54.95%, so they're still comparable if you're talking about just a single tank; its when you start stacking them up that a problem arises. The rest of them are 45%/55% on the nose.

What's of more use is the full mass of fuel tanks versus their dry mass - again, except for the Oscar-B and Round-8, the full-to-dry ratio for fuel tanks in KSP is 9:1 (for the Oscar-B it's 5.245:1; for the Round-8 it's 5.44:1). Since it's only the mass of the fuel you're carrying that changes when you're burning a rocket, that 9:1 ratio gives you a handy substitution you can put into the rocket equation -

delta-V = ln(M/Mo)*Isp*Go = ln(9Mo+X/Mo+X)*Isp*Go, where X is the mass of everything in your rocket that isn't a gas tank.

Again, that works only if you leave out Round-8s and Oscar-Bs.

Why is that useful? Well, if you've got a target delta-V in mind and you know the Isp of the engine you're going to use (say 800 for a nuke in space), you can solve that equation for Mo, multiply the result by 9 to get the full mass (i.e. the amount of gas you're going to need for the amount of delta-V you want), and then divide that by 0.5625 (the full mass of an FL-T100 tank) to figure out what configuration of tanks you're going to want (an FL-T200 is the same as 2 FL-T100s, an FL-T400 is the same as 4 FL-T100s, an FL-T800/X200-8 is the same as 8 FL-T100s, an X200-16 is the same as 16 FL-T100s, an X200-32 is the same as 32 FL-T100s, and a Jumbo-64 (the big orange tank) is the same as 64 FL-T100s - a bit of a pattern there).

Edited February 14, 2014 by capi3101

[Virtualgenius]

Thanks for the answer not sure how you got the 9x=11000

[capi3101]

Captain Sierra was answering the example question in your original post directly - You have 1000 units of Liquid Fuel and wanted to know how much Oxidizer you wanted. X is the amount of oxidizer and the ratio is 9/11, therefore:

9/11 = 1000 / x

9 * x = 11 * 1000

x = (11 * 1000 / 9) = 1,222.22

Thus if you have 1000 units of Liquid Fuel, you should have 1,222.22 units of Oxidizer.

[Virtualgenius]

Thanks guys its helped fix a problem with stages not decoupling properly due to left over fuel 0.1 to be exact however its now working perfectly

[kBob]

Or you could just do 1000*11/9 on a calculator substitute whatever you want for the 1000. That is: Liquid Fuel*1.222 should be accurate enough.

Edit: And just flip the ratio to do the reverse i.e. Ox*9/11=LF or Ox*.818 gets you Liquid Fuel.

Edited February 15, 2014 by kBob

[Virtualgenius]

A nice little gui app you punched the required fuel numbers into would be fantastic gogo developers