Why do fuel and oxidizer units have such an odd ratio?

[Vaporo]

This has always been something that's bugged me:

Fuel and oxidizer units are set in a very odd (1:1.22?) ratio. Why was this done? Why wasn't the mass of one unit of oxidizer or one unit of LF tweaked so that the LF+O unit ratio is 1:1? It's very annoying when I'm transferring fuel to have to try and balance my fuel and oxidizer when the units don't line up quite right.

I am not suggesting that this be changed, but I am wondering why it was done this way. Any thoughts?

[Kegereneku]

Well, it doesn't matter in game-design but realistically, fuel and oxidizer in rocket are not needed in the same quantity.

From a gameplay perspective of course you could do 1:1 just as well as you could do an absurd 1:22 ratio and justify it however you want.

You might hear some persons here claiming that respect of real-world density, fuel ratio, fuel name, planet size, atmosphere size, reaction-wheel saturation/lock...etc is paramount to KSP "fun".

But as far I'm concerned this is a way for the Developer to emulate the "real world" disparity while balancing mass in Spaceplane design.

btw, I think it would be better to show the LF/O with unit of mass.

[Vanamonde]

As this is not a suggestion but a question about the nuts-and-bolts of the simulation, it has been moved to Gameplay Questions.

[Gaarst]

Ratio is 9 LF for 11 Ox (in units and mass) which does not correspond to any real LF/Ox ratio

LF and Ox have the same mass density: 5 kg/L or 5 kg/unit (LF and Ox units are L)

[NathanKell]

How do you know the units are L?

If they're L, they sure don't use much of the tank...

[Rhomphaia]

This has always been something that's bugged me:

Fuel and oxidizer units are set in a very odd (1:1.22?) ratio. Why was this done? Why wasn't the mass of one unit of oxidizer or one unit of LF tweaked so that the LF+O unit ratio is 1:1? It's very annoying when I'm transferring fuel to have to try and balance my fuel and oxidizer when the units don't line up quite right.

I am not suggesting that this be changed, but I am wondering why it was done this way. Any thoughts?

Well having the units correspond to an equal mass makes it easier to manually calculate Delta-V on the fly. Just add them together and divide by 200 to get the mass of propellant. Would just be annoying if they has chosen to go with a 1:1 unit ratio where the mass of a unit was different for fuel and oxidizer.

[Gaarst]

How do you know the units are L?

If they're L, they sure don't use much of the tank...

LF and Ox density is 5 kg/L according to the wiki.

720 + 880 = 1600 units of LF+Ox weigh 8 t (X200-16 fuel tank)

So 1 t of fuel is 200 units, 1 kg of fuel is 0.2 units; so 1 unit is 5 kg

Density is then 5 kg/unit

1 unit = 1 L

[Hannu]

1000 kg fuel tank has diameter of 1.25 m and height of about 1 m. Volume of such tank is 1200 l but volume of liquids is only 200 l (110 l oxidizer and 90 l fuel). Real tanks have thermal insulation, elliptical ends and mechanical structures which takes volume but ratio is very much larger. Liters and densities are somewhat arbitrary.

[impyre]

To answer the OP question... the reason is to simulate the effects of stoichiometry ratios. Anything that reacts has an ideal ratio of reactants, this ratio is called the stoichiometric ratio... and it's based on an understanding of the reacting molecules and the reaction istelf. For example, Hydrogen and Oxygen can combine and combust. The stable product is water, so each reaction would ideally require two hydrogens and one oxygen. So in terms of mols, it's a simple 1:2 ratio. Due to different masses, the mass ratio is much different. In terms of atomic mass it's approximately 88.89% by mass oxygen and about 11.12% by mass hydrogen. About a 1:8 ratio. There are tons of different ratios... one for every reaction that could possibly happen. HHO requires an oxidizer-to-fuel ratio of about 8:1, while RP-1 (kerosene-based rocket propellant) only requires a ratio of about 2.56:1. There's no innate benefit of any given ratio by itself. The real advantages of different types of fuels depends on energy density (how much energy is released by reacting a given mass of reactants) and exhaust velocity (which affects ISP). Also, toxicity is a commonly looked-at factor... RP-1 is quite safe compared to hydrazine. There's not much point in simulating any given ratio... the fact that they did it at all just adds some "flavor" to the game.

[NathanKell]

Almost nothing runs stoich, though; US engines usually run fuel-rich, both open and closed-cycle (the famous kerolox engines were about 2.25:1, hydrolox at ~5.5 or 6 to 1), whereas the Russians run ox-rich for their open-cycle engines and very ox-rich for closed.