What kind of fuel does KSP use for LiquidFuel?

[horndgmium]

Title says it all. I need to know for science!

As well as what oxidizer I suppose.

[TheCanadianVendingMachine]

Probably Hydrogen, and Liquid Oxygen

[R0cketC0der]

I would say it's RP-1 for some engines and LH2 for some others. The oxidizer is probably LOX as it is the most common used one in real-life.

[sal_vager]

It doesn't really have a defined real world analogue, it can just as easily be liquefied Kerbals as it can be fermented grass or a petroleum based fuel

[horndgmium]

Haha, I kind of like the idea of liquefied Kerbals... "Biofuels", which is actually kind of why I'm interested.

Turns out it's incredibly hard (not impossible) to synthesize oxidizer using bacteria because oxidizing agents typically have the unfortunate side effect of death.

[DeathFromBelow]

Don't the small aircraft fuselage tanks say kerosene on them? I've always assumed the rockets use the same thing since the fuel is interchangeable.

[sal_vager]

Ahh well now, oxidiser is something else again, Kerbin does have an oxygen atmosphere so cooling that down until it liquefies is a possibility.

But oxidisers can be many things including nitrates, I ask you, have you seen any Kerbal sewer systems? I haven't, all that er, matter has to go somewhere

[Zaak]

The masses and volumes appear to indicate liquid oxygen and some kind of hydrocarbon, though as with most things in stock KSP there's no strict correspondence to real life.

Take a look at http://forum.kerbalspaceprogram.com/threads/64118-0-23-Real-Fuels-v5-3-21-14 if you're interested in real-life rocket fuels in KSP.

[EtherDragon]

Probably Hydrogen, and Liquid Oxygen

Probably Hydroken and Likuid Oxyken, in accordance with the "add 'K' to every word" rule.

[sal_vager]

the "add 'K' to every word" rule.

Like with Mun, Duna, Moho, Eve....

[Kryxal]

Haha, I kind of like the idea of liquefied Kerbals... "Biofuels", which is actually kind of why I'm interested.

Turns out it's incredibly hard (not impossible) to synthesize oxidizer using bacteria because oxidizing agents typically have the unfortunate side effect of death.

No, the liquefied Kerbals are from the Kethane mod, part KE-WAITNONOSTOP-01...

[EtherDragon]

Like with Mun, Duna, Moho, Eve....

Clearly, those planets were named by some famous 19th century scientist before the "add 'K' to every word" rule was codified in the konstitution. =p

[cantab]

It's meant as a generic liquid fuel and not anything in particular. But our three knowns are the oxidizer:fuel mass ratio, the maximum vacuum Isp, and the fuel and oxidizer density.

Density is a bit uncertain. It's often thought that the units for fuel are litres, but I really don't think they can be. The FL-T200 has an overall volume of 780 or 1500 litres, depending on if you take the diameter as 1.25 m or 1 m. For it to only hold 200 litres of propellants is absurd. I believe the fuel units are gallons. The FL-T200 has a volume of 170 to 400 gallons, depending on whether you use the smaller or larger size and the imperial or US gallon.

This, then, gives a fuel and oxidizer density of 5 tons per thousand gallons, or between 1000 and 1300 kg per cubic metre, depending on which ton and which gallon is used. A far more sensible value than the 5000 kg per cubic metre you get from assuming the units are litres.

(Note that choosing different volume units for the fuel does not affect the relationship between volumetric fuel flow, specific impulse, and thrust. The volume units cancel out.)

With that issue out of the way, we can pick some candidate fuel/oxidizer combos. A list is here:

http://en.wikipedia.org/wiki/Liquid_rocket_fuel#Bipropellants

And after some looking and thinking, I have a best candidate:

Monomethylhydrazine/Beryllium fuel

Dinitrogen tetroxide oxidizer

The mixing of beryllium into the MMH lowers the ox:fuel ratio and raises the Isp. The density remains around 1200 kg/m³. It's not an exact match, but it's plenty close enough for KSP! The combination is also I assume hypergolic, which fits with the engines being repeatedly re-ignitable, and what I see as the general simplicity of the Kerbal Space Program.

[zarakon]

Probably munshine

[duncan1297]

ive noticed recently that the jet fuel fuselages say "Caution Kerosene Fuel" on them.

[karolus10]

Probably munshine

Actually Redstone rockets (used for first mercury suborbital flights) used mixture of alcohol and liquid oxygen .

[Streetwind]

Density is a bit uncertain. It's often thought that the units for fuel are litres, but I really don't think they can be. (...) I believe the fuel units are gallons.

KSP uses the metric system exclusively. There is not a single imperial unit anywhere in the game, so don't expect them to make an exception here.

Far more probable is that it's neither liters nor gallons nor any other RL unit of volume. It could be "kerbalnaut helmet fillings" for all we know.

As for the density, remember these Kerbals live on a planet that has the gravity of something with a radius 11 times larger for no particular reason. Considering that, you'd almost expect fuel to be even more dense than it is right now Everything on Kerbin and Kerbal-related is just super dense. Including the brains of Kerbals, of course, which is why they have a stupidity meter.

[Dispatcher]

I think the jet fuel is well defined (the warnings on jet fuselages). As for rockets, perhaps the color(s) of the lit exhaust might indicate possible fuel/ oxidizer combinations.

[cantab]

As for the density, remember these Kerbals live on a planet that has the gravity of something with a radius 11 times larger for no particular reason. Considering that, you'd almost expect fuel to be even more dense than it is right now

My objection to this, though, is not the density per se. It's that if the fuel has a density of 5000 kg/m³, then the tanks we see are mostly empty space. I'm sorry, but in my view that's just too silly even for KSP.

[Anton P. Nym]

I think the jet fuel is well defined (the warnings on jet fuselages). As for rockets, perhaps the color(s) of the lit exhaust might indicate possible fuel/ oxidizer combinations.

Yes, with the warning on the jet fuel tanks I think that the Liquid Fuel resource is kerosene, or more precisely RP-1 given that it's interchangeable with the rocket tanks. If it's RP-1, the most likely oxidiser is liquid oxygen.

Now, to determine how much of each fuel tank's mass is RP-1 and LOx and how much is tank, plumbing, and insulation...

-- Steve

[Dispatcher]

Interesting that RP-1 combustion temperature is given as 3670K. Current stock engine max temperature is for most engines 3600, including the RAPIER. The aerospike is 3400 and the LV-N is the only one to exceed the others, at 4000.

Also, the RP-1 oxidizer to fuel ratio is 2.56. Currently, stock tanks have a ratio of essentially close to 1 to 1 by volume in liters (according to the KSP Wiki), with the mix being a little oxygen rich. This makes KSP mixes more similar to Russian mixes rather than US mixes, which tend to run fuel rich. By mass, the hydrocarbon fraction is probably somewhat greater than the oxygen fraction.

Using the currently large orange tank as an example (and the proportions seem consistent with the stock tanks), the dry tank mass is 4 tons and the full mass is 36 tons, so the fuel/ oxidizer mass is 32 tons. So the dry mass is about 11% of the total full mass. Further, this might mean a maximum fraction of 5.5% for the oxidizer component system and likewise for the fuel component system; although they can trade off as long as the 11% is not exceeded. This assumes that the outer tank hull mass is included in with the above fractions. (My fueled rocket adapters and nose caps use the outer hull as the fuel tank, with the oxidizer tank [edit: and pumps] being immersed within it.)

So it seems to me that the RP-1 is not an exact match for the KSP fuel, but has similarities.

Edited March 29, 2014 by Dispatcher

[Bobnova]

The temperatures are OK with me, few to no rocket engines can actually take the full combustion temperature of their fuel. There's usually cooling of some sort.

[horndgmium]

If there's one thing I want in the Kerbal space program it's legitimate metric units for all measurements.

[Red Iron Crown]

Density is a bit uncertain. It's often thought that the units for fuel are litres, but I really don't think they can be. The FL-T200 has an overall volume of 780 or 1500 litres, depending on if you take the diameter as 1.25 m or 1 m. For it to only hold 200 litres of propellants is absurd. I believe the fuel units are gallons. The FL-T200 has a volume of 170 to 400 gallons, depending on whether you use the smaller or larger size and the imperial or US gallon.

I've come to think the units of liquid fuel and oxidizer are not volume units at all, unless they both happen to have exactly the same very high density which I consider unlikely. Remove a unit of fuel or oxidizer from a tank and the mass decreases equally. I just think of them as 5Kg units. You might notice that if you assume the entire tank volume is filled with fuel, the density of the fuel and oxidizer becomes more realistic than the stupidly dense 5000Kg/m³.

I think liquid fuel itself is a bit of an abstraction. It burns in jet engines, which makes me think it's kerosene. It gives an incredibly high I_sp in the LV-N, which makes me think it's LH₂. According to the Wiki, it's 5000Kg/m³, which makes me think it's liquified selenium or something.

[Dispatcher]

If there's one thing I want in the Kerbal space program it's legitimate metric units for all measurements.

I'm not aware of any missing metric particulars, but this may be due to my play style and limited mod creation. I am aware that there seem to be discrepancies between the Wiki use of the liter for fuel unit and the stated density.