What fuel / oxidizer mix?

[Orc]

Hi all

What fuel / oxidizer mix are the Kerbal liquid fuel engines supposed to be burning?

The fuel to oxidizer ratio is close to 1:1.

 

According to http://www.astronautix.com/

Hydrogen LOX is 1:4.

Kerosene LOX is 1:2.56

H2O2 (hydrogen peroxide) Kerosene is 1 : 7.07.

 

So what are Kerbals burning? Any ideas? Any word from the Dev team?

 

Inquiring minds want to know.

Kind regards

Orc

[cantab]

A long time ago I found an exotic fuel mixture that matched the LF:O ratio and the engine performance well, but that was before the efficiencies took a nerf. I'm pretty sure it was never "supposed" to be anything other than generic rocket fuel. indeed early versions of KSP didn't even have oxidizer as a separate thing.

[Streetwind]

10 minutes ago, Orc said:

So what are Kerbals burning? Any ideas? Any word from the Dev team?

They're burning "liquid fuel" and "oxidizer". It does not represent any realworld propellant and was never meant to.

The ratio, by the way, is 9 to 11 (or 1:1.2222...)

 

Edited December 9, 2015 by Streetwind

[GoSlash27]

Potassium krakenate and kerbium tetroxide in a 9:11 mix. It's hypergolic and highly volatile. It leaches into metals. Anything it contaminates (which is pretty much everything) becomes susceptible to detonation due to excessive heat or shock.

Best,

-Slashy

Edited December 9, 2015 by GoSlash27

[A35K]

The Interstellar mod assumes that it is H LOX, but the density doesn't really match up, neither does the ratio. I wonder if it could be a hydrogen isotope, such as deuterium? Would that even be possible? 

 

Edited December 9, 2015 by A35K

[NecroBones]

2 hours ago, A35K said:

The Interstellar mod assumes that it is H LOX, but the density doesn't really match up, neither does the ratio. I wonder if it could be a hydrogen isotope, such as deuterium? Would that even be possible? 

 

Theoretically, sure. But I don't see the benefit. Deuterium will behave the same way as regular hydrogen, chemically speaking. However, the increased mass does a couple of things-- Generally heavier propellants allow for higher thrust, but lower ISP, since the propellant doesn't have the opportunity to accelerate as much as lighter materials do. And if the goal is to aim for higher thrust, such as in a lower stage, it's considerably easier and cheaper to use RP-1 (kerosene) since it's non-cryogenic.

[steuben]

I'm with F₂O₂ and a nitro-carbane. Only way to explain some of the booms. 

[NathanKell]

The thermal properties, and engine Isps, are modeled on NTO and Aerozine 50. Densities and mixture ratios (note, however, KSP uses volume ratio not the mass ratio in which mixtures are usually given) were left unchanged for backwards compatibility.

[Jimbodiah]

"It's Kerbals!"

- soylent green

[OhioBob]

18 hours ago, NathanKell said:

(note, however, KSP uses volume ratio not the mass ratio in which mixtures are usually given)

Well, we know that fuel by itself has the same density as oxidizer/fuel combined, therefore the density of oxidizer must be the same as the fuel.  That would make both volume ratio and mass ratio the same.

There are several oxidizer/fuel combinations that would have mixtures ratios around 1.1, though none that are commonly used.  There's a chart near the bottom of this page that gives several examples.  The ones closest to 1.1 are:  LOX/MMH, LOX/Aerozine 50, and NTO/hydrazine.

(edit)  Correction, that mixture ratio should be 1.22, not 1.1.  That's a little high for LOX/Aerozine 50, and NTO/hydrazine, but is getting closer to LOX/(ethanol+water) and RFNA/hydrazine

Edited December 10, 2015 by OhioBob

[NecroBones]

7 hours ago, OhioBob said:

Well, we know that fuel by itself has the same density as oxidizer/fuel combined, therefore the density of oxidizer must be the same as the fuel.  That would make both volume ratio and mass ratio the same.

There are several oxidizer/fuel combinations that would have mixtures ratios around 1.1, though none that are commonly used.  There's a chart near the bottom of this page that gives several examples.  The ones closest to 1.1 are:  LOX/MMH, LOX/Aerozine 50, and NTO/hydrazine.

 

Yes, agreed. In ResourcesGeneric.cfg both the LiquidFuel and Oxidizer resources have the same density (0.005 tons), so it's really the same in any case. This makes each unit (of either resource) 5kg in mass. Unfortunately there's nothing to determine actual volume measurements for these, short of estimating the volume of the corresponding tanks as they appear in the game.

 

As an interesting data point, some quick googling revealed NTO/Aerozine 50 to use a ratio of around 1.6 (possibly as high as 1.9 in some cases). The densities are NTO: 1.450 g/cc, and Aerozine 50: 0.903 g/cc.

Edited December 10, 2015 by NecroBones

[OhioBob]

1 hour ago, NecroBones said:

As an interesting data point, some quick googling revealed NTO/Aerozine 50 to use a ratio of around 1.6 (possibly as high as 1.9 in some cases).

Yes, that's correct.  Aerozine 50 was developed by Aerojet for use in the Titan missile.  In the Titan the mixture ratio was about 1.9.  Aerozine 50 was also used in the Apollo service and lunar modules, where the mixture ratio was 1.6.  One of the reasons for the 1.6 mixture ratio is because at this proportion the oxidizer and fuel have the exact same volume, so the same tanks can be used for both.  This can clearly be seen in the LM ascent stage as the two spherical tanks are the same diameter (see below).  However, since the fuel has less mass, it's tank is mounted farther from the centerline so the LM remains in balance.

This asymmetry wasn't an issue in the descent stage or service module because each had two oxidizer tanks and two fuel tanks, which could be arranged so that similar tanks were opposite each other to balance the mass.

Interestingly, my own computations show that NTO/Aerozine 50 produces a higher specific impulse at a mixture ratio of 1.6 then at 1.9.  I've never been able to find a good explanation for why Aerojet went with a 1.9 mixture ratio for the Titan. 

 

[NathanKell]

Yep, as KSP is currently, with same-density LF and Ox, the distinction between mass ratio and volume ratio is moot. Just wanted to make that point clear, however, since if, say, someone did Monoprop + Ox, or added resources, there it would be important, and it's something many modders have missed.