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Post by n2maniac on Nov 27, 2016 18:25:15 GMT
Noticed an interesting discrepancy in the combustion temperature between RP-1/LOX (1400K, lol wat) and Decane/LOX (~4000K, seems about right), decided to dig deeper. After all, these are both long chain hydrocarbons and should combust nearly identically. Error seems to be here:
Material RP-1
Elements C H
ElementCount 1 2
Density_kg__m3 769
EnthalpyOfFormation_kJ__mol -462.2
MeltingPoint_K 283.1
BoilingPoint_K 560
Pretty close to the properties of C16H34. However, reducing the molar mass to 14 (CH2 formula) while keeping the same heat of formation makes it effectively 16x more thermodynamically stable. So, if RP-1 performance was a mystery to anyone, here is the answer. FYI. |
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Post by dajuice on Dec 4, 2016 1:14:13 GMT
Any idea how to mod this? Just changing the elements count doesn't do the trick.
Edit:
Found it: You have to modify the formula in combustibles.txt in addition to the reaction formula in ChemicalReactions.txt. Lox/RP finally has a more realistic exhaust velocity.
Now for the next issue: LOX/H2 in reality achieves significantly better isp as in game...
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Post by lieste on Dec 4, 2016 1:21:28 GMT
Any idea how to mod this? Just changing the elements count doesn't do the trick. Presumably dividing the enthalpyofformation in ratio. (i.e. roughly x1/16th) using only the data given. |
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Post by dajuice on Dec 4, 2016 1:45:33 GMT
Any idea how to mod this? Just changing the elements count doesn't do the trick. Presumably dividing the enthalpyofformation in ratio. (i.e. roughly x1/16th) using only the data given. Works too, breaks one of the stock engines though. I settle for C13H28 as the formula for RP-1 for now. |
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Post by zuthal on Dec 5, 2016 13:06:16 GMT
From the boiling point spec given in the official NASA specifications for RP-1 (end point of the distillation at maximally 525 °F or 274 °C), it seems like the single alkane that is the best fit would be pentadecane, C15H32, with a boiling point of 270 °C. C13H28 boils at 235.4 °C and C16H34 at 286.8 °C.
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Post by pokington on Dec 6, 2016 21:52:51 GMT
See my bug report here. Basically, you can drop the enthalpy of formation or increase the element count to make it dodecane. Dodecane is 15.25x heavier than the "molecule" present in the raws, so it has the same net effect as a propellant and makes more sense from a modeling perspective. |
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Post by qswitched on Dec 10, 2016 2:03:47 GMT
The next patch will have pokington's fix treating RP-1 as dodecane. Sorry for the delay, and thanks!
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