Post by pokington on Oct 1, 2016 12:51:15 GMT
The performance of combustion rockets in-game appear to have some issues squaring with real-life examples.
Firing up the rocket editor, the only way to adjust exhaust velocity is to play with the expansion ratio and stoichiometric ratio. These parameters are pretty simple to tweak to get an optimum ISp for a particular propellant combination in-game. This is usually stoichiometric ratio of 1 and always expansion ratio of 1000 and angle of 1 degree. The issue is that even their completely unpractical maximal performances (that nozzle looks hilarious) are often significantly less than many real-life examples of rocket engines using the same propellants.
Additionally, throat area is not a factor in specific impulse, which AFAIK is not correct.
(as an aside, RP-1 appears to be borked as a propellant anyway, since it you can't even make fuel tanks with RP-1 in them)
IRL values were pulled from Wikipedia's handy propellant table, using the vacuum engine with a 40:1 expansion ratio values.
It appears to me that the oxidizer to fuel ratio is very wrong as well -- it should be pretty easy to trade that perfect stoichiometric burn for increased thrust at a loss in specific impulse, but with the current system in game generally any ratio other than 1 results in horrendous performance. Real rocket engines almost never burn at the perfect stoichoimetric ratio because with many fuel combinations it's actually less performant anyway due to the higher molecular mass of the reactants.
Firing up the rocket editor, the only way to adjust exhaust velocity is to play with the expansion ratio and stoichiometric ratio. These parameters are pretty simple to tweak to get an optimum ISp for a particular propellant combination in-game. This is usually stoichiometric ratio of 1 and always expansion ratio of 1000 and angle of 1 degree. The issue is that even their completely unpractical maximal performances (that nozzle looks hilarious) are often significantly less than many real-life examples of rocket engines using the same propellants.
Additionally, throat area is not a factor in specific impulse, which AFAIK is not correct.
| Propellant | In-Game Ve Max | Optimal In-Game O2 Mass Ratio | IRL Ve | IRL O2 Mass Ratio |
| LOx/RP-1 | 1.91 km/s | 3.99 | 3.46 km/s | 2.77 |
| LOx/Methane | 3.45 km/s | 3.99 | 3.61 km/s | 3.45 |
| LOx/H2 | 4.63 km/s | 7.92 | 4.46 km/s | 4.83 |
(as an aside, RP-1 appears to be borked as a propellant anyway, since it you can't even make fuel tanks with RP-1 in them)
IRL values were pulled from Wikipedia's handy propellant table, using the vacuum engine with a 40:1 expansion ratio values.
It appears to me that the oxidizer to fuel ratio is very wrong as well -- it should be pretty easy to trade that perfect stoichiometric burn for increased thrust at a loss in specific impulse, but with the current system in game generally any ratio other than 1 results in horrendous performance. Real rocket engines almost never burn at the perfect stoichoimetric ratio because with many fuel combinations it's actually less performant anyway due to the higher molecular mass of the reactants.
