Hydrolox vs metholox vs kerolox revisited
This and previous blog were really inspired by "raging" arguments I have read why methane based fuel is so much better than kerosene. With speculations related to future propulsion of SpaceX BFR and MCT based on methane and liquid oxygen (metholox), is so much better than standard kerosene/RP-1 and liquid oxygen combination. This article tries to put a new light to this argument, primary from assumptions about rocket equation expressed through Isp and propellant density.
The key difference is viewing rocket capability through its propellant volume, not its mass. This was heavily influenced by the analysis done by Dr. Bruce Dunn in his article Alternate propellants for SSTO Launchers (archived here). In this paper Dr. Dunn used a fixed volume SSTO and analyzed its performance from volume constrained perspective. The results (at least for me) were highly unexpected: the paper showed that liquid hydrogen and methane are not the best possible propellant combinations - they are actually the worst possible fuels for SSTO launchers.
The issue of kerolox vx metholox is further cluttered with proposed/expected mini-Raptor as new upper stage engine with reported Isp of 380, which seems to be a huge improvement compared to Merlin 1DVac Isp of 340. But most of this "upgrade" has nothing to do with different fuel and its inherently higher specific impulse. Most of this increase can be attributed to more advanced staged combustion cycle. The table below compares metholox vs kerolox performance, based on information from Dr. Dunn's article:
As a baseline, we shall assume that we need to design a hypothetical second stage of Falcon 9 vehicle, which has 100m3 of propellant volume available. For the provided propellant volume we shall assume dry weight of 6000kg.
Now using the formulation of the rocket equation from the previous article, we can calculate the performance of our stage:
The results are surprising, showing that hydrolox is by far the worst possible propellant. Even solid HTPB significantly outperforms it in all possible scenarios. Using given comparison, it is obvious that overall winners are RP-1/LOX and RP-1/H2O2 mainly due high density. Note that HTPB cannot have such performance in real life, because solid stages have to sustain much larger pressures (they are more like pressure-fed stages and thus would much higher have structural density). Thus when considering volumes and densities as real cost drivers for rockets, hydrolox and metholox should be avoided.
The key difference is viewing rocket capability through its propellant volume, not its mass. This was heavily influenced by the analysis done by Dr. Bruce Dunn in his article Alternate propellants for SSTO Launchers (archived here). In this paper Dr. Dunn used a fixed volume SSTO and analyzed its performance from volume constrained perspective. The results (at least for me) were highly unexpected: the paper showed that liquid hydrogen and methane are not the best possible propellant combinations - they are actually the worst possible fuels for SSTO launchers.
The issue of kerolox vx metholox is further cluttered with proposed/expected mini-Raptor as new upper stage engine with reported Isp of 380, which seems to be a huge improvement compared to Merlin 1DVac Isp of 340. But most of this "upgrade" has nothing to do with different fuel and its inherently higher specific impulse. Most of this increase can be attributed to more advanced staged combustion cycle. The table below compares metholox vs kerolox performance, based on information from Dr. Dunn's article:
| Fuel/oxidizer | Temp (K) | MR | Overall density | Isp (vac) |
|---|---|---|---|---|
| H2/O2 | 20/90 | 6 | 358 | 469.2 |
| H2/O2 | 14/64 | 6 | 395 | 469.2 |
| CH4/O2 | 112/90 | 3.5 | 828 | 386.4 |
| CH4/O2 | 101/64 | 3.5 | 890 | 386.4 |
| RP-1/O2 | 298/90 | 2.7 | 1031 | 375.9 |
| RP-1/O2 | 234/64 | 2.7 | 1124 | 375.9 |
| RP-1/H2O2 | 298/298 | 7.2 | 1312 | 343.7 |
| MMH/N2O4 | 273/273 | 2.1 | 1200 | 336.0 |
| HTPB | 298 | 1760 | 301.0 |
As a baseline, we shall assume that we need to design a hypothetical second stage of Falcon 9 vehicle, which has 100m3 of propellant volume available. For the provided propellant volume we shall assume dry weight of 6000kg.
Now using the formulation of the rocket equation from the previous article, we can calculate the performance of our stage:
The results are surprising, showing that hydrolox is by far the worst possible propellant. Even solid HTPB significantly outperforms it in all possible scenarios. Using given comparison, it is obvious that overall winners are RP-1/LOX and RP-1/H2O2 mainly due high density. Note that HTPB cannot have such performance in real life, because solid stages have to sustain much larger pressures (they are more like pressure-fed stages and thus would much higher have structural density). Thus when considering volumes and densities as real cost drivers for rockets, hydrolox and metholox should be avoided.
Well that hardly a fair comparison. What you should have compaired is using a second stage with a fixed Mass
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