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Long winded introduction in spoiler: I put together a hypothetical situation to compare the Space Shuttle ET mounted with 1 SSME to a Metholox powered stage with 1 engine. For this exercise, I was interested in what the mass of a Metholox powered stage would come in at. In other words, attach 1 SSME to ET, fire, how much dV? (14.3 km/s) Use the rocket equation to figure out a Metholox stage with the same dV and compare mass. Raptor: Thrust: 1900 kN TWR: 198.5 Mass: 977 kg ISP: 380 s Fuel Mix: 3.8 SSME: Thrust: 2279 kN TWR: 65.9 Mass: 3527 kg ISP: 452 s Fuel Mix: 5.9 ET: Gross mass: 760 000 kg Dry mass: 26 500 kg Notes: Metholox stage: 119 331 kg including tank, fuel, and engine. o_O this can't be right... Nobody shoots an empty fuel tank into space for fun so I decided to account for a payload. You can see in the plot below how the stage mass changes as payload increases. There are a couple of things going on behind this plot, first the dry mass of the SSME + ET doesn't change (about 30t), second the Metholox stage dry mass does change (4.4t to 13.6t). At the cross point of the blue and purple lines (20t payload) the Metholox stage is 11 385 kg close to 1/3 of the dry weight. Interesting that there is a payload range where Metholox has the advantage. I suspect that it has to do with the dry mass to payload ratio. Also, TWR for this whole deal is about 0.25 with a 20t payload. Interesting... what if we add... MOAR BOOSTERS: Changing up the plot a bit, here we have a 3 engine hypothetical situation. For the ET+SSME contraption, it's probably not too far off of Shuttle-C with no SRBs. With more engines the cross point moves up to 30t and the TWR is 0.73 at the cross point which is in the range of common upper stages. Anyway, there was no real science here and I suppose that if we want to go someplace in a hurry, Metholox might be a decent strategy. Maybe if you wanted to send the Hubble Space Telescope on an escape trajectory... Closing thoughts: this situation is kind of unrealistic, could I come up with a more realistic situation to better reflect a hypothetical Metholox upper stage? Yes, probably. I think the point remains that there is possibly a situation where a higher density fuel makes more sense from a mass standpoint than a hydrogen powered one though.