There are several other use cases that Starship is well optimized for. Not surprisingly it's also fairly optimal for building large LEO constellations, It's a little bit large for the current Starlink satellite size, but if the Starlink satellites get significantly larger and more capable, Starship will be fairly optimal.

Given SpaceX's business, it seems safe to assume that Starlink was a design goal with at least a similar priority to the Mars goal.

Another use case it'd work fabulous for would be a LEO space hotel business.

Finally, it's also a great rocket for any use case that involves returning large masses, even if the return is from higher than LEO. Yes, it'll be a thirsty beast requiring many refuelling trips, but the tyranny of the rocket equation makes it hard to do any better. If you want to return dozens of tons from the moon or elsewhere you'd be hard pressed to do better than Starship.

Any large fully-reusable rocket basically fits those goals, but methane might not be ideal for the second stage, if you don't intend to refuel on Mars. Hydrogen has better ISP, so your mass budget isn't so tight. It's also six times better as a coolant, so if you want to try SpaceX's original plan of using evaporative cooling in place of heavy tiles, it'll work better. Blue Origin and Stoke Space both use a methane first stage, hydrogen second stage. (Stoke Space has planned evaporative cooling from the start, Blue Origin put in a patent application for their version later.)

Then again, Musk is also big on reusing components as much as possible, so he might have opposed multiple fuels on principle.

> Hydrogen has better ISP, so your mass budget isn't so tight.

You need much larger tanks, so the mass advantage is pretty much completely eliminated. Hydrogen engines generally have much lower thrust for a given size too. Falcon9 or Starship style staging is infeasible with a hydrogen second stage. Rockets that use hydrogen for their second stage separate a lot higher and faster than Falcon9/Starship to make up for this reduced thrust. This makes Falcon/Starship style 1st stage recovery impossible.

Hydrogen would be great for a 3rd stage. If you want it to be recoverable, design a third stage that fits within the Starship enclosure. This would be a fabulous way to do small BEO missions without requiring a whole bunch of refueling.

How are you distinguishing Falcon/Starship style recovery? Blue Origin actually attempted first-stage propulsive recovery with their first orbital test, with a hydrogen second stage. (It didn't work, but neither did SpaceX's first attempt.)

Yes, "impossible" was unreasonable hyperbole on my part.

Blue Glenn stages much higher and further than Falcon; it's caught much further off shore. More significantly, it doesn't have a return to launch site option like Falcon does and Starship always uses.

One of my favorite science fiction threads is the ulta rich building personal homes in LEO, and creating a service industry as a result. I find it more believable than colonizing Mars for altruism.

Not altruism

Disagree. Falcon 9 is likewise not hydrolox. Hydrolox unquestionably gives you the most payload going the farthest for a booster of a given size. But why is booster size even in the equation?

For most purposes the customer does not care one iota about the booster, they are interested in the cost per kg to get where they are going. For low orbit hydrolox imposes more handling nightmare costs than it saves in amount of rocket, it is not the fuel of choice unless you're trying to impress.

(Now, things change considerably when you looking at deep space. But methalox or even kerolox fueled in orbit still beats hydrolox fueled on the ground. And hydrolox is much less storable--your rocket costs weight, necessary to reach orbit but once you're up there a smaller engine means less wasted mass. The only advantage to a bigger engine is Oberth and that is only truly relevant if you either care about time (Apollo took an inefficient path for this reason), or because you are going to carry velocity into deep space. Look at the flight path of the Webb. The booster flew higher than the maximum efficiency path because the deep space stage was puny. It wasn't powerful enough to circularize normally, the telescope fell back quite a bit before the engine had built up enough velocity to stay up. But it was worth wasting some energy on that in order to not lift as much engine away from the Earth.)

With the possible exception of the fuel choice, all of the others are requirements for a recoverable rocket that needs to land back safely on Earth. Mars and Earth are not that different, so anything that helps you land and take back off easily from Earth also help with Mars.