A lot of the Moon mission hinges on Starship being a reliable machine that does all of the things Musk promised. I have no doubt that SpaceX in time could build something good, they do have a lot of good people working there. But the time lines given by Musk to the government in order to get the contracts weren’t viable. And as usual Musk overpromised about the capabilities.

One of the biggest doubts at the moment is about Starship being able to go to the Moon. The plan was to send up a Starship into LEO, then send up another Starship to refuel the first one. That way it would have enough fuel to go to the Moon and work as a lander there. It doesn’t need to do much, just get to the Moon, take the people to the surface and get them back into Lunar orbit.

The issue with this is, a lot of things need to go right in order for this to work. You’d need two functioning Starships, they need to both launch into the correct orbit and rendezvous. Then they need to dock and transfer fuel, undock and separate. This is pretty much never been done, so they would be doing something new, but in theory it can be done. Hard and experimental, but in principle achievable.

However when calculations were made, it turns out once you put a Starship in the right orbit it’s not possible for it to have enough fuel to fully refuel another Starship. So Musk said they would simply stretch a Starship and use it as a fuel station. Nobody is really sure if this stretching is even possible, as this wasn’t part of the original design, but let’s say it is. Now the mission become more complicated still, you’d need the Starship that does the Moon mission. Then you’d need the fuel station ship and another ship to fuel that station. And all of this has to work and be timed properly for the Moon mission to work.

But then further calculations were made and nobody is sure how many Starship launches would be required to fill up that fuel station. Partly because Starship isn’t finalized, so the exact specs are unknown. But back of the napkin calculations put the figure at something like 6 launches. A big problem is the fuel used is very hard to store for any amount of time. As it’s cryogenic, it needs to be kept cold. On Earth this is done by using very thick and sturdy pressure vessels, combined with a bunch of machinery and off-gassing. But in space this gets harder, since the pressure vessels need to be light, they can’t be as sturdy. And there isn’t room (both in volume and weight) for all of the cooling machines, which would require too much power and cooling themselves to even work. So we end up with only off-gassing to maintain temperature. This usually doesn’t matter, on Earth the fuel that’s lost gets replaced right away up to the point of liftoff. After that the fuel is used to fly the mission and usually the rocket’s main fuel tank is empty after that. This puts a lot of time pressure on the whole thing, that fuel station in orbit is losing fuel all of the time. So it’s a race to fill it up faster than it’s losing fuel. So those 6 missions need to be flown within a day or maybe two. And if it turns out the amount of fuel being delivered is lower than expected or the loss is higher, there would need to be 12 fueling missions within a day. Not strictly impossible, but not exactly easy. And the not knowing is making people nervous.

They are so far behind schedule, on a system that hasn’t been finalized, let alone tested, it’s very doubtful they could do it anywhere in the near future. Nasa has since asked other companies if they could build a lander if SpaceX can’t do it. But canceling the whole landing part is an option as well.