> They do this on earth every time they fuel the rocket. I understand it will be more difficult in space, but I don’t see why specifically this problem is the real engineering target over say, reuse.
The article goes into this in some detail. In particular:
* You have to get the propellant into space. This is going to take a large number of flights (~15) at a pace that has not been done before for a vehicle of that size (a launch every six days)
* You need to launch at pace because otherwise the propellant will boil off, which is another issue - you need to shade or insulate the propellant for a much longer period of time in much harsher conditions
* There is no gravity: whereas on earth the propellant separates relatively cleanly into liquid and gas this isn't the case in space
Yes, the article lists a few reasons, none of them convincing. Specifically:
> You have to get the propellant into space. This is going to take a large number of flights (~15) at a pace that has not been done before for a vehicle of that size (a launch every six days)
SpaceX has done 2 Falcon 9 launches in 1 day, and they would have done 3 if the third one had not have been scrubbed [1]. I really don't think that launching Starship is going to be any different, especially as it was specifically designed for reuse, unlike Falcon 9.
> You need to launch at pace because otherwise the propellant will boil off, which is another issue - you need to shade or insulate the propellant for a much longer period of time in much harsher conditions
First part is same argument as above. Second part (shading) - again, I don't see why it is harder than other hard things. Just add more insulation. Possibly do some passive or active cooling.
> There is no gravity: whereas on earth the propellant separates relatively cleanly into liquid and gas this isn't the case in space
Very similar problem to how you feed liquid propellant into a rocket engine when it relights in zero gravity. You use a small ullage thruster for this.
Yeah, a 9 meter diameter one, which adds mass and volume and complexity and detracts from the payload.
Instead what they do is use thrust to accelerate the whole vehicle a little, which presses all the liquid into one end of its tank where it can be pumped out. Instead of carrying special settling thrusters, they originally planned to use ullage gas for this but it's not clear that can work.
pretty much everything, including and especially plastic, becomes a fuel when it comes into contact with liquid oxygen. With liquid oxygen in contact with a fuel you're virtually guaranteed a fire at some point as it takes very little heat to start the combustion. This is why when rockets tip over it's an explosion and not just a broken airframe with fuel/oxidizer leaking out.
Most plastics are very brittle at the cryogenic temperatures. Also if you are using that method for a liquid oxygen tank, you need to make sure that the plastic you choose doesn't spontaneously combust on contact with LOX.
Cryogenic temperatures make most materials more brittle, hard to get a material that works at a wide enough range of temperatures to make a balloon to work correctly.
If you go for a narrower range of temperatures (ie. not structurally stable above 0C), it would need to be manufactured, transported, stored, tested and installed at seriously low temps which probably negates the possible advantage with the added technical complexity.
The article goes into this in some detail. In particular:
* You have to get the propellant into space. This is going to take a large number of flights (~15) at a pace that has not been done before for a vehicle of that size (a launch every six days)
* You need to launch at pace because otherwise the propellant will boil off, which is another issue - you need to shade or insulate the propellant for a much longer period of time in much harsher conditions
* There is no gravity: whereas on earth the propellant separates relatively cleanly into liquid and gas this isn't the case in space