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
can you use a plunger, instead of a pump? more like a syringe?
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.
deeper discussion with math: https://forum.nasaspaceflight.com/index.php?topic=60124.60
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.
> 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.
[1] https://news.satnews.com/2024/03/31/spacex-enjoys-two-out-of...
> Like a lot of space technology, orbital refueling sounds simple, has never been attempted, and can’t be adequately simulated on Earth.[18] The crux of the problem is that liquid and gas phases in microgravity jumble up into a three-dimensional mess, so that even measuring the quantity of propellant in a tank becomes difficult.
And for cryogenic propellents specifically:
> Getting this plan to work requires solving a second engineering problem, how to keep cryogenic propellants cold in space. Low earth orbit is a toasty place, and without special measures, the cryogenic propellants Starship uses will quickly vent off into space.
Fuel transfer and storage in orbit is problematic in many respects.