I’m a space systems propulsion design engineer by profession. I worked on a project which I will not name that requires on-orbit refuelling. (It’s not this one and I don’t and will never work for Elon Musk).
The technology for in-orbit refuelling doesn’t exist, and there’s a whole lot of new technology required. Remotely docking is akin to self-driving in complexity; don’t forget to factor in the signal delay if you’re in a lunar or translunar orbit. If you make this a crewed activity only, then the problem becomes one of pneumatics. A pressure system that can reliably contain and transfer pressure up to the levels of spacecraft fuel (around 300 psi for liquid, 3000 for gas) repeatedly, in both directions is very, very heavy. The valves are heavy, the tanks are heavy, the control systems are heavy. Too heavy to be considered viable for spaceflight. Even less so for a mission whose payload is “as much transfer fuel as we can possibly get up there”. A huge amount of innovation has to take place before this can become real. As of 2022, when I last worked on this, none of the technology was even being researched, that is to say it was not even at TRL 3. Typically these things take on the order of a decade or so to get to TRL 9, if they are successful and quick.
I’m not saying it’s impossible, I’m saying I’ll be fascinated to see which solutions they come up with, and that I’m sceptical that they do have current solutions which are feasible and useful, rather than something like a one-shot refueling subsystem that weighs 250kg and delivers 15 litres of hydrazine.
I’m a space systems propulsion design engineer by profession. I worked on a project which I will not name that requires on-orbit refuelling. (It’s not this one and I don’t and will never work for Elon Musk).
I’m just an enthusiast so I trust your professional opinion more than my own knowledge.
The technology for in-orbit refuelling doesn’t exist, and there’s a whole lot of new technology required.
I’m assuming your referring to the SpaceX on-orbit refueling of cryogenic propellants requirement, which I agree hasn’t happened yet in history by anyone. However, there is a regular on-orbit refueling of cryogenic propellants for years on a regular basis with Progress spacecraft refueling the 400 liters worth of tanks (860kg of hydrazine) on the ISS for its orbit raising rocket engine located on the Zvezda module of the ISS. Cryogenic propellants would be a whole different animal however.
I’m sceptical that they do have current solutions which are feasible and useful, rather than something like a one-shot refueling subsystem that weighs 250kg and delivers 15 litres of hydrazine.
As this technology is in regular use in space today, I don’t doubt SpaceX could do this, but we agree this doesn’t accomplish anything close to what is needed for the lunar mission SpaceX signed up for.
Don’t forget ESA’s ATV and the NASA RRM for refuelling the ISS. As was the case with launchers for a while, the Europeans and the Americans have beautiful, expensive and awesome solutions, while the Russians just get the job done (often by waiving safety standards)
Anyway, the ISS is a different beast, it’s in LEO and it didn’t need to be launched in one go, so you can send up heavy equipment and integrate it on-orbit, activities which require Gantt charts so autistic that my eyes bleed when I think about them. Starship-to-Starship refuelling would mean sending a single spacecraft up with all the necessary equipment to do propellant transfer, which is what I was thinking of when I wrote my comment, as you say.
Yeah the only stuff I could find about actual orbital refueling was basically some tests with cryogenic fuel pumping on the ISS, which had some fairly serious problems, and a few times that basically a small satellite was refueled / serviced by another small satellite, which yeah as you say, just deliver a tiny amount of hydrazine, an exceptionally less volatile and easier fuel to deal with.
I’m a space systems propulsion design engineer by profession. I worked on a project which I will not name that requires on-orbit refuelling. (It’s not this one and I don’t and will never work for Elon Musk).
The technology for in-orbit refuelling doesn’t exist, and there’s a whole lot of new technology required. Remotely docking is akin to self-driving in complexity; don’t forget to factor in the signal delay if you’re in a lunar or translunar orbit. If you make this a crewed activity only, then the problem becomes one of pneumatics. A pressure system that can reliably contain and transfer pressure up to the levels of spacecraft fuel (around 300 psi for liquid, 3000 for gas) repeatedly, in both directions is very, very heavy. The valves are heavy, the tanks are heavy, the control systems are heavy. Too heavy to be considered viable for spaceflight. Even less so for a mission whose payload is “as much transfer fuel as we can possibly get up there”. A huge amount of innovation has to take place before this can become real. As of 2022, when I last worked on this, none of the technology was even being researched, that is to say it was not even at TRL 3. Typically these things take on the order of a decade or so to get to TRL 9, if they are successful and quick.
I’m not saying it’s impossible, I’m saying I’ll be fascinated to see which solutions they come up with, and that I’m sceptical that they do have current solutions which are feasible and useful, rather than something like a one-shot refueling subsystem that weighs 250kg and delivers 15 litres of hydrazine.
First, musk is an ass. No defense of him.
Spaceflight, however, is awesome.
I’m just an enthusiast so I trust your professional opinion more than my own knowledge.
I’m assuming your referring to the SpaceX on-orbit refueling of cryogenic propellants requirement, which I agree hasn’t happened yet in history by anyone. However, there is a regular on-orbit refueling of cryogenic propellants for years on a regular basis with Progress spacecraft refueling the 400 liters worth of tanks (860kg of hydrazine) on the ISS for its orbit raising rocket engine located on the Zvezda module of the ISS. Cryogenic propellants would be a whole different animal however.
As this technology is in regular use in space today, I don’t doubt SpaceX could do this, but we agree this doesn’t accomplish anything close to what is needed for the lunar mission SpaceX signed up for.
Don’t forget ESA’s ATV and the NASA RRM for refuelling the ISS. As was the case with launchers for a while, the Europeans and the Americans have beautiful, expensive and awesome solutions, while the Russians just get the job done (often by waiving safety standards)
Anyway, the ISS is a different beast, it’s in LEO and it didn’t need to be launched in one go, so you can send up heavy equipment and integrate it on-orbit, activities which require Gantt charts so autistic that my eyes bleed when I think about them. Starship-to-Starship refuelling would mean sending a single spacecraft up with all the necessary equipment to do propellant transfer, which is what I was thinking of when I wrote my comment, as you say.
I appreciate your expert input!
Yeah the only stuff I could find about actual orbital refueling was basically some tests with cryogenic fuel pumping on the ISS, which had some fairly serious problems, and a few times that basically a small satellite was refueled / serviced by another small satellite, which yeah as you say, just deliver a tiny amount of hydrazine, an exceptionally less volatile and easier fuel to deal with.
Hydrazine isn’t less volatile or easy to deal with. Not unless your point of comparison is that one fluorine test rocket.
Here’s the MSDS on hydrazine: https://nj.gov/health/eoh/rtkweb/documents/fs/1006.pdf
It’s easier to store. Aside from it being the single most hazardous chemical substance known to man, that is