I think I cover this in a footnote? ISS gets refueled, and there have been satellite experiments like Orbital Express, but no one has attempted bulk rocket-to-rocket propellant transfer.
No, they're both pretty significant. In the ISS case you have propellant mass moving around that's just a tiny fraction of the total system mass, while in the rocket case a sizable portion of the total mass gets shifted.
Moreover, in refueling ISS you can use something like a flexible bladder and pressure differential to simplify the job of moving liquid from container A into container B. But in the rocket-to-rocket case, you might be moving propellant from an almost-empty Starship into an almost-full depot rocket. In that case, you're trying to hunt around for liquid in an almost empty fuel tank, and push it into an almost full one.
You can't use a bladder because Starship is too big, and it's hard to maintain a big pressure difference (unless you're willing to vent a lot of propellant in the process).
The problem would be very hard even without cryogens.
> Moreover, in refueling ISS you can use something like a flexible bladder and pressure differential to simplify the job of moving liquid from container A into container B. But in the rocket-to-rocket case, you might be moving propellant from an almost-empty Starship into an almost-full depot rocket. In that case, you're trying to hunt around for liquid in an almost empty fuel tank, and push it into an almost full one.
> You can't use a bladder because Starship is too big, and it's hard to maintain a big pressure difference (unless you're willing to vent a lot of propellant in the process).
You just put a baffle in the tank so the volume with remaining propellant is small and close to full. Also eliminates sloshing issues that you'd need to deal with anyways.
In orbit fluid transfer is a reasonably solved problem, and there are many ways to do it. In addition to using bladders, you can also use ullage motors, centrifugal propellant settling, capillary tubes, etc. Cryogenics are harder because cryogenic pumps are just generally more challenging than standard pumps, but luckily the pumping requirements for propellant transfer are much less demanding than for engine restart - rocket engines need high flow rates and can't tolerate entrained gas, propellant transfer can use slower but more robust pumps.
I don't mean this to diminish the accomplishment of the engineers who spent quite a bit of time solving this problem, but rather to point out that people have been working on this for decades and have had significant success. Sure we haven't done a perfect 1:1 dress rehearsal in orbit where large quantities of cryogenic propellants are transferred between docked spacecraft, but we've done everything shy of that.
The lack of bladder seems directly driven by the cryogenic temp. Whats stopping you from using large (or many) bladders for warm fuels?
I don’t see the hard problem of “hunting” for fuel in a rigid container. Yes you need ullage, but how is this worse than what you need typically to feed an engine?
I'll defer to people who know more about rocket design about why you couldn't have a huge stretchy bladder holding RP-1 (for example) in a rocket stage.
The problem with hunting is that a liquid/gas system forms a bunch of weird intermixed 3D blobs in microgravity. You either need to accelerate the docked rockets (so the liquids pool at one end) or you need some apparatus to separate liquid and gas in microgravity. Both are hard to do.
Engines never have to worry about the microgravity case, there are always ullage motors or some other mechanism to accelerate the rocket before engine ignition so that fluid and gas separate.
> I'll defer to people who know more about rocket design about why you couldn't have a huge stretchy bladder holding RP-1 (for example) in a rocket stage.
Do you mean you have a cite to this claim? Would love to read.
> You either need to accelerate the docked rockets (so the liquids pool at one end)
Right, this is what I was referring to as “ullage”.
> Engines never have to worry about the microgravity case, there are always ullage motors
My point was that standard ullage motors can and will be used by SpaceX for the transfer. Why do you think this is harder than the fairly standard case of starting an engine in microgravity?
> Do you mean you have a cite to this claim? Would love to read.
No, I mean that I have a handwavy explanation for why you can't put 500 tons of kerosene in a big stretchy bladder inside your rocket, but I'm hoping that someone who knows more about rocket design than I do will comment here.
> My point was that standard ullage motors can and will be used by SpaceX for the transfer. Why do you think this is harder than the fairly standard case of starting an engine in microgravity?
Because the motors have to run for much longer (many minutes instead of a few seconds) and the mass distribution of the docked system is rapidly changing during that entire time.
