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.
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?