The problem I see with this is rockets tanks are usually not empty. The are stringers and other supports inside otherwise the wall thickness would have to increase to give the tank enough rigidity.
Quote from: jak Kennedy link=topic=43522.msg1710332#msg1710332 The problem I see with this is rockets tanks are usually not empty. The are stringers and other supports inside otherwise the wall thickness would have to increase to give the tank enough rigidity.Good point.Hence separate tanks for fuel to be transferred on top of the craft and non-pressurised should help reduce the load. Cylindrical shape will increase strength of outer wall, with the biggest weakness being the bottom, so it should probably be rounded in bottom (even if not all fuel can squeeze out). Bottom will have to sustain ~200 ton at 5-10G, which can be reinforced from below.
Thrusting is the way, that was determined long ago.
As far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.
Quote from: DreamyPickle on 08/07/2017 08:27 amAs far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.Settling of cryogenic propellants on orbit has been done though.
Quote from: savuporo on 08/07/2017 08:33 amQuote from: DreamyPickle on 08/07/2017 08:27 amAs far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.Settling of cryogenic propellants on orbit has been done though. exactly
Is thrust ullage for the entire duration of a multi kiloton fuel transfer really easier than burning up (and down) a minor spin ullage?From first principles, I would think that spin ullage would be a better fit for slower, longer duration fuel transfers, (thrust the whole time vs thrust once to spin up and once to spin down) which due to square/cube on transfer pipes should scale better to larger craft.
Quote from: rakaydos on 08/07/2017 03:49 pmIs thrust ullage for the entire duration of a multi kiloton fuel transfer really easier than burning up (and down) a minor spin ullage?From first principles, I would think that spin ullage would be a better fit for slower, longer duration fuel transfers, (thrust the whole time vs thrust once to spin up and once to spin down) which due to square/cube on transfer pipes should scale better to larger craft.Thrusting is easier. No attitude changes, no additional stresses, no worry about changing moments of inertia, etc.Boil off gases can be used for the thrusting.
You do realize that for maintaining the same acceleration constant for longer time you need to throttle up the trusters exponentially.
The thrusters can be shut off while spinning.
Quote from: Peter.Colin on 08/07/2017 06:09 pmYou do realize that for maintaining the same acceleration constant for longer time you need to throttle up the trusters exponentially.There is no need for the same acceleration rate, just a constant acceleration.But your point is wrong anyways. There is no mass loss or gain. The "system" is the two spacecraft and they are joined and the mass transfer is internal to the "system". So the same thrust is the same acceleration.Quote from: Peter.Colin on 08/07/2017 06:09 pmThe thrusters can be shut off while spinning.No, they can't. A rotating system will have changing moments of inertia and likely require a need for additional thruster firings for control.
Same trust is not same acceleration (= not same g-force)An object moving twice as fast has four times the energy.going from 50km/h to a 100km/h takes 3 times more thrust than than going from 0km/h to 50km/h.This adds up exponentially.
Also a rotating craft would need additional thruster firings to stop the rotation after the transfer is complete.
It seems like we are trying to solve a problem that has already been solved, as fuel transfer from two docked craft does not seem that different from fuel transfer to the engines.