Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around. Additionally, you need BOTH fuel and oxidizer to get combustion in space. This is one way where refueling in space is actually easier than on the ground where there’s ambient pressure and the atmosphere already has oxygen in it, so oxygen alone is sufficient.The materials science of sealing isn’t really any harder than on the ground (vacuum welding is a thing, but it’s actually rare for this to happen). But there’s additional advantages in that you don’t have to worry about ice (or even liquefied air) condensation, which at cryogenic temperatures is as strong as like epoxy. On the ground, condensation and frost can build up which interferes with a seal or makes it difficult to disengage. Not so in vacuum.
Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around.
With tower catch checked off, I thought it's a good time to revive the thread.Someone somewhere mentioned the idea of co-rotating nose-docked ships (for propellant settling) and got me thinking...
That is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.
Quote from: Greg Hullender on 10/23/2024 11:21 pmThat is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.I thought the consensus was that such a mission plan would require fully fueling in HEEO, not LEO.
Quote from: Twark_Main on 10/24/2024 07:28 amQuote from: Greg Hullender on 10/23/2024 11:21 pmThat is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.I thought the consensus was that such a mission plan would require fully fueling in HEEO, not LEO.I thought that was just with the V1 Starship. I still have your "spreadsheet of doom," by the way; I just don't have the numbers to put into it.
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.Ships dock back-to-back. Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship. Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.After transfer, flywheels cancel out spin of the vessels, and vessels unlock.It may be possible to so something similar using only thrusters instead of flywheels.
Quote from: Herbie on 10/25/2024 03:34 pmI have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.Ships dock back-to-back. Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship. Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.After transfer, flywheels cancel out spin of the vessels, and vessels unlock.It may be possible to so something similar using only thrusters instead of flywheels.Interesting concept, the problem is that any axis not aligned with the maximum moment of inertia is unstable. The two ships will start tumbling until they are rotating in the only stable configuration which should be around an axis perpendicular to the symmetry plane going through both ships long axis (and that plane will want to be parallell to the orbital plane).That configuration might work with your suggestion instead as most of the propellant would pool on the dorsal top and bottom corners of their respective tanks.
Quote from: eriblo on 10/25/2024 04:24 pmQuote from: Herbie on 10/25/2024 03:34 pmI have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.Ships dock back-to-back. Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship. Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.After transfer, flywheels cancel out spin of the vessels, and vessels unlock.It may be possible to so something similar using only thrusters instead of flywheels.Interesting concept, the problem is that any axis not aligned with the maximum moment of inertia is unstable. The two ships will start tumbling until they are rotating in the only stable configuration which should be around an axis perpendicular to the symmetry plane going through both ships long axis (and that plane will want to be parallell to the orbital plane).That configuration might work with your suggestion instead as most of the propellant would pool on the dorsal top and bottom corners of their respective tanks.I was wondering about the rotational stability, but the tennis racket theorem suggests that rotation is stable about the principal axis associated with the minimum moment of inertia. Of course, that result assumes fixed moments of inertia, which wouldnot apply given the dynamic nature of the mass distribution in the system.
I was wondering about the rotational stability, but the tennis racket theorem suggests that rotation is stable about the principal axis associated with the minimum moment of inertia. Of course, that result assumes fixed moments of inertia, which wouldnot apply given the dynamic nature of the mass distribution in the system.
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.
Quote from: Herbie on 10/25/2024 03:34 pmI have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.Gee, it's only 131 pages over the past five years--read faster! :-)Seriously, the things that soured me on all rotational approaches in the foreseeable future were:a) You only need maybe 50 micro-g of acceleration to settle the tanks, so even a very long ullage burn (e.g. hours) won't waste very much fuel.b) Cryogenic plumbing is the very devil to make work, so anything that needs new plumbing is probably off the table for a long time. Any practical system needs to work with Starships more-or-less as they are today--no new pipes and/or outlets. You definitely want to avoid anything that would add extra mass to each and every tanker Starship--adding mass just to the depot is probably okay.c) No one has even tried to make a rotating system work in space, much less one that would potentially involve huge changes in moment of inertia from docking, fuel transfer, and sloshing. You could imagine it taking many years to get the kinks worked out.The ullage-burn approach is so easy and so cheap it's just impossible to beat. Or so it seems, anyway. We'll know more when they start actually trying to do it.