“ If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.”
– Henry David Thoreau
“ If we are serious about this, then our objective must be more than a disconnected series of missions, each conducted at huge expense and risk, and none building a lasting infrastructure to reduce the expense and risk of future operations . If we are serious, we will build capability, not just on the ground but in space. And our objective must be to make the use of space for human purposes a routine function.” –John Marburger
Famous Heinlein quote: “Once you're in orbit, you're halfway to anywhere.”
Corollary: As hard as getting to orbit is, it’s only half the challenge of getting anywhere else.
While microgravity handling and transfer of storable propellants is a proven technology, there remain some challenges and technical risk for orbital cryogenic storage and transfer.
Microgravity Cryogenic Fluid Management Issues
Propellant orientation not constrained as in a 1G field
Boil-off venting problems: you don’t want to vent liquid
Engine feed/propellant transfer problems: you want to avoid accidentally ingesting gas in your feedlines
Mass gauging: how do you tell how much you have left if you don’t know what the orientation is?
Boil-off: Cryogens like to be cold, sunlight is hot, vacuum is a lousy heat sink, and boiling cryogens build pressure in your tanks. Also vented propellant boil-off is mass you have to ship at great cost out of a gravity well that is wasted.
Propellant Settling: A Key Enabler to Microgravity Cryogenic Fluid Management
If you can provide a sufficient force to the fluid to force it to take an orientation of your choosing, many of these problems become much easier.
This is called “propellant settling”
Methods of propellant settling
Propulsive Settling (Apollo, Centaur, and others)
Diaphragms (common with storable propellants but some work has been done for cryo propellants)
Tethers and Gravity Gradients
Surface Tension Devices
Magnetic Propellant Positioning
If you can settle the propellant, propellant transfer and storage become very similar to terrestrial cryogenic handling.