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This topic is inspired by the work of GW Johnson.
It is offered in contrast to:
Movable Partition Cryogenic Liquid Pumping Method by tahanson43206
The rotating method described by GW Johnson would take advantage of centrifugal force to cause cryogenic liquids to adhere to the inner wall of a rotating tank.
Because the liquid is collected on the inner wall of the delivery tank using this method, it can be collected by suitable pumping mechanisms, and pushed toward a customer tank.
The Rotating Tank Cryogenic Liquid Pumping Method has challenges, which members of the forum are welcome to address.
A preliminary list would include:
1) The delivery tank and the customer tank must be perfectly symmetrical and able to rotate without wobble.
2) The rotation operation will take place in open space, near a space station where the rotating delivery tank is based.
3) The space station must have positive control of the rotating systems at all times
4) Fail-safe methods must be in place to insure the rotating equipment does not suddenly accelerate into the station, or into nearby customer vessels.
5) The time required for completion of propellant transfer will include:
a) time to fill the delivery tank with cryogenic liquid
b) time to dock the customer vessel to the delivery tank including securing the liquid transfer portal
c) time to release the delivery tank and attached customer vessel from the station and move to a safe distance
d) time to accelerate the combined systems around the longitudinal axis and to correct for any wobble that may occur
e) time for the transfer of liquid from delivery tank to customer tank, while maintaining perfect symmetry in both vehicles
f) time to return the connected systems to the docking port on the station
g) time to disconnect the customer vessel from the delivery tank
This process must be repeated for each cryogenic liquid to be transferred.
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Even if the propellant tank was perfectly balanced when manufactured, which can be surprisingly difficult to actually do, there is no ability to rotate a partially filled propellant tank containing a liquid, without any wobble-inducing sloshing. By definition of what you're doing, it's going to be at least partially imbalanced during propellant transfer. We're talking about a million kilograms of propellant. Applying a relatively minor amount of centrifugal force / artificial gravity is going to generate a very large moment of inertia when the masses involved are so great.
Someone needs to do back-of-envelope moment calculations to evaluate how large the forces can become, and if this is practical to do. For example, compute the moments for 1/10th the force of gravity to determine how great the stress will be on the bearings for specific "out-of-balance" conditions, to see if this is practical. We can damp-out some level of imbalance using heavy masses attached to external hydro-pneumatic pistons that vary the position of the weight.
The "Fluidampr" is an example of such a system attached to high performance or heavy duty engines. However, this tech has an associated non-trivial mass penalty at the scale required. It is relatively simple, long-lasting, and it works well enough. The "Fluidampr" system is a completely sealed flywheel-like weight "floating" in liquid Silicone. To reduce engine damaging vibration ("wobble"), it shifts position inside the housing to damp-out vibration as heavy components move around inside the engine at high speed, creating out-of-balance conditions.
We may need to use gas (highly pressurized Argon) vs Silicone for this system. We will obviously have to change the size of the device and mass involved. It needs to be "super-sized" for this application. It may be beneficial to use Tungsten or Depleted Uranium as the moveable mass inside the housing. Tungsten is big money. DU is much cheaper. The housing needs to be some kind of steel that is little affected by gross temperature swings, such as Titanium or stainless. Aluminum will suffer from fatigue failures, which is why it's not used. CFRP would be the absolute lightest solution for the housing material, but again, big money (might still be worth the cost). CFRP is the least temperature affected material to use. Titanium and stainless are all going to expand / contract more, but they're easier to test and don't require molding equipment.
Housing material and lifespan requires a trade study and FEA to "math-out" all the relevant factors affecting the long-term durability of the solution. Durability matters greatly here, because safety-of-flight and the ability to operate for many years and survive many spin-up / spin-down cycles for propellant transfer is a design consideration.
I would opine that the aerospace majors will have an easier time with tooling for machining metal vs molding and curing a composite (will be far less expensive to make / iterate one-off metal vs composite housings), especially since we're unlikely to mass-manufacture this component. We require 1 or perhaps 2 propellant depots containing a small number of very large propellant tanks. Perhaps the lowest orbit depot should be for a LOX-only depot (the majority of the propellant mass for use with any kind of fuel, from LH2 to RP-1), whereas a modestly higher orbit fuel depot will contain the fuel (LH2, LCH4, RP-1, etc).
I would further opine that electromagnets are quite suitable for inducing rotation (for filling and draining) and LOX tank wall propellant adhesion for the LOX depot, due to the strong paramagnetism associated with LOX. All common hydrocarbon fuels, and Hydrogen itself, are only weakly diamagnetic, so they're much poorer candidates for the use of electromagnetism to induce adhesion and flow.
Mass, complexity, and longevity are all significant factors. The propellant tanks will likely be stainless.
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