What do you think is the best method to transfer Methalox fuel from one ship to another in space?Some options to separate liquid from gas in the tank could be: spinning ships, accelerating ships, making use of a gravity gradient induced by a planet? magnetic force (liquid Oxygen is attracted by magnets), creating an internal vortex, etc.
Do you envision the rotation to settle the fuel at the bottom or at the side of the rocket?
The paper chose rotation as the method for settling propellants, but now I would favor acceleration.
Two cyndrelical rockets docking belly to belly, then introducing a minor (sub-1 RPM) rotation to settle the tanks. Within a quarter rotation, all propellant in both tanks should be interacting with a tank wall and gaining the effects of centrifical force, pooling on the side of the tanks opposide the docking port. Then just pump fuel and oxyser into the opposite pressure vessel.
Quote from: rakaydos on 08/05/2017 05:32 pmTwo cyndrelical rockets docking belly to belly, then introducing a minor (sub-1 RPM) rotation to settle the tanks. Within a quarter rotation, all propellant in both tanks should be interacting with a tank wall and gaining the effects of centrifical force, pooling on the side of the tanks opposide the docking port. Then just pump fuel and oxyser into the opposite pressure vessel.But then you have a shifting center of mass. Eventually you have a mostly full spacecraft and a mostly empty spacecraft docked together with a center of mass mostly in the full one. It might work but it might put a big strain on the docking interface.
Rotation could be a little tricky when your center of mass is constantly changing.
But then you have a shifting center of mass. Eventually you have a mostly full spacecraft and a mostly empty spacecraft docked together with a center of mass mostly in the full one. It might work but it might put a big strain on the docking interface.
Quote from: Ionmars on 08/05/2017 04:03 pmThe paper chose rotation as the method for settling propellants, but now I would favor acceleration.If you're already planning to be accelerating towards a destination then this method would make a lot of sense. And it's not like rotational methods don't use any fuel, since they have to start the rotation of the two spacecraft at the very least, and depending on the fueling methods and docking systems they might have to de-rotate before undocking too. Spinning up and spinning down would consume some of the fuel that otherwise could have gone towards acceleration.I'm not sure what the percentage of use cases are where that would be preferable, but it's worth remembering as an option.Nice job on the AIAA paper too - it's one thing to talk about something on a forum, but it's a whole lot harder to write a paper with the intent to be published on AIAA.
Quote from: Coastal Ron on 08/05/2017 07:19 pmQuote from: Ionmars on 08/05/2017 04:03 pmThe paper chose rotation as the method for settling propellants, but now I would favor acceleration.If you're already planning to be accelerating towards a destination then this method would make a lot of sense. And it's not like rotational methods don't use any fuel, since they have to start the rotation of the two spacecraft at the very least, and depending on the fueling methods and docking systems they might have to de-rotate before undocking too. Spinning up and spinning down would consume some of the fuel that otherwise could have gone towards acceleration.I'm not sure what the percentage of use cases are where that would be preferable, but it's worth remembering as an option.Nice job on the AIAA paper too - it's one thing to talk about something on a forum, but it's a whole lot harder to write a paper with the intent to be published on AIAA.I agree with your analysis. In the case of the large depot, it consists of a lightweight framework with up to six spaceships attached to it, all in parallel with one another. Imagine another spacecraft approaching from "behind," intending to attach itself to an empty berth (parking space). As it approaches, it sees the framework and ships (the depot) as revolving around their common center. In order to park, it needs to match the motion of the empty berth. So it begins to spiral towards the depot whereby the radius of the spiral is a little greater than the radius of the spinning berth. Its rotational speed also matches the rotational speed of the depot. It comes to hover just "above" the empty berth. Now it must also revolve on its own axis slowly, so that its dorsal surface with fuel connectors is always facing the berth. Then it can connect to the berth framework.This maneuvering is a complex choreography using GNC thrusters. More important, it requires energy (fuel) that could be applied to a better use. So I now think it would be better to not rotate the depot, but allow spaceships to approach and park in a simpler manner. When it is time to transfer fuel, a slight acceleration (10E-5 g) would be applied to the depot, including the ship providing fuel and the ship receiving fuel. The direction of acceleration is important because we want to steer the depot towards a higher orbit that contributes toward TMI for all ships going to Mars. The others (presumably tankers) would return to Earth for reuse.
......Another method could be to temporarily rotate the fuel inside the large depot tank with a small electric motor connected to a mechanical stirrer inside the tank.If there's no rotation of the fuel needed, the rotational energy could be converted back to electric energy trough regenerative braking.
Quote from: Peter.Colin on 08/05/2017 08:49 pm......Another method could be to temporarily rotate the fuel inside the large depot tank with a small electric motor connected to a mechanical stirrer inside the tank.If there's no rotation of the fuel needed, the rotational energy could be converted back to electric energy trough regenerative braking.It is not intuitively obvious to me that this would work. The interior motor would begin a rotational motion of the fluid, but it seems to me that its energetic action would also cause some droplets to spray into the empty portion of the tank, adding to fuel dispersion as well as conglomeration.
I believe the easiest and least complex way to do this is by designing the tanker with a separate tank for filling the spacecraft (ie not shared with propulsion tank). This tank is made of some ALU compound and is shaped like a cylinder with a piston inside the tank, not pressurised and with some kind of drivebar in the middle for driving the piston. Motor and mechanics are sealed inside the tank, eg no movable parts that go into the tank. Piston ring seals around and a pressure valve for backfilling the other side of the piston with gas to prevent under-pressure.The methane is easy as it doesn't burn without oxygen (like a car gasoline pump floating in petrol). The oxygen is perhaps a bit harder due to fire hazard.The vehicle being filled requires no such device.Then it's a simple matter of matching orbit, connect a hose and squeezing the content over to the other vehicle.I believe rotation or acceleration will complicate matters more than wished.Edit: Actually a much simpler design would simply be to leave the piston loose. Just push it down using pressurised methane or oxygen in gas form from the rear side (from boil-off). No center pole or motor needed.