Orbital refuelling

[pavelcherepan]

This has puzzled me for a while - why haven't space refueling and orbital fuel depots become widespread? It seems like a no-brainer, especially for long missions and putting satellites on GSO. Also for deep space manned exploration it should be much easier to send fuel and the craft separately using a couple smaller rockets rather than a proposed monstrosity of SLS (in a sense that it's huge and super-duper expensive).

[swansont]

What kind of fuel? Volatile things tend to degrade fairly readily.

 

Why would two smaller rockets be better than one big one?

[pavelcherepan]

What kind of fuel? Volatile things tend to degrade fairly readily.

 

Why would two smaller rockets be better than one big one?

 

Cheaper for one. A large rocket like planned SLS works on the very edge of technology and everything in it is extremely expensive. For example, Falcon 9 costs ~$70 million per launch and can take ~13 tonnes to LEO. SLS is predicted to have a launch cost anywhere from $500 million and up to several billion. We can safely assume that the lowest estimate is for the lowest payload version that can take about 70 tonnes to LEO. Then using 5 launches of Falcon 9 we can send the same amount of cargo at a cost of some $350 million, which is only 70% of the cost of SLS launch.

 

Additionally, smaller rockets like Falcon 9 or Soyuz or Ariane 5 are built in relatively large numbers which lowers the cost of any single vehicle. SLS, on the other hand is likely to be relatively rare (there are not many uses for a launcher with such a great payload) and that will keep prices up. Falcon 9 also have recently finally managed to safely return the first stage back and if that becomes a norm it will drop prices by a lot.

 

Also a larger rocket will either experience higher losses due to drag (if accelerating fast) or higher gravitational losses (if accelerating slower). It will probably have to use a less energy efficient trajectory with a later gravity turn compared to smaller rockets, which can start turning very early on.

 

As far as boil-off of cryogenic fuels is concerned, some tank designs allow very low rates of boil-off of like 0.13% per day for LH₂ and as low as 0.016% per day for LOX. Obviously, there is option of using non-cryogenic fuels like rocket-grade kerosene or hydrazine.

 

http://www.sciencedirect.com/science/article/pii/0360319988900559

https://tfaws.nasa.gov/TFAWS01/NASA/13Spacecr/dHULL.PDF

 

EDIT: Also, if you're going on a long mission, such as a trip to Mars or outer Solar system, you wouldn't use cryogenic fuels anyway, because if you travel to Mars in 6 months then by the time you get there even with best tanks available you'd lose somewhere between 22-30% of your original H_2.

Edited December 26, 2015 by pavelcherepan

[swansont]

You still need the big rocket if you're going beyond LEO, and I thought that was the point of this exercise. You aren't going to send a Falcon 9 to Mars.

 

Kerosene freezes at -40 ºC. Not the best thing to leave out in some orbiting fuel depot. Also, how much fuel will you waste getting into the same orbit and docking with the depot?

 

The most straightforward way I can think of to make this useful is to have an entire stage of a rocket waiting in orbit, so you can dock with it and fire its engines. Which means you need a rocket capable of lifting that stage into orbit.

[Greg H.]

Realistically, large scale orbital facilities like this aren't going to become wide spread until orbital manufacturing using materials gathered from space becomes a thing. That significantly lowers the cost because you don't have to spend all that energy climbing up out of the Earth's gravity well for the structural pieces - you just ship up the electronics and specialized equipment, and build the heavy bits in situ.

[Tsiolkovsky]

Part of it is probably that we aren't doing enough in space at this moment to provide the motivation for building those types of things. Hopefully that changes soon.

[pavelcherepan]

You still need the big rocket if you're going beyond LEO, and I thought that was the point of this exercise. You aren't going to send a Falcon 9 to Mars.

 

 

I agree, but why can't we make space exploration vehicles modular so that they can dock on LEO, load fuel on LEO and then go on their way? Also, even if we want to use a big rocket to lift the actual spacecraft we can still use smaller rockets to launch fuel for it, that means that instead of launching something like Block 2 SLS with predicted launch costs in excess of $1 billion, one could launch the fuel using commercial rockets and then launch the spacecraft using a cheaper Block 1B. For the price of extra $500+ million that is supposed to be the difference between Block 1B and Block 2, you could launch in excess of 100 tonnes of fuel into orbit using commercial launchers.

 

 

 

Also, how much fuel will you waste getting into the same orbit and docking with the depot?

 

Soyuz spacecraft have delta-v between 250-390 m/s depending on the version and they dock with ISS with plenty of fuel to spare (or at least enough for later de-orbit). Found some hard numbers - for Soyuz TMA-19 the entire rendezvous required ~66.1 m/s of delta-v and there-s no hard numbers for the docking but they had to change relative velocity from ~45 m/s to 0 so around 60 m/s probably. So all in all, about 120 m/s for the entire rendezvous and docking, which is not much given that for something like a return mission to Mars you'd need in excess of 10000 m/s of delta-v.

 

http://spaceflight101.com/soyuz-tma-19m/soyuz-tma-19m-flight-profile/

 

 

 

Kerosene freezes at -40 ºC. Not the best thing to leave out in some orbiting fuel depot.

 

First of all, it's in vacuum which is not very conductive to heat, it's in insulated tank and would probably have solar cells and that way you could use electricity to keep it from freezing. In any case, if you're going on a mission somewhere you'd still have to take fuel with you and you'd still need to make sure that it doesn't boil off or freeze during the course of the mission, and as a result these engineering issues will have to be solved anyway, so I am confused as to why you think this is so important.

 

Also, wouldn't overheating be a bigger issue than the possibility of fuel freezing?

 

 

 

Realistically, large scale orbital facilities like this aren't going to become wide spread until orbital manufacturing using materials gathered from space becomes a thing. That significantly lowers the cost because you don't have to spend all that energy climbing up out of the Earth's gravity well for the structural pieces - you just ship up the electronics and specialized equipment, and build the heavy bits in situ.

 

Or vice versa. Having functioning fuel depots in orbit will allow for cheaper space exploration, mining and manufacturing, which in turn would lower the costs of creating and supplying more fuel depots at various orbits and ultimately would lower the price even more. At the same time, fuel tanks for non-cryogenic fuels like RP1 are pretty lightweight (compared to hydrogen tanks) and so you won't be carrying a lot of dead weight from the gravity well, most of the weight will be in very useful fuel.

 

EDIT: Back to kerosene. Even if we send a tank with 10 tonnes of kerosene into space and this tank has no insulation, is completely transparent to infrared radiation and completely ignore irradiation by the Sun, we will still end with a total cooling time to freezing point of at least 1.19 days by Stefan-Boltzmann law. Obviously, with all those other factors in place it will take much-much longer.

 

http://www.wolframalpha.com/input/?i=%282.84*10%5E28*1.38*10%5E-23%29*%28233%5E-3-273%5E-3%29%2F%282*1*5.67*10%5E-8*60*60*24%29

Edited December 27, 2015 by pavelcherepan

[swansont]

 

 

 

EDIT: Back to kerosene. Even if we send a tank with 10 tonnes of kerosene into space and this tank has no insulation, is completely transparent to infrared radiation and completely ignore irradiation by the Sun, we will still end with a total cooling time to freezing point of at least 1.19 days by Stefan-Boltzmann law. Obviously, with all those other factors in place it will take much-much longer.

 

 

If this plan is going to work, you aren't going to launch the fuel a couple of days before the main launch. It's going to be much earlier.

[pavelcherepan]

 

If this plan is going to work, you aren't going to launch the fuel a couple of days before the main launch. It's going to be much earlier.

 

As I said I was describing absolutely a worst-case scenario. And also I will repeat my question, since you might have missed it - wouldn't overheating be a bigger issue than the possibility of fuel freezing?

 

Again, let's look at numbers. Say, we send 10 tonnes of kerosene in a cylindrical tank 2 meters in diameter. It would be 3.89 m long approximately and will have a surface area of 30.72 m². Say, when the tank is on the sunny side of the Earth, some 30% of it's area is effectively exposed to solar irradiation (best-case scenario). Then, given that solar power in the vicinity of the Earth is 1367 W/m² we have some 13998.08 W of solar irradiation that the tank is exposed to.

 

By Stefan-Boltzmann law, using a temperature of the fuel at 273 K and full area of 30.72 m² we have 9675.09 W of radiative power that the tank is emitting (in case of no insulation at all). If it's on a 500-km circular orbit with a period of 1.57 hours, Earth's shadow is something around 2.36 radians, or ~0.376 of the total orbital period.

 

Then, if we maintain orientation of the tank so that it gets the maximum amount of the sunlight at all time when not in the shadow for one orbit we get a total power:

 

P_T = (1-0.376)*13998.08 - 1*9675.09 = 8734.8 - 9675.09 = -940.29 W/orbit or -598.91 W/hr

 

And that amount of power will only allow us to cool down our 10000 kg of kerosene by less than 0.1 ^oC/hr so then it will take some 400 hours or 16 days for it to freeze. And again, this calculation is for the case with no insulation. The real tank will obviously be built with some proper multi-layer insulation and then heat losses will be negligible.

Edited December 28, 2015 by pavelcherepan

[swansont]

Fuel depots (i.e. plural) won't all be at that orbit, and insulation isn't magic. It impedes heat output, but it also impedes heat input. Maintaining a certain orientation implies fuel use.

 

Watts per hour?

[pavelcherepan]

Fuel depots (i.e. plural) won't all be at that orbit

 

I'm not quite sure why this is important. Could you please elaborate?

 

 

and insulation isn't magic. It impedes heat output, but it also impedes heat input.

 

Yes, but if we have a proper multi-layer insulation it will, say, reduce both input and output heat by a factor of 20 (for example). Ratio of input/output heat will stay the same, but the actual amount of heat lost will be lowered by that same factor, wouldn't it?

 

 

Watts per hour?

 

Oooops! W*hr of course.

 

Anyway, once again I want to bring up the fact that kerosene was just an example and regardless of which fuel is used in depot it will require exactly the same handling to prevent freezing/boil-off as it will when it's on board spacecraft on a potentially very long mission. If you're going to Mars, it doesn't matter whether you use fuel depots or go via conventional means, you'd still have to make sure that your fuel is still in working condition by the time you get there and will need to perform maneuvers for capture, and landing, and trip back. And in this case we're talking months. Since long-term missions work and new ones are being prepared, you can safely assume that there is an engineering solution to this issue that you seem so focused on.

 

Anyway, apart from fuel freezing or flying away, are there any major issues with fuel depot strategy?

 

I didn't start this topic to promote any agenda, rather the contrary - I believe that in NASA and other space exploration organisations there are thousands of people who are heaps smarter than me and they are all don't seem to be planning on using any fuel stations, although on paper the idea seems rather good. Hence the question, why don't they?

[swansont]

 

I'm not quite sure why this is important. Could you please elaborate?

 

 

The inverse square law of the sun's output. If you are going to put fueling depots along the path of a rocket, some are going to be far from earth.

 

But again, you will have to accelerate to dock with them, and then accelerate to continue on your way, so I wonder how much fuel this wastes and how much that reduces your proposed cost savings.

[pavelcherepan]

 

If you are going to put fueling depots along the path of a rocket, some are going to be far from earth.

 

But again, you will have to accelerate to dock with them, and then accelerate to continue on your way, so I wonder how much fuel this wastes and how much that reduces your proposed cost savings.

 

Not quite. There's no need to put depots along the path of the rocket. Just at LEO or some place nearby so that we can lift lighter, empty rocket into space, fuel it up there and send it on it's way. A cost difference in launch costs between a launcher that can lift 70 tonnes and a 100 tonnes is huge, based on the numbers I've seen. At the same time lifting 30 tonnes of fuel into orbit using commercial launchers would set you back probably around $100 million. It would still be cheaper than building an enormous rocket to do it in one go.

 

I've already spoken about rendezvous fuel costs in post #7. It's not much considering that you have lighter spacecraft to begin with (very little fuel) and the delta-v numbers are very small compared to what you'd need in total for a Mars mission for example.

 

 

The inverse square law of the sun's output.

 

It doesn't matter. Spacecraft all over the place use engines after extremely long travel times and fuel does not freeze. Take Curiosity landing or Rosetta's capture maneuvers as examples. And in this example Rosetta uses liquid fuel MMH+nitrogen tetroxide and it hasn't frozen in 11 years!

 

It's just a matter of choosing correct fuel for the mission and engineering tweaks to make sure it stays in shape. If done properly fuel never freezes so it's not an issue here.

Edited December 28, 2015 by pavelcherepan

[Enthalpy]

Fuel Propellant depots are a well-studied option, with some (one?) strong advocates at Nasa.

 

They permit to split the launch mass, which can be useful by itself:

• If the existing launchers are too small, it may sometimes avoid to develop a new one;
• But if a new launcher is necessary anyway, a bigger one isn't much more difficult, and developing costs more than recurrent manufacturing.
• In my scenario for a manned Mars mission, I split a trip over three SLS-class launches, so a Falcon wouldn't have coped with the needs.
  http://www.scienceforums.net/topic/83289-manned-mars-mission/

They offer advantages beyond splitting the mass:

• If a manned mission must travel faster than the economical (say Hohmann) path, then presetting heavy hardware (like fuel) by a slow and economical means saves much launch mass.
• Newer propulsion, like ionic or my sunheat engine
  http://www.scienceforums.net/topic/76627-solar-thermal-rocket/
  save much launch mass over chemical propulsion but their faint thrust takes more time to reach a location, say a high orbit. They can serve to preset fuel for a crew, say to land on the Moon and come back.
• If you produce propellant in-situ, say oxygen by electrolysis of Mars' atmosphere or Moon's regolith or an asteroid's ice, you have to store it. Storing liquid oxygen in Lunar orbit or at a Lagrange point would reduce the launch mass (but cost??) because it's easier to put there starting from the Moon, and it can serve to send a vessel or craft that dives to and accelerates near Earth to take advantage of the Oberth effect
  https://en.wikipedia.org/wiki/Oberth_effect
  you might consider doing the same at Mars' moons.

Whether you store only the fuel or as well the tank, the engines, the vessel or craft - that depends on mission needs and choices.Moving propellants between tanks hasn't been done in space up to now, but I suppose we need only to decide it. I've no firm opinion about swapping the tank instead of moving the liquid.

 

----------

 

Nitrogen tetroxide and hydrazine and its derivates are stored in space since ever, despite hydrazine freezes so easily. Solar-powered resistors keep it warm. Though, we want to get rid of these toxic and not so efficient propellants.

 

We can insulate a tank well
http://www.scienceforums.net/topic/60359-extruded-rocket-structure/page-2#entry761740(and nearby messages)

and this suffices to store hydrogen for a week or if needed a month, enough to reach the geosynchronous orbit directly or for a short manned Moon mission. Easy, light, cheap.

 

We can design cold tanks by making them white or putting sunshades. This suffices at Sun-Earth distance to store passively oxygen, methane, cyclopropane and spiropentane, farnesane and most propellants. It doesn't suffice for hydrogen, but hydrogen is very much necessary, say to put craft in orbit around Uranus and Neptune, and to fuel my sunheat engine.

 

To my eye, active cooling is a better choice. It works for hydrogen too, near Venus and Mercury as well, and more easily on low orbit around the lukewarm Earth. A big tank needs only 100W cold. The cooler can consume very little of the propellants or run with sunlight. Again, it needs only to be decided

http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051

(mess there, some relates to a generator, some (May 17, 2010) with a cryocooler using the same hardware)

with redundancy I'd rely on active cooling.

 

Oxygen is the first and necessary step to avoid toxic propellants and improve efficiency, so storage must focus on cold propellants. Hypergolic propellants aren't necessary at attitude thrusters, say for a lander, as I describe an igniter there

http://forum.nasaspaceflight.com/index.php?topic=27308.0on 01/13/2012

 

I consider long-term storage as a fundamental enabling technology for space exploration, both of oxygen and hydrogen (hence active), and it's easy.

[pavelcherepan]

Thanks Enthalpy! I was hoping you'd join the discussion.

 

I totally agree with all your points, but then there is the question - if it's really such a good idea and it's relatively easy to accomplish, why after 55+ years of space exploration still no one uses propellant depots, nor are there any plans to create any (at least not as far as I know)?

[rangerx]

Space Shuttle Endeavor (STS-57) conducted what was called: Super Fluid Helium On Orbit Transfer (SHOOT) experiment to investigate resupply of liquid helium containers in space.

 

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880018700.pdf

 

I gather the main problem they had to overcome was the managing the residual volume of gas in each the donor and recipient containers. In micro gravity, these mix rather than separate and give rise to cavitation.

 

Apparently it required a lot of different instrumentation to measure. Even then accuracy was an issue and they often depended upon estimations rather than hard data.

 

The test concluded by suggesting it's mission goals were "to verify components and techniques that have not been demonstrated in space before and cannot be demonstrated on the ground".

 

That said, we need a lot more missions just to test this, no less perform it in a reliable or viable manner. I suppose once we figure out where we want to go into deep space we'll try harder, but in the meanwhile it seems rather expensive and complicated to learn something in the absence of an objective. Even something as routine as resupplying the space station is not without it's perils and cost.

 

It's a terrific topic for discussion, though. Thank you for the OP.

Added note: by "these mix", I meant to say liquid and gas, not both containers.

[pavelcherepan]

Space Shuttle Endeavor (STS-57) conducted what was called: Super Fluid Helium On Orbit Transfer (SHOOT) experiment to investigate resupply of liquid helium containers in space.

 

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880018700.pdf

 

 

Thanks for your link. It was an interesting read, although I might say that it might not be 100% relevant. Obviously helium can't be used as a propellant and none of the propellants used nowadays are ever in superfluid state. I guess, this experiment was more focusing on transferring helium from one spacecraft to another to be used as a coolant.

 

Had a look around internets and, well there's been plenty of small-scale orbital refueling done and in many cases these were not experiments, it was done on a regular basis, for example, soviet Salyut 6 and Mir space stations had attitude control engines that were regularly refueled by Progress spacecraft, then there was Orbital Express test by DARPA and NASA and there are two other missions in the works - both like Orbital Express aimed at extending the service life of satellites.

 

https://en.wikipedia.org/wiki/Salyut_6

http://www.hightechscience.org/mir_podu_refueling_control_panel.htm

https://en.wikipedia.org/wiki/Orbital_Express

https://en.wikipedia.org/wiki/Space_Infrastructure_Servicing

https://en.wikipedia.org/wiki/Mission_Extension_Vehicle

 

So you see, it has been done with liquid fuels many times on relatively small scale, but I can't see that there would be any major issues with scaling it up.

[rangerx]

 

So you see, it has been done with liquid fuels many times on relatively small scale, but I can't see that there would be any major issues with scaling it up.

 

Thank you for your response and links.

 

I agree the Soviets and subsequent Russians became adept at refueling the orbital station Mir in micro gravity and yes it was small scale from Progress to the end use.

 

What was the propellant tinvolved? Hydrazine?

 

It's already been discussed, but without considering the cost of things I'm inclined to think the moon is a better platform for up-scaled storage and transfer depots. Would you agree some gravity is better than micro gravity?

 

It seems a lot simpler than dealing with shielding, heating, cooling, instrumentation and other physical boundaries. Then again, I supposed you'd have the added burden of building landers. In that vain, Space-X has been successful in re-entering a rocket recently. Would this be more practical than orbital stations?

 

As to micro meteriorites and other space debris, would the moon (or other planet) based operations be much safer than orbital stations in this regard?

 

I guess the next question would be the type of use? I suppose there are two. One being a single use craft sent in advance of the main launch which would rendevous along the way then be abandoned and the other being a service station for multiple spacecraft on an ongoing basis. The latter would undoubtedly be more elaborate than the former even though the internal mechanisms are the same.

 

Likewise, orbital objects require "launch windows". Would lunar launches have longer windows of opportunity?

 

Sorry to discuss with so many questions, but as an amateur space buff I find this topic intriguing.

[pavelcherepan]

It's already been discussed, but without considering the cost of things I'm inclined to think the moon is a better platform for up-scaled storage and transfer depots. Would you agree some gravity is better than micro gravity?

 

 

There would be advantages in having depots on the Moon, but all of these are completely negated by the prohibitive energy costs. Going LEO->surface of the Moon->Low Lunar Orbit requires ~7.8 km/s of delta-v and this is huge amount of energy. LEO to low-Mars orbit requires ~7.2 km/s. In essence if a spacecraft has to fuel up on the surface of the Moon it would have to carry the amount of fuel that can otherwise get it into Mars capture orbit. Lack of gravity only requires some rather simple engineering solutions, presence of gravity on the other hand comes with a lot of delta-v strings attached.

 

 

 

What was the propellant tinvolved? Hydrazine?

 

I believe it was UDMH and nitrogen teroxide:

 

http://spaceflight.nasa.gov/history/shuttle-mir/spacecraft/s-mir-detailed-main.htm

 

 

 

It seems a lot simpler than dealing with shielding, heating, cooling, instrumentation and other physical boundaries.

 

On the surface of the Moon you'd still need insulation, shielding and cooling, depending on the propellant of your choice.

 

 

As to micro meteriorites and other space debris, would the moon (or other planet) based operations be much safer than orbital stations in this regard?

 

As I see it, there wouldn't be a significant difference in micrometeorite danger between surface of the Moon and LEO simply because Moon has no atmosphere so there's nothing to prevent micrometeorites from hitting the depot on the surface.

 

 

 

I guess the next question would be the type of use? I suppose there are two. One being a single use craft sent in advance of the main launch which would rendevous along the way then be abandoned and the other being a service station for multiple spacecraft on an ongoing basis. The latter would undoubtedly be more elaborate than the former even though the internal mechanisms are the same.

 

In addition to the two you've mentioned I see a third option. A craft with fuel on board sent in advance of the main launch then later it docks with the main craft and accompanies it on it's trip. That way you don't need to carry empty fuel tanks on the main launch which will increase the launch mass and costs.

 

 

Likewise, orbital objects require "launch windows". Would lunar launches have longer windows of opportunity?

 

There will be more launch windows for a craft rendezvousing with the propellant depot in LEO compared to the Moon, because the orbital period of the depot is ~1.5-3 hours compared to 28 days. The lunar launch windows will likely be a bit longer. I can't say which one will come on top, probably LEO is still better here.

 

https://en.wikipedia.org/wiki/Launch_window

Edited January 11, 2016 by pavelcherepan

[rangerx]

Fascinating, really.

 

Without straying from the topic, I'll get back to the point in a moment.

 

I'm an amateur radio operator and spend much of my time monitoring and communicating through space objects, including Mir before it was re-entered and the ISS. I was particularly glued to my equipment when the Soviet Union fell and Cosmonaut Sergei Krikalev (U5MIR) was forced to a longer duration flight. I have several recordings, where he (and other cosmonauts) would speak candidly about about the government emergencies and his orders from flight controllers. In one instance he was talking about growing crystals in metallic ampules which results would "be investigated on earth" but went on to mention he was having some trouble with his oven, so he was starting new experiments. Hydrodynamics.

 

http://wh03.droa.com/~c1706149/audio/U5mir.mp3

 

Do you think this would have been about fuel transfers?

[Enthalpy]

Resupplying Mir and Saliout: up to now I believed they docked a (full) Progress or Soyuz to the stations, didn't transfer any propellant, and let the Progress or Soyuz with its own engines push the station. Is there a source that indicates a propellants transfer, in which case I'd be happy to change my belief?

 

Moon's surface: it offers no protection against micrometeorites as there is no atmosphere. A very strong disadvantage of the Moon are its extreme ground temperatures, from >+100°C during the day to -200°C during the long night. That's only the ground temperature, from which you can insulate a craft during the day and adjust the sunlight absorption and heat emission - but night is damned difficult as you have no light. You need your own heat source, or a huge and compact tank (and already full!), very well insulated, to avoid freezing after 2 weeks without sunlight.

 

Apollo missions stayed there only during daytime, Yutu carried a radioisotope heat generator (but failed at night I believe), and my scenario to bring lunar samples back

http://www.scienceforums.net/topic/85103-mission-to-bring-back-moon-samples/

is temporarily stopped at this difficulty. There are solutions without plutonium, but they impose much of the craft's design, so they must be decided very early in the design.

 

In orbit as opposed, you have sunlight to adjust a craft's temperature, and eclipses last for 1/2h or 1h, not 2 weeks.

 

So in my eyes, propellants on the Moon's surface are right only if one has to go there anyway and needs them there. Even to go to Mars by producing oxygen on the Moon (provided this is cheaper, not just lighter), it's better to store the lunar propellant in lunar orbit, Earth orbit or at a Lagrange point, because the propellants to go there from the Moon's surface is already used, so the remaining tank is smaller hence lighter.

 

Though, I've already been wrong on the tricky subject of propellant depots, so maybe some smart scenario makes use on an other advantage I haven't seen.

 

Transferring propellants: superfluid helium was looking for trouble the big way. It's already counter-intuitive and difficult on Earth with a human operator. Sound fluids like oxygen and kerosene or spiropentane are muuuuuch easier. To separate the liquid from the gas (which isn't always the vapour! It can be nitrogen, helium...), I suppose one simple means would be a sort of centrifugal impeller that puts the liquid against the shell at the pipe's sucking end. Maybe the pipe's end can move through the tank to seek the liquid were it is, or we can make a gentle blow of the gas through the tank - that's nearly a vacuum cleaner. Or we let the whole tank rotate, if it's a mere depot, or let both craft accelerate.

 

What about a fabric or a sponge that goes through the almost emtpy cylindrical tank to catch the liquid? It's not tight, so it can be thin metal fibres, easy at cold.

 

I wouldn't have a membrane between the liquid and the gas, because I suppose it's impossible with hydrogen, quite difficult with oxygen, and heavy in any case.

[pavelcherepan]

Resupplying Mir and Saliout: up to now I believed they docked a (full) Progress or Soyuz to the stations, didn't transfer any propellant, and let the Progress or Soyuz with its own engines push the station. Is there a source that indicates a propellants transfer, in which case I'd be happy to change my belief?

 

There's a quote from wiki about Salyut station:

 

 

The rearward of the two ports was fitted with plumbing to allow the station to be refueled by unmanned Progress spacecraft. These freighters, which brought supplies and extra equipment to keep the station replenished, helped ensure that the crew were always able to carry out useful scientific work aboard the station. In all, twelve Progress flights delivered over 20 tonnes of equipment, supplies and fuel.

 

Also I can't find a source for the propellant transfer for Mir station but I see that it had a fuel transfer control panel:

 

http://www.hightechscience.org/mir_podu_refueling_control_panel.htm

Edited January 13, 2016 by pavelcherepan