I believe this (mating tanks given multiple missions) is what they were considering. Imagine you have your reusable falcon 9 boost into orbit a 'fuel only' second stage. Land the first stage, put another one on it, and send it up again. Once you have your 'boost pack' up in orbit, presumably docking with each other into a single unit, you launch your crew which attaches to the 'tug' that takes them to the moon.

ULA has a great writeup of how you might do this if you have on orbit refueling capabilities.

And on the one hand it seems like total science fiction but on the other, Elon is talking about next year.

This can be applied to much more than just fuel, mind you. Interplanetary manned vehicles might very well end up looking like space stations, with a bunch of modules docked together in order to provide the necessary creature comforts to sustain a crew for months on end.

In fact, a permanently-in-orbit transfer vehicle might make sense here; have this vehicle run the actual Earth<->Mars transfers, then just shuttle in fuel/supplies/crew on each Earth arrival.

Either way, a Falcon-like reusable launch system makes plenty of sense here in terms of making this sort of thing possible.

> a permanently-in-orbit transfer vehicle might make sense here

Buzz Aldrin is a big proponent of this approach: https://buzzaldrin.com/space-vision/rocket_science/aldrin-ma...

Jet fighters routinely use so much fuel taking off that the first thing they do after gaining altitude is refuel. Rockets can use the same idea.

Yes but as the ULA whitepaper pointed out there are some challenges to doing so in orbit. For one, the lack of gravity makes it hard to pump fuel. You can spin the tanker to achieve fuel settling but now you are both spinning bodies. Then there is handling cryogenic propellants in space, you get nominal bleed off from warming and again ULA had a really interesting design of an internal combustion engine using the bleed off of Hydrogen and Oxygen as a chiller pump. But suffice it to say, its not as easy as it is flying a jet behind a slow re-purposed jetliner, and doing that is already difficult.

> the lack of gravity makes it hard to pump fuel

Evidently this problem is solved, because liquid fuel rockets have been working fine in space since the 60s. (Liquid fuel rockets have their propellants pumped into the chamber.)

> nominal bleed off from warming

Launch the tanker just beforehand. There won't be time for the fuel to bleed off.

Liquid fuel rockets use Surface tension against the slosh baffles to hold fuel in partially full fuel tanks in place.

Ullage motors are used to settle the fuel at the bottom of the tanks, so the main motor pumps can get to it.

Once the main motor starts providing acceleration, the ullage motors are no longer required.

For cross tanking, keeping the ullage motors running for long enough to transfer all the fuel, without the main motors running might be prohibitive.

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

I didn't know about ullage motors, thanks for the link. The link also says that only very tiny accelerations are needed - so this shouldn't be a big problem for a tanker. Like I hypothesized, the problem has been solved.

Bearing in mind that the ullage motors need to be of a type that won't suffer from ullage problems. That implies they cant use the main motor fuel.

The saturn 5 used small solid rockets as ullage motors. Alternatively you could use externally pressurised bladder tanks for a range of non-cryogenic fuels. Cold gas or possibly h2o2 as a monopropellant come to mind. Either option has a comparatively low specific impulse, so even running them at very low overall accelerations for extended time periods is likely to be cost-prohibitive.

I like the suggestion elsewhere of spinning the tank. You could also dock, then spin the whole combination ship. If you abandon the fuel station idea, you could just take the extra fuel tank with you, transferring fuel to your internal tank during a main engine burn, then abandoning the empty tank in a wierd orbit.

it's only been solved if it's demonstrated, which it wasn't.

Do the fuels have to remain in a cryogenic state in space? Can much larger containers with warmer fuel be used once the fuel is in orbit? I have no idea if this makes any sense.

A major problem is pressure. As the fuel heats up and boils off the pressure increases and the tanks have to bleed it off or they'll burst eventually. Also the engine design for cryogenic stages assume a liquid fuel and moving enough gaseous fuel and oxidizer isn't in the current designs.

That's a good question. A big bag of fuel hanging outside is a fine place to store it if you don't care about temperature or pressure. You would need some kind of mechanical means to squish the bag to get the fuel out though, so maybe an accordion with a motorized retractor?

I think you are underestimating expansion ratios (ratio of volume of liquid and gas at reasonable pressure). The ratio for water vapour at sea level is ~1000x. The "large bag" might end up being so large that it's weight is a significant fraction of the weight of the fuel it contains.

Even if you brought up a bag large enough to contain gaseous fuel and oxydizer, before you could actuall use the fuel you'd have to compress it again.

This is partly because they don't always take off with full tanks which allows for shorter runways and/or safer takeoff. Depending on the ordinance that a jet may have to carry, it starts making more sense to stay a bit lighter until a minimum airspeed is achieved.

The idea of docking modular components in space is a good one though. Perhaps all you really need to do is lift a bunch of fuel tanks since there is plenty of time to perform burns with smaller boosters on longer distance trips.

The SR71 literally leaked fuel until it reached operating temperature. It had something crazy like 2 minutes to rendevous with a tanker after takeoff.

Using "milch cows" seems like a cost effective solution to fueling/supplying a moon run. Of course, the green-eyeshade crews will get to have the last say.

It's not as advantageous as you'd expect, because your ships now have to have docking collars extra sensors, and extra maneuvering capability. The rendezvous will take extra time and fuel, and the extra time means you need more life support for the crew.

Worse, the extra steps involved create extra risk, both for mission failure and to the lives of the crew. You could get to your fuel depot and discover the battery sytem has failed and your depot is tumbling, or a leak caused ice buildup that prevents you from docking. Or the chillers failed and enough oxydizer got vented into space that you can't continue the mission.