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Fuel weight is a major limitation on the range of manned spacecraft.
It might make sense to pre-position fuel tanks (establish depot/depots) in orbit with different planets and moons to serve as docking stations to refuel manned craft.
Are there any plans/studies on establishing depots in space?
Yes. This is considered in human interplanetary missions in the form of pre-placement of chemical rocket stages by solar-electric propulsion (SEP) systems. SEP is very efficient, but slow, whereas chemical rockets are inefficient, but provide fast transit times. You want fast transit times for humans to minimize radiation problems and to reduce the required supplies. SEP lets you get the chemical stages out there efficiently to wait for the humans to go use them.
As for why stages, a rocket engine is a small fraction of the mass of a stage. So if you're going to pre-position propellants, you might as well attach an engine to the tanks to have a ready-to-go stage.
The chemical propellants considered are usually cryogenic, which adds the difficulty of keeping the propellant cold. There are technology development efforts underway to address that problem.
Landau and Strange found that pre-positioning chemical stages in high-Earth orbit actually gets the propellant most of the way anywhere energetically, is convenient for stable storage and access (with the help of our Moon), and the propellant can be used very efficiently during a perigee pass close to Earth.
There are lots of studies. Are any serious? Probably not at this time. That is, nothing is funded. The US is unlikely to do so, due to the fixation on the SLS which is basically a design that says "We don't need no stinkin depots". The Russians seem to have no great ambitions at this time. The Chinese are not yet at a sufficient state to consider this. Probably no one else in the world has a sufficiently serious program to be worth considering.
That is the direct answer. As an aside, there are many benefits to depots.
One that is important is that it opens a market for competition. That is, if someone needs a depot for a mission (be it a government or a company) they have just created a market. They need something. They are willing to pay for it. Now, companies can compete to provide payload to it.
Depots are great examples for this, since really the cargo being launched is dirt cheap. LOX, LH2, Kerosene, or water or whatever. So any launcher can deliver it.
The cheapest one will win. Reliability interestingly is not that important if the payload is cheap and the launcher cost is cheap, and there is no immediate time dependency.
Thus if the US for example, eschewed the SLS approach and went with a depot approach some interesting things could happen.
They could stimulate an industry for launch. Doesn't matter if it takes 100 Falcon 1e launches to fill the depot, if they are cheap enough, and launch frequently enough. Nor does it matter if you fill the depot with a Delta 4-Heavy launch. What matters is cost, and ability to deliver in time.
I can think of three uses for fuel depots:
Prelaunched and placed on the surface of, or in the orbit around, the mission target, like Mars.
In geostationary orbit, in order to refill multiple satellites there.
The idea of having gas stations cruise around between different orbits cannot be economical. It takes too much fuel to alter orbital inclination in LEO or to reach different GPS-satellites. For such orbits refueling must be satellite specific. Even a placement in some orbit between Earth and Mars won't work, because if the launch is delayed by a day because of weather, it would miss the depot.
In geostationary orbit, however, the delta-v between different satellites might be small enough for one fuel depot to service multiple satellites which have been designed to be refueled. The gain would be the satellite owner's option to prolong the lifetime or not, depending on how the technical lifetime and competition and whatnot develops in the future. This could be commercially feasible, the other two alternatives would be for scientifical purposes and governmentally financed. #2 is described in NASA's Design Reference Mission 3.0 for Mars, but it is not on any schedule or budget. To some degree #1 is what is done with the ISS.
Lots of plans.
1) The Canadians are apparently working on a spacecraft that is part space tug, part robo-mechanic & part filling station:
After years of planning, Canadian company MacDonald Dettwiler and Associates (MDA) announced it is building the first space gas station, with plans for a 2015 launch. The Space Infrastructure Servicing vehicle will fly in geosynchronous orbit, where it can reach several key commercial and government satellites. It will be able to bring them extra fuel as well as reposition them or perform basic maintenance, according to MDA.
The communications satellite company Intelsat, which has the most geosynchronous satellites, will be the first client.
Early last year Dextre tested a package sent to the ISS with (it looks like) dozens of different satellite parts: http://www.spaceflight101.com/robotic-refueling-mission.html
2) Lots of plans and studies from NASA and others:
In August 2011, NASA made a significant contractual commitment to the development of propellant depot technology by funding four aerospace companies to "define demonstration missions that would validate the concept of storing cryogenic propellants in space to reduce the need for large launch vehicles for deep-space exploration." These study contracts for storing/transferring cryogenic propellants and cryogenic depots were signed with Analytical Mechanics Associates, Boeing, Lockheed Martin and Ball Aerospace. Each company will receive US$600,000 under the contract.
3) One of the main rationales behind John Hunter's light gas gun was to shoot fuel cylinders into orbit (with the help of a rocket assist near the top of the projectile's parabola). These would be collected at depots at a predetermined orbit and then be available to craft to refuel for injection burns and/or to satellites for enhanced stationkeeping.
Interestingly, a fuel depot is one of the objectives of the Lunar Space Elevator project that a company called LiftPort is pursuing.
The idea is that an elevator could be constructed from the lunar surface to EML1 using COTS technology available today. Using a solar-powered lifter to transit payloads from EML1 to the lunar surface and back solves the problem of the large ΔV required to soft land a payload on the lunar surface or return it to lunar orbit.
According to Michael Laine, the LiftPort CEO, lunar regolith could be processed into oxygen and hydrogen. He plans to extract that oxygen and hydrogen, then store it either at EML1, or at the Lunar Elevator counterweight, which would be 250,000 km from the lunar surface. At that point it could be used as fuel for any planetary or deep space mission.
I don't have the technical background to know how feasible this plan is and would enjoy hearing others' insights into this intriguing project.
