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NASA Looking To Build 'Gas' Stations In Space
coondoggie writes "Fuel is a major issue when it comes to long-duration spaceflights — its weight is a problem for launch and once a spacecraft runs out of fuel there's no place to get more. That's where in-space 'gas' stations located at strategic spots along a route would be a boon to spaceflight. Which is exactly what NASA is looking to do by beginning to solicit proposals for what it calls an In-Space Cryogenic Propellant Storage and Transfer Demonstration that will lay the groundwork for humans to safely reach multiple destinations, including the Moon, asteroids, Lagrange points and Mars."
You're misinterpreting how most current spaceflight is done. At present only asteroid/comet/deep space missions use any form of continuous thrust, in the form of low-thrust ion or Hall-effect thrusters. Anything to a major gravitational body will still rely primarily on high-thrust impulses from traditional chemical rocket motors. Though technology on the horizon may be changing that, it is the current state of affairs.
The path to Mars using chemical thrusters is very straightforward -- if you look up the Hohmann transfer, thats basically the way its done. Leave Earth orbit so that your sun centered orbit is elliptical and touches the Mars orbit. When you get to Mars speed up again to catch up (in practice you do a capture burn and do it in a frame where it looks like your slowing down, but nonetheless). If you want to be really clever sometimes you do a major maneuver in the middle to allow you smooth out some of the problems that occur because the planes of the orbits aren't quite the same. All throughout there you do small maneuvers to keep on course. If you want to go faster, you can do faster shorter transfers, but it requires bigger burns on both sides.
However, in order to do this with chemical thrusters, you need a lot of fuel. A 1500 kg probe requires an extra 1100 kg of fuel just for the catching up maneuver, and probably > 3000 kg for the departure burn (I don't have data on that at hand right now). Its logarithmic so if you wanted to get that probe back to Earth you'd have to bump those measures up by factor of 2 or 3. Throw in landing and departing the Martian surface and it just gets uglier. This is why a Mars Sample Return mission is so hard -- you just can't stack that much mass on top of a launch vehicle.
Imagine instead though, that you had a cheap way to get fuel to orbit. 'Space Guns' and other such ideas are primarily ridiculous because they apply 100s of Gs that would kill a person or most hardware. Fuel won't care though -- so use high-cost rockets to get the people and high-value equipment to orbit, fill up empty (expandable?) fuel tanks there with cheap fuel launches, and then get on your way. Maybe ship some more fuel to Mars, but I'm not sure the numbers make sense for that. However, you could definitely use this technology with technology to extract fuel from the Martian environment to make the return leg easier though.
Thats why fuel depots are interesting for space exploration.