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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."
Rather than
Advantages: putting the heavy lifting on the booster on Earth (where logistics is easier), don't waste energy stopping/pausing and restarting the trajectory.
Disadvantages: You better be sure you can refuel in flight.
The raw materials available for building components in space via NEOs (near Earth objects) are orders of magnitude greater than they are on earth. The problems lie more in the host of technologies that do not yet exist for harvesting, transforming, and manufacturing in space. There are further logistical issues regarding getting to the materials. Flying out to an asteroid isn't cheap, neither is returning with the goods.
All that said, the rewards for conquering these technological hurdles is mind boggling. To date we've only been getting our toes wet with respect to researching technology leading to the industrialization of space. Because of which much of this seems more sci-fi than anything. The short-term thinking majority can't conceive of any kind of substantial future in space. But they are the same kind of visionless people that wouldn't have bothered trying to industrialize Earth because it was too hard with solutions difficult to imagine. Explaining our vision for humanity in space to such people is like trying to explain your vision for creating what we know today as a smartphone to a pre-industrialization era person. All you'd get is mockery and ridicule about your pie in the sky, day dreaming flights of fancy.
Two of my imaginary friends reproduced once
Is it actually cheaper to send up a bunch of smaller rockets with fuel as payload than it is to simply send a bigger rocket with enough fuel on it?
Yes.
Flight rate is generally more important to launch costs than size. A small rocket you can fly a thousand times a year will cost you far less than a big one you fly once a year simply because you can mass produce them and reuse them several times before you throw them away, rather than custom-building a new one every time you launch it.
If I remember correctly, the plans for reusing Saturn V stages made no financial sense until it was flying about once a month, for example; at NASA's actual launch rates the savings from reusing them would be less than the costs of developing the technology to do so.
A further issue is that by splitting your hardware and fuel across multiple launches, one exploding rocket doesn't lose your entire multi-billion dollar Mars mission. A near guarantee of losing one payload out of a hundred launches is likely to be better than a 1% chance of losing the entire thing.
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.
You pretty much nailed it all on the head. The only thing that I wanted to add was that there has been one probe to move between two massive bodies (Earth and the Moon) using a continuous thrust system: the SMART-1 probe with its Ion engine. The downside: it took 13 months (it only took the Apollo astronauts a couple days) and used a series of really strange, constantly expanding orbits (basically a spiral), on the plus side it only took 1/10th of the total propellant mass that a chemically powered spacecraft would.
Ion/Hall/Plasma thrusters are great for moving cargo where you don't care too much about how long it takes (especially in the beginning of the mission). This type of technology could easily be used to move fuel to one of these "Gas Stations" in earth, moon, sun, or mars orbit. You could start this years before the need date (before you get busy testing out the manned space craft) then the chemical fuel could already be there when you're ready to launch the manned space craft.