(Believe it or not, I pulled this from a sample chapter from Indistinguishable from Magic on the Baen website. The bulk of the easy to find articles on the topic seem to end up citing that book and this paper. Who am I to break with convention?)
Properly speaking, I should have talked about Space Fountains before I talked about Launch Loops. The launch loop is essentially a refinement of the Space Fountain idea that‘s more focused on getting payloads up to speed than it is on maintaining a space elevator without the tether. But the Space Fountain ties in better with Mass Drivers I think, so it pairs up well with last week’s highlight.
The Space Fountain idea came out of frustration with the complete lack of suitable materials for building a Space Elevator. While I am personally more worried about maintenance costs and the creation of a massive earth orbit navigational hazard, many space elevator fans are convinced that the real problem with space elevators is that we literally do not have the technology to build one for Earth yet. I guess that’s a reasonable point.
But what if we built our space elevator and just left out the cable?
I’ll pretend I know what you’re thinking. The cable’s the thing that holds our counterweight station in place and it’s the thing that our payloads ride to get up to the station. The cable is more or less the entire point of the space elevator. Why would we even want to get rid of it? How could we?
Well why’s easy. Like I keep saying, we can’t build the cable. We’re still at the phase where the only materials we can make that are strong enough can at most be made in lengths best measured in centimeters when for a space elevator we really want them in lengths measured in megameters.
So we’re left with how. And the answer here, much like in most of life, is a gun.
Mass drivers are linear electric motors repurposed as guns. There’s two things about that that make them useful for our space fountain needs. First, when a motor pushes something the motor itself feels an equal and opposite push. Second, motors can act as generators by slowing things down to generate electricity rather than spending electricity to speed things up.
At the top a station can hover, even if it’s travelling at a speed below the orbital velocity for its altitude, by reflecting pellets or other masses that have been launched up to it from Earth.
In between the ground and the station a tower can be built with very weak materials. Instead of relying on its own strength, it uses mass drivers in the tower to lift itself by slowing pellets on the way up and speeding them up on the way down.
Payloads can either climb a tower that is supported by the pellets, or be launched as part of the pellet stream.
The whole thing sounds energy intensive, and it is, but essentially all the energy spent getting the pellets up to speed is recovered when they’re slowed down. The big energy cost is the overcoming friction and dealing with the inefficiencies of the mass drivers. And even friction can be mostly ignored if the tower is built in a way that maintains a vacuum along the entire projectile path.
So do I think it’s a good idea?
Oh god no.
Screw up your aim, lose power for too long, get a bit too much wind pushing against the tower, really any little thing going wrong means catastrophic failure.
Much like the Launch loop, even if things work perfectly the Space Fountain can never afford to lie idle. With expensive reusable rockets, or orbital airships, or hybrid air-breathing rockets, or lightcraft, or mass drivers, it’s bad news financially if you have to stick them in a warehouse for a quarter because you don’t have enough clients. With active structures like Launch Loops and Space Fountains, you can’t stick them in a warehouse. You need to keep them running all the time whether or not you have customers paying to use them. For that matter, once you consider erosion concerns from space debris, Space Elevators have the same problem to a certain degree.
The Space Fountain is a neat idea, and it could probably be made to work. I just don’t see it being worth the trouble.
