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How to build railguns and coilguns, continued...
Why EML might make space launch cheap and easy
 

Conventional guns and rockets both use chemical reactions to propel them. The basic idea of either an explosive or a rocket propellant (which is sort of like a controlled explosion) is to make some chemicals burn into hot gasses. These gasses expand and it is the force of this expansion that propels a rocket, bullet or cannon shell.

railgun launch to space
Figure 1: EML launch to space. Artwork courtesy SAIC.

The trouble with using hot gas to make something go fast is that gas can only expand as fast as the speed of sound in that gas. It can't expand at a supersonic speed. For the hot gasses that propel artillery shells, even the ones with the highest speed of sound that are used with artillery shells can only go about 1.5 km/s (5,400 km/hr or 3,240 miler per hour). This is the muzzle velocity of the M1 Abrams tank gun. It's a lot better than the sspeed of sound of plain old air, but way short of hypervelocity.

Rockets go faster than the speed of their exhaust gasses by carrying the fuel to carry the fuel and so on. However, means the faster you want a rocket to go, the amount of fuel it needs grows exponentially.

This explains why rockets can that achieve low speeds are cheap and easy; but space launch is staggeringly difficult. If not for Pioneer X’s gravity assist from Jupiter, it would never have achieved the trajectory that today is carrying it out of the solar system. If a rocket couldn't make gravity assists to escape the solar system, it would have to be approximately 98% fuel. This is impossible because the rocket would look like a balloon and be too weak to get up to to solar system escapoe velocity, which is 16 km/s (57,600 km per hour). Throwing away stages helps, but not much.

By contrast, when you shoot a projectile out of a gun, it doesn't have to burn fuel to carry any fuel with it. This means it takes far less fuel, only that needed to generate the electricity foor the launch.

OK, this does create a slight problem. if you use electromagnetism to shoot something out a gun at 57,600 km per hour, well, that's a good way to get mashed flat from the acceleration. I sure wouldn't want to go from sitting still to 57,600 km/hr in a fraction of a second. However, we don't have to mash people flat when they fly into space from an electromagnetic gun. Unlike a gun powered by an explosion, an electromagnetic gun doesn't have to come up to speed in an instant. We could build a really long electromagnetic gun, maybe hundreds of km long, and gradually pour on the speed.

Meanwhile, from the mid-1970s on, as other EML researchers were wrestling with and failing to solve the problems of achieving hypervelocity coil guns, Rod Hyde of Lawrence Livermore Laboratory was calculating what he could do presuming that other people resolved the “just a matter of engineering” details.

Elevators into Space

Hyde’s Space Fountain is a case in point. A coil gun would fire conducting rings from the surface of the Earth up through an evacuated “beanstalk” to a semi-orbiting platform which would deflect the stream back down, forming a continuous loop. The idea is to use the kinetic energy of this coil gun to hold up a space platform against gravity, and then hang an elevator from the platform. , The advantage over an elevator anchored in geosynchronous orbit is that Hyde’s stalk doesn’t need to be constructed from unobtanium. Space Fountain made Hyde a sensation among science fiction fandom, but the EML community mostly ignored him.

Of course there are other ways to run an elevator up and down a geosynchronous beanstalk. For example, a simple linear synchronous motor, sort of like a Maglev that goes stright up. As with elevators, a counterweight could take most of the force -- just a matter of engineering (groan). For more on space elevators and a solution to unobtanium, see “A Hoist to the Heavens” in the Aug. 2005 IEEE Spectrum. You can read here about a scale model Space Elevator that just might scale up -- just a matter or engineering.

If you want to watch a competition among space elevator builders yourself, keep an eye on the X-Prize website for their next competition. Their 2007 event was lots of fun, if you like to see things blowing up and crashing. They allow spectators -- all you need is nerves of steel!

That's why many researchers have concentrated on how to create a beanstalk's unobtanium. The reason we call it unobtainium is that we don't knwo how to obtain it today, but maybe tomorrow! Candidate materials that are strong enough and light enough to hang all the way to Earth from geosynchronous orbit without breaking from its own weight include diamond fibers and buckytubes (also called nanotubes).

Other researchers look to more conventional EML launch to space. Could it achieve the $25 per pound to orbit that NASA once thought it could deliver by 2000? Miles Palmer, who ran the gun launch to space studies for SDIO’s Brilliant Pebbles, once favored railguns. Today he is bullish today on coil guns -- perhaps something like a super long maglev train. “A superconducting system has the best chance,” Palmer says, but such a project might cost “$20 to $50 billion.”

Palmer says that commercial producers of high temperature superconductors project that in a few years they will be able to manufacture wires that, according to his calculations, could even power the coils of a maglev capable of Earth-to-space launch. He would avoid the switching and stability problems of other coilguns by gradually accelerating the payload, but notes this would require a track tens of km long. He says it would take a maglev track of 100 km to launch people if they could tolerate 15 gees of acceleration. For the less physically fit, a peak load of 3 gees would require about 500 km of track.

However, the U.S. Air Force Office of Scientific Research recently chose instead to give the IAT a contract to develop a space launch railgun that would use a plasma for its armature.


“Beanstalks,” a chapter from Indistinguishable from Magic, by Robert L. Forward (Baen, 1995).

"Earthbreak: Earth to Space Transportation," by Rod Hyde, Defense Science 2003+ Vol. 4, No. 4, 1985, pp. 78-92.

 

 
       © 2013 Carolyn Meinel