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Magnetic Propulsion Pellet Gun Achieves 20km/s
"Researchers at Sandia National Laboratories have used their Z Accelerator, a magnetic accelerator used in equations of state research, to accelerate pellets to speeds of about 20 km/s (about twenty times faster than a high-powered rifle bullet). Full story is here." Uses projected for this technology include simulating the impact of space junk, and, Yes, as the core of a hyper-velocity weapon.
(I've been forwarded a copy of the research paper, so here's how this thing really works.)
Ok. The Journal of Applied Physics article (Feb. 2001) does not describe this as a "gun" at all.
The device uses the Z-machine current source to send a large amount of current through two concentric pipes that are connected at one end. Current goes up the outer pipe and down the inner one.
This sets up a very strong magnetic field between the pipes, which pushes the two pipes apart (crushing the inner one and pushing outwards on the outer one). This is the same kind of effect that you get in a loop of wire that carries current (motor principle).
Samples on plates are stuck to the sides of the outer pipe. Magnetic forces accelerate these plates outwards rapidly, and the samples deform. This deformation is measured, giving a lot of useful materials information (the purpose of the experiment).
It's unclear from the article whether the plates, the samples, or neither go flying. The velocity quote is probably just the maximum velocity achieved while the pipe is expanding outwards under pressure. Letting the plates fly would give a somewhat better experiment, but would cause practical problems (they'd destroy whatever part of the machine they finally smacked into).
Physics-wise, this works on exactly the same principles as a railgun (motor principle with DC current). It's optimized for pressure experiments, not for firing projectiles; you could probably build a railgun that was more efficient at the second task.
Inductive effects do occur (this is a short current pulse), but are considered a source of error in the experiment, and so presumably aren't the dominant effect.
This or any of a number of other magnetic cannon techniques could work, or even a compressed gas gun (like the "Super Gun" from years ago).
The problems are twofold:
To reach orbit, you need an energy of about 30 MJ/kg (for a velocity of about 8 km/sec). Energy is force (mass times acceleration) times distance, so for a given acceleration, you can calculate the barrel length of the gun required. For a 3g accelerator for humans, you get a gun 1000 km long. If you can overcome the engineering problems and build a gun that accelerates cargo at *3000* gravities... you still have a gun 1 km long (about 0.62 miles, if you were wondering).
This can be built, but it's *expensive*. There would have to be a very strong incentive to build it, and amortizing the cost of the gun will raise your launch costs by quite a bit.
You'll notice that the experiment discussed in the article was done in vacuum.
Your gun will have to have an enclosed barrel (even if it's a magnetic gun). Otherwise, the mach-16 sonic boom will tear the firing coils off their mountings or otherwise harm the gun's structure.
Your gun (and anything near it!) is still going to have a lot of wear from the effects of a hypersonic projectile smacking into the atmosphere on leaving its muzzle.
Lastly, your projectile is going to have to have shielding around it to keep it from burning up while leaving the atmosphere. This will add weight to the projectile, and require a higher muzzle velocity (because the projectile will slow down).
All of these problems are manageable, but they increase the cost of the gun and the cost per kilogram of payload even more.
In summary, yes, we can scale up this and other guns like it; however, they're a royal pain to build and use, and there isn't enough demand at present to make them practical.
OTOH, this is one of the best ways known to get material off the moon. Required energy (and hence barrel length) is much lower, and there's no atmosphere to cause problems.
The range on this thing wouldn't be stupendous, I bet. Particularly with small particles (ones with a low kinetic energy/cross sectional area) aerodynamic drag is going to really kill the velocity quickly. At these velocities, you have to play fun games with the shape of the projectile to cool the thing, so it doesn't just ablate away.
My aerodynamics prof at UT Austin was working on a railgun project for the Army at Balcones a couple years ago. He brought in some videos of the experiments they did with projectile shaping. Basically, the idea was to make a circular indentation in the front of the projectile (think like a hollow-point bullet) whose depth was tuned to the expected velocity of the projectile. (insert obscenely complex mathematics here) The idea was to set up a standing pressure wave in the nose cavity, so the moving air would cool the projectile. The tests I saw were with a plastic blank, but the full-on projectiles were going to be 2" diameter tungsten rods. I can't even really imagine enough heat to ablate a tungsten rod, but damn if that's not just what happens at those velocities.
The computer models I watched of these rods impacting armor plate were amazing. It looks just like those slo-mo milk drop photos, except instead of milk it's molten steel. Wow.
Why yes, I AM a rocket scientist!
About time?!
What percent of the federal budget do you think is spent on 'defense'?
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Soma: because a gramme is better than a damn.
Now I know why there is an energy crisis in California.
BTW, this DOESN'T sound like a rail gun. A rail gun uses a series of magnets to propel a projectile. This sounds like it uses one rapidly expanding magnetic field.