Friday, November 07, 2008


Researching the metal bismuth, I found myself sidetracked to ultra-high power electromagnets.  Producing fields of up to 60 Tesla[1] and popular for, among other things, frog levitation (don't ask -- oh, okay), they are the strongest electromagnets made; it turns out above a certain flux density, superconductivity ceases and there you are, back in room-temperature physics again.

     The gadgets are Bitter Electromagnets, named after the inventor, Francis Bitter.  How interesting that a solenoid design[2] that can handle very large energies, suitable (perhaps) for a coilgun or Gauss rifle, would be named "bitter."

     Resistive heating of the electromagnet is one issue with such projectile weapons; it is possible the liquid-cooled Bitter design could be scaled down and use thermosiphoning or even be adapted for air cooling.  That leaves magnetic saturation of the thrown slug itself as the last big issue to cling to.  (Energy source?  Look to your cordless drill's battery packs, already working much like a magazine!)

     Lead, brass and powder is just one way.  There are others. 

     (If you think this is wild, wait'll you see electric rocket engines!)
1. I am reliably informed this is not the magnetic force required to compress 60 Teslas down to the volume normally occupied by one Tesla.  'Kay?
2. While the classic large design uses a series of stacked plates (overlapping and interleaved conductor and insulator) in the form of single turns of a helix, I wonder if one might machine the thing from a billet of copper and a chunk of PTFE or similar, perhaps with the longitudinal cooling and compression-bolt holes predrilled.  Then again, the gain in energy handling might not be worth it -- see "magnetic saturation."


Anonymous said...

My fantasy has been to build a gun that shoots a blob of ionized metal. A small (or not so small depending on the magnitude of your power supply) piece of metal foil is vaporized by the current which continues to flow through the plasma. The current through the plasma in a the magnetic field exerts a force on the plasma propelling it out the muzzle. Under the right conditions (MASSIVE high voltage capacitors) velocities in the neighborhood of 20 km/s have been achieved (this was a military contract in the mid-80s).

Even if a handheld version could only achieve one km/s it would have it's uses.

Alas, I have the interest, the skill set, and the money to explore such a device but I just don't have the time. Or if I made the time I wouldn't have the money.

Tam said...


You have to promise that it will have an iridium barrel, to keep us David Drake fans happy.

alan said...

It always comes down to energy density.

But I'd be all over a plasma pistol.

Then I could light my patio torches from 50 feet away.

(Extra points awarded for those who get the literary reference)

The Captain said...

This is one of the geekier posts I've ever started my day with.

J.R.Shirley said...

I really, truly *need* a rail gun.

Anonymous said...

Tam, there are seven David Drake books available at Which one(s) should I start with?

As for the iridium barrel... I'd make it out of whatever was the best value. But for you, if iridium wasn't the best choice of material, I'd plate the barrel with iridium.

Anonymous said...

I... I think I need a cigarette now.

Word ver: tackin. "Well, I was out tackin' prarie dogs with my bitter when outta nowhere comes..."

BobG said...

"I wonder if one might machine the thing from a billet of copper and a chunk of PTFE or similar, perhaps with the longitudinal cooling and compression-bolt holes predrilled."

Not a good idea. I used to work with high density magnetic fields when I worked with large motors and generators (one of my last jobs was repairing and rewinding an exciter for Hoover Dam), and one of the biggest problems we had was with eddy currents heating up sections of the stack where there had been hot spots burning the insulation, welding of the laminations from blown windings, or specks of copper imbedded in the stack. We used to calculate the saturation point (this was back in the seventies, when we had to use slide rules), set up a magnetic field, and see if there were hot spots. Sometimes you would see areas go immediately cherry red, other times you looked at it through an IR viewer.

Anonymous said...

I keep telling people. We need to de-mothball the Iowas and put nuclear reactors in 'em.

BBNs. With rail guns. You know it must be done.

Roberta X said...

Bob, I'm thinking of very scaled-down versions, cooled by longitudinal circulation of a nonconducting media through the flat, edge-wound coil, replacing the conventional small solenoid coils spaced along the acceleration path of the projectile. Better cooling would do more than allow maximum useful flux density: it would support a higher sustained rate of fire.

I'm sure somebody's done the analysis: given the basic principle, there's probably an optimal projectile mass, velocity (ooo, caliber wars -- again) and rate of fire somewhere between a nasty small-cal buzzgun that behaves more like a ray gun up to an electomagnetic repeating shotgun.

I don't even have a good handle on the scale here -- the mental image that started me musing pushes the (first) coil so hard it gets treated like cartridge brass: one use and out. If they carried the projectile as well, you'd have a very bullet-like thingie. But single-coil designs are generally puny. (This also points down another path: EMP grenades. Now there's good clean honest fun!)

PDB's giant railguns on ballteships is a whole other school of thought -- one that gets even meaner of they're plasma railguns. I think we'd want skywriting drones, too. ("It says 'Surrender Dorothy,' Great Leader...")

Turk Turon said...

I want a plasma gun that can create skywriting from the ground by means of focussing energy at high altitude, creating a vacuum, water vapor rushes in and freezes, causing white writing to appear. Make a full-color version and you could "sky-paint".

Captcha: synglay

Anonymous said...

Roberta - why did you have to point this out? Now you got me eyeing the Clausing and thinking "Will I burn down the house"?

And PTFE is a nono - good for chemical resistance(They make high purity acid pumps from it), and low friction, but for this, you'd want something with high temp resistance, good mechanical strength(something PTFE lacks) and good electrical resitance. Or you could skip the plastic alltogether, and use refractory.

PDB: We shouldn't bother. What we should do now that they're enlarging the locks on the Panama Canal, is dust off the Plans for the Montana class, and update 'em for rail guns and fission reactors. 12 big guns beats 9, and it's alot easier than tearing a ship completely apart and then putting them back together again. :)