From J. F. ALLEN
This rail gun thing (Technology, 17 July) reminded me of long ago in
Cambridge, late 1939 or early 1940, when a chap took over my research room
and built a small rail gun with rails a metre or so long, and firing pellets
of maybe 50 grams. I was annoyed because the pellets kept taking chips
off my nice panelled door.
He finally had a display for some top brass, which I attended, on a
common near Cambridge, with a bigger model and a projectile of perhaps 200
grams and a van full of capacitors. We stood respectfully well back, and,
on firing, the shot flew all of 7 metres, but the driving band-cum-armature
flew about 100 metres. I never saw him or heard of him again.
In 1945, in mid-July, shortly after V-E Day, I joined a group of three
or four others on a search in Germany for secret weapons. We landed on a
temporary air strip at Frankfurt and drove in an uncomfortable American
ammo lorry to Munich and on into the Bavarian Highlands. In a remote big
country house we found a small group with a larger rail gun with a projectile
of half a kilo or so, rails about 4 metres long, plus a large van filled
with several cubic metres of capacitors and a generator. It had never worked
because the armature always welded itself to the rails on passage of the
pulse of a million amps or so. The boss probably had friends in high places
and was put there to keep him out of the way.
Silly ideas often don’t die, they just fade for a few decades and then
come back as something new and wonderful. So we have the Kirkcudbright rail
gun firing a solid tungsten shot, not a shell, and with a muzzle velocity
of, say, Mach 10.
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Such a gun certainly has problems. The sliding armature attached to
the shot and bridging the rails must give a sliding contact both of low
friction and low electrical contact resistance. Difficult. I doubt if the
shot will be in flight long enough for air resistance to melt it, but it
must have fins to give it stable flight since it is nonrotating, and these
could easily melt.
A big problem is the huge volume of capacitors required for energy storage,
and also the electrical generator. I doubt if it will ever be possible
to squeeze it into a tank.
From my experience, I would not put any money on the gun’s success,
but it will undoubtedly keep the team of scientists and engineers in hot
dinners for some years.
J. F. Allen University of St Andrews
