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From: rsiatl!jgd@gatech.edu (John G. De Armond)
Newsgroups: sci.military
Subject: Re: Particle Gun (was Particle Beam Gun)
Message-ID: <10620@cbnews.ATT.COM>
Date: 26 Oct 89 01:28:02 GMT
References: <10315@cbnews.ATT.COM> <10412@cbnews.ATT.COM> <10571@cbnews.ATT.COM>
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From: rsiatl!jgd@gatech.edu (John G. De Armond)

In article <10571@cbnews.ATT.COM> bnr-fos!.uucp!hwt@watmath.waterloo.edu (Henry Troup) writes:
>
>
>From: bnr-fos!.uucp!hwt@watmath.waterloo.edu (Henry Troup)
>
>Various objections were raised on the basis of resistance proportional to
>square of velocity, heating, etc.
>
>However, let's say I fire a pellet at .7c aimed at a target one kilometre
>away.  It takes .7 / 300,000 second to get there, or 2.3 microseconds.
>
>I don't think that's long enough for the pellet to vaporize; and it's 
>certainly not long enough for the vapor to spread much.  So I hit the 
>target with a very dense plasma 1cm across - that still has much of the
>starting energy.
> 

When I was in college in the early '70s, I worked on a NASA research 
project involving micrometeor impact.  We were chartered with developing
methods of stoping the particles.  A micrometeor is a particle in the
size range of sand traveling in the tens of thousands of feet per second.

We built a gun that would fire a small pellet about half the size of
a BB.  This pellet was capable of penetrating up to about 18" of steel
plate.  The hole was several inches in diameter and looked as if it had
been burned through.

Normally the tests were conducted in a vacuum.  High speed photographs
would show that the pellet was intact and spherical until the moment
of impact.  Upon impact, the pellet would turn to plasma and proceed
to burn a  hole in the steel.

Once, toward the end of the experiment, we fired a pellet with atmospheric
pressure in the test chamber.  The first thing we learned was that the
shock waves damaged our sensors :-)  The second thing we observed was that
by the time the pellet had traversed the span from the gun to the target
(about 3 feet), it had become a plasma and had lost a goodly proportion of
its velocity.  There was damage to the target but nowhere nearly as much
as in the vacuum.  The pellet quite possibly would have dissipated as
vapor if it had traveled across the room.  This does not bode well for
weapons applications.

Worse, the solution to stopping these particles proved to be very simple.
A composite consisting of several layers of paper and aluminum foil stopped
the particles completely.  The principle involved is that the outer layer
started the plasma process.  The paper served as an ablative substance
whose vaporization carried off energy.  The aluminum reflected this
heat outward.  Several layers are typically penetrated but a composite
of perhaps 30 layers, no more than a eight inch thick was completely
effective at stopping the particles.  This technology is why space
vehicles look like they are wrapped in gold foil. (for heat control too).

I don't think hypervelocity projectiles will ever be feasible for surface
combat.  I'm certainly no ballistitian but I suspect that there is 
an optimal combination of density and velocity for a given penetration
and range requirement.  

John