hwt@watmath.waterloo.edu (Henry Troup) (10/25/89)
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. A more serious objection is that this had better be the main gun on a tank, because the recoil is going to be large. If the pellet is .1 g, the recoil force is 30 MN. If the tank weighs 30 tonnes, it will acquire a velocity of .7 metre/second. Anyone want to fire one of these from their shoulder ? utgpu!bnr-vpa!bnr-fos!hwt%bmerh490 | BNR is not | All that evil requires hwt@bnr.ca (BITNET/NETNORTH) | responsible for | is that good men do (613) 765-2337 (Voice) | my opinions | nothing.
jgd@gatech.edu (John G. De Armond) (10/26/89)
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
raymond@ganymede.ame.arizona.edu (Raymond Man) (10/27/89)
From: raymond@ganymede.ame.arizona.edu (Raymond Man) Above a certain speed, nowhere comparable to speed of light, Reynolds number and Mach number will cease to have any significant meaning. The physics will no longer be of continum flow. Just call me `Man'. Uh-oh. I don't know. raymond@jupiter.ame.arizona.edu
d9bertil@dtek.chalmers.se (Bertil Jonell) (10/27/89)
From: Bertil Jonell <d9bertil@dtek.chalmers.se> > A more serious objection is that this had better be the main gun on a > tank, because the recoil is going to be large. If the pellet is .1 g, the > recoil force is 30 MN. If the tank weighs 30 tonnes, it will acquire a > velocity of .7 metre/second. The main problem is the acceleration that will work on the tank and its crew. In the above example (.1g pellet at .7c) that acceleration, even if you assume an extremely long barrle length, will be around 3 700 000 g's ! (Barrle length of 20 metres gives an flight time in the barrle of 19 microsecond ie the tank will accelerate to 0.7 m/s on 19 microseconds. thats bad for the health!) The energy required for this feat is about 2.2 TJ (Tera-joule). A quick calculation gives that this approximates the energy in an 0.5 kt warhead. ( aorund 4.2e9 J in one ton of TNT if my memory is correct) [mod.note: Paul F. Dietz (dietz@cs.rochester.edu) confirms this figure. - Bill ] And you got to pump all those 2.2 TJ into the gun (and the projectile) in just 19 microseconds!!! That will be around 1.2e20 Watts! An *astronomical* figure! This kind of guns is better adapted to fleet air defence: short flight time to target, great energy requirement and the large mass of naval vessels (to counter act recoil) all contribute to that. Also, Whats wrong with lesser speeds? a APFSDSDU at 10km/s will do damage enough to kill anything that moves on the ground. Relativistic speeds aren't necessary to make an "impression" on the target. (APFSDSDU (my resarvation for the order, could be APDSFSDU): Armor Piercing Fin Stabilized Discarding Sabot Depleted Uranium: Anti-armor munition) > Anyone want to fire one of these from their > shoulder ? Only if it was available in an recoilless configuration B^) > utgpu!bnr-vpa!bnr-fos!hwt%bmerh490 | BNR is not | All that evil requires > hwt@bnr.ca (BITNET/NETNORTH) | responsible for | is that good men do > (613) 765-2337 (Voice) | my opinions | nothing. -- Bertil K K Jonell @ Chalmers University of Technology, Gothenburg NET: d9bertil@dtek.chalmers.se VOICE: +46 31 723971 / +46 300 61004 "Don`t worry,I`ve got Pilot-7" SNAILMAIL: Box 154,S-43900 Onsala,SWEDEN (Famous last words) "There's Medellin. Drugs 'R Us, all in that one little city in the valley. One nuke could settle things..." Tom Clancy 'Clear and Present Danger'