From: Kent Nordstrom <knord@q...>
Date: Fri, 17 Sep 1999 10:27:00 -0400
Subject: [increasingly OT] How much is a kT? (and some RG stuff)
In response to a long series of posts about railguns, which has come down to a discussion of how much energy a kiloton of TNT is, Robert Smith writes: > Just to be pedantic (hey, it's a Friday), it's worth stating that the Well, I'll match my slow-Friday pedantry against anyone's. A kiloton is a metric unit which is defined as 10^12 calories (a nice, round number). Convert it to Joules, and you get the 4.18 x 10^12 figure recently mentioned in this thread. The exact amount of energy a kg of TNT gives off when it goes *pop* varies from about 980-1100 calories, depending on things like the charge's precise density, temperature, and confinement. As a result, you can plug in either a short ton (2000 pounds) or a metric ton (1000kg) and get a value around 10^12 per kiloton depending on what you choose for how energetic TNT is. And in any case, the "kiloton" is already a metric unit of measure, so it should suffice admirably for our discussions of a future, all-metric world of the 2100s. (And I think the FT background of the US rejoined with Britain is about the only way SAE units of measure will ever go away on this side of the pond). A comment on targetting: some have mentioned that even with a slug moving at a good fraction of c, it'll take several seconds to hit a moving target a long range. Well, the hit numbers for a railgun *do* go down with range in FT! Also, consider that the RGs are probably firing a burst of projectiles (or at least firing steadily over the 10-20 minutes a FT turn lasts), probably with many different aimpoints selected by the targetting AI to hit as many possible trajectories of the target as it can. And as previous folks' figures have shown, you don't need to hit with too many of those projectiles moving at.1c to cause some big hurt... A comment on RGs: building something that can accelerate even a 10g mass to 0.1c would be pretty long and heavy (which is probably which they're mostly 1-arc weapons...). Something more reasonable might be a RG with heavier, slower projectiles and a higher ROF, hoping to solve it's targetting troubles by putting a cloud of metal where some part of it'll intersect the target's path. 1kg projectiles going at 2x10^6 m/s (a bit less than 0.01c) would have 1kT of kinetic energy. If 1" is a million km, it'd take around 2 minutes for a projectile to get to max range of 30". Well, you're only hitting on a "6" anyhow, and 2 minutes is still short compared to the length of a turn (you can fire a heck of a lot of projectiles over a whole turn... and you can fit plenty of 1kg slugs (a metal cube about 5cm a side, since metals have densities around 10... smaller if you make 'em out of something really dense) on a big ol' KV tub). (Yet another side note: you'd want the projectiles to be very precisely machined... at least massing very close to some standard amount. Any variation in weight would be a variation in velocity (although the F/C computer might be able to compensate a little bit as it fires the RG), which would be a big deal in determining whether you hit that millions-of-km-away target...). Another important consideration is barrel wear. A big problem in current RG research is that the rails take a heck of a lot of abuse from the projectile going out so damned fast. The slug isn't levitated in the middle of the barrel like most folks' ideas of a gauss gun. An electric current passes from one rail to the other through the projectile to make it go (well, through a plasma just behind the projecile). If your projectile is ripping down the barrel at 0.1c, you're going to be changing a lot of barrels! If I were a KV ordie, I'd much rather have RGs shooting at 0.01c--they wouldn't wear out as often. (To add yet another gratuitous parenthetical, if 1" is some smaller distance, like thousands of km, it makes even more sense to have RGs fire slower, heavier projectiles. At 1"=1000km, it takes a 0.01c projectile only about a second to get to maximum range. Now we can assume a much slower rate of fire for added drama, "Railgun capacitors still charging! We can open fire on the Hu'Man in 5 minutes!"). Of course, one can always apply appropriate PSB of super-tough conductors or magno-plasma isolation or whatever to prevent barrels from wearing out in spite of relativistic muzzle velocities. (I almost wrote "mango-plasma isolation" above... who knows what uses fruits will have in the weaponry of the future? We've already got the weenie gun, after all...) Keep 'Em Flying,