[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
4.184
>GJ value is the energy released by one _short_ ton (2000lb) of TNT...
>[...and in the future we'd be using metric tons.]

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,

Kent