High speed projectiles

Tony Hayes:

>I've been reading the posts on hurling various things at other things
>as an attack, interesting but problematic.  There seem to be two
>classes of objects to be thrown, kg sized at moderate velocities (10's
>of km/sec) and gm sized at fractional light velocities (10,000's of
>km/sec).  There would then be two classes of targets, ships and
>planets (space stations are so small compared to planets that they
>will be treated as ships).

Actually, there are *four* classes:
1. Large & slow (asteroid?)
	mass: million of tons and up
	speed: 1-50 km/sec
	delivery system: requires installing ship class drives on a
		suitable "rock" and using *lots* of fuel
	detection: easily detected by a medium tech level civilization
		(but *we* would be in trouble!)
	defense: difficult to destroy, you tend to get a *nasty* meteor
		shower. Deflection is easier *if* time permits.
		Atmosphere is not a defense!

2. medium and fast (ship?)
	mass: hundreds of tons
	speed: 50-500 km/sec
	delivery system: Requires an expendable ship, and a lot of
		fuel. Maybe drop tanks and an AI?
	detection: normal
	defense: somewhat more difficult than against a normal ship

3. small and fast
	mass: tens of kg
	speed: 10-100 kn/sec
	delivery system: Requires a mass driver we could build *now*.
		And which would fit in a medium size ship
		(disassembled). You set it up on a convenient asteriod
		or build extra bracing and attach it to a comet
		nucleues (free fuel!)
	detection: *very* difficult. An 8 kg steel projectile occupies
		approximately one liter. that's a ridiculously small
		cross section for most detector systems. And this thing
		will be at ambient temperature, and could easily have
		an anti-radar coating.
	defense: unlikely. This will hit as hard as a large missile.
		And the odds of a *natural* impact in this range is so
		rare that non-military vessels and installations won't
		be armored against it.  (note that in the (low* end of
		this speed range a *paint flake* blasted a 1-cm pit
		into a shuttle window). Planetary atmosphere should be
		a decent defense, but not guaranteed. (They'd be like
		an artillery barrage if they hit the ground intact,
		which *is* doable!)

4. very small and oh-my-god!
	mass: grams
	speed: 1000+ km/sec
	delivery system: requires one *hell* of a mass driver or
		railgun, but possibly doable even now. Maybe
		installable in a large ship.
	detection: impossible in any practical sense.
	defense: impossible in any practical sense. Atmosphere is a
		defense except for projectiles at a high fraction of
		c. They will not only cause blast effects, but massive
		radiation problems. (at above 1/10th c this is
		effectively one *big* cosmic ray, and will generate a
		shower of secondaries that will be lethal on the
		surface!)

Adrian Hurt:
>Just how much acceleration can a good mass-driver deliver?  If the
ship is
>on the order of 100's of metres long and the mass-driver is spinally
mounted
>(so it is as long as the ship), then the acceleration needed is on the
order
>of 10**11 m/s**2.

I don't have any figures handy. I *may* try bothering a casual
acquaintance at Lawrence Livermore Labs who has worked on railguns and
massdrivers. I've seen an (unused) test projectile. A 1 cm x 1 cm x
.4 cm  block of glass reinforced by some sort of fibers (boron?). They
don't use steel because it *flows* at these accelerations... (If I
remember something from an unrelated bit of trivia properly, this means
they must be doing better than 10^7 gs, as a 10^7 g centrifuge exists
and is built of steel...)

I *think* they were getting 10-100 kn/s out of something less than 10
meters long. And dangerous velocities out of a *one* meter unit.

Any materials sciense folks out there care to look up what pressure it
takes to make steel flow noticeably in *milliseconds*?

The real trick is that a railgun *eats* power. All that kinetic energy
has to be fed into the gun as power. On the other hand, it's very
eficient at making the conversion.