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From: dietz%USC-CSE@USC-ECL.ARPA
Newsgroups: net.space
Subject: Orbital Artillery
Message-ID: <16648@sri-arpa.UUCP>
Date: Mon, 13-Feb-84 09:34:00 EST
Article-I.D.: sri-arpa.16648
Posted: Mon Feb 13 09:34:00 1984
Date-Received: Sat, 18-Feb-84 04:11:04 EST
Lines: 77

Watching news accounts of the New Jersey's shelling of Lebanon, it
occurs to me that this job could be done from space with mass drivers.

A kilogram of mass, falling to earth from infinity, has a kinetic
energy of over 62 million joules, about the same as the explosive
energy liberated by 15 kilograms of high explosive.  The energy from
geosynchronous orbit is almost as high.  To put this number in
perspective, 62 million joules is enough energy to lift 62 metric tons
of matter 100 meters, if it were to be converted into potential energy.
That 1 kilogram of mass will dig a big crater, assuming it survives the
atmosphere.

Mass drivers being discussed for lunar mining have a mass flow of about
10 kilogram/second at a velocity of about 2 km/sec, with mass being
accelerated in 1 kilogram chunks.  Orbital velocity at geosynchronous
orbit is about 3 km/sec, so a mass driver with an exhaust velocity of
around this much is needed to put mass onto trajectories intersecting
earth.

Assuming a 10 kg/sec mass flow, the geosynchronous mass driver can
deliver the equivalent of 150 kg of high explosive to the earth each
second, or .54 kilotons per hour.

In contrast, a shell from the New Jersey's 16" guns has a mass of about
a ton; around 500 shells were dumped on Lebanon in a 12 hour period.
(I'm not sure how many of these shells were 5" shells.)  This
comparison isn't really fair because a large number of small explosions
will do more damage than if the same explosive force is detonated in a
smaller number of large explosions; blast damage scales as the 2/3
power of explosion energy.  Taking this law at face value, and assuming
the New Jersey can fire one 16" shell every 30 seconds, the mass driver
has about 18 times the firepower.

At 11 km/sec, the ultrahypersonic projectiles will be in the atmosphere
for only a few seconds.  Some mass loss to ablation is acceptable. The
projectiles should be long and thin to minimize drag forces, yet should
be designed to fragment when a solid surface is encountered so their
energy is deposited near the surface.

>From 40,000 km away, accuracy is a problem.  Assuming an average
velocity of 5 km/sec, the projectiles will reach Earth in under three
hours.  This time can be reduced at the cost of a larger mass driver
and higher power requirements, but remember that projected mass drivers
for asteroidal mining have exhaust velocities of ~10 km/sec.  Engineers
working on lunar mass drivers are confident that lateral velocity
errors of only a few meters per hour are achievable, leading to an
error of perhaps 10 meters after three hours.  Longitudinal velocity
errors are harder to correct.  An error of 1 meter/second will cause
the projectile to arrive about 1 second off the target time; the
earth's motion will cause the target to move 450 meters during this
second.  This problem can probably be solved by arranging for
"achromatic" trajectories that focus projectiles of slightly varying
velocities.  Errors due to atmospheric forces will be neglible if the
projectiles enter head-on; maintaining projectile orientation could be
a problem (spin?  tail fins?).  All other errors (deviations from
symmetry in earth's geoid, lunar and solar gravity, magnetic fields,
light pressure) can be accounted for fairly precisely.

Assume that most of the projectiles land in 1 km x 1 km area.  During a
24 hour bombardment, some 864,000 projectiles will be launched, or one
projectile for every 1 or 2 square meters of target area.  Saturation
bombing indeed.

Mass for projectiles would most likely come from nickel-iron asteroids
(for density).  Retrieving the asteroid is no problem with the mass
driver assumed here.  The asteroids would also yield ballast mass to
anchor the mass driver in orbit against the reaction produced when the
it is fired, and to shield the mass driver against hostile attack.

Power requirements are not excessive.  Launching 10 kg/sec at 5 km/sec
requires 125 megawatts of power (at 100% efficiency).  A nuclear source
is probably best.

The mass driver could defend itself against attack by shooting clouds
of sand at oncoming missiles or satellites, or by shooting projectiles
at larger targets.  Sand injected into earth-intersecting orbits would
be a nice touch for the 4th of July.