Okay, lets get the record straight on the Livermore gas gun.  
The project manager is Dr. John Hunter, and he works for the
Laser group at Livermore.  What, you may ask, does gas guns
have to do with lasers? Nothing, really, but the gun is physically
located across the road from the Free Electron Laser building,
and the FEL building has a heavily shielded control room (thick walls)
from which the gun firings are controlled.  So I suspect that the
office he works for is an administrative convenience.

I visited Hunter at the beginning of Feb. and we toured the gun.
At the time I was working on gas gun R&D at Boeing, where I work,
but I am now doing other things (helping to save the space station),

The gun uses a methane-air mixture, which is burned in a chamber
about 200 ft long by 16 inch ID (i.e. it looks like a pipe).
The chamber holds a 1 ton piston which is propelled at several
hundred m/s down the chamber.  On the other side of the piston
is hudrogen gas, initially at room temperature andsome tens
of atmospheres.

The piston compresses and heats the hydrogen ahead of it until
a stainless steel burst diaphragm ruptures, at around 50,000 psi.
The barrel of the gun is about 100 feet long and has a 4 inch
bore.  It is mounted at right angles to the chamber (i.e. they
intersect).  This was done so that in the future, the barrel
could be raised and the gun fired into the air without having to
move the larger and heavier chamber.  The projectile being used
in testing is a 5 kg cylinder of Lexan plastic, 4 in in diameter
and about 50 cm long.

All of the acceleration comes from the expansion of the hydrogen
gas from 50,000 psi downwards until the projectile leaves the
barrel.  The barrel is evacuated, and the end is sealed with a
sheet of plastic film (a little thicker than Saran wrap).  The
plastic is blown off by the small amount of residual air trapped
in the barrel ahead of the projectile.  

The gun is fired into a bunker filled with sandbags and plastic
water jugs.  In the early testing fragments of the plastic
projectile were found.  At the higher speeds in later testing,
the projectile vaporizes.

The testing is into a bunker because the Livermore test range is
about 3 miles across, and the projectile would go 100-200 km
if fired for maximum range.  The intent is to move the whole gun
to Vandenberg AFB after the testing is complete, where they can
fire into the Pacific Ocean, and use the tracking radar at VAFB
to follow the projectiles.

The design goal of the gun is to throw a 5 kg projectile at 4
km/s (half of orbital speed).  So far they have reached 2 km/s,
and the gun is currently down for repairs, as on the last test
they blew a seal and damaged some of the hardware (I think it
had to do with the methane-air more detonating than burning, but
I haven't had a chance to talk to Hunter directly on this).

There are people waiting to test scramjet components in this
gun by firing then out of the gun into the air (at Mach 12=
4 km/s), since the most you can get in wind tunnels is Mach 8.

This gun cost about 4 million to develop, and is basically
a proof-of-concept for a bigger gun capable of firing useful-
sized payloads into space.  This would require on the order of
100 kg projectiles, which deliver on the order of 20 kg
useful payload to orbit.

Dani Eder
