The Light Gas Gun
The light gas gun is an apparatus for physics experiments, a highly specialized gun designed to generate very high velocities. It is usually used to study high speed impact phenomena (hypervelocity research), such as the formation of impact craters by meteorites or the erosion of materials by micrometeoroids. Some basic materials research relies on projectile impact to create high pressure: such systems are capable of forcing liquid hydrogen into a metallic state.
Light Gas Gun Operation
A light gas gun works on the same principle as a spring piston
airgun. A large diameter piston is used to force a gaseous working
fluid through a smaller diameter barrel containing the projectile to
be accelerated. This reduction in diameter acts like a lever,
increasing the speed while decreasing the force. In an airgun, the
large piston is powered by a spring or compressed air, and the
working fluid is atmospheric air. In a light gas gun, the piston is
powered by a chemical reaction (usually gunpowder), and the working
fluid is a lighter gas, such as helium or hydrogen (hydrogen offers
the best performance as explained below, but helium is much safer to
work with. Hydrogen also causes a lesser amount of launch tube
erosion). One addition that a light gas gun adds to the airgun is a
rupture disk, which is a carefully calibrated disk (usually metal)
designed to act as a valve. When the pressure builds up to the
desired level behind the disk, the disk tears open, allowing the
high pressure light gas to pass into the barrel. This ensures that
the maximum amount of energy is available when the projectile begins
moving.
One particular light gas gun used by NASA uses a modified 40 mm
cannon for power. The cannon uses gunpowder to propel a plastic
(usually HDPE) piston down the cannon barrel, which is filled with
high pressure hydrogen gas. At the end of the cannon barrel is a
conical section, leading down to the 5 mm barrel that fires the
projectile. In this conical section is a stainless steel disk
approximately 2 mm thick, with an "x" pattern scored into the
surface in the middle. When the hydrogen develops sufficient
pressure to burst the scored section of the disk, the hydrogen flows
though the hole and accelerates the projectile to a velocity of 6000
m/s in a distance of about a meter.
NASA also operates light gas guns with launch tube sizes ranging
from 0.170” to 1.5” at Ames Research Center. These guns have been
used in support of various missions beginning with Apollo reentry
studies in the 1960’s and most recently for high-speed thermal
imaging. Velocities ranging from 1 km/S up to 7 km/S can be
achieved. The largest of these involves a 6.25" diameter piston
weighing more than 46lbs. to compress the hydrogen.
Design physics
The limiting factor on the speed of an airgun, firearm, or light gas
gun is the speed of sound in the working fluid — the air, burning
gunpowder, or a light gas. This is essentially because the
projectile is accelerated by the pressure difference between its
ends, and such a pressure wave cannot propagate any faster than the
speed of sound in the medium. The speed of sound of helium is about
3 times that of air, and the speed of sound in hydrogen is 3.8 times
that of air. The speed of sound also increases with the temperature
of the fluid (but is independent of the pressure), so the heat
formed by the compression of the working fluid serves to increase
the maximum possible speed. Spring piston airguns heat the air
enough to combust some of the piston lubricant; this raises the
speed of sound in the compressed air enough to overcome frictional
and other efficiency losses and propel the projectile at more than
the speed of sound in the ambient conditions. Light gas guns have
been built that are capable of propelling projectiles at speeds of
up to 7000 m/s, over 5 times the velocity of which small-bore
firearms are capable.
Impact profile
When the projectile fired by a gas gun impacts its target, the
pressure applied depends upon the mass of the projectile. Obviously,
a dense projectile will apply more pressure overall than a light
one, but researchers have recently begun to vary their projectiles'
density as a function of length. Since the projectiles travel at a
known velocity, changes in density as a function of length have a
predictable relationship to the impact pressure applied as a
function of time. With materials in a wide range of densities (from
tungsten powder to glass microspheres) applied in thin layers,
carefully-made projectiles can be used in constant-pressure
experiments, or even controlled compression-expansion-compression
sequences.