Learn something new every day |

Subscribe and treat yourself to a FREE gift


19 September 1992

Technology: US ‘supergun’ sets sights on space

In the next few weeks, the US will fire the first shots from its own
‘supergun’. Projectiles weighing 5 kilograms will hurtle from its barrel
at 4 kilometres a second, but will only travel for 30 metres before they
slam into a target of sandbags. The aim of the Super High Altitude Research
Project is not to produce a weapon, but to launch materials into space
more cheaply than with a rocket.

The SHARP gun, which is nearing completion at the US government’s Lawrence
Livermore National Laboratory in California, has cost about $4 million to
build over three years. It is fundamentally different from the guns Saddam
Hussein attempted to build to bombard Israel. The Iraqi superguns were similar
to conventional artillery: a projectile is forced up the barrel by an explosive

The SHARP gun is like an enormous air rifle. It is known as a light
gas gun because hydrogen is compressed to an extremely high pressure and
then released into the bottom of the barrel beneath the projectile, forcing
it up the tube. Hydrogen is used because it expands more rapidly than heavier
gases and so gives the projectile a greater velocity. Light gas guns are
commonly used in high velocity research laboratories, but never before
has such a large gun of this type been built.

Soon after firing the first test shots at Livermore, John Hunter, head
of the SHARP project, hopes to move the gun to Vandenberg Air Force Base
at Lompoc, California, where it will be mounted with its barrel pointing
in the air to test its ability to launch projectiles into space. Hunter
calculates that if launched upwards, the projectile would reach an altitude
of 450 kilometres.


Hunter and his team believe that it would be possible to build much
larger light gas guns which could launch heavier payloads into space much
more cheaply than an expendable rocket or shuttle. Although it would need
major advances in technology to achieve, the team calculate that for $7
billion they could build a gun capable of launching 10-tonne projectiles
into low-Earth orbit.

If such a gun operated for ten years, this method of launching would
cost $500 per kilogram of payload. Shuttle launches cost around $20 000
per kilogram. Hunter says 90 per cent of the materials that are needed for
space exploration could be launched in this way; the only things that need
to be launched on a shuttle are those that cannot survive an acceleration
of 1500 g.

The SHARP gun consists of two tubes connected at right angles to each
other. The larger tube, which is 82 metres long and 36 centimetres in diameter,
is the pump tube where the hydrogen is compressed. The smaller launch tube
is 47 metres long and 10 centimetres in diameter.

To fire a shot, a steel piston weighing one tonne is put in the pump
tube near the end farthest from the launch tube. A mixture of methane and
air is put into the space between the piston and the end of the tube and
the rest of the tube is filled with hydrogen. The methane-air mixture is
ignited and the explosion forces the piston down the tube compressing the

The hydrogen quickly reaches a pressure of 4100 atmospheres (60 000
pounds per square inch). The pressure breaks a restraint in the coupling
between the two tubes and the hydrogen gas rushes into the launch tube
pushing the projectile along. To stop air in the launch tube slowing the
projectile down, the air is pumped out before firing and the far end of
the tube sealed with a thin piece of plastic. The projectile bursts through
the plastic at a speed of 4 kilometres per second and the hot, compressed
hydrogen following it instantly bursts into flames as it meets the air.

To absorb the shock of the compression in the pump tube, the plugs at
each end of the tube push against 100-tonne weights mounted on sleds. These
slide along their tracks by about 3 metres after the impact. The recoil
of the launch tube is absorbed by a 10-tonne weight of similar design
and also by a container full of a jelly-like substance. The recoil squirts
the jelly out through slots in the container with enough force to injure
a bystander.

Designing the chamber that connects the two tubes was the hardest part
of the project, according to Hunter. It weighs 40 tonnes and is made of
concentric rings of a cobalt-nickel-steel alloy, the toughest alloy available.
Building larger guns will require much higher pressures and stronger materials.
Larger guns may also have extra hydrogen pumped into the launch tube at
one or more points to give the projectile an extra kick.