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Technology: US 'supergun' sets sights on space

作者:鄢推    发布时间:2019-02-28 05:18:13    

By DANIEL CLERY 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 charge. 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 hydrogen. 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,

 

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