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SPACE: Space Guns

Vectors For May 1998
[email protected]">greg goebel / in the public domain

* One of the simplest ideas for putting payloads into space is also the oldest. The idea of blasting an object into orbit goes back to the 17th century and Isaac Newton's classic treatise on math and physics, PRINCIPIA MATHEMATICA.

Newton was not serious about space flight. His famous illustration of how a cannon mounted on top of a mountain could, if given a big enough charge, fire a cannonball that went clear around the Earth was simply an illustration of elementary orbital mechanics. However, in the 19th century, French romantic novelist Jules Verne imagined sending humans to the Moon using a giant cannon.

Verne's giant cannon was impractical, and illustrated some of the problems with the idea of simply shooting an object into space with a gun. Unlike a rocket, an artillery shell fired upward loses energy continuously after launch, which means that it must have a tremendous muzzle velocity. Since the length of a "space gun" is necessarily limited, this implies thousands of gees of acceleration -- and the large muzzle velocity also means that the projectile will have to endure severe friction and heating effects while trying to fly up through the dense lower atmosphere.

In any case, 19th-century artillery was too primitive to make the prospect of putting a payload into orbit a serious proposition. However, development or large and powerful artillery pieces progressed rapidly after the turn of the century.

By 1918 Germany had developed an artillery piece of unprecedented range. This weapon was known as the Wilhelmgeshuetze, or Paris Gun. It had a bore of 357 millimeters and a barrel length of 30 meters.

The Paris Gun fired a 106 kilogram shell, driven by an explosive charge of 200 kilograms that produced an acceleration of 7,500 gees and a muzzle velocity of almost 6,000 kilometers per hour. The gun's maximum range was 126 kilometers, with the shell reaching a peak altitude of almost 42 kilometers during its three minutes of flight.

While similar large long-range artillery pieces were used as late as World War II, the development of aircraft and rockets provided a much more effective way to deliver munitions over long distances, and the development of bigger and more powerful artillery pieces ended.

Use of such large guns for space launch remained a possibility, however. The maximum possible muzzle velocity of an artillery piece charged with nitrocellulose explosives is sufficient to launch small probes to high altitude for atmospheric sounding applications, and in the mid-1960s experiments along this line were performed using lengthened US Navy surplus 406 millimeter (16 inch) guns.

The effort was designation HARP, for High Altitude Research Project, and was the brainchild of Gerald Bull and a group at McGill University in Canada -- with support by Charles Murphy of the US Army Research Office and Aberdeen Proving Ground.

Bull's group devised a fin-stabilized projectile named Martlet for cannon launch. As the Martlet had a much smaller diameter than the cannon bore, it was fired using a snug-fitting "sabot", or shoe, that was discarded after the Martlet left the muzzle.

About 200 Martlet 2s were launched with the 406 millimeter guns, with most of the launches from the island of Barbados in the Carribbean but a few from Yuma Proving Ground in Arizona. The Martlet 2s carried various payloads, including chemical releases and ruggedized instruments. They were fired to altitudes of up to 180 kilometers.

Smaller projectiles were launched from 127 millimeter and 178 millimeter (5 and 7 inch) guns to altitudes of about 75 kilometers from Yuma and the US National Aeronautics & Space Administration's (NASA's) launch facility at Wallops Island, Virginia. A total of about 570 ballistic projectiles were launched in the course of HARP.

While HARP blasted projectiles into space, the McGill group was driving the development of cannon-launched rockets to put payloads into orbit. Their Martlet 3 design was a discarding-sabot solid-propellant rocket with a diameter of 190 millimeters (7.5 inches), and was to be launched from a 406 millimeter gun.

The Martlet 3 was to lead to the Martlet 4, which was to be a multistage cannon-launched rocket with a launch mass of 1.2 tonnes and a payload capacity of 90 kilograms to low Earth orbit (LEO); it would be given a muzzle velocity of 5,400 KPH. The McGill group also considered a three-stage rocket design that could put 295 kilograms into a 185 kilometer orbit using all solid fuel, or 590 kilograms into a 1,100 kilometer orbit using all liquid fuel. This vehicle would be launched from a 813 millimeter (32 inch) gun.

Development of these cannon-launched projectiles proceeded to the point where subsystems were test-launched, demonstrating survival under accelerations of up to 10,000 gees. Subsystems included solid-rocket motors, an IR horizon sensor, a spin-rate sensor, Sun sensors, NiCad batteries, a solenoid-operated cold gas thruster, and various support electronics modules.

The McGill group eventually concentrated on a rocket-propelled variant of the Martlet 2, named the Martlet 2G-1, as a minimum alternative to the ambitious Martlet 4. The Martlet 2G-1 would have been able to put a two kilogram payload into LEO, making it an excellent demonstrator for a cannon-based "nanosatellite" launch system.

Unfortunately, funding for HARP eventually dried up and disappeared, even though the Martlet 2G-1 and various Martlet 3 rockets had been designed and were under construction.

Although HARP was discontinued, it was the most impressive effort ever made to blast payloads into space using a cannon -- and in fact appears to be the only project that ever succeeded in doing so. It was also groundbreaking in developing rocket technology for launch by artillery, and in developing instrument and guidance systems that could withstand the stresses of being fired out of a gun.

Eventually, guided munitions that could be fired out of cannon, such as the American Copperhead laser-seeking 155 millimeter round, were developed and deployed, but Bull's dream of using a cannon to put a payload into orbit remains unrealized.

* The story of Gerald Bull didn't end with HARP, however, and took a turn straight out of James Bond (and in fact was dramatized in a movie made for US TV). Bull was embittered by the termination of HARP, and in 1980 served a short term in a US prison as part of a plea bargain for charges of smuggling arms to South Africa.

After he was released, Bull was unable to interest anyone in the US in his superguns, and so moved to Brussels and peddled his designs to anyone who would pay -- first to the Chinese, then to the Iraqis. This was a fatal mistake in judgement; Bull was gunned down in front of his home in early 1990, apparently by agents of the Israeli Mossad intelligence agency.

Three weeks after Bull's death, British customs seized components of an extremely large-caliber gun that were being readied for shipment to Iraq, disguised as pipe sections. After the Iraqi defeat in the Gulf War in 1991, UN inspectors operating in Iraq discovered an incomplete 350 millimeter supergun with a fixed elevation, and parts for an even bigger 1,000 millimeter supergun.

* The muzzle velocities that can be obtained with a cannon driven by nitrocellulose explosives are limited, and so research has been conducted on alternatives.

One such alternative is the light gas gun, which was invented in the postwar period as a means of performing hypersonic experiments with missile warhead reentry vehicle designs, and studying the risks of space debris to spacecraft.

Obtaining high velocities in a cannon requires a gas with a high speed of sound, exerting high pressures on the base of a projectile through a long barrel. The speed of sound squared varies inversely with the molecular weight of the gas and directly with the gas temperature, meaning that a hot gas of low molecular weight makes an excellent propellant for a space gun.

A light gas gun uses a piston to rapidly compress a reservoir of helium gas. This reservoir is sealed off from the gun barrel by a diaphragm; when the diaphragm breaks, the hot gas expands rapidly and blasts a projectile down the barrel. Light gas guns using hydrogen instead of helium are expected to have even better performance.

There have been several research programs conducted on light gas guns. One of the most significant was led by John Hunter of the US Lawrence Livermore National Laboratory. Hunter is now promoting a commercial scheme for a light gas gun, appropriately named the Jules Verne Launcher, for delivering small payloads to orbit.

* Electromagnetic guns have been one of the most prominent alternative technologies for space cannon. Research has been conducted on two different approaches: railguns and coilguns.

A railgun consists of a pair of copper rails, mounted in an insulating barrel, with the rails connected to a rapidly switched high current source. An armature on the projectile to be fired completes the circuit, resulting in a magnetic force that drives the projectile down the barrel. This armature is usually actually a plasma arc ignited at the base of the projectile.

Switching such high currents has proven tricky in practice. Railguns also suffer from erosion of the rails after a few launches, and the designs based on plasma arcs have difficulties with uncontrolled arcing around the projectile or to the muzzle. Railgun enthusiasts have proposed designs that they claim will be able to boost a ten kilogram projectile to 36,000 KPH, but so far railguns have been restricted to lab-scale systems with muzzle velocities no greater than 21,600 KPH.

Coilguns are a little more intuitive in design. They consist of a series of pulsed electromagnetic coils that accelerate a projectile to high velocity. They are more mechanically complicated than railguns, but since there is no direct contact between the projectile and the coils they avoid the erosion and arc-over problems of railguns.

"Mass drivers" based on coilguns were considered for launching payloads from the Moon at least as far back as the 1960s, and small-scale models have been built for decades. NASA has designed a coilgun that can accelerate 10 kilograms to 39,600 KPH; an enhanced version of this device has been proposed to boost a 300 kilogram rocket to 36,000 KPH, allowing it to put a 150 kilogram payload into LEO.

However, so far coilguns have lagged railguns in performance. A major drawback to both railguns and coilguns is that any facility using them would be big and very expensive.

* A new alternative for a space gun, the ram accelerator, has been promoted by Abraham Hertzberg and colleagues at the University of Washington since 1988.

The ram accelerator consists of a long, sealed tube filled with a mixture of fuel and oxidizer, such as hydrogen and oxygen. A projectile resembling the centerbody of a ramjet is shot into the tube at a velocity of about 3,600 KPH, igniting the mixture and blasting the projectile down the tube, which acts like the outer cowling of a ramjet.

It is possible to accelerate the projectile in several distinct modes by varying the fuel-oxidizer mix in different sections of the launch tube, with the sections isolated by thin diaphragms that are ruptured by the projectile as it speeds up the tube.

While there have been proposals to build ram accelerators to launch one-tonne projectiles for delivering supplies to LEO, so far these devices have remained lab experiments. The University of Washington group is currently operating a three stage, 120 millimeter ram accelerator that launches 4.3 kilogram projectiles with a muzzle velocity of 4,320 KPH.

* All the options for space cannon face the same constraints that Jules Verne's Moon gun would have had to contend with.

The fact that a projectile leaving the muzzle of the space cannon loses energy from that instant on means that it has its highest velocity during the part of its flight path that moves through the densest parts of the atmosphere.

As a result, the projectile must be able to withstand frictional heating and must also be given additional muzzle velocity to overcome the losses it will suffer. A simple calculation based on a 1-kilogram cubic projectile launched at a muzzle velocity of 39,600 KPH at sea level shows that it will lose 20% of its velocity and a good part of its ablative thermal protection in the first 16 meters of flight.

One way of minimizing these losses is to launch the projectiles from the top of a mountain. Calculations show that launch energy requirements are cut by almost a third if the cannon's muzzle is placed on a mountaintop at an altitude of 4.6 kilometers (15,000 feet).

The energy requirements for launching large payloads with a space gun are still extreme, however, and so the approach appears best suited to launch of large numbers of small "hardened" payloads. Constellations of "nanosatellites" for communications or similar applications could be placed in orbit at relatively low cost, using the space gun as a "first stage" for launch of a rocket-boosted projectile. Such a projectile would weigh about a tonne and carry about a 60 kilogram payload; the space gun would have to accelerate it to a muzzle velocity of 9,000 to 14,400 KPH.

None of the space gun technologies investigated to date have been scaled up to this size, and doing so would require a major capital investment that would demand high launch rates for a long period of time to break even. However, building such a gun does not require the development of any major new technologies and remains an interesting possibility for the future.