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[4.0] Space Guns

v1.1.7 / chapter 4 of 7 / 01 feb 14 / greg goebel / 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 well-known illustration of how a cannon mounted on top of a mountain could, if given a big enough powder charge, fire a cannonball that went clear around the Earth was simply an illustration of elementary orbital mechanics. However, with the development of modern spaceflight, the idea of using a cannon to blast payloads into orbit has fascinated many space enthusiasts, leading to interesting and sometimes bizarre results. This chapter outlines the history of the space gun, and its future prospects.

Babylon gun segment


[4.1] JULES VERNE'S MOON CANNON / THE PARIS GUN
[4.2] THE V-3 PUMP GUN
[4.3] GERALD BULL & HARP
[4.4] GERALD BULL, SADDAM HUSSEIN, & THE BABYLON GUN
[4.5] ELECTROMAGNETIC GUNS
[4.6] THE SHARP LIGHT GAS GUN / RAM ACCELERATOR

[4.1] JULES VERNE'S MOON CANNON / THE PARIS GUN

* The first detailed description of a giant gun to launch payloads into space was in Jules Verne's 1865 novel, FROM THE EARTH TO THE MOON. The novel described a "Moon Gun", built in Florida. It was buried in the ground, with a length of 274 meters and a bore of 2.74 meters. It used 122 tonnes of guncotton (nitrocellulose explosive) to launch a passenger-carrying shell made out of aluminum with walls 30 centimeters thick. The passengers were protected from launch acceleration with shock absorbers.

Verne's Moon Gun 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, which would crush the crew into a thin film on the floor, and the large muzzle velocity also means that the projectile would have to endure severe friction heating while trying to fly up through the dense lower atmosphere.

* In any case, 19th-century artillery was too primitive to make even a smaller unmanned space gun realistic. However, development or large and powerful artillery pieces progressed rapidly after the turn of the century, helping lead to the entirely unexpected carnage of the First World War.

At 7:18 AM on 23 March 1918, a mysterious explosion occurred in Paris. People thought the explosion was from a bomb dropped by a German aircraft, but no aircraft had been heard or spotted. 21 more explosions occurred that day, killing 15 people and wounding 36. Recovery and analysis of munition fragments indicated that the explosions were caused by artillery shells, fired by a weapon with extreme range. The best conventional artillery of the time had a range of about 37 kilometers, and the front was about 113 kilometers away.

The Germans had long wanted a way to attack Paris, in hopes of demoralizing the French public or at least disrupting French war planning. Aerial bombing was attempted, but the raids proved costly and ineffective. In 1916, the German Army approached the Krupp manufacturing firm about the possibility of building an artillery piece that could hit Paris from behind German lines. Krupp's engineers considered the problem, and had a weapon ready for testing by the spring of 1917.

The weapon was known as the "Wilhelmgeschuetze (Kaiser Wilhelm Cannon)", or more generally as the "ParisKanone (Paris Gun)". It was derived from a 380 millimeter naval gun, into which was inserted a 210 millimeter liner. A long smooth-bore extension was added, providing a barrel with a total length of 34 meters. The barrel was so long, in fact, that a suspension cable had to be strung over a superstructure on the top to keep it straight.

Paris Gun

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; the Paris Gun's shells were the first objects ever sent by humans out of the Earth's atmosphere and into space. The large powder charge melted the lining of the gun slightly every time it was fired. That meant that the shells had to be built in a numbered series, in a sequence of increasing diameters, to be fired in that order until the barrel was replaced and the cycle began again. The barrels were swapped out on a monthly basis.

Apparently a total of seven Paris Guns was built, with three put into action. The Germans obtained targeting information from spies in Paris, who relayed information through Switzerland on where the shells struck the city. The Paris Guns fired 350 rounds from March through August 1918, killing 256 people and wounding over 600. The Allies tried desperately to destroy the guns with artillery and aircraft, but the Germans camouflaged the guns and protected them in concrete emplacements when they were not in use. After the Armistice, the Allies tried to seize the big guns, but the Germans managed to spirit them away. To this day nobody is exactly sure what happened to them; since they were never used in combat in the Second World War, it seems likely they were ultimately scrapped.

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[4.2] THE V-3 PUMP GUN

* The Paris Gun had the longest range of any pure artillery piece ever used in combat. Large naval guns, coastal guns, and railroad guns were developed between the wars and used in World War II, but although these guns fired very big shells, they had nowhere near the range of the Paris Gun.

There were attempts to better the Paris Gun. Hitler was frustrated at his inability to strike effectively at England, while England became a launch platform for air raids on the Reich. The Nazi regime investigated a number of "V weapons" -- where "V" stood for "Vergeltungswaffe (Vengeance Weapon)" -- as a means of hitting back. The V-1 cruise missile and the V-2 ballistic missile are well known, but the Nazi regime also investigated an extremely long range artillery piece, the "high-pressure pump gun", known as the "V-3". This was a bizarre weapon that consisted of a very long barrel, about 127 meters in length, with firing chambers spliced into its sides periodically along its length. The idea was that each chamber would have a propellant charge, and each charge would be fired in sequence to push the shell up the barrel. Workers building the weapon called it the "Fleissiges Lieschen (Busy Lizzie)" or "Millipede".

The basic concept had actually been thought up by two Americans named Lyman and Haskell in 1870. The German Frankfurt arsenal had tried to build a weapon based on their ideas, but could not get it to work properly, and the idea was abandoned for the time being. In 1938, a Czech named Lugendhat rediscovered the concept and tried to sell his "Multiplex Gun" proposal to the British, but they turned him down. Within a few years the concept had been revived once more, this time by a German engineer named Conders, chief engineer at the Roechling Stahlwerk company, which built a number of different types of fin-stabilized concrete-penetrating shells for conventional artillery.

In May 1943, Conders demonstrated a 20 millimeter multichamber gun, and the authorities were interested enough to give the go-ahead for development of a full-scale version, 150 meters long. In September 1943, work was begun on building a network of tunnels hundreds of meters long into a mountain near Mimoyequecs in France, which were intended to accommodate five V-3 guns, elevated at a fixed angle of 50 degrees and pointed towards London. The fin-stabilized shells were relatively small, weighing roughly 70 kilograms each, but each gun was expected to be able to fire four rounds an hour.

Allied aerial reconnaissance quickly spotted the activity at the mountain. Allied intelligence couldn't figure out exactly what the Germans were doing, but it was clear that they were up to no good, and so the site was heavily bombed. The Allies also tried to use a secret weapon of their own to attack the site on 12 August 1944, in the form of a war-weary US Navy Consolidated Liberator patrol bomber that had been converted to fly under radio control and was packed with explosives. A pilot and copilot were to fly the aircraft to the vicinity of the target, set the radio control, and bail out while the aircraft flew on to crash into the target. The bomber exploded in mid-air, killing the pilot and copilot in a tremendous explosion. The pilot was Navy officer Joseph P. Kennedy JR, and his father, the wealthy Joseph P. Kennedy SR, had plans to cultivate him for the US presidency. The senior Kennedy's second eldest son, John F. Kennedy, would become president instead.

* The radio-controlled bomber hadn't even really been necessary, since the site had already been effectively shut down by the earlier attacks. Although a prototype of the V-3 was built to perform experiments and train gun crews, and two smaller Conders guns are believed to have been built, they never saw any real action. The idea sounded interesting on paper, but it wasn't very practical. Synchronizing the charges from the multiple firing cylinders and avoiding the effects of possibly disastrous back-pressure was a difficult, maybe impossible, task.

After the war, the days of big military artillery pieces were numbered. A big gun capable of firing an atomic shell was proposed by the US Army in 1944. The idea was dropped when the war ended, only to be revived as the Soviet threat became more visible. The Americans developed a 280 millimeter gun known as "Atomic Annie" to fire atomic shells. This weapon was developed in the mid-1950s. It was of generally conventional design, and could fire an atomic munition up to 29 kilometers. It weighed 68 tonnes, and was transported by twin semi-trucks, one attached to each end of the gun carriage. However, battlefield training exercises in Europe demonstrated that it was far too vulnerable to air strikes.

The big field gun was a thing of the past. Battlefield missiles were more useful for long-range atomic bombardment, and atomic shells were developed for standard 200 millimeter and 155 millimeter guns, with rocket boosters used to extend shell range when necessary, the loss of accuracy being irrelevant given the destructive radius of the munition. There are rumors that the Chinese developed long-range artillery for potential attacks on Taiwan, but otherwise the long-range artillery piece is a thing of the past.

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[4.3] GERALD BULL & HARP

* Although there was no longer any real need for a battlefield supergun in the postwar period, use of such large guns for space launch remained a possibility. In fact, even before the war, back in 1926, German rocket pioneers Hermann Oberth and Max Valier had performed a reasonably detailed paper analysis of a space gun based on Jules Verne's Moon Cannon, avoiding Verne's obvious blunders.

The Oberth-Valier space gun was 900 meters long and was built onto the side of a tall mountain to reduce the amount of atmosphere the projectile had to plow through. The barrel was evacuated before firing and sealed with a metal cap, which would be blasted off by the projectile launch. The projectile itself was made of thick tungsten steel, with a diameter of 1.2 meters and a length of 7.2 meters. The projectile was blasted out of the gun using a large charge of nitrocellulose propellant explosive. Oberth and Valier did their homework and their space gun was technically plausible. However, it was clearly a monstrously expensive proposition and it is unlikely that even they thought it would be built any time soon. And there is where matters stood, until Gerald Bull entered the scene a generation later.

* Bull was born in Ontario, Canada on 8 March 1928. He went to the University of Toronto as a student, and his technical abilities were such that he graduated with a PhD at the age of 23. Bull was fascinated with artillery and would become one of the world's most influential artillery designers.

In 1957, the Soviets put the first Earth satellite, Sputnik 1, into orbit, and Bull felt that Canada needed to orbit their own satellite in response. Building a satellite booster was beyond Canada's means, and so Bull decided to design a supergun that could boost small rockets into orbit. He had been a fan of Jules Verne as a boy, and FROM THE EARTH TO THE MOON had made a long-lasting impression on him.

Bull had already been tinkering with launching shells from small-caliber artillery for aerodynamic studies, and in 1960 he began work with the US Army on using a modified 120 millimeter gun, with the rifling bored out and fitted with a barrel extension, to fire weather sounding probes into the upper atmosphere, beyond the reach of weather balloons. The project was named the "High Altitude Research Program (HARP)". After some funding ups and downs, the scheme was finally implemented, with the modified HARP guns firing 10 kilogram probes using a 16 kilogram propellant charge. The probes either dispersed metal strips of "chaff" into the upper atmosphere for radar tracking, or released a sensor module called a "Metsonde (meteorological sonde)" that floated back to earth on a parachute while returning data by radio.

The 120 millimeter guns were transportable and performed soundings from Fort Greeley, Alaska; Highwater, Quebec; Yuma Proving Grounds, Arizona; the NASA Wallops Island facility, off the coast of Virginia; and the island of Barbados in the Caribbean. About 300 120 millimeter HARP projectiles were fired, reaching a maximum altitude of 72 kilometers.

With the 120 millimeter HARP guns proving successful, Bull went on to modify several 175 millimeter guns into transportable HARP guns. These larger guns could fire an 18 kilogram projectile to an altitude of up to 100 kilometers with a 50 kilogram propellant charge. About 60 175 millimeter HARP projectiles were fired.

The projectiles fired by the 120 millimeter and 175 millimeter HARP guns were essentially darts with fins that were placed inside a four-piece, spool-shaped wooden fitting called a "sabot" (French for "shoe" and pronounced "SAY-bow") to allow them to fit into the gun tubes, and backed by a wooden baseplate. The sabot and baseplate fell away as the projectile left the barrel. Electronic payloads were shockproofed by potting them in a composite of epoxy and sand.

* Of course, Bull wanted to do more than just launch weather probes: over the long run he wanted to put payloads in orbit. To this end, he also developed a "supergun", built of two 400 millimeter naval guns welded together in tandem and supported by an external structure, with a total length of 30 meters. It was of course not transportable in any reasonable sense of the word, and was set up at a fixed site on Barbados, where it had a clear downrange launch path over the ocean.

The supergun projectiles were similar in concept to those launched by the 120 millimeter and 175 millimeter HARP guns, but of course they were substantially larger. They were known as "Martlets". Two proof-of-concept "Martlet 1s" were fired early in the program, but the remaining shots used the "Martlet 2". Although there were a number of Martlet 2 subvariants, they typically weighed 84 kilograms, were 13 centimeters (5 inches) in diameter, and were fitted inside the supergun barrel with a sabot. Payloads included chemical releases and ruggedized instruments. The launch charge was about 330 kilograms of propellant.

The first firing of the HARP supergun was in 1962. About 200 Martlet 2s were fired by the supergun, with each shot accompanied by a spectacular geyser of flame. On 19 November 1966, the HARP supergun blasted a Martlet 2 to an altitude of 180 kilometers. This record still stands as the highest launch of an artillery projectile.

* While the HARP supergun blasted projectiles into space, the McGill group was pushing forward with the development of cannon-launched rocket projectiles to put payloads into orbit. Their "Martlet 3" series of projectiles were discarding-sabot solid-propellant rockets with a diameter of 190 millimeters. The group also worked on a scaled-down "Martlet 3E" to be fired from the 175 millimeter HARP guns, to be used for meteorological sounding shots so the HARP supergun could be reserved for satellite launch work.

The Martlet 3 series was to lead to the "Martlet 4", a multistage cannon-launched rocket projectile with a launch mass of 450 kilograms and a payload capacity of up to 23 kilograms to low Earth orbit (LEO). It would have a muzzle velocity of 5,400 KPH and six pop-out fins for ascent stabilization. Various refinements were considered that could have quadrupled its payload capability.

The McGill group also performed a design study of a three-stage rocket projectile design that could put 295 kilograms into a 185 kilometer orbit using solid fuel rockets, or 590 kilograms into a 1,100 kilometer orbit using liquid fuel rockets. This vehicle would be launched from an 800 millimeter gun.

Development of these cannon-launched rocket 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 infrared horizon sensor, a spin-rate sensor, sun sensors, nicad batteries, a solenoid-operated cold gas thruster, and various support electronics modules.

As funding for the HARP program became more uncertain, the McGill group began to focus on a fast-track rocket projectile based on the Martlet 2, named the "Martlet 2G-1" AKA "GLO-1A", as a demonstrator. The Martlet 2G-1 would have been able to put a two-kilogram payload into LEO. Unfortunately for Bull, funding for HARP dried up and disappeared before the Martlet 2G-1 was ready. HARP was canceled in June 1967. Bull was crushed.

* Although HARP was discontinued, it was the most impressive effort ever made to launch space payloads 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 projectiles for launch by artillery, and in refining instrument and guidance systems that could withstand the stresses of being fired out of a heavy artillery piece. This data would prove useful in development of cannon-launched guided munitions developed in later decades. Apparently a souvenir of the effort still exists, since at last report the HARP supergun was sitting idle in Barbados, rusting away.

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[4.4] GERALD BULL, SADDAM HUSSEIN, & THE BABYLON GUN

* The story of Gerald Bull didn't end with HARP, however, taking a turn straight out of James Bond. Bull was still determined to develop a supergun for launching space payloads. To fund his work, he went into the arms trade, forming the Space Research Corporation (SRC), based in Vermont on the US-Canadian border. He acquired American citizenship, and the SRC became a very unusual company, handling transactions on either the US or Canadian side of the border, depending on how national laws affected the transaction.

SRC scored a big hit on the arms market by designing a new advanced 155 millimeter artillery shell, the "GC-45", and 155 millimeter guns to fire it. A shell leaves a low-pressure wake immediately behind it, which creates drag that slows down the shell. Bull figured out a scheme to bleed hot gas from the back of the GC-45 to eliminate this void. He also added aerodynamic improvements based on HARP research, making the shell more slender and streamlined, and added finlike bumps on the side of the shell to stabilize it. His improved 155 millimeter gun, using a slightly extended barrel and firing the GC-45, had 50% greater range than the competition. The use of the bleed system meant that the shell couldn't carry as much explosive charge, but the ability to hit an adversary while remaining out of range of counterfire made that a fair tradeoff.

The GC-45 led to a revolution in artillery design, and within a few decades guns and shells based on Bull's work were in widespread use. However, in 1980, Bull was arrested by the US Customs Service for violating the arms embargo against South Africa, and he did a seven-month stint in a US Federal penitentiary. The details of this misadventure remain mysterious, with some sources claiming that Bull was little more than a convenient scapegoat for a bungled deal set up by the US Central Intelligence Agency (CIA) to provide arms to Angolan rebels. In most other contexts that would sound like conspiracy paranoia, but Bull's unusual background makes it seem somewhat believable.

Whatever happened, Bull was embittered, and in 1981 he transferred the SRC to Brussels, Belgium, a center for the arms trade where he was unlikely to get into much trouble with the law -- though as it would turn out, the law would be the least of his worries. He was still infatuated with the space gun. In 1984, Bull and Charles Murphy wrote a book on the Paris Gun and HARP. Bull obtained a good deal of original documentation on the Paris Gun from friends at Krupp and ran computer analyses of its performance. Bull became fascinated with World War I in general and the Paris Gun in particular. He also continued paper design studies for a space gun and rocket projectiles that would be fired by it.

* Iraq had been fighting a long, grinding war with Iran through most of the 1980s and had made good use of Bull's improved artillery technology. In 1988, the Iran-Iraq war ended in an expensive victory for Iraq. The Iraqis began to look for new weapons for future conflicts, and in 1988 they approached Bull. Bull told them of his ambition to build a space launch gun, and pitched the idea of Iraq becoming a "space power", with the capability of placing small satellites in orbit for the equivalent of a few thousand dollars in launch costs.

Bull downplayed the usefulness of such a supergun as a weapon, though he did suggest that the Iraqis could use it to launch spy satellites. Iraqi dictator Saddam Hussein liked the idea, and the Iraqis began to provide Bull with financial backing for developing the supergun, under the codename of "Project Babylon" -- though the project was hidden under the cover designation of "Petrochemical Complex 2 (PC2)".

Bull's Babylon supergun was to weigh 1,900 tonnes and was to be 156 meters long, with the barrel built in segments and supported by four huge shock absorbers. The tube walls would be 30 centimeters thick at the base, tapering to 6.5 centimeters at the muzzle. It would have a bore of a full meter, and would be capable of launching a two-tonne discarding-sabot rocket projectile, an evolved descendant of the Martlet 4, that could put a 200 kilogram payload into orbit.

Bull's son Michael objected to the project, suggesting to his father that it might make him dangerous enemies. Gerry Bull didn't abandon the project, but he did try to go underground with it. He ordered segments and components of the supergun from separate fabricators in the UK, Spain, the Netherlands, and Switzerland, telling them that they were elements of a petrochemical plant. However, Michael Bull said later: "My dad was probably the worst secret-keeper in the world." Gerry Bull was an enthusiastic man who spoke energetically, even boasted, about his dreams and ambitions, and as parts of the supergun arrived in Iraq, word got out about what he was up to.

As part of the supergun effort, Bull built a smaller prototype version of the supergun. The prototype was named "Baby Babylon", was 40 meters long, and had a bore of 35 centimeters. The Baby Babylon was assembled at a site in central Iraq in 1989. It was first test-fired horizontally, and then erected on a steep hillside, pointing it at an angle of 45 degrees, for long-range shots.

Bull was also assisting the Iraqis in other weapons projects, in particular a long-range missile based on clustering the Soviet-designed Scud battlefield missile. If completed, this missile would have the capability to hit Israel, and Israeli intelligence was following the project closely. Officials from the Israeli embassy in Paris paid a visit to Bull in Brussels and had a private chat with him. There is no record of what was said, but Bull's associates claim he began to show signs of stress, and odd things started to happen. One day, Bull went home to his apartment to find that his furniture had been rearranged.

On 22 March 1990, Bull was unlocking the door to his apartment when an unknown assassin with a silenced pistol shot him five times in the neck and back of the head, killing him on the spot. Bull was carrying $20,000 USD in a briefcase, but the case and the money were still there when his body was found. Who murdered Bull is not known. The Israelis are the prime suspects, but it is possible that the Iraqis had come to regard Bull as a security risk and a liability that had to be eliminated, despite the investment that they had made in him.

Some writers have speculated that if the Israelis did kill Gerald Bull, it was for his work on long-range Scuds. The Babylon supergun would have been much too good a target for a counterstrike to have lasted long as a weapon, and Saddam Hussein himself may not have seen it in that role. The idea of being the first Arab leader to put a satellite into space no doubt appealed to him, and there's speculation that he may have wanted it to attack American spy satellites. The facts of the case remain unclear.

In August 1990, the Iraqis invaded Kuwait, beginning the Gulf War. In November 1990, British customs received an anonymous tip and seized the last eight sections of Bull's supergun, which had been ready for shipment before the war broke out. After the Gulf War ended in defeat for Iraq in early 1991, United Nations (UN) inspectors searched through Iraq for weapons of mass destruction. They found the Baby Babylon gun intact, and also discovered sections of the supergun parked in a stockpile, awaiting assembly. They were destroyed by UN teams.

It is very unlikely that anybody will try to build anything much like the Babylon supergun again. Bull was committed to conventional artillery concepts, using explosives to blast a projectile out of a tube. Later investigations into building a space gun to blast projectiles into orbit suggested that better schemes are available.

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[4.5] ELECTROMAGNETIC GUNS

* 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":

Although both coilguns and railguns sound exotic, they are not new ideas. Speculations about coilguns for use as military artillery predate the First World War, and in 1917 a Frenchman named Fauchon-Villeplee actually built a working model of a railgun, with the shells fitted with "wings" that served as an armature. In 1937, an employee of the German Siemens company named Otto Muck started looking at railguns again, and in 1943 proposed the construction of a long-range railgun that could fire twelve 200 kilogram shells every minute, driven by a 100 megawatt power station. The German military was already committed to the development of long-range missiles and other V-weapons, and the project was not funded.

In 1944, another German engineer named Hansler proposed the development of a 40 millimeter antiaircraft railgun, and the Luftwaffe awarded him a development contract. However, with Germany collapsing under the weight of Allied armies, nothing came of Hansler's weapon. The Allies investigated the railgun concept after the war, but quickly discovered the limitations of the technology and gave up on it for a few decades.

* The major problems with both railguns and coilguns are that they require very large power supplies, possibly unrealistically large for a space launch application, and also have to switch very large amounts of power in very short times. In addition, railguns are inclined to 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. The simple violence of firing a railgun also tends to impose destructive stresses on it.

Railgun enthusiasts have proposed designs that they claim will be able to boost a 10 kilogram projectile to 36,000 KPH, but work towards practical weapons has been focused on much more modest muzzle velocities in the thousands of KPH. A rapid-firing combat railgun using a 10 kilogram "kinetic-kill" projectile seems to be within the realm of possibility -- at least for a warship big enough to carry an adequate power supply -- but a 50 kilogram projectile appears unrealistic.

Coilguns do not suffer from the erosion and arc-over problems that plague railguns, and scaling up coilguns does not seem to be as difficult. "Mass drivers" based on coilguns were considered for launching payloads from the Moon 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 projectile to 36,000 KPH, allowing it to put a 150 kilogram payload into LEO. There have also been suggestions for using coilguns as a space propulsion scheme, for example to alter the orbits of asteroids by blasting off masses encased in metal cans.

* A less ambitious scheme, though not by much, is the "magnetic levitation" or "maglev" launch system, which, like a coilgun, uses magnetic forces to drive a payload. Maglev technology is also not a new idea; it has been used in experimental trains for decades, and basically involves a scheme where an electric motor is turned "inside out", with the stator coils that drive the "motor" laid out along a linear track, and the "armature" of the "motor" attached to the train. Phased currents through the track coils accelerate the train down the track, and since the train floats above the track, there is no direct contact, permitting high speeds.

Maglev systems have not been considered for building space guns, but there is active interest in using them to build "sleds" that could provide initial acceleration for conventional spacecraft, possibly driving them up the side of a tall mountain. Maglev systems have been much more thoroughly investigated than railguns and coilguns. In the space launch applications envisioned, the final sled velocity would be only hundreds of kilometers per hour, and the power requirements would be manageable.

NASA's Marshall Space Flight Center built two small proof-of-concept maglev tracks. Marshall engineers believed that a maglev sled could be used to accelerate a reusable launch vehicle (RLV) to up to 970 KPH, using only about $75 USD worth of electricity and reducing the size of the RLV by 20%. The RLV would begin takeoff about halfway down the track, and the sled would be braked magnetically, allowing some of the power to be recovered, or to allow a clean recovery of the RLV in case of launch failure.

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[4.6] THE SHARP LIGHT GAS GUN / RAM ACCELERATOR

* Even as Bull was working on his Babylon supergun, John W. Hunter of the US Lawrence Livermore National Laboratory (LLNL) was developing a "light gas gun" to test concepts for space launch systems.

The light gas gun was invented in the 1950s for lab studies of hypersonic "reentry vehicles" for intercontinental ballistic missiles, and has been used as a high-speed research tool since that time. The name of the LLNL project was SHARP, for "Super HARP", and it was far bigger than any other light gas gun ever built.

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 such as hydrogen or helium makes an excellent propellant for a space gun.

There are several different schemes for light gas guns, but one of the more common uses a piston to rapidly compress a "pump tube" of hot light gas. The pump tube is sealed off from the "launch tube" by a diaphragm. When the diaphragm breaks, the hot light gas expands rapidly into the launch tube, blasting the projectile out the muzzle. Hunter described the scheme as something like a "giant BB gun".

Most light gas guns built before SHARP were small benchtop devices for testing high-speed aerodynamics, but SHARP was much more ambitious. SHARP was built at Lawrence Livermore and conducted its first test shot in December 1992. It featured a pump-tube system 82 meters long and with a bore of 36 centimeters, connected at a right angle to the launch tube, which was 47 meters long and had a bore of 10 centimeters. Three large counterweights were mounted on railroad cars to absorb the recoil, which occurred in both directions along the pump tube and backwards along the launch tube.

SHARP's pump tube contained a methane-air fuel mixture on one side of a 900 kilogram piston and hydrogen on the other. Igniting the fuel mixture caused the piston to blast down the pump tube and compress the hydrogen to 2,000 atmospheres, ultimately breaking the diaphragm into the launch tube and launching the projectile. SHARP could propel a 5 kilogram projectile to a muzzle velocity of 14,500 KPH.

SHARP was laid out in a horizontal configuration for test firings. The next stage was to relocate the gun to a mountainside and use it for high-altitude shots in preparation for a practical space-launch version, but funding dried up in 1996. At last notice SHARP was still in place at Lawrence Livermore, being used for occasional test shots.

Hunter then pursued a commercial scheme for a light gas gun, appropriately named the "Jules Verne Launcher (JVL)", for delivering small payloads to orbit. Hunter envisioned the JVL as more than three kilometers long and more than a meter in diameter. Simply scaling up SHARP wouldn't have worked, however, since that would have required a piston ten thousand times bigger and huge counterweights. JVL borrowed the idea behind the V-3 Busy Lizzie gun, gating high-pressure hydrogen from multiple compression chambers at into the launch tube at intervals along its length. It is hard to say if they would have encountered the same sorts of sequencing problems that plagued the V-3 and its brethren, since nobody with the means was seriously interested in funding it.

* Interestingly, a concept for something similar to the JVL had been proposed in 1961 by Jules Gram and Charles Smith of Babcox & Wilcox (B&W), a manufacturer of boiler systems, steam plants, nuclear power facilities, and the like. Gram and Smith envisioned digging a 3,050 meter long gun barrel 6.4 meters in diameter straight down into the top of a tall mountain. A hot-hydrogen chamber 25.9 meters in diameter and 427 meters long was dug below the bottom of the barrel, with the two connected by 70 fast-opening valves, each 1.2 meters in diameter. A spherical steam chamber 95 meters in diameter was dug below the hot-hydrogen chamber in turn, with the two also connected by 70 similar valves.

A network of tunnels and chambers was dug into the mountain to take in natural gas and water and produce hot hydrogen and steam. When the steam reached maximum pressure of about 510 atmospheres at a temperature of 540 degrees Celsius, it was vented into the hydrogen chamber through the valves.

There would be little mixing between the light hydrogen and heavy steam as the hydrogen was compressed, and so no piston was required. When the hydrogen reached maximum pressure of about 320 atmospheres at a temperature of 1,730 degrees Celsius, it was released into the barrel through the valves, blasting a rocket sitting on a sabot baseplate up the barrel and out the top of the mountain.

Foam plastic sabot segments kept the rocket upright in the barrel. The barrel was sealed at the top with a diaphragm, and partially evacuated by filling it with steam that then condensed. The rocket simply punched through the diaphragm on the way out, followed by a blazing pillar of hydrogen and then a huge mushroom cloud of steam. Gram and Smith estimated cost for their space gun as $270 million USD in contemporary dollars. NASA didn't bite on the proposal and it never happened either, but it certainly looked spectacular on paper.

* Another scheme for a space gun, the "ram accelerator", was developed by Abraham Hertzberg and colleagues at the University of Washington in the late 1980s.

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 spindle of a ramjet is shot into the tube with a conventional gun, 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 operated a three stage, 120 millimeter ram accelerator that launched 4.3 kilogram projectiles with a muzzle velocity of 4,320 KPH.

* Space guns seem to be one of those ideas that go in and out of fashion on a cyclical basis. Right now the idea seems to be out of fashion, but it will be no surprise if it revives in ten years or so. Who knows? Maybe one of these days someone will actually put something into orbit with one.

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