The HARP gun from the 1960s US Navy Project

Advanced HARP Launch Cannon
Tech Level: 9
Nuclear Launch Cannon
Tech Level: 9
Ram Accelerator Launch Gun
Tech Level: 12
Railgun Electromagnetic Launcher
Tech Level: 12
Coilgun Electromagnetic Launcher
Tech Level: 13
Circular Coilgun Manned Vehicle Launcher
Tech level: 14

Launch Guns are an idea that can trace itself back all the way to the Age of Reason, when Sir Isaac Newton used an imaginary super cannon on top of a mountain to illustrate how a cannonball could circle the Earth if given a big enough powder charge. Even centuries later, when people were first dreaming of serious space travel, their notions turned toward guns instead of rockets or flying machines. Jules Verne’s From Earth to the Moon and H.G. Wells’ Things To Come basically used enormous scaled-up cannons to launch mankind’s first forays into the Great Dark.

Launch guns have a number of major engineering obstacles. The projectile will lose velocity continuously on ascent, so its initial muzzle velocity has to be enormous. And because gun muzzles can only be made so long, this means the projectile will have to endure tremendous g-forces on launch, measuring in the thousands of g’s. Obviously, this completely rules out using much of this technology for manned missions.

The projectile must also be built to withstand the extreme frictional heating it will undergo in the dense lower atmosphere, as well as the accompanying air resistance. Some calculations estimate that a projectile launched from sea level would lose over 20% of its muzzle velocity just in the first 16 meters of light. It would be far more practical to build a launch gun at high altitude (at least some fifteen thousand feet up, where the lesser air density would reduce the muzzle energy needed to obtain orbital velocity by about a third.)

Protective launch canisters and jackets would be used to the protect many payloads, and can be shed after launch to boost velocity, much like oversized discarding sabot ammunition. Launch guns can also be used this way as a "first stage" for launching independently powered vehicles, such as small rockets.

Launch guns are best suited for microsatellite launches, putting payloads of 100 kilograms or less into orbit.

Tech Level: 9

In the 1960s, the US Navy used surplus 16-inch guns for its HARP (High Altitude Research Project) program. The goal was to launch small probes to high altitude for atmospheric sounding applications. HARP used fin-stabilized projectiles with a discarding sabot. These projectiles eventually achieved an ultimate altitude of 180 kilometers, past the edge of space. Electronic components and sensors used in HARP projectiles demonstrated that such things could be built to withstand the 10,000 g’s of force they had to endure upon launch.

Unfortunately, funding died for HARP before it could implement much more ambitious designs (some of which were already being built when the project was cancelled) that could have put payloads into actual orbit. The Martlet 2G-1 projectile design used a discarding-sabot single stage rocket that could have delivered a two kilogram microsatellite into orbit. The Martlet 3 and Martlet 4 projectiles, in the design stages at the end of the program, were two and three stage rockets respectively that could have put payloads of up to a ton into LEO.

The HARP project is significant in that today almost any moderately-industrialized country (or even wealthier private interests) could recreate the project and move ahead with its more advanced designs. It could, in fact, be one of the cheaper means for a non-space power to "bootstrap" itself up quickly into competition with the known space powers, at least in the field of small satellite launches. However, building a "supergun" in today’s political climate, especially after the Iraqi debacle with one in the early 1990s, might be a dicey prospect for any polity even if the gun is meant purely for space launch purposes.

A variation of the HARP gun is the light gas gun. Instead of using a conventional chemical explosive, the light gas gun uses highly compressed "light" gasses such as helium or hydrogen to propel a projectile.

Tech Level: 9

Any country capable of building nuclear weapons has a door into space. An expensive and politically and environmentally dangerous door, but a door nonetheless.

In 1957, the US engaged in underground nuclear testing of a device called Pascal-B as an experiment to contain radioactive fallout. It was routine as far as such things go, except that Pascal-B was placed at the bottom of a narrow 500-foot shaft, and was topped with concrete and a four-inch-thick steel plate "cap." When the bomb detonated, a high-speed camera recorded the metal cap blasting upward into the atmosphere. Some estimate that if it survived passage through the atmosphere, it had enough velocity to launch out of the solar system. Its estimated that today the Pascal-B cap, if its still intact somewhere out there, may have passed the orbit of Pluto and is heading into interstellar space.

The Pascal-B incident is as clear as one can get to demonstrating the feasibility of launching payloads with a nuclear-powered "cannon." Basically, a small nuke (typically 5 kilotons or less, but larger bombs can be used) is detonated at the bottom of a long, narrow, reinforced vertical launch shaft, its force used to blast a multi-ton payload into space. Like with the HARP guns, the projectile would be wrapped in a protective sleeve which it would shed after launch, like a discarding sabot round. The projectile may also be an independently powered vehicle such as a small rocket for increased velocity.

This may seem a potentially destructive way of launching a spacecraft, but this isn’t the case. In 1954, two steel spheres covered with a protective coating of graphite were suspended only a few meters from ground zero of a nuclear bomb. After the explosion, the spheres were found fully intact miles away, with only a thin layer of the graphite ablated away. (This self same incident, incidentally, also indirectly led to the creation of Project Orion.) The protective sleeve of a hardened sabot would be more than enough to protect most payloads.

The cannon itself would not be so lucky. In order to channel the blast properly, the detonation chamber and "muzzle" would have to be fairly constrictive, and no construction material would be able to survive even a small nuclear detonation that close. So even though the projectile could launch without major damage, the cannon itself would most likely self-destruct after only a single shot. There would also be the problem of nuclear fall-out, though diminished somewhat from most of the explosion being contained underground.

But even though nuclear launch cannons would be one-shot affairs, their advantage is that they can launch much heavier payloads than any other type of launch gun mentioned here, perhaps even matching the capacities of modern rockets, depending on the yield of the launching bomb.

Its highly unlikely anyone today would build a nuclear launch cannon with all the less expensive, less dangerous, and less controversial options available. However, they could be useful in certain specialized circumstances. They’re fairly easy to construct assuming one already has the bombs, and since they’re primarily underground they would be relatively easy to conceal or pass off as another type of construction. They therefore could be useful in launching large concentrations of military assets quickly into space all at once, for planetary defense or for a surprise first strike. They could also be a relatively inexpensive way of launching large payloads from an airless, high-gravity world, where their environmental damage would be a non-issue.

Tech Level: 12

The Ram Accelerator scheme was first proposed by Abraham Hertzberg and colleagues at the University of Washington in 1983. Though ram accelerators have been experimented with in laboratories, no field tests have yet been tried.

The ram accelerator consists of a long, sealed tube filled with a mixture of fuel and oxidizer, such as hydrogen and oxygen. The projectile zooms through the tube, compressing the fuel/oxidizer mix against the sides of the tube and combusting them to produce thrust, much like a sealed-system ramjet. A thin membrane on the end keeps the fuel mix in the tube but is easily penetrated by the projectile.

The longer the tube can be made, the more acceleration the projectile can pile on. Different sections of the tube can be separated by thin diaphragms, in order to take advantage of different fuel-oxidizer mixtures that work better at different velocities and pressures.

Because there is no tremendous build up of explosive pressure, Ram Accelerators have the potential to be used more rapidly for repeat launches than HARP launch guns, and would have a longer overall working life. The University of Washington group, which has so far been able to use a Ram Accelerator to launch a 4 kilogram payload to over 4000 kph, hopes to use the technology someday to launch microsatellites into low Earth orbit.

Tech Level: 12

A simplified diagram of railgun operation

A railgun consists of a pair of long, electrically conductive rails, mounted in an insulating barrel, with the rails connected to a rapidly switching 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. More simply put, the projectile "rides" the magnetic field it creates as it connects these two rapidly-flicking electrical sources down the length of the barrel. Think of the electrical arcs riding up paired antennae in a mad scientist’s laboratory, only moving much faster and propelling a bullet at the top of its arc.

The muzzle velocities railguns are capable of are astonishing. Railgun systems in laboratories have achieved projectile speeds exceeding 21,000 kph.

However, railguns have proven to have a number of drawbacks and the designs based on plasma arcs have difficulties with uncontrolled arcing around the projectile or in the muzzle. Switching such high currents has proven tricky in practice. But the biggest obstacle to developing railguns for practical use is the fact that the rails suffer from deformation and erosion after only a few launches at best, meaning they would have to be constantly replaced. Having to constantly replace the rails could easily prove to be a logistics and financial nightmare in launch gun version, making them an unlikely candidate to ever be scaled up for this use.

Tech Level: 13

Coilgun operation

Coilguns consist of a series of pulsed electromagnetic coils that accelerate a metal 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.

Each coil section along the barrel’s length is switched on rapidly in sequence, pulling the projectile forward, then switched off as the projectile passes so the next coil section can grab it with its magnetic field. In some advanced designs, the coils behind the projectile also switch polarity, using magnetic repulsion to further accelerate the projectile along.

Unlike railguns, coilguns can be made arbitrarily long, allowing for greater potential velocities using gentler accelerations. The main engineering obstacle to this technology is not so much producing enough power or strong enough magnetic fields, but overcoming timing and switching problems.

Because the projectile zooms so rapidly through the barrel, the magnetic fields switching on and off have to be precisely timed. Also, the current and voltage needed to produce the fields do themselves take time to build to strength and to fade away, especially in the bullet-time microseconds the projectile will typically be in the launch barrel. This can result in a loss of velocity, both from less than optimal field strength as well as slow-fading fields behind the projectile tugging on it and slowing it down. Precision timing programs and hardware are therefore an essential component of any coilgun, and one of the main reasons why they have proven much harder to engineer than their conceptual cousin, the railgun.

NASA has designed and built an experimental coilgun that can accelerate a 10 kilogram projectile to 39,600 kph.

Tech Level: 14

The main problem preventing manned missions from using launch guns are of course the massive g-forces a projectile would have to endure upon launch.

However, coilguns do not have to necessarily be constructed in straight lines. In fact, circular particle accelerators use similar principles and are a decades-old technology. It would therefore be possible to have a manned ship accelerated slowly in a very large circular coilgun, then diverted to a straight track when its ready for launch. However, the radius of such a launcher would have to be huge, in order to mitigate the effects of motion sickness and vertigo on the crew, on the order of at least a few kilometers in diameter. In fact, the larger the radius is the better. In one science fiction story, one such coilgun wrapped around the moon’s circumference, in order to launch crews and cargo to destinations into near-interstellar space at small fractions of lightspeed.

It might also take quite a while to build up the velocity needed for launch--several hours to several days, depending on the exact velocity desired and how gradual the acceleration is. The circumlunar coilgun described above took over a month to accelerate a manned vehicle up to launch speed.

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