Space gun
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A space gun is a method of launching an object into space using a large gun, or cannon. Though it is the earliest envisioned method of space launch, a space gun has never been successfully used to launch an object into orbit. The highest altitude ever reached by this method is 180 km, by Project HARP.
A space gun cannot, by itself, put an object into stable orbit around the planet from which it is launched. Since Kepler's laws of planetary motion were discovered, it has been known that any projectile fired from a planet's surface would follow an elliptical path with one focus of the ellipse at the center of the planet. This ellipse will obviously contact the planet's surface at the point of launch, and at least one other point. This means that a ballistic payload without orbital correction will always strike the planet within its first orbit unless it reaches vacuum at escape velocity, in which case its trajectory would be a hyperbola.
Isaac Newton avoided this objection in his thought experiment by positing an impossibly tall mountain peak, above the sensible atmosphere, from which his cannon would be fired horizontally. The projectile, however, would still circle the planet and return to the point of launch, though Newton's notional mountain peak possibly would have moved on because of the rotation of the earth.
As a result, all payloads intended to reach stable orbit would have to perform some sort of course correction to move very soon to another orbit that does not intersect the planet's surface (or result in aerocapture if the planet has a significant atmosphere.) The added fuel needed for a kick stage would reduce the payload-to-fuel ratio, decreasing the efficiency and increasing the complexity of such a system. It is conceivable that in a multi-body gravitational system, like the earth-moon system, a trajectory from earth could be found that does not re-intersect the earth, although these paths would likely not be very simple, frequent or desirable, and would require more energy.
High velocity passage through the atmosphere would make it difficult to control the trajectory of the projectile, would subject the projectile to high pressures and heat loading, and cause significant momentum losses early in flight.
If acceptable solutions to these problems can be found, a space gun could offer access to space at costs significantly lower than those seen with rocket launch. However, the very high accelerations experienced by a ballistic projectile mean that a space gun would be incapable of launching humans or delicate instruments intact, restricting its applications to orbiting freight with high acceleration tolerance and perhaps highly rugged satellites.
Perhaps the most famous representation of a space gun is Jules Verne's novel, From the Earth to the Moon (made into a silent movie, Le Voyage dans la Lune), in which astronauts fly to the moon aboard a ship launched from a cannon. Isaac Newton once used as an example a cannon placed on top of a mountain and fired with the correct velocity as a demonstration of his law of universal gravity, and the concept was featured in films as late as 1967 in films like Rocket to the Moon. Another famous example of a space gun is the hydrogen accelerator cannon used by the Martians to launch their invasion in HG Wells' book The War of the Worlds.
On the practical side, the most prominent and successful attempt to make a space gun was ballistics and cannon genius Gerald Bull's Project Babylon. During Project Babylon, Bull used his experience from Project HARP to build a massive cannon for Saddam Hussein of Iraq. This gun, had it been completed, would have been the first true space gun capable of launching objects into space. However, it appears that he was assassinated before its completion.
Since Bull's death, few have seriously attempted to build a space gun. Perhaps most promisingly, the US Ballistic Missile Defense programme sponsored the Super High Altitude Research Project in the 1980s. Developed at Lawrence Livermore Laboratory, it is a light gas gun and has been used to test fire objects at Mach 9. One of the lead developers John Hunter has since founded the Jules Verne Launcher Company in 1996, though has been unable to find funding for the multi-billion dollar project.
Ram accelerators have also been proposed as an alternative to light gas guns. Other proposals use electromagnetic techniques for accelerating the payload, such as coilguns and railguns.
Critics worry that a space gun, even one built for peaceful purposes, could easily be converted into a devastating weapon. Any gun capable of launching a payload into orbit could also be used to fire a heavier projectile at earthbound targets. However, a space gun would likely be large (in order to minimize accelerations,) making it difficult to move, and possibly forcing it to be fixed in place, which would only allow striking targets along an unchangeable path.
