Space gun
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A space gun is a method of launching an object into outer space using a large gun, or cannon. It provides a method of non-rocket spacelaunch. Though it is the earliest envisioned method of space launch, a space gun has never been successfully used to launch an object into orbit.
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[edit] Technical issues
The large accelerations experienced by a ballistic projectile would likely mean that a space gun would be incapable of safely launching humans or delicate instruments, rather being restricted to freight or ruggedized satellites.
Atmospheric drag also makes it more difficult to control the trajectory of any projectile launched, subjects the projectile to extremely high forces, and causes severe energy losses that may not be easily overcome. A space gun with a "gun barrel" reaching above the lower troposphere, where the atmosphere is most densely packed may mitigate the issue.
A space gun, by itself, is generally not capable of placing objects into stable orbit around the planet.
If acceptable solutions to these fundamental issues could be achieved, a space gun could offer access to space at an unprecedented low cost.
[edit] Getting to orbit
A space gun, by itself, is generally not capable of placing objects into stable orbit. The laws of gravitation make it nearly impossible to reach a stable orbit without an active payload which performs orbital correction burns to change the shape of its orbit after launch.
Since Kepler's laws of planetary motion were discovered, it has been known that any payload 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 possibly one other point. This means that an uncorrected, ballistic payload will always strike the planet within its first orbit unless the velocity was so high as to reach 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 from which his cannon was fired. The projectile, however, would still tend to circle the planet and strike the point of launch.
As a result, all payloads intended to reach orbit would have to perform some sort of course correction to create another orbit that does not intersect the planet's surface. The amount of fuel carried would thus 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, that a trajectory could be found that does not re-intersect the earth's surface, although these paths would likely not be very simple nor desirable, and would require much more energy.
This is not a concern for projects destined to leave the planet entirely.
[edit] Acceleration
With a space gun with a "gun barrel" with the length (l), and the needed velocity (ve), the acceleration (a) is provided by the following formula:
For instance, with a space gun with a vertical "gun barrel" through both the Earth's crust and the troposphere, totalling ~60km of length (l), and a velocity (ve) enough to escape the Earth's gravity (escape velocity, which is 11.2 km/s on Earth), the acceleration (a) would theoretically be more than 1000 m/s2, which is more than 100 g-forces, which is about 3 times the human tolerance to g-forces of maximum 20 to 35 g[1] during the ~10 seconds such a firing would take.
Any doubling of the barrel length would theoretically cut the generated g-force by half, and vice versa. Such a solution would be to have it non-vertical. Such a solution, however, is on the verge of being a launch track.
[edit] Practical attempts
On the practical side, the most prominent 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, Bull was assassinated before the project was completed.
Since Bull's death, few have seriously attempted to build a space gun. Perhaps most promisingly, the US Ballistic Missile Defense program 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 as yet 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.
[edit] Criticisms
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 sub-orbital speeds that could reach any target on the Earth's surface.
[edit] In fiction
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 H. G. Wells' book The War of the Worlds. Wells also used the concept of a space gun for the climax of his movie Things to Come.
