Space gun

A space gun by itself is not capable of placing objects into stable orbit around the object (planet or otherwise) from which it launches them. The orbit is a parabolic orbit, a hyperbolic orbit, or part of an elliptic orbit which ends at the planet's surface at the point of launch or another 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 or exceed escape velocity. As a result, all payloads intended to reach a closed orbit need at least to perform some sort of course correction to create another orbit that does not intersect the planet's surface.

A rocket can be used for additional boost, as planned in both Project HARP and the Quicklaunch project. The magnitude of such correction may be small; for instance, the StarTram Generation 1 reference design involves a total of 0.6 km/s (1,300 mph) of rocket burn to raise perigee well above the atmosphere when entering an 8 km/s (18,000 mph) low Earth orbit.[1]

In a three-body or larger system, a gravity assist trajectory might be available such that a carefully aimed escape velocity projectile would have its trajectory modified by the gravitational fields of other bodies in the system such that the projectile would eventually return to orbit the initial planet using only the launch delta-v.[2][3]

Isaac Newton avoided this objection in his thought experiment by positing an impossibly tall mountain from which his cannon was fired. If in a stable orbit, the projectile would circle the planet and return to the altitude of launch after one orbit (see Newton's cannonball).[4]

For a space gun with a gun barrel of length (${\displaystyle l}$), and the needed velocity (${\displaystyle v_{e}}$), the acceleration (${\displaystyle a}$) is provided by the following formula:

${\displaystyle a={\frac {v_{e}^{2}}{2l}}}$

For instance, with a space gun with a vertical "gun barrel" through both the Earth's crust and the troposphere, totalling ~60 km (37 miles) of length (${\displaystyle l}$), and a velocity (${\displaystyle v_{e}}$) enough to escape the Earth's gravity (escape velocity, which is 11.2 km/s or 25,000 mph on Earth), the acceleration (${\displaystyle a}$) would theoretically be more than 1,000 m/s² (3,300 ft/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[5] during the ~10 seconds such a firing would take.

The most prominent recent attempt to make a space gun was artillery engineer Gerald Bull's Project Babylon, which was also known as the 'Iraqi supergun' by the media. During Project Babylon, Bull used his experience from Project HARP to build a massive cannon for Saddam Hussein, leader of Ba'athist Iraq. Bull was assassinated before the project was completed.[7]

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 (SHARP) 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.

After cancellation of SHARP, lead developers John Hunter founded the Jules Verne Launcher Company in 1996 and the Quicklaunch company. As of September 2012, Quicklaunch was seeking to raise $500 million to build a gun that could refuel a propellant depot or send bulk materials into space.[8][9][10]

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.