Valier-Oberth Moon gun
In the 1920s members of the Verein für Raumschiffahrt amused themselves by redesigning Jules Verne's Moon gun, the Columbiad. In 1926 rocket pioneers Max Valier and Hermann Oberth designed a gun that would correct Verne's technical mistakes and be capable of firing a projectile to the Moon. The projectile would be made be of tungsten steel, practically solid, with a diameter of 1.2 meters and a length of 7.2 meters. Even using the latest gun propellants, a barrel length of 900 meters would be necessary. To eliminate the compression of air in the barrel during acceleration, it was proposed that the barrel itself be evacuated to a near-vacuum, with a metal seal at the top of the barrel. Residual air would provide enough pressure to blast this seal aside before the shell exited the gun. To minimize drag losses in getting through the atmosphere, it was proposed to put the mouth of the gun above most of Earth's atmosphere: it would be drilled into a high mountain of at least 4,900 meters altitude.
See also space cannon.