Hydrogen Gas Gun
Cargo could be launched from Luna with hydrogen gas gun. NASA's White Sands Test Facility uses a hydrogen gas gun to fire projectiles up to 7500 meters per second.[1] but to launch things to lunar escape velocity only requires 2400 meters per second. The task seems doable. The hydrogen must be recycled on Luna and they catch the hydrogen from their gun at White Sands too. Of course the device would need to be scaled up a bit to use it for launching cargo. White sands uses projectiles on the order of a gram. It would be more reasonable to launch cargo from Luna in 78 kilogram projectiles. An acceleration length of barrel of about 3840 meters (2.4 miles) is more than they use at White Sands, but on Luna that length would be helpful for reducing launch acceleration in this example to 750 meters per second. Then instead of a two stage gas gun, the cargo launching gun would be a transverse injected hydrogen gas gun. The shell would be 15.25 cm diameter (6 in.) 183 cm long (6 feet) with the tail 38 cm (15 in) as a cone. That is torpedo shape with no fins. The hydrogen would be injected cold behind the torpedo at the breech. Later transverse injection of hydrogen would be heated to bring the speed of sound in the injected hydrogen up to 0.95 times the speed of the shell. This would cause the shell to leave the injected gas behind as it moves down the barrel. The hydrogen injected from side ports would act as an gas bearing preventing the shell from contacting the barrel. This would lessen maintenance caused by generating gas for a centering force for the shell by having a plastic sabot wear away. Also plastic sabots are more difficult to get on Luna.
In a transverse injected hydrogen gas gun the injected hydrogen is not given any forward velocity by expansion. It expands against the tail cone of the shell as the shell passes to accelerate the shell. The hydrogen gas is only drawn forward down the barrel by skin drag from the shell. So recycling the hydrogen should be simple in concept. A door over the muzzle end of the barrel is closed after every shot and the barrel pumped down to vacuum. A whiff of hydrogen would be lost with every shot.
With the given dimensions the shell would have a volume of 28900 cm3 for a solid aluminum shell that would be 78 kg of cargo. For a steel tank with a 0.2 cm thick wall and a 15.05 cm inside diameter full of liquid oxygen the steel weighs about 13 kg and the oxygen weighs about 32 kg. Shells could hold spools of fiber glass or sifted sand of various types. Most things that could be mined or manufactured on Luna could be exported to L2 with such a gun. This is just a rough hypothetical example. A more refined device would actually be produced.
