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. 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 squared (m/s^2). 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 barrel would be evacuated before launch. Cold hydrogen would be injected behind the shell at the breech to start it moving. Later transverse injection of hydrogen would be heated as necessary 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 a gas bearing preventing the shell from contacting the barrel. This would lessen maintenance needed, compared to generating gas for a centering force for the shell by having a plastic sabot wear away. Plastic sabots are more difficult to get on Luna. The power to inject hydrogen at high pressures during the launch of the projectile is provided by burning a hydrogen oxygen mixture in cylinders outside of the barrel and driving pistons through the cylinders to force hydrogen through ports into the barrel. Electronics for timing the firing the various cylinders will be critical for the operation of the gun. For ports at every 12.7 cm (5 in.) over 30,000 diametrically opposed pairs of injection ports would be required. The ports could be associated with over 30,000 cylinders or with 7600 cylinders if the ports are ganged 4 pair to a cylinder. The transverse injected gas gun is considerably more complicated than a two stage gas gun with the gas all injected at the breech of the gun. The advantage of transverse injection is being able to maintain reasonably constant acceleration down the length of the barrel.
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 shell that is 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, about 70% oxygen. 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.
Transverse injected hydrogen gas guns are mentioned on a NASA associated web page. An advantage of a version of such a gun as described in this article is that it does not need a high rate of electric power at launch. It can use stored chemical power. Launching of some cargo could start before the high power rate is achieved.
As above noted, hydrogen gas guns are used on Earth for research with projectile size typically less than a gram. This type of gun also is considered for use in launching to orbit from Earth in which case the projectile should be more than ten metric tons in order to punch through the atmosphere effectively. On Luna the transverse injected hydrogen gas gun can have a moderate size projectile of 65 or 78 kilograms and function effectively. On Luna the amount of heating required to bring the speed of sound in hydrogen up to 2280 meters per second is a mild 687 C, on Earth to bring a shell up to circular orbit velocity would require bringing the speed of sound in hydrogen up to 7390 meters per second. This would require heating the hydrogen into the region in which dissociation must be accounted. So additional engineering concerns would need to be addressed.
- White Sands Test Facility Two-Stage Light Gas Guns
- Web pages on this topic can be found by searching for "Transverse gas guns". The particular page I found had its web address fixed so it cannot be linked from a wiki.