Launching material off the moon for use in space is a problem in itself. Many methods are feasible technically. Far less are economically sound. Sending the propellant from Earth for instance is something that should only be done for the very early missions for extreme value payloads like an astronaut or a pure helium 3. The more that can be done with an in situ technique the better. Especially if it can be done on the real cheap, relatively of course.
Launch cannons are discussed for Earth launch on occasion and dismissed just about as often. Lunar gun launch is a different story. With an extruded tube twenty kilometers long it is possible to reach Lunar orbital velocity with accelerations averaging under ten gee. One bar of pressure could accelerate three tons at ten gee with a tube two meters in diameter. More or less pressure would be used for different masses with a thirty ton payload only needing ten bars pressure to hit the ten gee average acceleration.
Two hundred meters of launch tube would be sufficient for hardy payloads capable of tolerating thousand gee accelerations. Early in colony development chunks of frozen oxygen might fit that bill with the metal containers as part of the product delivered to a Lunar orbiting factory. Firing the gun against vacuum instead of an atmosphere really allows guns to show their best side.
All that is hardly new. One thing I haven’t seen as much of is methods of conserving the drive gas from the launch. I suggest that a reusable sabot be part of the standard equipment to support the payload and seal against gas leakage. That also has been thought of. I don’t know of any proposals to conserve all the drive gas for use again and again.
What may be original is the concept of using electromagnetic braking to stop the sabot inside the tube after it has imparted sufficient velocity to the payload. Electromagnetic braking can be done without the massive electrical generation equipment of the various mass drivers often suggested. It can in fact generate electricity while stopping the sabot. The massive power generated during that braking second or two should be usable somewhere in a colony that is still under construction.
The drive gas will start to be evacuated from the gun barrel into storage tanks as soon as the payload reaches design velocity. The gas scavenging should continue as rapidly as possible to relieve pressure on the sabot that hopefully extended the mechanical brakes as soon as possible to reach a complete stop well before the end of the gun barrel. The scavenging tanks should have large volume at low pressure to evacuate the barrel quickly. The drive tanks should be high pressure and low volume to facilitate good acceleration control. The sabot is returned to the gun breach area after the barrel has been evacuated to set up for the next shot. The drive tanks can be repressurized from the scavenge tanks at leisure as long as it is quick enough for the next shot.
The extrusion process should be able to turn out a gun barrel of continuously increasing length early in a colony development. As soon as the first two hundred meter section can handle ten atmospheres it could be used to send 300 kg payloads to lunar orbital velocity. The payload would either need to be caught into a circular orbit or provide a circularizing burn in order to avoid lithobraking at the launch site. The payloads would gradually increase in mass as the barrel extended until reaching the 20 kilometer length that would allow the sabot four kilometers of braking room after loosing the payload.
Muzzle loading air rifle for lunar launch with 100% volatile recovery. Low tech and cheap enough enough for you?