Capture tube is a simple tubular device which captures cargo and accelerates it by magnetic drag or friction. The minimum mass is only 100 tons.
There are three designs of the capture tube: elastic tubes (electrowheel and electrotube), telescopic tube, and rigid tube described in this file.
The rigid tube is filled with argon gas and loose dust or snow. A ball-shaped cargo enters the tube through gate valves, silencer shaped baffles, and plasma window. These devices reduce gas leakage into the outer space. The cargo is launched from a gun, a sling, or a sounding rocket before it enters the capture tube. GPS guides the projectile into the capture tube with the precision of 20 centimeters. When the cargo enters the tube, it is accelerated to the velocity of the tube by drag against the gas and dust. A gyro mounted in the capture tube helps stabilize it.
If the cargo is brought from the Earth and the Moon, the momentum of cargo (raw materials) transported from the Moon is balanced by the momentum of cargo delivered from the Earth. No additional means of replenishing orbital energy is needed. See details of a similar idea in Moon-Earth momentum exchange.
The rigid capture tube will buckle when its length is greater than its diameter multiplied by 100. Buckling limits length of the rigid tube to several kilometers, which corresponds with cargo acceleration inside the capture tube on the order of 1000 g.
A very cheap capture tube can be made from empty fuel tanks joined with silicone rubber. The tanks are filled with dust only (no argon gas), so the expensive plasma window is not needed. Electrodes stir up the dust to ensure that its density is uniform.
Edward F. Marwick (Northfield, Illinois), "Crashportation and Edportation," Proceedings of the Seventh Princeton/AIAA/SSI Conference, May 8-11, 1985.