Space tethers and guns can greatly reduce the amount of propellant that must be carried by the rocket launchers and they can reduce the reentry velocity as well.

When the rocket is launched, it has just enough propellant to accelerate itself and its cargo to 3 km/s. When it has reached this velocity, it docks with the umbrella projectile which carries all the propellant that accelerates the rocket and its cargo to 6 km/s. The projectile is fired from the two stage light gas gun. (The optimum flight profile of the first stage of a conventional rocket launcher is similar to the flight path of the ballistic gun projectile.)

When the rocket reaches 6 km/s, it deposits its cargo near the rotating end of a bolo. A winch riding on the bolo pulls its cargo to the center of the bolo's mass. The empty rocket slides to the end of the bolo, which rotates at 3 km/s relative to the bolo's center of mass. The empty rocket rides on the bolo until it is released near the rocket launch site.

The empty rocket reenters the atmosphere at 5 km/s, so it does not need expensive thermal shield to survive the reentry. This rocket does not need staging and expensive turbopumps, but it has to be sturdy enough to survive reentry and splashdown. The best rocket design is the engine cluster.

The gun is a fixed structural part of a submarine, so it has the ability to sail to any latitude and aim at any angle. The gun is so large that it has to be cast in small segments. A recoilless gun is cheaper than a recoil gun because the axial force is lower and the joints between the segments do not have to be strong. The umbrella projectile is filled with liquid oxygen and liquid methane. It is placed inside a strong, reusable sabot to minimize its stress during the firing. The umbrella projectile does not tumble above the atmosphere, so the rocket can dock with it quickly. Erosion of the internal surface of the gun is reduced by encasing the sabot in Teflon.

A bolo made of high-strength plastic would be vulnerable to space radiation and thermal fatigue. Perhaps the best material for the bolo is a rope made of strong (6.5GPa) carbon fibers coated with a thin layer of aluminum and fused together in a hot press. The aluminum layer is used as the electrodynamic tether. The electrodynamic tether is powered by solar cells and used to replenish the orbital energy of the bolo. The bolo must be attached to massive piece of orbital junk so that it does not deorbit after cargo capture.

The optimum flight profile

The optimum flight profile of a conventional rocket launcher (Ariane 44 L)

Recoilless submarine gun

Recoilless submarine gun