Eddy Current Brake to Orbit
This works by having a long slotted aluminum tube in orbit and having the shuttle vehicle that is launched to orbit enter this slot while the tube is moving by at orbital speed. Magnetic flux from permanent magnets deployed by the shuttle is directed at the walls of the tube causing a repulsive force and a retarding force.[1] By reducing the relative speed of the orbiting aluminum tube and the shuttle, the shuttle is brought up to near orbital speed. Since eddy current braking loses effectiveness at low relative speeds, friction braking is used to give the shuttle the last 18 meters per second to reach orbital speed and match velocity with the aluminum tube. During all the time that the relative speed of the shuttle and the aluminum tube is greater than 18 meters per second, there is no contact between the material of the shuttle and the material of the aluminum tube.
The aluminum tube requires electrically enhanced orientational stability because the tube needs to have extremely slight deviation from the absolute orbital path to not communicate radial or transverse motions to the shuttle. This is achieved by shifting masses along the length of the tube to counter any tendency to pitch up or down. Electrically enhanced rigidity is achieved with electromagnets varying the tension on selected tension members in the structure as is determined to be necessary by continuous laser measurements. Other sorts of transducers could also provide the needed effect. Fine yaw control is provided by reaction wheels sized for adjusting the yaw of the spacecraft carrier. Course yaw control is provided by steering with the electric thrusters to unload the reaction wheels.
The structural material holding the aluminum in place varies from aluminum to steel as necessary to avoid having the speed of sound in the structure match the speed of the shuttle as it moves through that portion of the tube. This avoids having a destructive shock wave move through the structure.
The tube requires high specific impulse electric thrusters to maintain orbital momentum when donating some momentum to the less massive shuttle. Thrusters of sufficient force are possible because they can be made very massive since they would not be supporting their own weight, just restoring the small number of meters per second the tube loses by interaction with the shuttle. A tube in orbit around Luna would use oxygen as reaction mass for the electric thrust because oxygen is readily available on Luna. VASIMR thrusters can use oxygen as a reaction mass because the VASIMR thruster has no electrode in contact with the heated plasma and will not have the problem of electrode oxidation. A magnetoplasmadynamic thruster using hydrogen that NASA is developing would have problems with electrode oxidation if switched to oxygen reaction mass. The use of oxygen will also have an effect of lowering the maximum achievable specific impulse. A guestimate is 25,000 m/sec exhaust velocity instead of the 100,000 m/sec anticipated for NASA's thruster. This is the result of oxygen having 16 times the atomic weight of hydrogen. Still, the oxygen fueled VASIMR would be able to generate sufficient momentum to bring a 2.5 metric ton shuttle up to orbital speed using only 170 kg of oxygen reaction mass. So bringing up oxygen from Luna to fuel the thrusters should not be an intractable problem. A spacecraft carrier orbiting Earth would use hydrogen fueled thrusters as above.
This sort of device is possible for the moon, Mars, and Earth. A similar device would work for moving from orbital speed to a stop on the lunar surface.
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Power Conservation
Replacing the simple aluminum of the tube with electrically conductive wires, insulation, and electrical power storage would recover some of the energy lost in the momentum transfer from the tube to the shuttle. The eddy current brake to orbit device could be built when it becomes economical to ship large masses of construction material from Luna to lunar orbit.
There have been objections to mass drivers because of large current switching requirements needed to have drive coils first attract the bucket then turn off to not retard the bucket on the way to orbital velocity. There is no such current switching requirement for the eddy current brake to orbit device.
Shifting mass from the ends toward the middle of the tube causes a decrease in the moment of inertia, so the angular rate of the tube (which averages about one rotation per orbit) increases to maintain constant angular momentum. This causes a pitch down movement relative to the horizon. Shifting mass from the middle towards the ends causes the opposite effects.
Because the entire tube moves at the speed of the center of the tube rather than at the speed dictated by the radial distance form the center of the elliptical orbit, the cross sections of the tube from leading end to trailing end all follow similar orbits but with the perilune shifted forward for leading cross sections and rearward for trailing cross sections. The slotted tube that interacts with the shuttle occupies a circular arc centered on the center of Luna at all times to maintain unstable equilibrium with respect to pitch.
Landing on Luna
Setting down on the lunar surface by eddy current braking has some advantages over using a tether. With a tether the vehicle to be landed must achieve the correct orbital speed at the correct position at the correct time to rendezvous with the tether and be dropped to the moon. With the eddy current braking device, the vehicle to be landed only requires the correct position and correct direction of movement with respect to the slotted aluminum tube to properly enter the eddy current braking device and brake to a stop. It is much like landing an airplane on a runway. The differences are that an airplane does not have such a high relative speed to the runway, the runway is somewhat wider than the eddy current braking slot, and the airplane must contend with varying wind conditions which would not affect eddy current braking on Luna. Also for an eddy braking slot there is no requirement for super strong structural materials as for a tether.
If the orbiting tube imparts velocity to the shuttle at a rate of 30 meter per second per second, then such a tube for Luna should be about 59 kilometers (37 miles) long. With the same three g acceleration such a spacecraft carrier in orbit about Earth would need to be 1030 kilometers (644 miles) long. As with other low cost per unit mass systems of launching to orbit, a high mass rate of launch is needed to realize the potential low cost per unit mass.
Potential for future expansion
A shuttle only needs to reach orbital altitude to land on a spacecraft carrier and thus obtain orbit. This requirement is modest enough that a fully reusable shuttle is possible for Earth. If the market for space transportation at reduced costs is large enough, there could be a dozen spacecraft carriers in low equatorial orbit about Earth and dozens more in orbits 40 degrees inclined to the equator with such perigee and apogee as necessary to avoid collisions. Fail-safe engineering can be a feature in spacecraft carriers orbiting Earth as long as they are confined to equatorial orbit or some single orbital plane. The structure can be mostly aluminum which burns on re-entry to the atmosphere. It should be made to break into small pieces if it experiences runaway pitch instability. The pieces should break up from tidal force and the heat of re-entry so as to cause minimal damage on the ground. When there are more than a threshold number of spacecraft carriers in 40 degree inclined orbits the whole system would be subject to a cascade of debris forming collisions caused by debris. This may be beyond the capability of fail-safe engineering to mitigate.
