# Driving on the Moon

## Galactic Wheeled Vehicle Speed Record

On Luna it is possible for wheeled vehicles to go faster than on Earth or Mars, because there is no air resistance. One reaches a limit caused by wheels flying apart from centrifugal force at high speeds. That limit can be circumvented by having a large diameter nonrotating hub support a relatively thin ring of a wheel by magnetic force. At 2000 miles per hour, 894 meters per second, the rim of a 30 meter diameter wheel needs a 26,641 meters per second squared (2700 g) pull from the hub to keep it going around in a circle instead of flying off on a tangent. If the steel rim (or steel tire) has a cross sectional area of 6.35 square inches (41 cm^2) then an inch length (2.54cm) of the rim masses 819 grams (1.8 pounds) and requires a force of 4860 pounds (21800 newtons). That sort of magnetic force could support a steel rim from a non rotating hub. It might be possible with specially strong steel wheel tapering from a thick hub to a thin rim to have a solid wheel hold together at that size and speed. But if we go to the ultimate case a solid rotating wheel will definitely not work.

## Wheel Launch to Orbit

The ultimate case is a hub sixty kilometers in diameter in low orbit around Luna at the equator. Instead of a nonrotating hub, the hub is built in orbit and rotates once per orbit to keep the bottom side down and the top side up. The wheel would be a circular ring, 60 kilometers inside diameter and 60.005 kilometers outside diameter. It would run on a track on the rim of the hub at 1635 meters per second relative to the hub and stationary relative to Luna wherever it is at the bottom of the hub. It would touch Luna's high spots with feet on a suspesion system with springs. The wheel would experience 92.31 meters per second squared (about 9.42 g's) including centrifugal and tidal forces. The hub would experience the stress from supporting the wheel and 0.0816 meters per second squared tidal force near Luna's surface and 0.0784 meters per second squared at the top. Tidal forces are zero at an altitude of 30 kilometers. The hub can be made as thick as it needs to be to support the wheel without calling for any super strong materials because the added stress from the hub's additional thickness is based upon the weak tidal forces it must resist. Initially it would be possible to use just a small arc of the wheel and spin it up to soft land cargo on Luna and pick up cargo. Eventually mountains could be lowered and valleys filled in so a wheel would be constantly rolling down the road maintaining orbital speed and altitude for the hub while two other wheels would spin up and spin down on the same hub to transfer cargo to and from the first wheel or directly to and from Luna.

Another means of maintaining orbital momentum for the wheel is for it to extend a grapple to a portion of track ahead of the wheel from the rim and pull on the grapple while pushing on a portion of track behind the wheel. This would convert electrical energy directly into increased orbital velocity. To decrease orbital velocity the grapple would extend to a portion of the track to the rear of the wheel on a line from the rim that would turn an electrical generator as it extends. A rod extended to the track in front would turn the gears of a generator as it was pushed closer to the track as the wheel rolled. Such rods and grapple lines would need to be nearly ten meters long if they were to extend from the wheel to a portion of the track one degree ahead of and one degree behind the wheel.

While wheel launch to orbit is heavier and more complicated than eddy current braking to orbit, all of the necessary technology seems to be ready. Magnetic levitation would need to work at higher relative speeds than on Earth's railroads, but the trend is that there is an increasing lift to drag ratio at higher speeds, so it should work better than on Earthly railroads.