A Lift Engine Elevator

In the online literature, most elevators consist of a fixed tether of some 100,000 km radius, with a counterweight at the top. Payloads climb the tether from the base to the counterweight under their own power, which is beamed up to them by lasers. Usually there are two tethers: one for rising payloads, and the other for descending payloads.

But another approach would be to install a lift engine on the counterweight, and haul up the tether below, on which payloads would would be attached. Rather than have the payloads work to climb the tether, the entire tether with attached payloads would be winched up to the counterweight.

Several objections immediately present themselves, however. Such a system would require a continuous feed of new tether material at the base of the elevator, and an equal loss of winched up tether from the lift engine. How is new tether to be supplied? And what is to be done with the winched-up tether at the counterweight? There is a simple answer to this: the tether forms a loop rather than a single strand, with one half going up, and the other going down. The tether would turn on pulley wheels at each end. And the pulley wheel at the counterweight would be powered by the lift engine.

But if there is a pulley wheel both at bottom and top, the logical place to put the lift engine is not at the top, but at the bottom. And this would make supplying power to the lift engine very simple. It could be any sort of engine: an electric motor, a diesel engine, a steam engine.

But a further objection to this proposal is that a fixed tether is usually tapered along its length, reaching a maximum cross-sectional area at the synchronous point, where tether tension is at a maximum. How can a loop of material be tapered? Clearly it cannot, and must be of constant cross-sectuonal area along its entire length.

One possible solution is to put further loops of tether inside the main loop, such that they meshed with the rising and falling main loop tether. And since these inner loops would have one side rising at the main loop speed, and the other side falling, they would remain in place within the main loop. In this manner, layers of tether could be meshed together to produce an effective taper with a maximum cross-section at synchronous.

There would perhaps be no need for the inner loops to be connected to the Earth. So long as the several loops mesh together, one side will be rising as rapidly as the other descends, and inner loops will remain in place. Further pulley wheels might only be required to keep the rising and descending tethers separated.

But there is a clear problem of how to get multiple tether loops to bind together to form a single 'up' or 'down' tether ribbon. They are likely to slide over each other, losing traction. Either such multiple loop tether ribbons would have to have cogged surfaces, or vices would need to be attached along the conjoined regions to hold separate ribbons tightly together. Both solutions present mechanical difficulties.

The tether loop system could be slowed, stopped, and even reversed if necessary. The lift engine could be slowed and stopped to allow payloads to be attached or detached from the tether loop. If tether attachment points were sited at even intervals along the tether, it could be arranged that a payload would be detached from the tether at the top at the same time as a payload would be attached at the bottom.

A tether loop would also allow relatively easy maintenance and replacement. With the lift engine stopped, old, worn, or damaged tether loops could be cut, and a new tether length fixed to the ends of the the old tether. When the lift engine was restarted, the new tether would be drawn up, while the old tether was payed out. When the new tether had completely replaced the old tether, the lift engine would be stopped, the old tether detached from the new tether, and the two ends of the new tether bonded together to complete the new tether loop. Replacing the inner tether loops would be a bit trickier, perhaps.

Furthermore, since it appears that space elevators tend to sway east and west if there is a greater flow of mass up them than down, a tether loop system might provide a little countering stiffness. And if the tether loop system was augmented with an adjacent cantilever truss, considerably more stiffness would be provided. Such a truss would provide access to the entire tether loop system, and also provide anchor points for pulley wheels.

Such a tether loop system, on the scale of an elevator, would allow the essential components of a siphon to be developed and tested on a smaller scale than a siphon. A 65,000 km radius tether loop elevator could be extended to become a 250,000 km radius siphon. At which point an electrically powered lift engine would become an electricity generator. And in a siphon, there might be two or more tether loops

The simplest space elevator is clearly a single tether with a counterweight at the top, with payloads climbing up and down the tether. Since payloads travelling up and down are likely to collide on a single tether, either a single tether would have to alternate between traffic going up and traffic going down, with 'traffic lights' at top and bottom. Or two tethers would be provided, one for up traffic, and the other for down traffic.

The principal advantage of the tether loop system would seem to be that the power driving the system could be delivered at ground level, where it is easiest to provide it. Payloads would not have to ascend or descend under their own power, even if that power was remotely supplied by lasers or some other means. They would simply have to hold on to the moving tether. Another advantage is that the revolving tether systems are, in principle, fairly easily replaceable. A further advantage of a twin tether scheme would be that it would allow bracing between the two tethers to give the system some degree of stiffness, and allow the rotational energy of the Earth to be directly applied to the elevator. A stiffened elevator would not swing around, or swing around quite so much.