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Momentum Exchange Tether Systems
Moravec
investigated whether it was possible to design a rotovator for the Earth
and other planets[6]. Unfortunately, his analysis showed that unless very
strong tether materials can be found, the mass ratio of a rotovator for
the Earth or other large planet becomes too high to be practical to build.
This is unfortunate, because otherwise, it would be possible to build tether
transport systems that would allow travel between the surface of the Earth
and the surface of any solid body in the solar system without requiring
fuel, provided the mass flow in toward the Sun slightly exceeds the mass
flow outward. Moravec did find that rotovators were feasible for the Moon
and other small airless bodies. Rotovators could be designed that would
touch down from one to n times per orbit, but the tether mass was minimum
for a rotovator that had a total length one-third the diameter of the body,
was in an orbit with an altitude of one-sixth the diameter of the body,
and rotated so that it touched down six times per orbit. In a later, unpublished
paper, Moravec designed a rotovator made of the Dupont fiber Kevlar for
use around the moon. Recent calculations indicate that with current materials such as Spectra 2000, with a tensile strength of 3.25 GPa and a density of 0.97 g/cc, or
By using tether reels and small thrusters on the "grapple" structure at the tip of the tether that attaches to the payload, the time for depositing the payload on the surface and picking up a new payload can be increased to many minutes. The payload would be "flown" in early by letting out cable from the tip reels and using a combination of rockets and lunar gravity to get to the surface and land earlier than the tip would normally arrive. As the payload sits on the ground and the tether descends, the grapple reels would reel in the excess cable. A well designed cable reeling system would not abruptly relax all tension in the cable as the payload touched the lunar surface, but would maintain most of the payload weight by cable tension so as to minimize transients in the main tether. After the nominal touchdown time has passed, the payload can remain on the ground for an additional time by merely releasing cable as the main tether starts to pull away. After the payload transfer has been safely completed, the rate of unreeling of cable would be decreased, and the payload lifted from the surface.
The
Cable Catapult
Another concept developed by Dr. Forward of Tethers Unlimited is the Cable
Catapult System. In this system, a very long tether is used as a launch
rail. A long tether is extended in space and pointed towards the target.
The payload is attached to a linear motor powered by an external electrical
source, and the linear motor "climbs" the tether, accelerating the payload
up to launch speed. At the launch point, the payload is released to travel
on to the destination while the linear motor is decelerated to a halt on
a shorter section of tether.
Figure 4. The Forward Cable Catapult System Concept
This concept has the potential to enable launch velocities 30x the characteristic speed of the tether material. With advanced materials, launch velocities of 30-100 km/s may be possible, enabling interplanetary travel with durations of months rather than years.
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