Momentum Exchange Tether Systems
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.
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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