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APPENDIX G

THE MASS CATCHER

The catcher is equipped with radar capable of detecting payloads 10 s before arrival, that is, 2000 m away. A signal from the radar is processed to locate the spot at which the payload will cross the catch area, and the net is manipulated into that position for interception. Having captured the payload, the net and reel assemblies (rigs) act to decelerate it from its incoming velocity of 200 m/s to 20 m/s. The payload is then released into a storage depot and the rigs return to their original position by means of the closed loop tracks. The estimated cycle time is 60 s.

With 60 such rigs on a single catcher, it can catch 0.32 X 10^6 t/yr on a 100 percent duty cycle. In order to catch 11 X 10^6 t in 10 yr, 3.5 catchers on the average have to be operational at all times, each catching from a separate stream of payloads shot from the same mass driver. The installation of 5 catchers at L2 provides adequate margin for downtime for maintenance.

The time history of a payload and the rigs is shown in a two-dimensional representation in figure 5-29 and is detailed as follows: The payload enters the catcher area with a velocity v = 200 m/s where it is decelerated constantly by 30 m/s^2. When its velocity reaches 20 m/s it is released to a storage depot attached to the rear end of the catcher frame. The reel assemblies and net motions are divided into three stages:

  1. From zero to 2.5 s, reels are accelerated by pull from the payload while cords are being released until the velocity of the rig matches that of the payload.
  2. From 2.5 to 6 s, both rig and payload are decelerated by a constant value of 30 m/s^2 until their velocity reaches 20 m/s. Energy is stored in spinning a flywheel in the reel assembly.
  3. From 6 to 9.57 s, the cables are reeled in and the reel assemblies accelerated until the net clears the payload. The net is then pulled toward the inside track. The payload then proceeds on its own with constant velocity to the storage depot, while the rig circles around to the return track.
The above analysis is made for payloads arriving at the center of the circle enclosed by the triangular frame. If a Gaussian distribution is assumed most of the payloads arrive in this neighborhood. For payloads arriving away from the center a more detailed analysis is needed. It is expected that for such payloads, centering takes place initially and that by the end of the deceleration period the payload has been brought very close to the center. It is possible that some parameters may have to be adjusted to avoid any possibility of snarling.

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