Segment-Based Elevator Architecture
Background
The idea of mechanical connectors was introduced by Andrew Price at the 2nd SE conference last year as a means to rapidly deploy a functioning full-thickness ribbon.
The technique was motivated by the desire to eliminate the in-space layer bonding technique that is at the heart of the seed elevator deployment design.
On closer examination, however, it became clear that the advantages offered by this design make it into a superior ribbon architecture, independent of the deployment scenario.
The segment based architecture makes the elevator much easier to construct and maintain, and therefore feasible just that much sooner.
Overview
The central premise of this architecture is the use of discrete segments and mechanical connectors as basic building blocks.
The elevator structure thus becomes modular, since segments can be replaced as they wear down or are damaged.
(A method for replacing segments under full tension while observing make-before-break is illustrated below)
In terms of development and fabrication, this architecture benefits us in several ways:
- Deployment: During deployment, the ribbon segments can be fully tested and qualified on the ground before being deployed, since the mechanical connection process is discrete and easily lends itself to full inspection in orbit.
- Operations: Segment replacement offers a viable approach to both ribbon wear and damage repair.
- Feasibility: The set of technical challenges required from the CNT material is reduced, since we do not have to develop any of the in-space layer-bonding technology on top of the CNT production technology.
- Development Schedule: The engineering associated with segment replacement can be developed in parallel with the material program, sine the connectors do not interact with the micro properties of the CNT material.
- Flexibility: Segments offer a natural way to deal with the different ribbon structures that will be required at different altitudes.
Segment Replacement
In order to achieve the self-maintenance objective, we need to come up with a method of replacing a ribbon segment under full load.
Below is a conceptual representation of two ribbon segment ends and a suitable connector system.
Figure 3-1: Conceptual connector mechanism
Note the ability of the triangular yoke to accept a third segment end, and to rotate in response to load changes.
For a sense of scale, the ribbon is 1m (~3 feet) wide, and the yoke is about 18" across. The individual segments are 100-1000 km long.
The diagram illustrates only the functional workings of the connector. We have several mechanical implementations for connector systems, not necessarily based on this 2-pin-and-a-triangle concept.
We are now ready to look at the replacement sequence.
In the sequence, we follow the following rules:
- Single climber operation.
- Make-before-break.
- No sudden load transfers.
- Climbers cannot drive across "y" junctions (where 3 segments are attached).
- Traction drives can support 20 tons (a climber's worth), but cannot hold full ribbon tension - for that purpose we must use a deployment-style accumulating spool.
Fully loaded segments are drawn continuous. Unloaded ones are drawn dotted. Half-loaded ones are drawn dashed.
Figure 3-2: Segment replacement sequence
And now, for the step by step action:
- Climber carries a replacement ribbon, plus an attached strain-relief tail, to the top-side connector.
- Climber attaches the ribbon to the top yoke, and travel towards the bottom yoke.
- Climber attaches to the lower corner of the lower yoke, (or top of the lower ribbon) and begins to spool in the strain-relief tail, until the ribbon-to-ribbon connection matches the right eyelet of the yoke.
- Climber unspools a little bit to relieve the tension, and disconnects the tail from the new segment.
- Tension is now distributed equally between old and new segments.
- Climber connects the tail to the old segment, and spools in a little bit to take on the load.
- Climber disconnects old segment from yoke, and begins to unspool the tail, until the tension on it disappears.
- Climber releases tail and travels towards the top-side yoke.
- Climber disconnects the old ribbon segment from the top yoke, and carries it away.
The strain relief tail is used to compensate temporarily for the difference in length between the stressed and unstressed segments. The spool mechanism must be able to handle full ribbon tension, which can reach several times the limit of what the climber traction drive is designed to handle.
The two parallel stretches of ribbon are a source of concern — they must not tangle or adhere to each other. Solutions to this problem will depend on the ribbon micro-properties.