Tether Climber Engineering

This page outlines one view of the Engineering challenges of a Space Elevator Climber : it describes general concepts for consideration but does not offer detailed design solutions.

FUNCTIONAL REQUIREMENTS

The design of any device or system must first focus on the Functional Requirements. These can be characterised as Primary or Secondary, or can be ranked in priority order.

For the Space Elevator climber the Primary Functional Requirements are :

1. Apply a Tractor Force to the Tether

2. Climb the Tether

   • At Speed, without damaging the tether

   • With high Durability and Reliability

3. Carry an Adequate Payload

Secondary Functional Requirements include :

1. Achieve Primary Requirements at an acceptable cost

2. Descend the Tether

( Travel from the Elevator GEO Node to the Apex Anchor is effectively a ‘descent’, so for this portion of the tether this Requirement becomes ‘Primary’. The Requirement to descend from GEO to the Earth’s surface must be confirmed by economic and operational analysis. )

TRACTION OPTIONS

There are two fundamental means by which the climber could apply a traction force to the tether.

1. By mechanical contact (Friction Drive)

2. With little or no mechanical contact (Electromagnetic)

These options are summarised in the slides below.

These two traction options impose Functional Requirements on the tether, summarised below.

It should be added that the electromagnetic drive options are likely to need less forceful contact with the tether, leading to a lower risk of causing damage.

These requirements highlight that neither the tether or the climber can be designed in isolation : a climber can only ascend the tether if the tether characteristics are compatible with the climber traction system. At present the properties of prospective Earth space elevator tether materials have not been measured on macro-scale samples : without these details it is difficult to finalise a climber design concept, and tether characteristics could limit the feasible climber mass.

Climbing at Speed

The second Functional Requirement for the climber is that it should ascend the tether at considerable speed. If the requirement is to ascend the 100,000 km Earth Space Elevator in two weeks then the average speed must be 82.7 m/sec (298 km/hr, 186 mph), but a constant speed is unlikely : if the climber power is limited then the maximum climber speed will increase at higher altitudes as the effective weight decreases ( see Papers on Climber Design and Power for discussion of this in detail ).

Speeds in excess of 100 m/sec are achieved on some terrestrial wheeled vehicles, although the engineering is challenging. Even higher tracked vehicle speeds are being achieved by using electromagnetic (LIM) drives, and such a solution may well be needed on the Earth Space Elevator to achieve target transit times.

As before, detailed tether material properties are required before climber concepts can be finalised. The high-traction friction drives needed at low altitudes may not be capable of high speed operation, whereas high-speed LIM drives may not be capable of providing sufficient traction at low altitudes with acceptable power levels.

Safety, durability and Reliability

A third critical climber Functional Requirement is that the climber is safe, durable and reliable : there must be a high demonstrated confidence that every climber departing the Earth Port will successfully reach its destination.

Climber reliability must be demonstrated during the Engineering development phase. No climber could be allowed to climb an Earth Elevator unless high reliability levels have already been demonstrated beforehand.

High levels of component redundancy have always been part of spacecraft design, and the climber will not be an exception. A single large climber may have many single-point failure modes : one mitigation option would be to have multiple smaller drive units. Multiple drive units would have other benefits, as shown in the second slide below.

The current initial system concept envisages a climber of 20 tonne gross mass, with 14 tonnes of payload and 6 tonnes of ‘climber’. The multiple drive unit concept would not change this, the ‘climber’ mass would include the multiple drive units and structural chassis. The size and number of drive units would depend on design optimisation, but could perhaps be ten units of mass 500kg. The climber power source, probably solar panels, could be mounted on either the chassis or drive modules.

Another benefit of the multiple drive module concept would be the relative ease of producing more or less massive climber assemblies to meet non-standard operational or payload needs.

Other Design Considerations

There are a number of other Design Considerations that arise from the Functional Requirements listed above, some of these are described in the slides below :

The ‘Parking Brake’ will also be necessary if the climber is solar powered and so needs to pause on ascent when in the shade of Earth. The duration of these night-time stops will become shorter as the climb progresses, with the sun only totally eclipsed at all altitudes for a few dates close to the equinoxes.