Challenges in Designing the Best Climber for the Space Elevator
One of the most critical components of the space elevator is the climber. It is second only to the ribbon in importance and perfection of design. As we will see shortly at the X-Prize Cup, building the perfect climber is not an easy task but getting close is well within grasp.
The baseline climber consists of a few basic components: the power system, the drive system, the payload and the infrastructure. We will get into each of these but before we do we need to understand the goal – hauling cargo, whatever it may be, up the ribbon. The climber meets this goal by lifting the maximum payload at a specified ascent speed. A quick glance at the basic layout of a climber and we quickly understand that to maximize the payload at a given speed we need to minimize the mass of the other components – the power system, the drive system and the infrastructure. This means optimizing the systems to work together and for their precise purpose.
In the available publications the baseline climber has been discussed and laid out. Variations on this are being proposed but we will work with the baseline climber today. To optimize this system we have to reduce mass everywhere possible without creating too much risk of failure. In the original designs and in most of the climbers at last year’s competition the climbers were rigid frame made from aluminum. The drive motors and rollers were hard mounted in these frames and the photovoltaic cells were set in rigid arrays sometimes backed with solid sheets of metal. The control units ranged from very simple switches to computers in boxes.
During the competition last year I had some detailed discussions with one of the teams after their first failed attempt. The discussions targeted the mass of their climber directly and after cutting off various components including a compressed gas braking system the climber passed breakeven and began to ascend the ribbon on the next attempt. For a first design they did well and I expect them to do well again in the coming event. But I hope to see a different design at the competition this year and believe it will lead to an optimized design that can eventually be used on the real space elevator.
To reduce the mass of the systems, advanced materials (composites) are needed in all structural components. For mounting the motors, fiber sandwich panels are needed. The motors themselves should have their cases replaced by thin composite shells where possible. The rollers might be spoked assemblies within composite tubes. The rollers should also lean toward the multiple offset roller design (even though I show a pinch roller design in the image below) to reduce the compression on the ribbon required and the losses there.
The primary frame running from the drive down to the secondary rollers and power array, if this design is used, should be thin composite components in a truss type design. All tension components such as the supports for the photovoltaic array need to be thin polymer threads (a CNT or metal thread will be needed on the real climbers due to radiation and UV damage) simply strong enough to hold the array flat. These same tensile components can provide the structural support for the photovoltaic array though careful handling will be required due to the fragile nature of the cells. The photovoltaic arrays should also be held in an open-backed structure to reduce mass and allow radiative (and in the competition conductive) cooling of the cells. The designs we have done along these lines look like they will meet the requirements for the competition and the real system.
For the control system, simple is best. Simple switches to allow the climber to fulfill its duties. The control can be done through RF controls designed for model airplanes.
The second target of optimization is in the power system. At last year’s competition a large spotlight was used due to it availability and ease of use. The problem with white light is that much of it falls outside the useable spectrum for silicon of GaAs cells. The result is low efficiency (<20%) and excess heating of the receiving array – enough to melt solder. The answer is a different system. A tuned laser can provide a receiver efficiency above 60% and minimal heating with silicon or GaAs and on short distances microwave can also be a viable energy source. By selecting the right system the array can be minimized and thermal issues eliminated. The second part is a highly efficient motor at all of the required speeds. Fortunately, electric motors have been tuned and perfected over years and regularly have efficiencies well over 80%. The key in the motor is minimizing the mass. The key is to minimize and optimize all the components and as we see here this is primarily focused on the mass and power.
