Aluminium framework system used in tether test rig

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Kevin Gingerich of Bosch Rexroth explains how his company's products and technologies are helping the Space Elevator to become a reality, rather than a subject of science fiction.

Hosted by the Spaceward Foundation, a non-profit organisation dedicated to furthering space science and technology, the Space Elevator Games unite academic, commercial and educational minds together for a series of challenges designed to rekindle the USA's interest in space. Since the inaugural event 2005, these annual games have grown in popularity but have still had no winner – the prize money in 2008 is $4million!

Essentially a space elevator is a fixed structure that uses a climbing mechanism to transport material from the earth's surface into space. Construction of such a structure may seem like a daunting task, but organisations like the Spaceward Foundation are hoping to advance the technology with contests like the Space Elevator Games.

One technique of erecting the fixed structure of the space elevator is by using a form of tethering method. The Space Elevator tether is a thin ribbon, with a cross-section area roughly half that of a pencil, extending from a ship-borne anchor to a counterweight well beyond geo-synchronous orbit. The climbing mechanism can then travel along the tether beyond Earth's gravitational pull without the use of rocket propulsion. Theoretically, the space elevator could then deliver cargo and people into orbit at a fraction of the cost of traditional launching methods.

A long way to go

Ben Shelef, aerospace engineer and co-founder and leader of the executive crew of the Spaceward Foundation, states: "Right now the technology to complete a space elevator does not exist. The purpose of the Space Elevator Games is to get the right people in industry and academics thinking about how we can make this work and to stimulate the development of new technologies to make the structure a reality."

The Strong Tether Competition – a $2million technology challenge backed by NASA – is just one of the events in the Space Elevator Games. Designed for developing a new class of super-strong tethers, a tether pull machine was developed specifically for testing and measuring the tensile strength of each team's proposed elevator tethers. The competition requires a 50 per cent improvement in breaking force from year to year, and started with a commercially available tether when the contest began.

Shelef explains: "The tether pull machine is designed to run a comparative test between two tether samples. One of the decisions made early on in the programme was that we wanted to make tether testing an exciting business to watch, so we created a head-to-head strength competition."

Test to destruction

For this purpose Shelef and his team designed what is essentially a tug-of-war machine. The tether pull machine grabs two tether samples at their respective ends, and then pulls their free ends towards each other. Since the pulling mechanism floats, the resultant force on the tethers is equal, and as the force increases, one will break first - "and with a very loud bang!" notes Shelef. The remaining tether, along with the pull mechanism, will lunge the other way and clearly indicate a winner.

The tether machine itself comprises a rectangular box structure about 12-feet long and 18-inches high on each side. Shelef says: "It has sliding parts, it needed to look sleek, and from our experience as designers, Rexroth's aluminium structural framing system was the natural way to go as the material to build the machine."

Using Rexroth bolt-together connectors, the tether pull structure can be quickly assembled by the team without special tools or skills and the extruded aluminium framing looks clean and attractive without painting or other finishing. The team could also choose from a broad selection of Rexroth accessories to extend the machine beyond a simple frame and base to a complete multi-functional structure. And, every aluminium structural framing component is reusable, which made it simple for the team to change the tether pull machine as their design needs developed and changed.

The Strong Tether challenge is conducted in two rounds. The first round pits tethers from two teams directly against each other to determine the team with the strongest tether. The second round then determines if the first-round winner is at least 50 per cent stronger than a 'house' tether that represents off-the-shelf materials. If it is, that team will win the competition.

Shelef comments: "I specifically requested the Rexroth aluminium structural framing because it could provide 60 times the strength of steel, yet it is also five times lighter than steel. The aluminium structural framing was able to meet our performance criteria while also allowing us to assemble the tether machine quickly and easily."

24-hour response

A last-minute request from Shelef, when the Spaceward Foundation team was passing through Phoenix on its way to the competition, put Bosch Rexroth's service levels and expertise to the test. Incredibly, the team had not yet finalised the project parts list and needed help fast. All in all it was a great joint effort between Bosch Rexroth and its local sales partner to come through for the team and have the aluminium extrusions machined and kitted in less than 24 hours.

At the 2007 Space Elevator Games, held at NASA's Ames Research Center in Mountain View, California, the Spaceward Foundation team again called upon one of Bosch Rexroth's local sales partners to supply the structural framing. In the months leading up to the competition, order changes and additions, as well as multiple short lead-time deliveries, were accommodated successfully, confirming their overall commitment to the project.

About the Space Elevator

The Space Elevator is a thin ribbon, with a cross-section area roughly half that of a pencil, extending from a ship-borne anchor to a counterweight well beyond geo-synchronous orbit. The ribbon is kept taut due to the rotation of the earth (and that of the counterweight around the earth). At its bottom, it pulls up on the anchor with a force of about 20 tons. The ribbon is 62,000 miles long, about three feet wide, and is thinner than a sheet of paper. It is made out of a carbon nanotube composite material.

Electric vehicles, called climbers, ascend the ribbon using electricity generated by solar panels and a ground-based booster light beam. In addition to lifting payloads from earth to orbit, the elevator can also release them directly into lunar-injection or earth-escape trajectories. The baseline system weighs about 1500 tons (including the counterweight) and can carry up to 15-ton payloads at a rate of one per day. The climbers travel at a steady 200 kilometres per hour (120 mph), do not undergo accelerations and vibrations, can carry large and fragile payloads, and have no propellant stored onboard.

Orbital debris is avoided by moving the anchor ship, and the ribbon itself is made resilient to local space debris damage. The elevator can be made larger by using itself to carry more ribbon pieces into place. There is no limit on how large a Space Elevator can be!

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