Preparing For The Space Station

As well as evaluating materials and technologies needed to build the space station, the astronauts - and Mission Specialist Sam Gemar in particular - assembled and tested a miniaturised model of one in Columbia's middeck. Known as the Middeck Zero-Gravity Dynamics Experiment (MODE), it consisted of miniaturised space station 'modules' and components for its girder-like truss structure, which would be used to support electricity-generating solar arrays. The intention of the experiment, which had also been carried on an earlier Shuttle mission in September 1991, was to explore the dynamic behaviour of large deployable structures in the microgravity environment.

In addition to the USMP-2 and OAST-2 operations, a number of other, 'secondary' payloads were accommodated on board the Shuttle. One of these was a nifty extension to the Canadian-built RMS arm called the Dexterous End Effector (DEE). Instead of employing the usual means of capturing targets in space - snaring them with wires that closed around a stubby grappling 'pin' - this experiment used a set of powerful electromagnets to generate an attraction force of 1,450 kg. It was expected that, if such a system achieved 'operational' status, it might increase the robotic arm's dexterity and alignment accuracy.

Furthermore, from the astronauts' points of view, it provided them with a sense of 'touch' with the arm and offered important benefits when building smaller, more compact grapple fixtures for future spacecraft. Although the experiment had been extensively tested on the ground before Columbia carried it into orbit, NASA officials were quick to stress that only a series of tests in space could truly demonstrate the electromagnet's capabilities. For STS-62, it comprised four components: a Magnetic End Effector (MEE), a Targeting and Reflective Alignment Concept (TRAC) camera, a Carrier Latch Assembly (CLA) and a Force Torque Sensor (FTS).

During the course of the mission, the new system was rigorously tested. The MEE employed a pair of U-shaped electromagnets to grab and release payloads fitted with a flat, ferrous grapple fixture or 'handle'. This provided a more reliable means of maintaining a good grip on a payload and was a safer method of releasing it in the

Preparing for the space station 217

Preparing for the space station 217

Marsha Ivins and Pierre Thuot during DEE tests on Columbia's aft flight deck. Note the joystick-like hand controller for the RMS on the panel in front of them.

event of a problem with the Shuttle or the RMS. If, for example, a mechanical failure occurred while the arm was grappling a large object, the satisfactory closure of Columbia's payload bay doors would be compromised and her return to Earth threatened.

MEE eliminated that risk because its electromagnet could be 'switched off simply by cutting power to it. It was also a much smaller and lighter device than the standard end effector and had fewer moving parts. Meanwhile, TRAC provided a simpler and faster means of manually or automatically aligning the electromagnet with a 'target' payload by using video images and mirrors. Crew members typically looked through a camera that pointed 'outward' from the centre of DEE at the target until the camera could see its own reflection, then finished the process by lining up a set of cross hairs.

As soon as alignment was achieved, the astronauts 'drove' the arm forward until magnetic forces mated its end effector to the target. The FTS provided feedback on the forces being applied by the arm, while the CLA provided a pair of dummy 'payloads' for use during the demonstration. Beginning on 4 March, within hours of reaching orbit, all three Mission Specialists - Thuot, Gemar and Ivins - took turns evaluating the 315-kg DEE in a series of eight tasks lasting about an hour apiece.

They commented that its ultra-fine guidance would prove useful during delicate space station construction tasks.

Later in the mission, on 15 March, Ivins - who would participate in a major station assembly flight in 2001 - jokingly challenged her colleagues to match the precision with which she had manoeuvred the RMS. Overall, the Mission Specialists returned with good reviews of the new device and used it to insert a series of 'pins' into sockets, with progressively smaller clearance rates from three millimetres to just three-quarters of a millimetre. Additionally, a 30-cm-wide flat beam was inserted into a slot and moved backwards and forwards to correlate readings from the FTS.

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