Flexing Canadarms Muscles

Just before 2:00 pm on 13 November, less than 23 hours into the mission, a major objective of STS-2 got spectacularly underway when Truly unlatched the $100-million RMS from its cradle along the port-side sill of the payload bay, rolled it out of its storage position and moved it to its outboard work station. It was Canada's contribution to the Shuttle - a contribution that dated to 1974 when Spar Space

Dick Truly (left) and Joe Engle in the Shuttle simulator's aft flight deck practise operating the RMS.

Robotics Corporation was contracted by the country's National Research Council to build a mechanical arm for deploying and retrieving satellites from orbit and ultimately assembling a space station.

The challenges involved in building an arm capable of such complexity and dexterity were enormous: it needed to operate automatically and under manual control and meet strict weight and safety requirements. Moreover, nothing quite like it had been built or used in space before, which made Spar's task yet more difficult. Although a horizontal floor rig was built to test the joints in 1977, the first real demonstration would not come until it was actually uncradled in orbit. The first space-rated RMS was delivered to KSC in April 1981 and, after a series of checks, installed on board Columbia on 20 June.

The arm was 15.2 m long, to enable it to reach the far end of the payload bay, and consisted - like a human arm - of shoulder, elbow and wrist joints, linked by two graphite-epoxy booms. Other components were made from titanium and stainless steel. To protect it from thermal extremes in space, the arm was covered in white insulation and fitted with heaters to maintain its temperature within required limits. Without a payload attached, the RMS could move at up to 60 cm/min, but this was reduced to a tenth of that time when fully loaded.

Ingeniously, the means by which the arm could 'pick up' and 'put down' objects was achieved by the so-called 'end-effector' - essentially a 'hand' that employed a kind of wire snare to capture a prong-like grapple fixture attached to deployable or retrievable payloads. Already, one of NASA's most important observatories - the Hubble Space Telescope, scheduled for launch in the mid-1980s - had an in-built grapple fixture that would enable it not only to be deployed, but also retrieved and repaired in space by future Shuttle crews.

During operational flights, astronauts would use two television cameras on the arm's wrist and elbow to guide the end-effector over a target's grapple fixture, before commanding three wire snares to close around it at just the right instant. When this was done, it would impart a force of 500 kg on the grapple fixture to allow the RMS to move the target into or out of the payload bay. No targets would be moved on STS-2, but on the next flight a desk-sized Induced Environmental Contamination Monitor (IECM) would be used to flex its robotic muscles.

''Its movements are much more flexible than they appeared during the training simulations,'' Truly told Sally Ride as the arm arched out of the payload bay. Although the RMS was controlled by the Shuttle's GPCs, its movements were commanded by Truly using a joystick in the aft flight deck. Under his control, he moved the shoulder and elbow joints up and down and left to right and pitched, yawed and rolled the wrist. As Truly issued each command, the GPCs examined them and determined the joints that needed to be moved, their direction and their speed and angle.

Meanwhile, the computers looked at each joint at 80-millisecond intervals and, in the event of a failure, could automatically apply a series of brakes and notify the astronauts. As Truly worked, a continuous flow of data on joint rates and speeds appeared on monitors in the flight deck. An hour after the RMS had been unfurled, Ride asked the men to establish the television feed, to which Engle replied,

''Transmitting. You people seeing anything down there yet?'' After a moment, Ride confirmed that, indeed, the first pictures of the arm in an inverted V-position had appeared on Mission Control's screens.

The images drew spontaneous applause from the Canadian delegation. For the next four hours, Truly and Engle took turns operating the RMS and testing it in all five control modes, using both the primary and backup software. Four days of tests were, through necessity, now crammed into a brief 24 hours. As Truly 'flew' the arm, Engle fired a series of bursts from the RCS thrusters to assess its performance under stress. The arm's television cameras were also evaluated and, on one occasion, showed a grinning Truly peering through Columbia's aft flight deck windows with a sign that read, 'Hi Mom'.

Only a couple of minor problems were encountered. The first was a failure in the arm's primary control mode, which the astronauts managed to bypass using the backup electronics (the cause would later be traced to a broken wire). Also, the elbow camera - one of six cameras affixed to the RMS - suffered a short circuit and failed towards the end of the mission. Otherwise, the first demonstration of the arm in orbit was a spectacular success and it performed within expectations under a wide range of temperature variations.

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