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CANADIAN ROBOT ARM TROUBLES ISS

By Rudolf Hofstätter

The technical problems of the International Space Station's robotic arm were still a cause of worry to both the resident crew and ground control quite some time after it had been assembled. In the middle of May its wrist joint jammed and on 23 May its shoulder joint failed to move in the proper manner. The cause of the trouble is attributed by the flight control centre to the backup power and control system in the shoulder joint, where it is suspected that one of the chips in the computer-controlled Arm Control Unit (ACU) has short-circuited.

Some new software should now enable the defect to be circumvented so that testing of the arm can at least continue despite the problem. Technicians from NASA and the Canadian Space Agency (CSA) have meanwhile begun work on developing the new computer program. If necessary, the ACU electronics box may have to be replaced by the astronauts from the outside.

The arm has both primary and backup systems. It is controlled by computer commands to the hand, elbow and shoulder joints from the Destiny module. It is only the reserve system that is a problem - the primary system is functioning normally - but both systems must be fully functional in order to attach the 12-tonne JAM airlock.

As the primary element of the expensive Canadian-built ISS manipulated system, the 17.5m (56ft) long robotic arm is indispensable to the assembly of additional components of the station. Up to now various modules, docking mechanisms, supply pylons and solar array paddles have all been attached to the space station using the 15m long Space Shuttle's robotic arm. In future, however, it is planned that the new arm, also known as a space crane, will move many other prefabricated components into their assigned positions, as the Shuttle arm is too short to reach all round the ISS.

The crane arm was only attached to the orbiting outpost in the middle of April during the most recent Shuttle mission. It would have been tried out for the first time on the next Shuttle feeder flight, when the US JAM airlock was to have been attached. This is intended for external work on the station when there is no Shuttle currently docked.

In the meantime a whole day of trials using the new component had been planned to practise the manoeuvres involved in attaching the airlock, which has now been postponed to July. It was hoped that during these practice trials it would have been possible to also solve another puzzle, for recently the arm unexpectedly suddenly started behaving normally.

The Canadarm 2 is too important to future expansion of the station through to 2006 for reliance to have to be placed on the random behaviour of a loose contact. It must operate faultlessly, otherwise expansion of the ISS by the West will have to be seriously delayed, at significant additional expense. The Russian modules on the other hand all dock directly onto the station and do not need the manipulator arm.

"If the arm cannot be repaired now, we shall be in serious trouble," the Russian Mir and ISS cosmonaut, Sergei Krikalev, told FLUG REVUE in Morgenröthe-Rautenkranz at the beginning of June. "It has already caused two Shuttle launches to be postponed. If it cannot be repaired in space, two extra missions will have to be planned, one to bring the arm back to Earth to repair it here and a second to take it back and reinstall it."

Then in the night of 7 June the shoulder joint failed to respond to the electronics control unit, only to resume normal working again afterwards. As it caused problems for two weeks it is important to avoid similar problems again in the future, otherwise the possibility that a module could get stuck halfway between the shuttle loading bay and the station during a lifting operation could not be excluded. While the arm does have a total of seven joints, it only takes one of them to fail during work and the whole arm automatically switches itself off.

The present resident crew of Yury Usachev, James Voss and Susan Helms was launched on 8 March. Their sojourn in space will now last a total of six months, four weeks longer than originally planned. The reason for this is the problems with the Canadarm.

Originally Discovery was to have transported the Italian-built Donatello Multi-Purpose Logistics Module and the American-Russian Expedition Crew 3, Frank Culbertson, Vladimir Dezhurov and Mikhail Turin, to the ISS and taken the present crew back to Earth again commencing in the middle of July. Because of the robotic arm, this mission has had to be postponed by a month to August.

Meanwhile on 8 June Yury Usachev and James Voss, clad in Russian Orlan spacesuits floated into the evacuated docking cone of the Zvesda module in the interior of the station. However, they did not climb out but remained inside to prepare the lower attachment point for the Russian DC 1 docking module that is due to be attached to Zvesda in the late summer.

This work had actually been planned for the first resident crew, but was postponed as the original launch date for the docking module in February was put back by six months. This was the first time such a manoeuvre had been performed on the ISS, although it had previously been tried in 1997 in the space station Mir when astronauts climbed into the evacuated Spektr module to search for breaches in the walls following impact with a Progress tanker.

In the middle of June NASA decided on the sequence of the next two Space Shuttle flights to the ISS. Transportation of the airlock by Atlantis had previously been planned for June. This launch has already had to be postponed five times, most recently to 12 July.

If the "arthritic" condition of the crane arm cannot be fixed, Atlantis will not take off with the airlock until the end of September. The next Shuttle mission will then be on 5 August, using Discovery, which is supposed to transport the third expedition crew to the station and bring its current inhabitants back to Earth.

It is also quite possible for other reasons that there will be no Shuttle flights for six weeks: between the middle of July and the beginning of August excessive temperature fluctuations in the Earth orbit would damage the docking mechanism and other cargo in the Shuttle loading bay, and from the middle of August to the middle September American ground stations are undergoing technical modifications which will take them intermittently out of service.

From page 42 of FLUG REVUE 8/2001