Refining Spacewalking Techniques

Although, in comparison to the crane and battery tests, the AERCam/Sprint run was a low-priority objective, it was nevertheless with disappointment that Columbia's crew realised they would not get an opportunity to test-fly it. ''It was supposed to be one [spacewalk planned],'' said Chawla after the mission, ''but we knew from our timeline that if we were going to fly Sprint, we wouldn't have time to do it. In all of our training runs, the tasks that we were supposed to do took longer than the timeline called for.''

It was with pleasant surprise that mission managers decided on 1 December to add a second EVA to the mission, although its primary focus was not to fly AERCam/Sprint, but rather to repeat and gather additional data on the crane tests from Scott and Doi's first spacewalk. According to Greg Harbaugh, planners had been unable to acquire all the data they needed from the spacewalk on 25 November. Specifically, Scott encountered difficulties connecting a mock ORU to the crane when its boom proved more flexible than anticipated.

''This crane, and the ORUs that are handled by it at the end of the boom, are fundamental to the success of the station,'' said Harbaugh, himself a seasoned spacewalker. ''They are used over and over again on the station. From our assessment, Winston had some difficulty mating the large ORU - the big battery - to the end of the boom. That was not entirely unexpected, but now we have an opportunity to go back and refine our techniques based on what we observed the first time.

''Winston did a lot of work. He tried it several times, with Takao assisting, with the technique we thought would work going in, which was basically Winston placing it down from above on top of the crane. When that didn't work, Winston pushed down and Takao tried to pitch up with the crane, a sort of'push-up/pull-down'-type sequence. And that didn't work. The technique that did work was where Winston got a little bit sideways and grabbed the crane from below and the box from above and 'squeezed' them together in a sort of sandwich technique. The concern with that technique is we are not confident that we're going to have the luxury at all the work sites on [the] space station of being in position to grab those two pieces and squeezing them together like that. We want to take advantage of the opportunity to really wring this out while we've got it on this mission.''

Scott and Doi's first focus was working with a mock station component, somewhat smaller than the battery they had handled on their 25 November excursion, known as a 'cable caddy'; this was another task originally planned for STS-80. The caddy, which was about a sixth as massive as the battery, was envisioned as a means of holding up to 20 m of replacement electrical cable. Scott began the task of mounting it onto the crane in the same way as he had done with the battery: pushing it down onto the crane's latches, then 'shoehorning' it into place.

He also rotated the assembly 90 degrees, using a tether to help to anchor it in place, then forced the caddy straight 'down'. His efforts produced no apparent problems. ''The first thing I'm going to do is try to dock the cable caddy to the grid facing upward out of the [payload] bay,'' he radioed for his crewmates' and Mission Control's benefit. ''Let's see how the soft-dock mechanism works ... I have a successful soft-dock! There is compliance in the crane; it moves a little bit, but not very much - I'd estimate maybe six or eight inches before I got soft-dock.''

Overall, he gave the crane an 'A' with the smaller cable caddy. Flight controllers asked him if the 'shoehorning' technique of mounting the caddy onto the crane might also work with larger components like the battery. ''Well,'' Scott replied, ''that's a possibility that [it] would have been successful had we thought about trying it. The problem with the large mass is controlling it. This one is so small, I can precisely position [it], then rock it forward. But the answer is 'yes', it is possible I might have been able to do it with the large ORU by rocking it.''

However, he cautioned against forcing large components into place on the crane. ''I think you'd get in trouble trying to use fast impulses with that large ORU,'' he explained. ''Remember, [the battery] is 500-600 lb and I deliberately kept my inputs low. I think if you put large inputs, or thought of slamming it home if you will, if it docked, you're going to have this large mass swinging back up at you because of the compliance in the boom. If it doesn't dock, you've got this large mass with a lot of rates. I would firmly not recommend that.''

Next, Scott tried locking the cable caddy onto the crane using a semi-rigid tether to keep the boom relatively steady. ''This tether technique is working pretty well,'' he said. ''It did prevent crane movement aft. If I add more force, it's nice and stable. I can even push against it if I need to. I really think that this could be a possibility with the large ORU, [but] the problem might be controlling the large ORU. I can restrain the crane from fore-and-aft movement with my tether, but part of the problem with the large ORU is up-and-down movement.''

''We need to come out of this mission with a clear understanding of the technique we expect to employ and confidence the system will be tolerant to that technique so we will be assured that we can move these boxes around,'' said Harbaugh. ''The implications for [the] space station are that we expect to do a lot of moving around of boxes. We're going to be moving them from one place to another - from the payload bay to various points on the station - and we expect this crane to help us greatly. If we can't move these boxes around and use them to replace boxes that have failed out there, then we have a fundamental design flaw in our thinking - our methodology -for how we're going to put this station together.''

Two different safety tethers were also used by Scott and Doi. One, the Body Restraint Tether (BRT), was designed to hold a spacewalker steady while clamped to a handrail; this had the benefit of freeing the astronaut's hands for working. Scott had already tested it during his previous spacewalk on STS-72 in January 1996 and Doi evaluated it on STS-87. The second tether was the Multi-Use Tether (MUT) which, although similar to BRT, had the capability of performing a wider range of tasks. For example, different 'end effectors' could be attached to it to grip ORUs, tools or handrails.

Scott releases AERCam/Sprint.
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