NASA working multiple array solutions

by Chris Bergin

NASA have approved a fourth spacewalk, should it be required, to aid the retraction of the solar arrays on the P6 truss, after efforts to ‘wiggle’ the tension out of the guide wires failed Friday.

A couple of IVA activities attempted to reduce the tension in the wires that run through the grommets, in order to retract the panels, without success. EVA 3 – which is proceeding with great success on Saturday – will include a closer look at the problem.

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Flight Day Seven was mainly an off duty day for the crew of Discovery, intermixed with media interviews – dominated by interest from the Swedish media in the continuing mission of their compatriot Christer Fuglesang.

As part of continuing cargo transfers – which was the other main element of FD7 – Suni Williams utilized time reserved for the transfer of the CGBA (Commercial Generic Bioprocessing Apparatus) payload from Discovery to the ISS.

‘CGBA comprises two educational experiments to be utilized by middle school students,’ noted today’s On Orbit status report. ‘One experiment will examine seed germination in micro-G including gravitropism (plant growth towards gravity) and phototropism (plant growth towards light). The second experiment investigates how micro-G affects the model organism Caenorhabditis elegans, a small nematode worm.]

Interest in the day’s events built up as troubleshooting on the retraction efforts with the P6 solar array opened with a ‘Wiggle Test’ – which involved 10 degrees of movement of the Beta gimble system on the truss. High speed video showed that the arrays did sway – but failed to help the guide wires spring into a desirable placement.

‘We’re still working with our array, trying to get it retracted,’ noted Deputy ISS program manager Kirk Shireman. ‘We wanted to do some motion on the arrays with the Wiggle.

‘We tried the single Wiggle. We tried the double Wiggle. We changed the timing on those Wiggles. All these ideas were designed to set up some motion in the arrays.

‘After we tried those activities, we went to deploy the array slightly, by one bay before attempting to retract. However, the guide wires hung up on the grommets, bunching some of the array panels together.’

This proved to be a problem as the one bay retraction that followed saw the bunch of panels slip over the ‘goalposts’ they are supposed to fold back into. NASA then decided to deploy by a quarter of a bay to ensure the tension in the guide wires was acceptable to leave the array in a stable configuration.

‘We didn’t make much progress in terms of retracting the bay, but we did learn a lot,’ added Derbyshire. ‘So in my opinion it was worthwhile.’

EVA 3, which will involve Williams and Robert ‘Beamer’ Curbeam, will complete the second half of work started by the re-wiring of the station’s power configuration on EVA 2. NASA also evaluated the possibility of using some get ahead tasks in EVA 3 to take a closer look at the P6 array via the spacewalkers.

While another attempt via IVA activities will take place on Sunday, the possibility is open for a fourth EVA to manually free the array for retraction. More news on this will follow at the weekend.

Looking ahead to Saturday’s EVA 3, ISS controllers and crew are preparing for the Channel 1/4 power-downs, which include reactivating the PDIM (Power/Data Interface Module) of the MedOps defibrillator equipment, aided by Thomas Reiter’s reconfiguration of the OpsLAN (Operations Local Area Network) by returning the SSC (Station Support Computer) laptops 4,5,7,8 and FS (File Server) to their original power sources,

This will be preceded by temporary shutoff of the A/L’s PCS (Portable Computer System) A31p laptop, and swapping the Lab CCAA (Common Cabin Air Assembly) air conditioner, from the port to the starboard unit, as a get-ahead for Saturday’s power systems (EPS) Channel 1/4 reconfiguring.

Mentions on both the On Orbit status report and NASA memos acquired by this site note the recent Solar Activity led to Discovery taking over attitude control of the ISS early Friday morning, via her small reaction control jets located at the rear of the vehicle.

‘Due to the solar events, the Earth’s atmospheric density at orbit altitude was 2.5 times greater than nominal,’ noted the On Orbit report. ‘This caused high momentum on ISS while being controlled by the U.S. Momentum Manager, with CMGs approaching saturation. The decision was made to hand control over to Orbiter VRCS (Vernier Reaction Control System) for the crew sleep period.’

It is not clear as to how long Discovery was in control of the attitude control, but NASA memos warned that Discovery may have to sacrifice the ‘flyaround’ of the ISS after undocking.

‘Holding attitude takes prop margin away from the Re-Rendezvous plan. After holding attitude for 17 hours, we will lose Re-Rendezvous’ noted the NASA memo. ‘Deleting the Fly Around will buy back 19 hrs of margin. Calculations will continue to be refined over time.’

Once those calculations are made known, a further report will follow on the relevance of any further attitude control requirements and a fourth EVA to the last few mission days for STS-116.

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