Fourth EVA possible for array issue

by Chris Bergin

The P6 solar array retraction has proved to be troublesome, with over 40 deploy and retracts of the solar panels performed to attempt a clean retraction on Flight Day 5 of STS-116.

Now at the 17.5 bay retraction point, the array has been withdrawn enough to avoid blocking the P3/P4 arrays from tracking the Sun. However, a fourth EVA – on Flight Day 9 or 10, is being discussed.

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The retraction event began at 12:28pm Central Time, but suffered folding and creasing during the withdrawal between the 19 bay point – more so up to the 17.5 bay point – which is about half of the full retraction length.

As stated by today’s On Orbit status report, the current length of retraction is acceptable.

‘A minimum of 40 percent of the array (~12.6 bays) must be retracted to provide enough clearance for activation of the P4 SARJ (Solar Alpha Rotary Joint) tested during STS-115. 

‘It will be redeployed after the P6 truss is relocated to the end of the P5 truss during 10A in 2007.  The P6 starboard array will be retracted during 13A.’

The P6 arrays – a set of two panels on either side – were suffering from a lack of required tension on one of the controlling wires, while folding problems could be observed during retraction.

The arrays have never previously been retracted, since arriving at the International Space Station in December 2000, on board Endeavour during STS-97.

Meanwhile, SARJ activation was completed for the newly arrived P4 arrays to begin solar tracking – which began at orbital noon, as controllers note their wish to continue with the Flight Day 5 timeline.

The new arrays, taken into orbit in September on mission STS-115, are stationed on the complex’s P4 truss segment. Those arrays need to rotate to track the sun and provide power to the orbiting outpost.

Contingency procedures will be utilized in the coming days to fully retract the solar array wing into its blanket box.

The most viable option is to fix the wing during a spacewalk, or EVA, which was intimated at by ground controllers, opening up the possibility of an EVA 4 on STS-116. The last resort NASA has pre-planned is a jettison of the array, as an absolute worst case senario, and very unlikely.

‘We sent some guys off to look at EVA options after EVA 3,’ said Kirk Shireman, deputy ISS manager, noting a spacewalk would allow some solutions to issues where only a helpful ‘poke’ would aid the retraction.

‘The hard part is that you look at the arrays and think ‘if you could touch it right there’ it would retract.’

However, NASA managers have to discuss the hazards involved with sending spacewalkers out to the array, to ensure any work on the panels can be carried out safely.

‘We have experts looking at the rules for when you send a spacewalker out there and touching the array with a tool,’ added Shireman. ‘These panels have glass in them and a charge – so there’s a shock hazard. So we’re going to do analysis before we do anything.’

This fourth EVA could be slotted in on Flight Day 9 – meaning it would be the day after EVA 3, or on Flight Day 10, which opens up the possibility that Discovery would remain docked to the ISS for one extra day.

‘We’ve made no decision concerning EVA 4, but we discussed when we need to make a decision for both FD9 or FD10,’ Shireman said. ‘We expect to make the decision in the next few days.’

As for what that spacewalk would involved, Shireman made a point of a desire to release some of the tension in the guidewire, incurred by the wires moving back and fourth through the grommets they run through.

‘Each array has a guidewire with one pound of tension, which go through grommets, which causes tension. We’re looking at ways where we can free the tension. We have teams of people looking into this to resolve these issues.’

Shireman also praised the NASA team for being prepared for such issues on orbit, and confirmed the array is currently in a safe configuration.

‘When these problems occurred today, we already had back up plans in place,’ he noted. ‘The configuration the vehicle is in right now allows of the power re-configuration (on the next two EVAs).

‘We’re in a good configuration for all the upcoming activities.’

Today was also the first of two big cargo transfer days for Joan Higginbotham, Nick Patrick, Mikhail Tyurin and Thomas Reiter, with several hours scheduled for Shuttle-to-ISS and ISS-to-Shuttle transfers.

‘Hardware deliveries include a new TVIS gyroscope, to be installed in the treadmill tomorrow, a SAFER (Simplified Aid for EVA Rescue) unit is being checked out by LA, and two full CWCs (Contingency Water Containers),’ added the On Orbit status report. ‘Return cargo is being configured in the Shuttle Middeck and in the Spacehab module.’

Flight Day 6, EVA 2, will be conducted as planned in the STS-116 schedule.

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