Opening EVA success – PRSD update

by Chris Bergin

In a near flawless opening EVA, spacewalkers Christer Fuglesang from Sweden and NASA’s Robert Curbeam have successfully carried out all the EVA-1 scheduled requirements – including some get-ahead tasks.

Meanwhile, on Discovery, an update on the PRSD (Power Reactant Storage and Distribution) has been listed by NASA, although the item has no mission impact and is classed as a stable condition.

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Fuglesang and Curbeam are carrying out what is being classed as the most complex set of jobs to be carried out via a Shuttle mission – consisting of three EVAs.

The six and a half hour spacewalk consisted of the installation of the P5 spacer, which involved the removal of launch locks, the guiding of the alignment and installation of P5 at the P4 truss by Sunni Williams on the Space Station Arm, and the tightening of the P5 attachment bolts with the PGT/Pistol Grip Tool).

They also removed the PVRGF (Photovoltaic Radiator Grapple Fixture) from P5 and stowed it for the time being on the MBS (Mobile Base System) until it can be transferred to the P6 aft radiator on a later spacewalk.

The final task was to remove and replace a TV camera located at port 3 of the S1 truss element, which will be used to view clearances during future truss installations.

The two spacewalkers were guided through their tasks under the able supervision of STS-114 astronaut Steve Robinson, who famously carried out a first-of-its-kind repair on Discovery. The first mission to launch post-Columbia required Robinson to remove two gap fillers from the belly of Discovery, which he carried out successfully.

Robinson, communicating with Fuglesang and Curbeam, aided the smooth transition through the tasks, using his own EVA experience to discuss and evaluate with the spacewalkers what the best course of action would be. Often he could be heard asking the EVAers what ‘they’ thought would be the next best step.

Fuglesang – who is receiving huge media coverage back in his native Sweden – also provided a space first for his homeland, uttering the first Swedish words in space, as he conducted part of the schedule tasks by counting out loud in his native tongue.

‘Nio, tolv,’ he was overheard saying, which translates to ‘nine, twelve,’ in English.

Meanwhile, on Discovery, the problems noted on Monday – all currently having no mission impact – have remained stable during Flight Day 4, with only one extra addition on the MER (Mission Evaluation Room) problem report.

That issue, taking the notes up to 11 items on the problem list – still very few for a Shuttle mission – is a PRSD (Power Reactant Storage and Distribution) O2 Tank 3 Check Valve Reseat issue.

The PRSD is one of three subsystems that make up the EPS (Electrical Power System). The PRSD subsystem stores the reactants (cryogenic hydrogen and oxygen) and supplies them to the three fuel cell power plants, which generate all the electrical power for the vehicle during all mission phases. In addition, cryogenic oxygen is supplied to the environmental control and life support system for crew cabin pressurization.

‘The PRSD oxygen tank 3 pressure increased with the heaters off, during the oxygen tank 4 heater pressurization cycle,’ noted a NASA memo acquired by this site’s L2 section. ‘This occurred after the O2 tank 3 heater switch was positioned to OFF (344:16:02:42 GMT), and the O2 tank 4 heater switch was positioned to AUTO (344:16:03 GMT).

‘The O2 tank 4 heaters immediately came on to pressurize the tank and manifolds. When the tank 4 pressure reached the tank 3 pressure, the tank 3 check valve should have closed, preventing the higher pressure fluid in the manifold from pressurizing tank 3. The tank 3 pressure tracked the tank 4 pressure for the first tank 4 heater cycle. The next two tank 4 heater cycles resulted in smaller pressure increases in tank 3.

‘Nominal operation resumed during the subsequent tank 4 heater cycles, indicated by small increases in tank 3 pressure due to environmental heat leak.’

This issue – possibly caused by transient contamination – won’t have a mission impact, but will be monitored, given a ‘next failure consequence’ would lead to the shutdown of Fuel Cell 3.

‘It is not a significant problem for a supply line check valve to temporarily fail to reseat for the first cycles of another tank’s heater cycles. If the tank supply line check valve failed to reseat, tank isolation would be lost,’ added the notes. ‘This would result in inadvertent use of the tank consumables.

‘Also, if an external leak developed upstream of the check valve (in the plumbing or in the tank) all of the PRSD oxygen tanks would feed this leak. Instead of the check valve isolating the leak, the manifold isolation valves would have to be closed, resulting in the loss of oxygen to fuel cell 3 (shutdown).

‘There are no impacts to the vehicle or mission, nor is there any required crew response. This is not critical because the check valve has demonstrated that it has reseated when the downstream pressure is greater than the upstream pressure.

The situation is classed as stable, and won’t require troubleshooting. It has been observed on previous flights, notably on STS-50 (O2 tank 7); STS-59 (H2 tank 5); STS-121 (O2 tank 3).

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