After an exhaustive effort to come up with a workaround to fix a balky valve in one of the International Space Station's two coolant loops, NASA managers decided Tuesday to change gears and press ahead with at least two and possibly three spacewalks to replace a refrigerator-size ammonia pump module.
Astronauts Rick Mastracchio, a spacewalk veteran, and first-time flier Mike Hopkins are scheduled to begin the first spacewalk, or EVA, Saturday around 7:10 a.m. EST (GMT-5). The second EVA is planned for Monday with the third, if necessary, on Christmas day.
The decision to carry out multiple spacewalks to repair coolant loop A means a delay for the planned launch of an Orbital Sciences Corp. Antares rocket carrying the company's Cygnus cargo capsule. Orbital engineers rolled the rocket to its seaside pad at the Mid-Atlantic Regional Spaceport at NASA's Wallops Island, Va., flight facility early Tuesday for a possible launch try Thursday night.
The flight now will be put on hold until next month.
"Orbital Sciences Cygnus spacecraft, atop its Antares rocket, now will launch no earlier than Jan. 13," NASA said in a statement. "The postponement of the Antares launch will allow ample time for the station crew to focus on repairing a faulty pump module that stopped working properly on Dec. 11."
While the station's six-member crew is not in any danger because of the coolant system problem, research activities have been curtailed and, more important, the lab has lost redundancy in a critical system. If a problem took down coolant loop B, the crew could be forced to evacuate and return to Earth aboard their Soyuz ferry craft.
"Our best position to be in is to have both those loops up and running and available to us," Kenny Todd, a senior space station manager at the Johnson Space Center in Houston, said earlier. "While we're sitting at one loop, I think we're somewhat vulnerable, and so clearly, from a program perspective, our intention would be to move sooner rather than later to recover that functionality."
The space station is equipped with two independent external coolant loops that use ammonia to dissipate the heat generated by the lab's electrical systems. Cooling is critical for station operations and while one loop can support critical systems and keep the lab operational, both are needed to avoid powerdowns of non-essential equipment.
The problem with coolant loop A developed last week when the flow control valve inside the station's starboard/loop A ammonia pump module malfunctioned, allowing the temperature of the coolant to drop below safety limits.
That did not affect loop A's ability to cool major electrical components mounted in the station's solar power truss. But to carry away heat generated by systems inside the station's habitable modules, where ammonia is not allowed, the coolant must flow through "interface heat exchangers" where it picks up heat carried by water that flows through cold plates where electrical gear is mounted.
"If we get the ammonia outside too cold, we are unable to integrate these interface heat exchangers with the internal loops because there's a risk of freezing the internal water loops," Flight Director Judd Frieling said Tuesday. "And if we freeze the internal water loops, that's a bad thing. ... If we freeze this, the water expands and would allow ammonia to come to the internal part of station and that's a really bad thing for the astronauts."
With loop A hobbled, flight controllers switched critical systems to coolant loop B and powered down non-essential equipment in the Harmony module, the European Space Agency's Columbus laboratory and Japan's Kibo research module to reduce the cooling required.
Engineers worked through the weekend to come up with an indirect fix, including a software patch to precisely control the operation of another valve in the system in a bid to regain temperature control.
Frieling said the software patch was intended to give flight controllers the ability to quickly make small changes in the position of a radiator return isolation valve in coolant loop A, a valve that is normally either fully open or tightly closed. By precisely controlling the valve's position as heat loads affect the system, flight controllers hoped to regulate the temperature and keep it within normal operating limits.
"We start (with the valve) at a known position, we tell it to drive and then a number of seconds after it drives, we pull the power from the valve," he said. "So that fixes it in a certain spot.
"The fidelity we have here on the ground to precisely control when that valve starts moving and stops is on the order of about .2 seconds, .3 seconds, somewhere in that range. We really need the fidelity to be much finer than that, we need it to be on the order of .1 seconds."
He said engineers had planned to uplink the patch late Tuesday, but NASA managers apparently decided otherwise, opting for the spacewalk repair job.
"NASA currently plans for two Expedition 38 astronauts to venture outside the space station Dec. 21, 23 and 25," NASA said in its statement. "NASA astronauts Rick Mastracchio and Mike Hopkins will remove a pump module that has a failed flow valve. They will replace it with an existing spare that is stored on an external stowage platform.
"The pump is associated with one of the station's two external cooling loops, which circulate ammonia outside the station to keep both internal and external equipment cool. Each of the three spacewalks will begin at 7:10 a.m. and is scheduled to last six and a half hours. NASA TV coverage will begin at 6:15 a.m."
The 780-pound pump module in coolant loop A is mounted on the forward face of the starboard S1 segment of the space station's power truss. The loop B pump module is mounted on the left side of the truss in a corresponding position.
The S1 pump module being replaced was installed during three spacewalks in August 2010 after the original unit suffered a short circuit that knocked its pump out of action. In this case, the pump in the replacement module is working normally, but the flow control valve is not regulating the ammonia temperature as required.
Three spare pump modules, supplied by Boeing, the station's prime contractor, are mounted on cargo pallets attached to the lab's power truss.
The pump module "is a difficult box to maneuver with, it's a big, unwieldy object," space station Flight Director Courtenay McMillan said before the 2010 pump replacement. "None of that part of it is technically difficult, but it's just very time consuming and takes a lot of focus."
Based on the 2010 repair work, spacewalkers would have to disconnect five power and data lines, three 1.5-inch ammonia lines and one half-inch coolant line. Two of those lines must be quickly connected to a "jumper box" to prevent pressure extremes in the ammonia supply as the station moves into and out of Earth's shadow.
A pump module replacement is one of several possible spacewalk repairs that all station astronauts are trained for before launch. Mastracchio and Hopkins spent the weekend reviewing procedures and preparing their spacesuits and tools for use if needed.
Lessons learned from the 2010 spacewalks will be incorporated in the upcoming EVAs and if all goes well, the astronauts should be able to remove the old pump module, install its replacement and make all the necessary connections in two outings. But NASA managers are holding a third spacewalk in reserve just in case.