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ISS managers mull coolant system repair options
Posted: December 12, 2013

Engineers are running tests to figure out what is affecting the operation of a critical valve inside a coolant pump aboard the International Space Station that has forced flight controllers to shut down non-essential systems, curtailing science operations and disrupting the crew's schedule.

Engineers are trying to figure out how to fix a balky flow control valve used to regulate the temperature of ammonia coolant in an external thermal control system loop aboard the International Space Station.The valve in question is located inside a pump module, noted above, that is part of coolant loop A, one of two critical systems used to keep station components from over heating. Credit: NASA
While the station's six-member crew is not in any danger, the loss of coolant loop A leaves the lab complex one failure away from a much more serious problem should coolant loop B, the lone operational system, run into difficulties of its own. Loop B is operating normally, but NASA managers want to restore the lost redundancy as soon as possible.

"Our best position to be in is to have both those loops up and running and available to us," said Kenny Todd, a senior space station manager at the Johnson Space Center in Houston. "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 flow control valve in question is used to regulate the temperature of ammonia flowing through coolant loop A. Depending on the results of on-going tests and troubleshooting, it may be possible to restore loop A to normal, or near-normal, operation remotely, by changing the way the suspect flow control valve is operated or modifying its control software.

But the valve is located inside a pump module on the station's solar power truss and if flight controllers cannot coax it back to normal operation, the astronauts likely would have to install a replacement during two or more spacewalks.

"This a tough one, because this is not hardware you can get to, it's external to the station so we can't go open it up, look at the valve and say well, it's a seal that's shifted, or it's a sensor that's come off or something," Todd said.

"That's not going to be obvious to us, and so the team is trying to manipulate this valve and draw some conclusions just based on secondary cues, what the temperature is doing, what the flow rate's doing, as they move this valve. Without putting eyes on valve, it's going to be tough to get specifically right at it."

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 was installed during three spacewalks in August 2010 after the original unit suffered a short circuit that knocked the 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. If a spacewalk is required, it likely would be carried out by Rick Mastracchio, a veteran spacewalker, and first-time flier Mike Hopkins, with Japanese astronaut Koichi Wakata operating the station's robot arm.

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.

But NASA managers will not order a spacewalk to resolve the current problem until all other repair options have been assessed.

Amid the troubleshooting, station planners also are considering what to do about the planned launch of an Orbital Sciences Corp. Antares rocket next week carrying a Cygnus cargo capsule on the company's first operational space station resupply mission. Several flight rules require two operational coolant loops for the capsule's berthing and if the problem is not resolved soon, NASA may be forced to delay the mission.

Launch is targeted for Dec. 18, but NASA managers met Thursday and deferred making a go/no-go decision "until we get a little bit more information on this particular issue," Todd said.

The space station is equipped with two coolant loops that use water and ammonia circulating through a complex arrangement of heat exchangers, pumps, valves and radiators to carry away the heat generated by the lab's myriad electronic systems.

Inside the station's habitable modules, electrical components are mounted on so-called cold plates that use water flowing through internal lines to keep equipment cool. The warmed water in the "moderate temperature loop," or MTL, is pumped to heat exchangers that transfer the thermal load to the ammonia coolant that circulates through the station's external thermal control system, or ETCS.

Powerful pumps in each coolant loop push the ammonia through an intricate system of valves and lines to large radiators mounted on the back side of the lab's main solar power truss where the heat is radiated to space. The cooled ammonia then is returned to the heat exchangers for another cooling cycle.

A flow control valve in each pump module opens and closes as required on computer command, mixing in ammonia from a secondary supply to keep temperatures in the lines warm enough to prevent water in the heat exchangers from freezing.

The problem developed Wednesday when sensors noted lower-than-allowable ammonia temperatures in coolant loop A, prompting the station's computer system to shut the loop down. Flight controllers later reactivated the system and discovered the flow control valve was not working properly to keep ammonia temperatures above pre-set limits.

The coolant loop was still running, engineers said, but it could not be used to reject heat. As a result, flight controllers worked through a long checklist to switch critical equipment to loop B.

While one loop can keep the station's critical systems operational, along with some non-essential gear, both are needed for full operation. Because of the flow control valve issue, non-essential systems in the forward part of the U.S. segment had to be shut down, including equipment in the European Space Agency's Columbus lab module, the Japanese Kibo laboratory and NASA's Harmony module, also known as Node 2.

"This is a position we don't want to be in long term," Todd said. "The team is continuing to work through the fault tree of what might be going on with this particular flow control valve and that work is ongoing as we speak. In the meanwhile, we've got a good, stable configuration, the crew's in good shape, no issues there."

In a morning tag-up with the station crew, Josh Matthew, the communicator in mission control, told the crew to expect a light timeline Thursday "due to us having to delete payloads that can't be supported in our current vehicle configuration."

"The loop A pump is still running, but the ammonia is too cold for us to be able to integrate any of the interface heat exchangers attached to loop Alpha," he said. "Overnight, we completed the reconfig we discussed with you guys (last night) to power down half of Node 2, JEM (Japanese Experiment Module) and Columbus due to our inability to reject heat from the Node 2 MTL. The ground teams also overnight worked to get the systems on board in a stable configuration with our limitation of being able to reject heat to loop Alpha."

He said engineers planned a series of tests "to understand what is actually going wrong with the loop A flow control valve. This morning, we've got a plan to command the valve to various positions to see how it affects the temperature of the loop ... so we can get further data for troubleshooting."