Station's carbon dioxide scrubber back on line
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 26, 2009
Worried the shuttle Endeavour might have to undock early because of a carbon dioxide removal problem aboard the International Space Station, engineers at the Johnson Space Center in Houston came up with a manual control work-around that successfully restored the CO2 removal system to near-normal operation.
With six full-time station astronauts and seven shuttle crew members on board the shuttle-station complex, carbon dioxide removal is a critical requirement. Without the CDRA, the Russian Vozdukh CO2 removal system and the CO2-scrubbing lithium hydroxide canisters aboard Endeavour would be over-taxed.
But engineers in Houston, led by environmental control and life support systems officer John Garr, came up with a work-around that allowed flight controllers to restart the CDRA Saturday evening.
"We saw higher temperatures than we typically see on the carbon dioxide removal assembly, which is located in the International Space Station laboratory module," space station Flight Director Derek Hassmann said early Sunday.
"We reacted to that by turning off heaters in that system. We didn't see the temperature decrease that we expected, so initially we thought that the heater was failed on, or stuck on, for some reason. As we were looking at that, we had one of the circuit breakers in the power system trip, which removed power from the system.
"So we went back and looked at the data and we think the problem lies somewhere in the heater controller assembly that actually tells the heater when to cycle on and off based on temperature data from the carbon dioxide removal assembly," he said. "And based on that assessment, we put together a procedure that basically allowed the ground to control the system in the way the software normally does."
"Normally, the carbon dioxide removal system operates completely on its own, scrubbing carbon dioxide from the air and then dumping that carbon dioxide overboard, and it's run by the software in the space station computers. But without that heater controller, and with that circuit breaker failed open, the system will no longer operate in that autonomous, or stand-alone, mode.
"So what we've done is, we built a set of commands and we put together a procedure that allows the folks on the ground in the control center to move the valves and command the system in the way the software normally does. It's not something that we want to do long term, for weeks for example, because (of) the number of commands we have to send from the ground. But in the short term, we've got the carbon dioxide removal system back up and running and operating at close to its normal capacity."
After Endeavour undocks Tuesday, the space station crew plans to replace the presumably faulty controller to restore CDRA to normal operation. A new carbon dioxide removal assembly is being launched on the next shuttle mission in late August.
"Bottom line, we're operating it manually with commands from the ground and we're scrubbing CO2 at a level we're happy with," Hassmann said. "After the shuttle leaves, we're going to talk about actually removing and replacing that heater controller."
At the Kennedy Space Center, meanwhile, the shuttle Discovery was hauled from its processing hangar at 7:22 a.m. EDT Sunday for the short trip to the Vehicle Assembly Building where it will be attached to an external tank and a set of solid-fuel rocket boosters. Rollout to launch pad 39A is targeted for Aug. 5, setting the stage for launch August 25 around 1:35 a.m.
NASA originally planned to to launch Discovery around Aug. 18, but conflict with a Delta rocket launch pushed the shuttle flight to Aug. 21 and then work to test the orbiter's external tank foam insulation triggered an additional delay.
During Endeavour's launch, an unusual amount of foam insulation fell away from external tank No. 131's central intertank section. To assess the condition of ET-132, the tank reserved for Discovery, engineers carried out two rounds of so-called plug-pull tests, drilling small cores in the intertank foam and then pulling on the insulation to make sure it was firmly bonded to the underlying structure.
Some 150 plug-pull tests were carried out and the foam behaved as expected. Analysis and repair of the foam cores is ongoing, but NASA managers felt confident enough about the health of the insulation to proceed with Discovery's roll over to the VAB.
Shuttle program managers plan to review Discovery's flight readiness Aug. 11 and 12, followed by an executive-level flight readiness review Aug. 18.
Aboard the space station Sunday, the astronauts planned to use the station and shuttle robot arms to remove a Japanese payload carrier pallet from the Kibo module's new external experiment platform and stow it back in Endeavour's cargo bay for return to Earth.
The combined 13-member crew will hold a traditional joint crew news conference at 2:28 p.m. to discuss the progress of the mission before reviewing the timeline and procedures for a fifth and final spacewalk Monday.
Astronauts Christopher Cassidy and Thomas Marshburn face a full plate for their final excursion. repairing insulation on the Canadian DEXTRE construction robot, rewiring a gyroscope control circuit, installing two television cameras on the Japanese experiment platform and deploying a spare parts attachment mechanism on the station's main power truss.
Cassidy and Marshburn plan to spend the night in the Quest airlock at a reduced pressure of 10.2 pounds per square inch to help purge nitrogen from their bloodstreams in preparation for working in NASA's 5-psi spacesuits.
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision M of the NASA television schedule; SRMS: shuttle robot arm; SSRMS: station robot arm; JLE: Japanese experiment pallet):
EDT........DD...HH...MM...EVENT 07/26 04:03 AM...10...10...00...STS/ISS crew wakeup 05:13 AM...10...11...10...ISS PAO event 06:03 AM...10...12...00...ISS daily planning conference 07:08 AM...10...13...05...SSRMS JLE grapple 07:08 AM...10...13...05...EVA-5: Tools configured 07:48 AM...10...13...45...JLE uninstall 08:18 AM...10...14...15...SRMS grapples JLE 08:38 AM...10...14...35...SSRMS releases JLE 08:38 AM...10...14...35...EVA-5: Spacesuit swap 08:58 AM...10...14...55...SRMS berths JLE 09:28 AM...10...15...25...EVA-5: Equipment lock preps 10:38 AM...10...16...35...Crew meals begin 11:00 AM...10...16...57...Mission status briefing 12:38 PM...10...18...35...EVA-5: Conference 02:28 PM...10...20...25...Joint crew news conference 03:08 PM...10...21...05...Crew photo 03:28 PM...10...21...25...EVA-5: Procedures review
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