Crew members on the international space station have ended their 10-day effort to repair a failed oxygen-generating unit, and in its place they have installed a jury-rigged unit that previously failed.
If it also fails, the station would have only 90 days of stored oxygen on board — and the outpost might have to be shut down. New hardware won't be available until the spring of 2005.
NASA's weekly status report on the space station, released late Friday, described how the newest device — Unit No. 7, in Russian parlance — was replaced by an older one that the crew had "refurbished last week using spare components." This device is referred to as Unit No. 5.
After the replacement, the crew briefly powered up the Russian-made Elektron oxygen generator, and then turned it off before they went to sleep. The device lacks the automatic monitoring and safety features required for continuous untended operations.
The status report cast the development in a positive light, saying that "almost two weeks of troubleshooting is paying off."
The refurbished Elektron is now the only working apparatus of its kind aboard the station, assembled from different components in an untested configuration. The key question is whether it will function well enough in the next three weeks to provide adequate confidence to launch the station's next crew.
The station's two crew members — NASA astronaut Mike Fincke and Russian cosmonaut Gennady Padalka — have been nursing along the balky oxygen-generating equipment for months. This month's breakdown of Unit No. 7 added significantly to the challenge.
NASA officials emphasized that Fincke and Padalka are not in any imminent danger, and engineers remain confident that the combination of backup procedures and in-flight repairs can keep the two breathing easy. Nevertheless, internal NASA reports and interviews with several space station officials have painted a picture of growing concern, interspersed with periods of relief and hope.
The latest problem first came to light on Sept. 8, when an internal NASA report said that "the Russian oxygen (O2) generator is still off." This Elektron generator uses electrical power to split water molecules into breathable oxygen and waste hydrogen. Russian space stations have used such devices, each about the size of a water heater, for more than 15 years.
New telemetry allowed Moscow Mission Control to rule out another recurrence of a familiar failure mode that involved gas inside a fluid container.
"The problem this time is apparently not with bubbles in the ‘Fluid Unit,’" the NASA report stated, "but with the unit's oxygen and hydrogen gas analyzer, which is a failure mode not seen before." This followed the occurrence a week earlier of another never-before-seen failure of a valve switch.
Before this latest failure, U.S. and Russian operators had hoped Unit No. 7 would operate "for at least another few months" before breaking down completely.
Last Friday, the station's astronauts succeeded in getting the unit restarted after three tries — but it shut down again within an hour. At that time, engineers in Moscow planned to try again. They now appear to have given up on restarting the broken unit.
The three Elektron units on board the space station are the last of their kind. The company that manufactured them has gone out of business, and the engineer who almost single-handedly made the final adjustments of flight units died several years ago. Reportedly he retained some "trade secret" about the final adjustments of the devices — and it died with him.
One of these units is "hard broken," NASA sources say, but Padalka was able to refurbish Unit No. 5 last week and installed it on Friday, replacing Unit No. 7.
Other oxygen sources
The two crewmen are breathing oxygen at a known rate and converting it to carbon dioxide, which is still being safely scrubbed from the air by Russian and U.S. devices.
Padalka and Fincke have backup oxygen-generation processes at their disposal, but these processes use up the station's limited supplies. The virtue of the Elektron system is that it is a regenerative system: It uses waste water, and it can operate indefinitely as long as there is water, electrical power — and no mechanical breakdowns.
Currently, according to NASA cargo manifests, there are 62 pounds (28 kilograms) of oxygen gas in a tank on board the Progress freighter now docked at the space station — enough for 16 days. Also, the station has 84 solid-fuel oxygen generators, or "candles," and at one man-day per canister, this would supply another 42 days of oxygen.
There is also a month's supply of oxygen in tanks on the U.S. air lock module — although if those tanks are used, that particular supply would be very difficult to replenish.
In all, about 90 days' worth of expendable oxygen would remain even if the last working oxygen generator broke down. That’s considered an adequate safety margin for the current crew. "They’re fat on gas," NASA spokesman Rob Navias insisted last week. However, a 90-day supply wouldn’t be enough for the next six-month mission.
The next robot resupply flight is now scheduled for Nov. 26, and additional expendable supplies could be sent up then. That flight's cargo manifest already has been modified to carry more "oxygen candles," and possibly even a new tank to carry 150 pounds of oxygen.
The oxygen problems aren't a surprise. Back on Aug. 9, an internal NASA status report stated that "the Elektron oxygen generator continues to be a major source of trouble."
Three days later, another report called it "the leading trouble spot aboard the ISS." That report continued: "Current efforts [in Moscow] merely focus on outlining and developing a more reliable procedure for reacting to the increasingly frequent failures."
In the longer run, help is on the way. "Three completely redesigned units are currently in production," a NASA source said on condition of anonymity. The new devices will not use the liquid-gel electrolyte system that has led to all previous units eventually breaking down. Instead, they will rely on a solid polymer electrolyte.
Delivery of the first of these units is expected "in March or April of 2005," the source said. How fast they really can be completed, tested and flight-qualified remains an open question for the severely underfunded Russian space program.
NASA is assessing the chances of speeding up a U.S.-built oxygen generator to have it available in four years, rather than the currently scheduled six years.