Having Too Much and Too Little Oxygen on the Space Station

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Several Russian Progress spacecraft were allowed to reenter Earth's atmosphere in 2003 carrying unused oxygen supplies. In one case, half of the oxygen being delivered to the ISS was dumped into the Pacific Ocean when these Progress spacecraft later reentered the Earth's atmosphere. Within months the ISS would face the prospect of a shortfall in oxygen supplies - and contingency scenarios, which included leaving the ISS unmanned as an option.

According to the current way that the ISS and Progress spacecraft, which resupply it, are designed this oxygen oversupply could not be kept on board as a reserve.

Speaking this morning at the 34th ICES (International Conference on Environmental Systems) annual meeting in Colorado Springs, David Williams from NASA JSC chronicled the experiences NASA went through in the aftermath of the Columbia accident. Without space shuttles to carry materials to and from the ISS, the U.S., Russia and the other ISS partners had to rely exclusively on Soyuz spacecraft to carry humans and Progress spacecraft to carry supplies and logistics.

Early studies (after the accident) of the implications of these constraints showed that the amount of water available would limit the crew complement to two instead of the three person crews which normally comprised an ISS Expedition crew.

The Russians had other concerns, somewhat different than the U.S. - concerns that oxygen would also be a problem.

After a Progress docks with the ISS, lines are supply lines are activated and high pressure tanks located external to the spacecraft are emptied into the habitable volume of the ISS. This is done until such time as the atmospheric concentration reaches a certain set point - including one for the amount of oxygen. Once these levels are reached the amount of gas introduced into the ISS is halted.

In the case of two Progress flights in 2003, internal atmospheric oxygen limits were reached before all oxygen could be unloaded. As such, unused oxygen was left inside the visiting Progress' tanks. For Progress 10P this amount was only 1 kg. However, for Progress 11P, 25 kg of oxygen was left onboard - half of the total amount of oxygen being delivered on that flight.

After this situation, which Williams referred to very clearly as a "debacle", the Russians "got a little bit smarter" with regard to how much oxygen was actually going to be needed. As a consequence the amount placed on board the next Progress flight 12P was adjusted to minimize wasted upmass with regard to oxygen. Given Williams' remarkably frank choice of words, it was also clear that he was heaping criticism on the whole system, not just one country - the sort of constructive self-criticism you would then want to learn from.

During 2003, the ISS experienced problems with its onboard ability to produced oxygen due to problems with the Russian Elektron unit's liquid unit. The Elektron uses electricity to cause the hydrolysis of water - splitting it into its two constituents - hydrogen and oxygen. The Elektron pumps the oxygen it produces into the cabin and dumps the hydrogen overboard.

These units have been balky on ISS and on Russia's previous space station, Mir. When a faulty Elektron liquid unit was replaced on ISS the spare unit operated for no more than a day, did not produce oxygen, and produced some odors.

A replacement Elektron liquid unit was delivered to the ISS on Progress 13P in 2004 to replace the failed unit. With the delivery of this unit, no spare Elektron units remain in space or on the ground. Russia is working to produce three spares, the first of which is planned for delivery in March 2005.

According to Williams, given the "very limited redundancy" on board the ISS, Boeing was asked to do a study to come up with contingency plans in case this Elektron unit failed as well. Had this unit failed, the ISS would have reached redline oxygen levels in July 2004 forcing NASA to either exceed these redlines or de-man the ISS.

Fortunately, that Elektron unit worked and continues to work. However, this lack of redundancy continues to present an operational risk until such time as the unit has spares on board the ISS.

Someone in the audience made note of the fact that the ISS went from having an oxygen surplus to a real threat of an oxygen undersupply in a short period of time and wondered why the Progress tanks could not have been kept on board the ISS for contingency use.

According to Williams, the location of these high pressure oxygen tanks on Progress spacecraft - external to the pressurized volume - would make it "nearly impossible" to bring them inside so as to use their contents.

When the logistics plan for the ISS program was constructed, using the existing shuttle, Soyuz, and Progress systems, many scenarios were taken into consideration. Some were prepared for, others were not. Of course, there is a cost associated with all changes to existing hardware.

Given the fact that the ISS is going to be in orbit for a long time, and interruptions in one or another logistics mode are inevitable, it would make sense to enhance the ability for previously incompatible - or inflexible systems, to be made more flexible.

The interesting thing about the session I attended ( and others at ICES) was the fact that a NASA representative was as open as he was about the problems facing the ISS - and how, for the most part, they have been solved along the way. Moreover, it was interesting to see how flexible the ISS program is becoming in its ability to deal with contingencies, the unexpected aspects of how things work in space, and the novel solutions that can arise when such situations call for quick solutions.

All of this will be needed if NASA is going back to the Moon and on to Mars.


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