Having Too Much and Too Little Oxygen and water on the International Space Station

Having too much and too little Oxygen and too little water on the International Space Station (ISS)

Novel solutions for the ISS after the Columbia loss

Tuesday, July 20, 2005

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.

David Williams from NASA JSC chronicles 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 the Russian Soyuz spacecraft to carry humans, and the Russian 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 one, instead of the three person crews which normally comprised an ISS Expedition crew. The water source is 40 kg flexible containers and is recycled from three sources, dehumidification of the air, human waste and fuel cell H2O by-product. Waste water is recycled through the catalytic oxidation reactor (COR) which recovers most of the effluent water, however it produces some by-products including brine. In the event, NASA located a British manufacturer who specializes in revolutionary compact marine desalination equipment. The company designed a modified unit to recycle the COR waste water brine and met a required upmass target of less than 3 kg. The water recovery unit was delivered to the ISS on Russian Progress 12P in 2004. This has enabled a crew compliment of two persons to be reliably maintained.

The water problem having been solved, 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, 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 of the Russian 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.

During 2003, the ISS experienced problems with its onboard ability to produce 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.

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 for us to enhance the ability for previously incompatible - or inflexible systems, to be made more flexible.

The interesting thing about the problems facing the ISS - is how, for the most part, they have been solved along the way. Moreover, it is 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 very novel solutions that can be found when such situations call for quick solutions.

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