Lessons Learned in Space Life Support System Testing

The earlier problems can be found and corrected, the easier and cheaper it is to fix them.
Doing less testing saves cost and time but doing too little testing increases the risk of
operational failures causing large costs and delays. Integrated test is necessary to determine if
the subsystems work together and the overall architecture performs as intended.

This report reviews the testing lessons learned from the NASA Systems Engineering
Handbook, a National Research Council report, and five reviews of International Space
Station (ISS) lessons learned. The five reviews all mention two important points. First, that
testing should be performed on the final integrated system, one as close as possible to the
intended flight system. Second, “test as you fly,” while operating as planned in an environment
as close as possible to the expected flight environment. Other lessons are the need for extensive preflight ground testing, the need to establish and defend an adequate budget, the problems using protoflight hardware on ISS, and the benefit of having ISS as a zero gravity test bed.

The major ISS life support systems, carbon dioxide removal, water recycling, and oxygen
recovery, were protoflight systems with little testing before launch to ISS. The failure rates of
these systems have been much greater than predicted and this has caused dissatisfaction with
the protoflight approach. The more costly traditional approach builds qualification and test
units in addition to flight units. The test units are used to find, analyze, and fix failure modes.
Other work shows that there is an optimum cost-effective amount of testing when redundant
systems must have a specified reliability and confidence.