Revisiting the Mark III/AX-5 Suit “Fly-Off”: Lessons Learned Applicable to Modern-Day Suits

At the start of the 1990’s, the upcoming Space Station appeared to be an opportunity to create the next-generation spacesuit, following ten years of flight experience with the shuttle Extravehicular Mobility Unit (EMU). Two suits were candidates for the “Space Station Suit”: the Mk. III from NASA Johnson Space Center, and the AX-5 from NASA Ames. The Mk. III continued the “conventional” hybrid approach of hard upper torso and soft goods for the arms and lower torso assembly, while the AX-5 was an all-metal “hard” suit design with sealed rotary bearings for mobility. The evaluation process in the “fly-off” between the two suits lasted almost two years, and is likely the most detailed head-to-head comparison of two spacesuits ever undertaken. Tests were performed joint-by-joint for each suit, and suited runs by four EVA-experienced astronauts were conducted in neutral buoyancy and parabolic flight. Wherever possible, each result was quantified to create a running “score” for the two suits, compared to an EMU as the control case. Crew subjective evaluations were presented and contested. In the end, no final conclusion was drawn nor formal report published. It was realized that no funding would be forthcoming for new suit development, and that the EMU would be the baseline U.S. suit for International Space Station. With the recent acquisition of all of the raw data and supporting information from the suit evaluations, this paper will take a “deep dive” into the process and strive to create an objective summary report of the process. The five volumes of data from the “fly-off” represent a wealth of information not only on those specific designs, but on the process of thoroughly testing and evaluating future EVA suits.