The essential Performance Requirements for the ISS Common Berthing Mechanism (CBM) were:
- The ability to acquire control authority over the in-coming module, parameterized by the number of (3) and separation between points of contact (90 +/- 30 degrees) between the on-orbit module and the in-coming module at the end of “capture”;
- The ability to establish and maintain a seal between the two modules, parameterized by the leak rate at their interface;
- The ability to withstand module- module loads resulting from station operations.
The seals responding to item (2) were required to function for long periods of time after mating (upwards of 10 years). At that time, NASA did not perform long-term missions, so was in the institutional habit of “gooping” their sealed interfaces (e.g., hatches) with high-viscosity, low-vapor-pressure grease in order to meet their acceptance and qualification leak rates. For short flights (weeks), this approach worked just fine.
During long-term use (years), however, even the high-viscosity grease can be sucked through imperfections in the seating of the seal. Furthermore, the CBM/CBM seal was a “field joint”, whereas NASA’s then-current practices were developed for “factory joints” that could be subjected to detailed testing and re-work if necessary: operation of the CBM had to be “fire-and-forget”.
Therefore, we didn’t permit that practice on the CBM: we required the seal performance to be established 1by the design of the seal, not by goop on the ground. As a result, most of the CBM-level testing used leak rate to verify that the “hard mate” process had completed properly. Furthermore, we required testing for permeation in order to fully discriminate the quality of the mated interface and provide leak rate data immediately relevant to the numerical value in the specification. The practice provided highly objective evidence of compliance with requirement (2) and a large data set with regard to the seal’s operational robustness.
The practice introduced far more leak rate testing than had previously been known to NASA (or to most of Boeing), and many of the old hands couldn’t (or didn’t want to) wrap their brains around it. The term (“…pervert”) was frequently applied in a derogatory manner, but those of us on the inside came to use it otherwise. When I use the expression “you sound like a leak test pervert to me”, I’m commenting on your attention to detail and the objectivity of the evidence you’re creating. It’s one of the highest compliments I have to offer.
Footnotes- As opposed to mere “through-leak” testing, which takes less time and requires less sensitive equipment.[↩]