to print. The DSCSll unit had 12-mm (0.5-inch) diameter signal rings with correspondingly short signal  brushes and 25-mm (1-inch) diameter power rings with longer power brushes. For various reasons (well  thought out) Windsat ended up with both signal and power rings at 38-mm diameter (1.5 inches). The  Windsat/DSCSII power brushes were much longer and had a much lower stiffness/spring rate. This made  the power brushes much less sensitive to variations in brush force from misalignment and thermal  excursions when compared to the shorter signal brushes. In hindsight, this was an obvious error it would  have been better to not blindly copy “heritage” and optimize the brush design for brush force sensitivity -  i.e., make all brushes like the power brushes.  Values of Thorouqh Testinq (vs. Dumb & Hamv)  This may seem obvious and not really a lesson learned, however, there is always budget and schedule  pressure to reduce testing. On Windsat we did not compromise when it came to thorough testing. This  had the extremely beneficial effect of catching many problems and fixing them before they were flown.  However, it also had the parasitic effect of “finding lots of blemishes under a microscope” that we might  have been just as happy to not know about. A highlighthecap of some of the things that were found by  extremely thorough testing that would not have been uncovered with “normal testing” follows. Some of  these had a big impact; others were just “interesting”.  - NRL test set designed to detect high-frequency noise finds problems not seen in typical slip ring  tests  Found at least some cold temp noise could be “burned thru” with current  Found high-frequency noise in air vs. GN2 & vacuum - -  -  - Found torque decreases significantly with vacuum level (<pressure=<friction)  Found & solved cold temperature noise during acceptance testing which helped diagnose on-  orbit anomaly (more on this later)  100% microscopic mapping of rings also helped diagnosis of anomaly & revealed cracks  previously unnoticed  Value of slip ring module level thermal cycle tests screening “bad bond”  On-Orbit Anomaly Lessons Learned  After 2 years / 33 million revolutions of nominal operation the Windsat BAPTA abruptly spun out of control  (Figure 11). After a lengthy (and confusing) investigation, telemetry analysis revealed that the BAPTA  resolver-based speed data was corrupt. Figure 1 1 shows the commanded speed, BAPTA resolver-based  speed estimate, and the “true” BAPTA speed derived from spacecraft rates from the spacecraft Inertial  Measurement Unit and conservation of momentum. In this figure, you can see the BAPTA speed going  unstable and the true speed diverging from the resolver-based speed. Once the instability grows, the  three-axis stable spacecraft begins to yaw uncontrollably, which triggers a commanded BAPTA spin-  down and spacecraft “safe hold” mode. Since the data shows that the resolver system appeared to be  corrupt, and the spin drive had redundant resolvers and electronics, the “B-side” was selected. The  redundant B-side, however, was also found to be unstable (Figure 12). After extensive unsuccessful (and  very confusing) troubleshooting, some troubleshooting commands were issued wherein the A&B sides of  the BAPTA electronics were simultaneously turned on and the stationary 16-bit resolver position was  queried. With the BAPTA stationary, both A and B side angles read identical (as they should) and they  also read the correct angle (as roughly determined via external means). It was recognized at this point  that all post-anomaly spin/troubleshooting was conducted at cold temperatures (around 5°C) as a result of  the Windsat payload being off from the BAPTA induced spacecraft safe-hold. From earlier experience  gained during cold temperature acceptance testing of the slip rings; it was theorized that cold temperature  related slip ring noise was likely corrupting the B-side resolver which had its “transmitter” coil power  passed across the slip rings. While the slip rings were able to start up at temperatures around 5°C during  the first two years of operation, it was postulated that the effects of two years (33 million revolutions) of  operation had resulted in a greater tendency for slip ring lubricant related noise to occur at cold  temperature. Additionally, with 20/20 hindsight it would have been better to size the slip ring heater larger,  but it was added late in the program when there was no more power left in the budget and no justification  for a change that would “break” the power budget as the slip rings worked fine at that time all the way  21 8