20.0055 degree. To provide a zero momentum payload a 189 Nms momentum wheel spins in the  opposite direction of the payload. During operation, the Windsat payload and BAPTA speed control is  “slaved to track the momentum wheel speed. This system performed nominally for 2 years on-orbit (33  million revolutions), until February, 2005 when the BAPTA abruptly spun out-of-control.  BAPTA / Slip Ring Design Overview  The BAPTA design (Figure 2) incorporates a “bolt-on” slip ring assembly which provided ease of  integration & test and “modular functionality” of the separate units. The BAPTA was designed with a  redundantly wound three phase brushless DC torque motor and redundant dual speed (1X & 64X)  resolvers. The resolvers provided 18-bit position resolution and better than 16 bit accuracy after  conversion via a resolver to digital converter. This also enhanced the servo “stiffness” to maintain the tight  20.05% speed control required. Two angular-contact machine spindle bearings supported the aluminum-  beryllium rotating housing, resulting in a very stiff, robust structural design.  Figure 2. BAPTNSlip Ring Assembly  The slip ring performance was the most challenging aspect of the BAPTA. The design life for Windsat is 3  years at 31.6 rpm, which translates to 50 million revolutions. Many spacecraft slip ring applications are for  solar array drives that have rotational life requirements several orders of magnitude lower. The Boeing  376, NGST/Ball DSCS II spun/despun satellites, and a few others, have the only slip rings with life  requirements in this class. The Boeing slip rings use silver rings with silver/MoS2 brushes, whereas the  DSCSll used a gold-on-gold slip ring design. However, unique to Windsat was the requirement to pass  high-rate 1553 digital data and resolver transmitter signals across the slip ring, which are much more  sensitive to high-frequency slip ring noise than these previous applications. Much life test data existed for  slip rings, but little if any high frequency noise data existed for slip rings at the time of Windsat design.  Fortunately, a 1968 vintage DSCS II program slip ring unit was found that had been through a 200 million  revolution life test in the 1970’s. This unit was tested in 1999 with a comprehensive slip ring test set,  designed to monitor & record high-frequency slip ring noise. After several hours of noisy operation, the  brushes pushed the significant amounts of wear debris to the side (Figure 3) and the slip ring noise went  away. This test data gave us confidence that this type of slip ring would work, while another life test on a  new unit closer to the flight design was run in parallel to the fabrication of the flight slip rings. The Windsat  & DSCS II slip ring designs are of the oil-lubricated, gold-brush / gold-ring variety, but differ from any  other gold-on-gold designs. The Windsat slip ring design consists of hard gold alloy brushes made from  wire that is swaged to a rectangular cross section (Figure 4). The brushes were swaged flat instead of  round in an attempt to accommodate more wear debris under the brush. Since Windsat & DSCS II spin in  only one direction, a pair of trailing rings sit in a “W” shaped contact groove compared to the more  common bidirectional “stubbing” & trailing round wire in a ‘‘V’ groove configuration. The “trailing only”  design is less prone to brush bounce compared to a “stubbing” design. Additional details on the DSCSll  slip rings can be found in reference 1. Since there was great concern for slip ring noise problems, 100%  parallel redundant rings were used for all signal rings resulting in 4-8 contacts per wire (4 vs. 8 contacts  21 0