GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Furthermore, a fourth electronic system, the Gyro Suspension System (GSS), underwent a completetransformation in 1996. Development of what was originally called the Gyro Suspension Unit (GSU) had beguna number of years earlier under the leadership of Richard Van Patten at Stanford. The original design was basedon a laboratory gyro suspension system using analog, rather than digital electronics, and a number of issues,including weight, power consumption, adherence to GP-B torque requirements and compatibility with the GP-B SQUID readout electronics, needed to be addressed in order to use this system in a spacecraft. As a result, theanalog suspension system was becoming increasingly complex, and senior GP-B managers were concernedabout the reliability and accuracy of this mission-critical system on orbit. With the GP-B launch date only foursixyears away, the senior program managers made the difficult decision to start over—that is, they decided todesign and construct a digital, rather than analog suspension system. A new collaborative GSS team, includingengineers and scientists from Stanford and Lockheed Martin was assembled at Stanford, under the leadership ofBill Bencze (who became post-flight GP-B Program Manager in 2005).It is a testament to the immense dedication and collaboration of the Stanford and LM teams that the three mostmission-critical electronics systems—the GSS, SRE and TRE—all performed flawlessly throughout the entiremission.6.4 Payload Integration, Testing and Repairs (1997-2002)In addition to continued development of the spacecraft, the Gyro Suspension System, and three key electronicssystems described in the previous section, the years from 1997 through 2002 were primarily focused on payloadintegration and testing. This included integrating the gyros, SQUIDs, telescope, etc. into the Science InstrumentAssembly (SIA), the SIA into the probe, and the probe into the dewar. The whole payload was then tested as aunified system. These years represented a critical period for GP-B, in which engineering design came face-tofacewith the realities of technology fabrication. Not surprisingly, some unexpected design and assembly issuesbegan to surface during tests of the integrated payload system. Addressing these issues used up the savings intime and money gained from streamlining the scope of the program, postponed the launch date from 2000 to2002 (then to 2003 and ultimately 2004), and resulted in greater NASA management oversight during thisperiod.6.4.1 Devising a Work-around for the Dewar Axial LockThe first of these issues was a problem with the dewar's axial locks, a set of three deployable clamps locatedaround the circumference of a pair of mating flanges in the center of the dewar—one flange at the top of thecryogenic portion of the probe and the other at the top of the dewar's helium-filled main tank. Each axial lockclamp had a bolt, that when tightened down, compressed the probe and dewar flanges together, ensuringpositive thermal contact between the lower, cryogenic portion of the probe and the dewar main tank.When Lockheed Martin delivered the final SMD to Stanford late in 1996, work began in 1997 testing andpreparing it to accept Probe C, which was also nearing completion. Preparation activities included filling thedewar with liquid helium and a painstaking process of inserting a sequential series of four nested lead bags,three of which were removed as part of the process, leaving a single, very effective cryogenic, superconducting,magnetic shield lining the probe cavity. Once lead bag installation was completed in the summer of 1997, ProbeB—the second prototype probe that had previously been delivered to Stanford—was inserted into the dewar as atest to ensure that the dewar-probe interface would function as expected when Probe C was delivered andintegrated into the dewar.In testing the SMD before delivery to Stanford, engineers at LM discovered some galling in the threads of one ofthe axial lock bolts, causing the bolt to seize up. At the time, LM engineers felt that this problem was isolated tothat individual bolt and that if it were not used, the remaining two axial locks could adequately hold the flanges156 March 2007 Chapter 6 — The GP-B Management Experiment