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

together. Thus, when the Stanford engineers inserted Probe B into the SMD for testing, they did not tighten thefaulty bolt, but they soon discovered that the same problem was occurring with the second axial lock, and theybecame concerned that if the bolts in either of the remaining two axial locks seized up completely, it would notbe possible to remove Probe B from the SMD.This was a critical issue, for which there were basically two solutions:1. Send the SMD (dewar) back to LMSC to be modified and re-built.2. Abandon the axial lock system entirely and modify Probe C, which was still at LMSC, to provide positivethermal contact between the two flanges using Belleville spring cup washers at the top (warm region) ofthe probe.It was clear that the latter solution, which became known as the Belleville Preload System (BPS), would be faster,less expensive, and less risky to implement. Thus, work began at LMSC to modify Probe C to use the BPS,delaying its delivery to Stanford by about eight months. This delay, and later others, required the dewar to bemaintained at a cryogenic temperature for over eight years, through the end of the mission—an amazing feat inits own right.6.4.2 Flight Probe C Integration & TestingThroughout Bradford Parkinson's tenure as Program Manager, Co-PI, John Turneaure, served as the GP-BHardware Manager, with responsibility for managing the day-to-day development of the GP-B flight hardware.In 1998, Parkinson decided to step aside as Program Manager to become acting CEO of Trimble NavigationLtd., a Silicon Valley navigation technology company, although still maintaining his position as a Co-PI, andalso a senior advisor to GP-B. Given his intimate knowledge about GP-B flight hardware and close workingrelationship with Parkinson, it was a natural transition for Turneaure to take over as Program Manager.Lockheed Martin delivered Probe C to Stanford in mid 1998, with Turneaure now serving as Program Manager,and immediately, testing began on the fit of Probe C into the SMD. The new BPS hardware was then used tospring-load the probe into the dewar, and tests were performed to verify thermal contact between the SIAportion of the probe and the dewar's main tank.Once it was established that the BPS dewar-probe locking interface was performing properly, it was necessary toextract Probe C and move it to a clean room at Stanford, where the Science Instrument Assembly (SIA),comprising the gyros, SQUIDs, and telescope, was then to be integrated into the lower, cryogenic cavity of theprobe. This work proceeded smoothly for several months, and by mid 1999, Probe C (also called the ScienceMission Probe or SMP) was ready to be inserted into the SMD for integrated payload testing.Testing of the integrated payload system commenced in August 1999, and during these tests, anotherunexpected thermal contact issue surfaced, as well as an unrelated problem with the pickup loop for gyro #4.Temperature measurements taken at various points in the probe and dewar revealed that a set of four copperbands, each attached to a corresponding window inside the neck of the probe, were not making thermal contactwith a matching set of copper bands on the outside of the probe neck, thereby preventing the probe windowsfrom intercepting thermal radiation propagating down the neck of the probe. The thermal interface between theinner and outer copper bands had worked well in the Probe A and B prototypes, but during the assembly ofProbe C, there apparently had been problems with the epoxy tape that secured these bands in position. Becauseinterception of thermal radiation in the probe neck was essential for maintaining a cryogenic environment inthe SIA, failure of this thermal interface was a serious problem that had to be solved. Consequently, the probehad to be removed from the dewar once again, further delaying the launch and mission.Collaborating closely, Turneaure and LM Lead Probe Engineer, Gary Reynolds devised a solution. Theydetermined that carefully drilling a series of holes through the outer bands and probe wall, into—but notthrough—the inner bands and then cementing small copper pegs into these holes with epoxy, would yieldGravity Probe B — Post Flight Analysis • Final Report March 2007 157