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

Figure 1-8. Clockwise, from top left: The dewar, a cross-section of the dewar, the porous plug, and one thrusterThrough a combination of multi-layer insulation, vapor-cooled shields, and a unique porous plug, the dewar'sinsulating chamber was maintained in a cryogenic vacuum throughout the mission. While virtually no heatcould penetrate from the outside wall through the vacuum and multi-layer insulation inside, a small amount ofheat was continually conducted from the neck of the Dewar and from the heat-trapping windows inside theprobe into the superfluid helium inside the dewar chamber. This very slight, but continual warming caused thesuperfluid helium to slowly vaporize into helium gas, which needed to be vented continually from the dewarchamber through the porous plug.Invented at Stanford, and engineered for space at the NASA Marshall Space Flight Center, Ball Aerospace, andthe Jet Propulsion Laboratory, the porous plug controls the flow of this evaporating helium gas, allowing it toescape from the Dewar, but retaining the superfluid helium inside. The plug is made of powdered titanium thathas been sintered (heated) until it coalesces into a sponge-like pumice with very tiny pores. The evaporatinghelium gas climbed the side of the dewar near the plug and collected on the plug’s surface, where it evaporatedthrough the pores in the plug, much like sweating in the human body, drawing heat out of the liquid heliumremaining in the Dewar, and thereby balancing the heat flow into the Dewar. The helium gas continuallyescaping through the porous plug was cycled past the shields in the outer layers of the Dewar, cooling them (thusthe name, “vapor-cooled shields.”) Moreover, it then was used as the propellant for eight pairs of micro thrusters,strategically located at the extremities of the spacecraft and used by the Attitude and Translation Control system(ATC) for high-precision pointing and positional control. Thus, not only did the dewar serve as the means ofmaintaining a cryogenic environment for the experiment, it was also the sole source of propellant for the ATCsystem.12 March 2007 Chapter 1 — Executive Summary