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

Figure 3-5. Cross sectional schematic of the dewarDuring the mission, the dewar's insulating chamber remained a vacuum, which limited the amount of heatpenetrating through the outside wall into the inner chamber containing the science instrument. In addition, itincludes several other devices for maintaining the necessary cryogenic temperature:1. Multilayer insulation—multiple reflective surfaces in the vacuum space to cut down radiation2. Vapor-cooled shields—metal barriers, suitably spaced, cooled by the escaping helium gas3. Porous Plug—invented at Stanford, and engineered for space at NASA Marshall Space Flight Center inHuntsville, AL, Ball Aerospace in Boulder, CO, and the Jet Propulsion Laboratory in Pasadena, CA. Thisplug allows helium gas to evaporate from the Dewar's inner chamber, while retaining the superfluidliquid helium inside. See section 3.2.3.1for a more complete description of the porous plug.When there was still helium in the dewar, virtually no heat could penetrate from the outside wall through thevacuum and multilayer insulation inside. However, a small amount of heat (about as much as is generated bythe message indicator lamp on a cell phone) did leak into the Dewar from two sources:1. Conduction of heat flowing from the top of the Dewar into the liquid helium2. Radiation leaking down through the telescope bore into the liquid helium.The porous plug controls the flow of this evaporating helium gas, allowing it to escape from the Dewar, butretaining the superfluid helium inside. The plug is made of a ground-up material resembling pumice. Theevaporating helium gas climbed the sides of the inner tank near the plug and collected on its surface, where itevaporated through the pores in the plug, much like sweating in the human body.The evaporating helium provided its own kind of refrigeration. As the helium gas evaporated at the surface ofthe porous plug, it drew heat out of the liquid helium remaining in the Dewar, thereby balancing the heat flowinto the Dewar. You can feel this effect on your skin when you swab your skin with water. As the liquidevaporates off your skin, it draws heat energy with it, leaving your skin a tiny bit cooler than before.The helium gas that escaped through the porous plug was cycled past the shields in the outer layers of theDewar, cooling them (thus the name, “vapor-cooled shields”), and then it was used as a propellant for eightpairs of micro thrusters that are strategically located around the spacecraft.Gravity Probe B — Post Flight Analysis • Final Report March 2007 73