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

The output of the pressure transducer as read by the onboard computer and sent to the ground throughtelemetry is in mV. The conversion from mV to psig (or torr) varies from transducer to transducer, but isguaranteed by the manufacturer to fall in the range of 200±50 mV/psig. Thus, each mV measured by the sensorcorresponds to roughly 0.20684 torr to 0.34473 torr.Figure 7-2 is a plot of twelve hours of restrictor pressure data recorded on-orbit in Thruster 5.A prominent feature of this plot is a lower bound on the pressure (at roughly 1.4 mV), which is sustained fornearly the full duration of the time period examined (Note: this behavior can also be observed in Thruster 7).There are occasional dips below this bound due to noise and perhaps transient conditions, but the overallinference to be made is that the thrusters can consistently control the restrictor pressure to a reading of 1.4 mV.Figure 7-2. 12 hours of thruster pressure data from 2004 day 184Since people in general do not think in terms of mN it is helpful to draw several analogies to put the figure of 1mN of thrust into perspective. For instance, a 1 lb weight produces a force equivalent to 4,442.8 mN and theaverage American male in his twenties weighs 168 lbs. or roughly 745,000 mN. Furthermore, a male bodybuilder capable of bench-pressing 500 lbs. could be shrunk to 1/2,200,00 th of his normal size and still be able tobench press a 1 mN weight. Also, if everyone at a sold out game in 3 Com Park (which has a maximum capacityof 64,450) held a 1 mN thruster underneath a 15 lb bowling ball, they would still need an extra 2,300 thrusters tolift it off the ground.In summary, ground testing and flight data indicate that the helium thrusters on Gravity Probe-B can belinearly controlled down to roughly 1 mN of thrust. This is a number that is so incredibly small, that it takescreative use of analogies for one to truly appreciate it and its role in the demanding task of testing Einstein.174 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis