RCS Quantity Monitor

The RCS quantity monitor sequence uses the general-purpose computer to calculate the usable percent of fuel and oxidizer in each RCS module. The RCS quantities are computed based on the pressure, volume and temperature method, which requires that pressure and temperature measurements be combined with a unique set of constants to calculate the percent remaining in each of the six propellant tanks. Correction factors are included for residual tank propellant at depletion, gauging inaccuracy and trapped line propellant. The computed quantity represents the usable (rather than total) quantity for each module and makes it possible to determine if the difference between each pair of tanks exceeds a preset tolerance (leakage detection).

The calculations include effects of helium gas compressibility, helium pressure vessel expansion at high pressure, oxidizer vapor pressure as a function of temperature, and oxidizer and fuel density as a function of temperature and pressure. The sequence assumes that helium flows to the propellant tanks to replace propellant leaving. As a result, the computed quantity remaining in a propellant tank will be decreased by normal usage, propellant leaks or helium leaks.

The left, right and forward RCS quantities are displayed to the flight crew on panel O3. When the rotary switch on panel O3 is positioned to the RCS fuel or oxid position, the RCS/OMS qty meters on panel O3 will indicate, in percent, the amount of fuel or oxidizer. If the switch is positioned to RCS lowest, the gauging system selects whichever is lower (fuel or oxidizer) for display on the RCS/OMS prplt qty, left, fwd and right meter.

The left, right and forward RCS quantities also are sent to the cathode ray tube, and in the event of failures, substitution of alternate measurements and the corresponding quantity will be displayed on the CRT. If no substitute is available, the quantity calculation for that tank is suspended with a fault message.

The sequence also provides automatic closure of the high-pressure helium isolation valves on orbit when the propellant tank ullage pressure is above 312 psia. The caution and warning red light on panel F7 is illuminated for the respective forward, left or right RCS, and a fault message is sent to the CRT. When the tank ullage pressure returns below this limit, the close command is removed.

Exceeding a preset absolute difference of 12.6 percent between the fuel and oxidizer propellant masses will illuminate the respective left RCS, right RCS or fwd RCS red caution and warning light on panel F7; activate the backup caution and warning light; and cause a fault message to be sent to the CRT. A bias of 12.6 percent is added when a leak is detected so that subsequent leaks in that same module may be detected.