A successful mission planning, in particular, at End-of-Life (EOL) depends on many factors including an accurate knowledge of remaining propellant, as a one of the major factors. Achieving the highest accuracy of propellant estimation is very important to a successful mission management. The current paper discusses different methods of propellant estimation, such as, book-keeping, PVT (Pressure, Volume, Temperature) and thermal gauging. The current paper shows that the Thermal Gauging Method (TGM) provides more accurate estimation of propellant remaining than bookkeeping and PVT methods at End-ofLife (EOL). The current paper also discusses the difference between the various thermal gauging methods, namely, TPGT, PGS and TGM . While the TGM consists of several steps, the paper discusses problems related to finding load of a propellant tank, e.g., how to isolate tank temperature rise due to tank heaters from rise due to other heat sources: sun, equipment, etc. A "toolbox" of software tools has been developed for pre and post processing and debugging. Some of the tools could be very useful for general thermal analyses. For satellites with multi-tank propulsion system, regardless of mono-or bipropellant, estimation uncertainty of individual tank load is another factor affecting remaining spacecraft life. Therefore, a total usable propellant load could be less than the sum of the individual tank propellant estimate. The current paper shows how to derive the expected usable propellant from load of each component and uncertainties associated with the load estimates. Nomenclature = standard deviation m = propellant load/mass p = model parameter which affects heating i = parameter index tot = total uncertainty fit = uncertainty related to curve fit BK
Comparison of accuracies of different propellant gauging methods is a tricky business unless the propellant tank(s) is completely depleted and an exact amount of consumed propellant is known. Satellite decommissioning is the most common case when tanks are drained to complete depletion. Recently, two GEOStar TM 1 satellites have been decommissioned and the propellant tanks were completely drained. This event provided a rare opportunity to compare predictions made by different methods with an actual tank load. In such a way, an actual accuracy of different methods of estimation was verified. This kind of comparison is almost absent in open literature. The current paper compares the accuracy of estimation of several gauging methods: Bookkeeping, PVT, Thermal Gauging Method (TGM) and Rapid Propellant Gauging (RPG), all of which have been developed and employed for propellant estimation for Orbital's GEOStar TM 1 satellites. It is shown that the TGM was the most accurate. The paper demonstrates that the RPG method is a useful tool for a quick independent evaluation of the Bookkeeping or PVT results.
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