Abstract: A method and system for use as a modeling tool accurately estimates propellant remaining aboard a spacecraft at the completion of a transfer orbit by iterating mass flowrate in a linear optimized fashion until the difference between a predicted .DELTA.V and observed .DELTA.V is minimized. Actual flight pressure telemetry data is used to calculate a corresponding mass flowrate which must have been produced by a liquid apogee motor (LAM) for the spacecraft to reach the desired position. A thruster specific impulse value (ISP) is derived from ground test data to allow computation of the amount of propellant consumed during transfer orbit. When this value is subtracted from the known initial propellant load, the propellant remaining at the beginning of life (BOL) of a satellite can be calculated.