Investigation into the effect of decreasing failure rate on long duration mission reliability estimates

Several previous spacecraft data studies, which have analyzed long duration missions, indicate a trend of decreasing failure rate such that subsystem reliability functions fit a Weibull distribution with a shape parameter less than one. The literature does not contain studies that compare reliability estimates of both a constant failure rate approximation, which is the usual assumptions, and a time dependent hazard function for actual launched spacecraft. Using the MESSENGER spacecraft configuration, launched 3 August 2004 to orbit and study the planet Mercury, this paper describes a comparison between mission reliability estimates using the constant failure rate approximation and a time dependent, decreasing failure rate based on previous long duration mission spacecraft data. Monte Carlo simulation of the spacecraft unreliability is also compared and contrasted with an exponential approximation and a Weibull approximation to the simulation results. The simulation agrees well with the Weibull approximation but increasingly differs from the exponential approximation as mission time increases. Accounting for this time dependent effect has implications with respect to spacecraft redundancy, weight, consumables and mission design.