Correlated reliability and an application: Propulsive landing on Mars

This article discusses reliability of landers and provides a review and examples of correlated reliability. Examples are cited to show generally beneficial effects of correlation in system reliability. Then, reliabilities of two near future landing systems are studied using two analytical (Downton, and Marshall & Olkin) bivariate exponential distributions and two simulation methods that incorporate correlation in reliability calculations. Both landing systems are composed of correlated two-unit subsystems. Numerical examples show mean system life, standard deviation of the system life, mean system life confidence interval, and reliability for each lander’s propulsive descent. Both simulation method results are in between the results obtained from the two analytical methods and Downton’s method yields the most conservative reliability. This article also shows how the Downton method–based reliability value can be predicted as a function of the reliabilities obtained from the other three methods. An up-to-date literature review of all related topics is also provided.