24-10-2012, 10:32 AM
Effect of Random Thermal Spikes on Stirling Convertor Heater Head Reliability
ABSTRACT
Onboard radioisotope power systems being developed to support future NASA exploration missions require reliable
design lifetimes of up to 14 yr and beyond. The structurally critical heater head of the high-efficiency developmental Stirling
power convertor has undergone extensive computational analysis of operating temperatures (up to 650 C), stresses, and creep
resistance of the thin-walled Inconel 718 bill of material. Additionally, assessment of the effect of uncertainties in the creep
behavior of the thin-walled heater head, the variation in the manufactured thickness, variation in control temperature, and
variation in pressure on the durability and reliability were performed. However, it is possible for the heater head to experience
rare incidences of random temperature spikes (excursions) of short duration. These incidences could occur randomly with
random magnitude and duration during the desired mission life. These rare incidences could affect the creep strain rate and
therefore the life. The paper accounts for these uncertainties and includes the effect of such rare incidences, random in nature,
on the reliability. The sensitivities of variables affecting the reliability are quantified and guidelines developed to improve the
reliability are outlined. Furthermore, the quantified reliability is being verified with test data from the accelerated benchmark
tests being conducted at the NASA Glenn Research Center.