Special Topics in Periodic Supply

The purpose of this chapter is to present several enhancements to the theory in Chapter 6. These are special topics that are important, but which would have been a distraction from the primary theoretical development. This is an opportunity to provide some further examples of analysis used for management decisions on the space station as well.

We begin with more discussion of availability, and some adjustments that need to be made when comparing resupply cycles of different length. Then we show how the degradation to availability due to maintenance removeand- replace time in orbit can be incorporated into an overall availability measurement.

Next we discuss a major topic, modeling of items whose demands are primarily due to wear out. There are several such items in the space station’s electrical power system, including batteries and solar arrays. The reliability engineer will typically estimate a failure rate (i.e. demand rate) that increases when the item approaches its expected lifetime. But to use our spares modeling techniques, the failure rate curve must be converted to a probability distribution for the time to failure. We show how it is possible to approximate any failure rate curve to any degree of accuracy, and compute analytically the distribution for time to failure.