by Melissa Winter
| September 25, 2014
I’ve recently had a number of users ask, “How do I model life cycle costs for a missile that just sits on a shelf?” I had never actually tried to model this, but of course I know it’s possible. So I turned to some of my fellow PRICE experts, and found that of course this is not the first time anyone has ever tried to model this kind of thing…
Many ordnance weapons such as mortar shells, torpedoes, bombs, missiles and various projectiles are stockpiled until they are actually needed. These weapons are commonly referred to as "wooden rounds" and theoretically, they never fail from usage. Consequently, their Mean Time between Failure (MTBF) calculations are of no value in computing life cycle costs. Since the customary failures never occur, the life cycle cost would appear to be insignificant. Many wooden rounds are, however, periodically removed from storage to be tested.
If the rounds test satisfactorily, they are returned to storage. This effort can represent considerable handling and testing costs over the life of a program. Also, we can expect a certain percentage of the tested rounds to fail the tests and require repairs and/or overhaul - or perhaps even be scrapped.
To model the periodic removal from storage, you merely replace the conventional MTBF value with the Mean Time Between Removal Rate (MTBR). For example, if EACH wooden round is to be removed from storage once every three years, the mean time between removals would be 3 x 8,766 = 26,298 hours (8,766 is average number of hours in one year).
You can account for the number of rounds that pass or fail the test using the False Failure Fraction in the System Folder input sheet, and you can calculate the labor to test the rounds by inputting Mean Time to Repair on Equipment. This should include the time taken to unpack and make the round ready for test, the time for the actual test, and the time taken to re-store the round. Finally you can also account for the cost to perform the actual repair based on your Maintenance Concept.
Like many other scenarios, shifting your philosophy can help you model many different situations with just one tool. And that is the beauty of parametrics!