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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Dan Mahoney
Date posted: Thu Oct 12 12:37:21 US/Eastern 2000
Subject: Missile Reliability
Say an expensive fielded missile system has demonstrated its flight reliability goal after 50 flights, but then 20% of the missile is upgraded with a new subsystem. How many new flights must the system undergo to get back to its demonstrated reliability value? Another 50?, 20% of 50?, something else?!? Can you somehow use 80% of the past experience? What if only 1 tenth of 1 percent of the missile was changed? Would that change the requirement? Any insights or guidance would be appreciated.


Subject: Missile Reliability
Reply Posted by: Ed Sherwin ( )
Organization: RAC
Date Posted: Fri Oct 13 11:12:33 US/Eastern 2000
Dan, you are asking a typical questions that others are asking in determining the reliability of "one-shot" devices. With the tighting of testing budgets and lack assets for testing, the past methods are not adequate. RAC has been doing some work in this area, I would like to talk to you on your specific request. Please email me your telephone number and I'll call you. Based on statistical approach, the number of sample required if a function of the number of allowed failures and the proportion defective and the confidence level desires. The table below is base on a proportion defective (p) equal to 0.1. Confidence Level 60% 80% 90% 95% 99% Failures Sample Size Required 0 9 16 22 29 45 1 20 29 39 47 65 2 31 42 53 63 83 3 41 55 66 76 98 To use this chart, let assume that you want to be 90% confidence that your missiles are only 10% defective or have a 90% probability of success. During the test you expect zero failures. If you tested 22 missile and had no failures you can accept the hypothesis that the missiles are 10% or less defective.

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