Completing the development process and getting to first flight has become a difficult hurdle for launch vehicles. Program cancellations in the last 30 years were largely due to cost overruns and schedule slips during the design, development, testing and evaluation (DDT&E) process. Unplanned rework cycles that occur during verification, validation, and testing (VVT) phases of development contribute significantly to these overruns, accounting for up to 75% of development cost. Current industry standard VVT planning is largely subjective with no method for evaluating the impact of rework. The goal of this research is to formulate and implement a method that will quantitatively capture the impact of unplanned rework by assessing the reliability growth that occurs throughout VVT. Typical reliability growth models for launch vehicles state that their respective models can be used during the testing phase, but are primarily used to model launch vehicle test flights where all failure modes are observable in a fully integrated system. Development testing for launch vehicles and launch vehicle subsystems does not always allow for fully integrated tests. Due to the extreme operating environment, lower fidelity testing is more common because it is less cost-prohibitive, and only a handful of flight tests are conducted. The fidelity level of development tests gets progressively higher as the system develops. This implies that each trial or test cannot be treated equally. To enable the use of a reliability growth model that assesses individual tests during development, when the fidelity of VVT activities varies, the fidelity level of each test is defined and the probability of rework between activities is modeled using a dependency structure matrix. A discrete event simulation then projects the occurrence of rework cycles during the testing process and predicts the reliability growth through an implementation of Halls framework. This method is applied to the Space Shuttle Main Engine to demonstrate how the impact of rework can be estimated during early design, using the actual test history as a baseline.
Framework for reliability growth and rework projections for launch vehicles during testing
2018-03-01
827404 byte
Conference paper
Electronic Resource
English
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