Space shuttle main engine plume diagnostics: OPAD approach to vehicle health monitoring

The process of applying spectroscopy to the Space Shuttle Main Engine(SSME) for plume diagnostics, as it exists today, originated at Marshall Space Flight Center in Huntsville, Alabama, and its implementation was assured largely through the efforts of Sverdrup, AEDC, in Tullahoma, Tennessee. This process, Optical Plume Anomaly Detection (OPAD), has formed the basis for various efforts in the development of in-flight plume spectroscopy and in addition produced a viable test stand vehicle health monitor. OPAD is based on the detection of anomalous atomic and molecular species in the SSME plume using two complete, stand-alone optical spectrometers. To-date OPAD has acquired data on 43 test firings of the SSME at the Technology Test Stand at MSFC. The purpose of this paper will be to provide an introduction to the OPAD system by discussing the process of obtaining data as well as the methods of examining and interpreting the data. It will encompass such issues as selection of instrumentation, correlation of data to nominal engine operation, investigation of SSME component erosion via OPAD spectral data, necessity and benefits of plume seeding, and the present status of efforts to quantify specie erosion utilizing standard plume and chemistry codes as well as radiative models currently under development.