The Solid Rocket Booster (SRB) Main Flame Deflector (MFD) at Launch Complex 39A was instrumented with sensors to measure heat rates, pressures, and temperatures on the final three Space Shuttle launches. Because the SRB plume is hot and erosive, a robust Tungsten Piston Calorimeter was developed to compliment measurements made by otT-theshelf sensors. Witness materials were installed and their melting and erosion response to the Mach 2 I 4000°F I 4-second duration plume was observed. The data show that the specification used for the design of the MFD thermal protection system over-predicts heat rates by a factor of 3 and under-predicts pressures by a factor of 2. These findings will be used to baseline NASA Computational Fluid Dynamics (CFD) models and develop innovative MFD designs for the Space Launch System (SLS) before this vehicle becomes operational in 2017.