We propose here to develop tunable diode laser spectroscopy as a diagnostic for the Space Shuttle main engines during test stand operations. These engines represent the state-of-the-art in rocket engine propulsion systems, and as such, they stretch available technology to the limit. The engines must be test-fired through several cycles prior to incorporation into the shuttle for flight operations. Diagnostic tests for the engines are extremely limited due to the harsh nature of the environment. We propose to develop diode laser instrumentation in order to measure temperature, velocity, surface erosion, and possibly efficiency in real time with an update rate of up to 1 kHz. The system technology will be based on wavelength multiplexed tunable diode laser spectroscopy which Zolo and Stanford have jointly developed to diagnose many types of aeropropulsion systems including SCRAMJETs, augmentors, and pulsed detonation engines. This project represents the first time that the wavelength-multiplexed technology will be tested on full-scale rocket engines.