A computer-controlled instrumentation system has been developed for use in measuring concentrations of various atomic species in the exhaust gases of a space-shuttle main engine on a test stand. The system is based on established techniques of atomic-absorption spectroscopy (AAS). Although the design of the system is specific to this rocket-engine-testing application, it may be adaptable to other applications that involve similar geometry, physical conditions, and chemical constituents of flowing gases.
To save time and money, the system was constructed largely from commercial equipment used previously in atomic-emission spectroscopy (AES). (The reason for choosing AAS instead of AES is that under the flow conditions in the specific application, the temperatures are too low to obtain adequate optical emission from the atomic species of interest.)
The elements Cr, Fe, Ni, Co, Cu, and Ag are introduced into the rocket exhaust through wear/erosion of the engine. The system schematic includes a source probe that generates an optical emission spectrum characteristic of the atomic species expected to be entrained in the rocket exhaust. The source probe contains a multielement hollow-cathode lamp containing the above elements plus neon as a fill gas and a collimating lens. Unlike traditional AAS systems, which chop the source signal to enable correction for background fluctuations and attenuation, the Stennis system ratios absorbing to nonabsorbing spectral lines. This method allows data to be continuously acquired and displayed.
The source probe is aligned so that the optical signal passes through the center line of the exhaust plume downstream from the rocket-engine nozzle where it is collected by the receiver probe. As the beam passes through the exhaust plume, metallic contaminants in the plume will alter the transmitted spectrum by absorbing light at specific wavelengths inherent to each atomic species of interest. The receiver probe collects the resulting signal, where internal optics transfer the signal to an optical fiber. The optical fiber transmits the signal to the detector/spectrometer assembly, where the signal is both dispersed into its component wavelengths and the signal levels are measured. An optical multichannel analyzer reads the data from the detector and sends the raw data to a remote computer for processing and near-real-time display.
This work was done by Gregory P. McVay of Lockheed Martin for Stennis Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category. SSC-00062