Spectral study of metallic molecular bands in hybrid rocket plumes

Abstract

A labscale hybrid rocket was used to study spectral bands produced by metal combustion. Bands in the ultraviolet visible region (300-750) are of interest. The rubber-like fuel, hydroxyl-terminated polybutadiene (HTPB), was doped with a metallic salt for introduction into the plume during combustion. When introduced, the metals produce atomic line emissions as well as molecular bands due to excited forms of metallic molecules formed in combustion. The most likely molecular band emissions are from the excited states of metal oxides or metal hydroxides formed by these metals in the presence of the oxygen flow of the hybrid rocket. As the concentration of metallic dopants increases in the flame, the molecular band emissions also increase. The fashion by which they increase is observed here. The high concentrations observed for these metals result in intensity versus concentration curves that alter from the expected linear progression for manganese, magnesium and strontium. The molecular band emissions observed for calcium, barium and copper in this study followed linear progression, as does the atomic line emission for barium. The line emissions for manganese, strontium and calcium lean toward the concentration axis. The curves are attributed to self-absorption or increased interactions among mixing species as metal concentration increases in the flame. A pattern-like combustion routine for each metal can be characterized with further study. Introduction Atomic spectral techniques have been used in the past to provide diagnostics for engine health monitoring. The National Aeronautics and Space Administration (NASA) and Stennis Space Center in particular have taken interest in these studies as health monitoring techniques for the Space Shuttle Main Engine (SSME).--' These techniques depend on the relationship of excited atomic species in the motor plume to the amounts introduced by failures in the engine system. It is important that a linear or otherwise describable and reproducible relationship exist, in order to be able to quantify wear or other elemental introduction factors in the motor or engine system. Molecular emissions as seen in the normal realm of atomic spectroscopy are viewed as interference. A classic example is that encountered with analysis of barium in the presence of calcium. The analytical atomic line of barium is swamped by the presence of an overwhelming molecular emission from calcium, such as CaOH. Steps are usually taken to minimize the presence of these molecular bands in such work. However, these type precautions are not applicable to the field of engine health monitoring or in combustion diagnostics when applied to exhaust plumes. Molecular emissions are present in rocket * Graduate Student, Student Member AIAA t Professor, Member of AIAA Copyright © 2000 by Keith Hudson. Published by the American Institute of Aeronautics and Astronautcs, Inc. with permission. 1 American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. combustion, and should be factored in when quantitative data is required. A thorough study of the effect of molecular emissions in exhaust plumes is necessary to determine interference, fraction of species present as molecular versus atomic and other parameters. The Hybrid Rocket Facility at the University of Arkansas at Little Rock (UALR) was constructed to provide combustion diagnostic testing, and uses a 2 X 10 inch labscale hybrid thruster.' Previous studies have revealed the usefulness of the labscale hybrid rocket system as a plume simulator for other propulsion systems, and characterized it for both atomic and molecular emissions.'' ' The presence of molecular bands was noted in these studies, both from the combustion of HTBP fuel and as formed by metallic dopants, such as manganese. In order to study tike molecular bands in rocket plumes, the labscale hybrid rocket fuel was doped with various levels of metallic salts. Combustion of these salts results in band emissions attributed to metal oxides or metal hydroxides. Some of the metals were chosen due to their presence in alloys used in certain engine components, and because they appear to have produced molecular bands in previous combustion studies.8'10'11'12'13 Other metals were added to the study based on their tendency to oxidize easily, thus are likely to produce refractory particles in combustion.