Terahertz Rayleigh Scattering of Particles in Rocket Exhaust Plumes
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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Characterization of particles in solid rocket plumes has been difficult due to the lack of real-time measurement techniques. This research proposes a new technique using terahertz radiation and Rayleigh scattering on alumina particulates in the plume. Previous techniques have involved collecting particles on a probe or mesh. Others have used Mie scattering using lasers, but this technique is limited to firing the beam straight across the plume. Terahertz radiation wavelengths in relation to alumina particle sizes, 1 micrometer to 100 micrometers, align with the Rayleigh criterion, requiring that the wavelength of light be at least ten times greater than the radius of the particle. Rayleigh scattering would allow for changing the location of the detector and even co-locating it with the source. A theoretical analysis of a notional setup involving a terahertz source and aluminum oxide particles inside a rocket plume was performed. The results from this analysis showed that a 1 kW source coupled with approximately 75 dB gain antennas would be required to perform this experiment. Because terahertz sources above 100 W are currently unavailable, an experiment was setup using microwaves at 2.45 GHz and properly scaled aluminum balls of radii ranging from 2.38 mm to 9.52 mm. A magnetron power source was used to provide a power range of 1-100 Watts into a microwave anechoic chamber. These results showed the expected trends with respect to source power and ball radius versus scattered power.
- Test Facilities, Equipment and Methods
- Combustion and Ignition
- Rocket Engines
- Rocket Propellants