The supersonic combustion ramjet (scramjet) engine is one of the more promising high-speed flight propulsion technologies. One of the reasons for this is the simplicity of the engine design, having no moving parts and requiring no external ignition source, and the fact that scramjets do not need to provide their own oxidizer. Despite this simplicity, several obstacles to the use of scramjet propulsion systems have become apparent, including the ability to produce sufficient fuel-air mixing at high speed, large total pressure losses, reduction in specific impulse with increasing flight Mach number, and the sensitivity of combustion to inlet temperature. This last problem can be very significant.

This work presents development of an oxygen-based diode laser absorption sensor designed to be used in a scramjet engine inlet. The sensor uses free-space propagation of light from a vertical-cavity surface-emitting laser (VCSEL) to determine the temperature, velocity, and angle of attack of an engine inlet in hypersonic flow.

A schematic of tunable diode laser absorption spectroscopy (TDLAS) optics for Hypersonic Inlet Measurements. The inlet configuration shows the laser passing four times through the freestream and the two shock layers.
The new sensor for the Mach number and angle of attack in a scramjet engine uses VCSEL-based absorption measurements. The method uses retro-reflection to very accurately determine the freestream velocity and the change in absorbance with line strength over the eight transitions available to the laser to determine the effect of angle of attack, determining the conditions using nonlinear least-squares fitting of the absorption spectrum against a database of computed spectra. The absorbance has been computed assuming two-dimensional analytical shock theory.

Simulation of absorption spectra with noise has been performed and has shown that the Mach number can be calculated very precisely for a given angle of attack, to less than ±0.02 from the correct value. The precision of the angle of attack measurement is lower, with an uncertainty at Mach 8 and α = 0.2 of ±0.25 degrees, for signals with no added random noise. The fits for angle of attack appear to be susceptible to convergence to a local, rather than a global, minimum using this fitting routine. This dependence can be minimized by selecting several start points for the least-squares fits, and choosing the final fit with the lowest residual. High-frequency Gaussian noise, up to 50% of the peak signal height, did not have a strong effect on the fitted velocity, but can have a significant effect on the temperature measurements. This sensitivity means that temperature measurements require a signal-to-noise ratio of 10 or greater to give a sensible value for angle of attack, because the peaks in the shock layers have approximately 10% of the signal in the freestream.

The system in its current form is capable of measuring both freestream temperature and velocity at measurement rates of 150 Hz. Sensitivity to vibration and very low absorption values prevented the system from being able to determine the angle of attack. It is possible that, with more attention to reducing the effect of vibration, some information about the angle of attack may be obtained.

Future work will be directed at reducing the susceptibility of the system to vibration, as this appears to be the largest source of uncertainty in the measurements. Vibration tests are being conducted to determine the cause of the susceptibility to vibration and reduce it by making the system move as a whole rather than having the two parts of the system move independently.

This work was done by Sean O’Byrne of the University of New South Wales, Australia, for the Asian Office of Aerospace Research and Development. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. AFRL-0164


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Diode Laser Sensor for Scramjet Inlets

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This article first appeared in the December, 2010 issue of Defense Tech Briefs Magazine.

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