Analysis of the photodiode boundary-layer transition indicator

Abstract

The photodiode transition indicator is a device that has been successfully used to determine the onset of boundary-layer transition on numerous hypersonic flight vehicles. The exact source of the electromagnetic radiation detected by the photodiode at transition was not understood. During some flight tests early saturation of the device occurred, and the device failed to detect transition. Analyses have been performed to determine the source of the radiation producing the photodiode signal. Four proposed radiation sources were investigated: 1) radiation from the quartz window material, 2) radiation from the heat shield material bordering the quartz window, 3) radiative emissions from the gases within the boundary and shock layers, and 4) radiation from ablation products. The results of these analyses indicate that the most likely source of the radiation is blackbody emission from the heat shield material bordering the quartz window of the device. Good agreement between flight data and calculations based on this radiation source has been obtained. Analyses also indicate that the most probable source of the radiation causing early saturation is blackbody radiation from carbon particles that break away from the nosetip during the ablation process.