Understanding the characteristics of the aeroheating and, specifically, the hypersonic boundary layer at transition can lead to more effective TPS design. Infrared thermography, which measures the thermal radiance of an object, is a useful technique to determine boundary-layer transition and surface temperature, as well as other flow phenomena. The principle of IR thermography is that all bodies with a temperature above absolute zero continuously emit, absorb, and reflect radiation in a characteristic manner. The emission intensity peaks at shorter wavelengths for higher temperatures and at longer wavelengths (for example, IR) for lower temperatures. At a given wavelength the emission intensity also typically increases with temperature. The intensity of the image of such an object also varies with other factors such as target surface emissivity (ε), path transmittance, and atmospheric radiance. With sufficient calibration, the IR intensity can be related back to an accurate surface temperature. If independent in situ surface temperature measurements are available the process is much simpler and more accurate. These in situ measurements serve as benchmarks for the rest of the image and thus, many of the aforementioned factors can be neglected. Blanchard et. al. (ref. 5) details the image-processing techniques and subsequent data analysis for both methods.
NASA/Missile Defense Agency Infrared Sensing Aeroheating Flight Experiment
An experiment using ground-based IR imagery to capture global windward surface temperatures on the Shuttle orbiter during boundary-layer transition was undertaken. The primary objective of ISAFE was to obtain hypersonic heating data in order to develop the capability to accurately determine and predict hypersonic boundary-layer transition in future vehicles. This project was a collaboration between NASA and MDA/ISTEF. ISAFE collected data on two Shuttle flights: STS-96 on June 6, 1999 (ref. 4), and STS-103 on December 27, 1999 (ref. 5). The Shuttle orbiter Discovery was the vehicle for both STS-96 and STS-103. Figure 1 shows the MDA/ISTEF Kineto Tracking Mount (KTM) which includes the 24-inch (61 cm) aperture telescope and figure 2 shows the MDA/ISTEF Small Transportable ISTEF Pedestal System (STRIPS) mount which includes the 12.5-inch (31.75 cm) aperture telescope, both used during ISAFE.
Figure 1. MDA/ISTEF KTM mount.
Figure 2. MDA/ISTEF STRIPS mount.