The final goal of this study is to develop an in-situ ablation sensing technique in flight for an ablative thermal protection system. By using an embedded ablation sensor, the receded and the charred fronts of an ablative heatshield of an entry vehicle are measured simultaneously. The ablation sensor has two measurement principles for this purpose. One principle is given by optical fiber sensing, in which the radiation emitted from an ablative material or from a shock layer is detected at different positions with several optical fibers. The time when the surface of the ablative material recedes at a depth is deduced from the temporal variation of the optical fiber output signal. Another is given by a resistive circuit. The resistance in the circuit varies when the sensor follows a charred state within an ablator during heating. The measured value of the resistance is converted to a depth. Each of the operational principles may be similar to the one developed in the U.S, but the combined usage of the principles in a single small sensor unit is believed to be new.
The cross-sectional view of an ablation sensor proposed in this study is shown in Fig.1. The central portion of the sensor, which consists of an ablative rod and optical fibers, is used for the measurement of the surface recession of a heatshield. The optical fibers are embedded into the grooves onto the ablative rod. Each tip of the fiber is set at a different in-depth position away from the tip of the ablative rod. The outer component, in which a resistive wire (called Karmalloy) is spirally wound onto a polyimide tube, is a resistive circuit. An electrical connection between the inner and outer wires is established via the carbonization of the polyimide tube during heating. The proposed ablation sensor is tested in an arcjet heating environment in order to validate the operational principles mentioned above. The sensor is embedded into an ablative test specimen. The test specimen is a low-density rigid carbon insulator impregnated by a polyimide resin.
A typical sensor output of the sensor is shown in Fig.2. From the figure, one can see from the time history of the output current for the optical fiber at 2, and 7mm that the value of the voltage increases from a given elapsed time, and that the value falls off after its peak one is attained. This tendency represents that the thermal radiation from the carbonaceous material increases high enough to detect the radiation by the optical fiber; and that when the optical fiber detects the shock layer radiation due to the surface recession of the test specimen, the voltage decreases rapidly. The qualitative characteristics of the voltage drop obtained from the resistive circuit component shows that the fabricated sensor works principally: the voltage drop decreases when a carbonized portion develops inside the sensor due to heating.
In the full paper, the result of a series of arcjet testing of the operational characteristics for our ablation sensor will be reported in detail, demonstrating an in-situ measurement technique of receded and charred fronts by our ablation sensor. In addition, the measured data will be analyzed to understand the ablation behaviors of the test specimen and will be compared with the ablative test specimen recovered from the testing to evaluate the prediction accuracy of the ablated depth by using the sensor.
