In-situ measurement of the char layer front of the resin-based ablative material using the resistance sensor

The in-situ acquisition of the char layer front (CLF) has significant importance for understanding the ablation process and predicting the temperature response. In this paper, two types of resistance sensors (RS) have been developed by means of “groove” (RS-1) and “laser etching” (RS-2) to measure the CLF. The ablation verification experiments were conducted using an oxyacetylene device. The results show that the polyimide (PI) insulating rod in RS-1 is relatively thick, which leads to circuits connected through charred PI rod rather than the CLF of ablative material. The thinner PI film is used as the insulating material in RS-2, and the charred PI film cannot be connected to circuits. As a result, the RS-1 measures the advance of an isotherm with the decomposition temperature of PI rod, while the RS-2 enables accurate measurements of the CLF. The maximum difference of CLF recession measured by RS-2 was 0.27 mm, with good repeatability. In addition, a method for quantitatively characterizing the CLF has been proposed to verify the accuracy of the RS. The ablative materials were processed into the small-sized samples and heated to different temperatures to obtain samples with different pyrolysis degrees. The pyrolysis degree of each sample was obtained by measuring the change of material density before and after heating, and the resistivity was measured by four probe techniques. The results show that the pyrolysis degree of the CLF is 90 % and the corresponding resistivity is less than 20 Ω·cm.