Are classical rain erosion experiments of infrared materials used in high-velocity seekers representative?

With increasing speed of missiles, rain erosion becomes a more and more worrying problem for seekers and particularly for infrared domes and windows, since infrared materials are relatively brittle. In the last twenty years some efforts have been done to measure the rain erosion resistance of these materials and to rank them. These measurements were performed at room temperature. In fact, the increase in velocity is followed by an increase in temperature of the infrared materials. Consequently, the real rain erosion resistance can be different of the one determined at room temperature. ONERA made in 1989 a first attempt to measure the influence of temperature by using a special holder in the SAAB-SCANIA rotating arm. The samples were heated at the extremity of the arm during the rain erosion experiments. Preliminary results were obtained but the temperature domain was limited to about 100 degrees Celsius by the high value of the convection transfert coefficient due to the arm rotation. More recently, such experiments were performed in our lab using a water jet generator. In this case there is no displacement of the sample, thus its temperature can be higher than in rotating arm experiments and well known. Rain erosion measurements were performed up to 200 degrees Celsius. The strong influence of temperature on the rain erosion resistance was confirmed but one can wonder if the decrease of rain erosion resistance with temperature is due to a decrease of the mechanical characteristics of the material or a thermal shock effect. Some basic experiments were done to answer this question and computations were performed to determine if, during a typical flight, there is a risk of thermal shock due to the difference of temperature between droplets and the window material.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.