Towards more realistic erosion simulation tests for high velocity EM and IR windows

Abstract

Numerous efforts have recently been made to determine quantitatively the rain erosion resistance of the materials used in radomes or IR domes of seekers. “Quantitatively” means that the conditions of water impingement achieved in the experiments are well known: droplets impact velocity, liquid water concentration (LWC), droplet impingement angle, eventually the capture coefficient of the holder. Measurements are performed using apparatus such as rotating arms, water jet generators or others, allowing an accurate ranking of the tested materials. Unfortunately, the current and future need of materials for heat-seekers of missiles flying up to very high Mach numbers, such as Mach 5 to 6, creates a difficulty for the measurement of the rain erosion resistance because it is impossible for conventional set-ups to reach these velocities. For example, rotating arms are limited to Mach 2–2.5. This limitation can be partially withdrawn by extrapolation of the power velocity dependence of the rain erosion curves from Mach 2 to Mach 5, with the inaccuracy linked to such an operation. But one must try also to take into account the possible change in rain erosion resistance due to the change in environmental conditions when flying at such high velocities, in particular the increase in temperature. For instance, aerothermal computation shows that the surface temperature of a 10 cm-diameter hemispherical Al 2O 3 dome goes up to 400°C after 10 s of flight at Mach 3. Different experiments have been performed using rotating arms and water jet generators, with samples heated up to 200°C, to determine the influence of temperature on the rain erosion resistance. These experiments have shown that a second mechanism can explain the degradation enhancement of the material: the thermal shock produced by the impingement of the cold droplets on the hot material. Recent results of rain erosion of materials used for high velocity heat-seekers are presented. In particular, emphasis is placed in the analysis of the possibility of the destruction of the droplets before impingement on the dome, computation of the temperature increase of the materials, measurements of the dependence on temperature of the rain erosion resistance of different materials including thermal shock effects. Finally, first experimental data concerning the internal temperature field inside the material during the impingement are presented.