Supersonic Rain Erosion Testing of Missile Radomes

S the advent of high speed aircraft and missiles, the damage to radomes and other parts from rain has become a problem requiring understanding and solution. Anticipating that this problem would become more important with increases in flight velocity, the materials laboratory of WADC sponsored a study of rain erosion testing techniques. One part of this study was undertaken by the Convair thermodynamics laboratories in 1952 and has continued since that time. A small group of engineers, under the direction of W. L. Dittmann, has been obtaining data on rain damage to radomes and radome materials at ever-increasing flight velocities. A large part of their task is to find methods that will accelerate the sample materials to high velocity without damage and with high probability of intact recovery. The first supersonic testing of materials was accomplished with a 20-mm aircraft cannon. Test specimens were mounted in the nose of a modified projectile and fired horizontally through 500 ft of simulated rainfall. Upon firing, a tracer element in the projectile was ignited and burned approximately f sec. A black powder charge then expelled the test specimen and parachute. The parachute checked the forward velocity of the test specimen within 10 ft and intact recovery was made. Speeds up to Mach 3.0 have thus been obtained. More recently, a 57-mm cannon was obtained. This increased the size of the test specimen to 2-in. diam. Data obtained from these studies show that erosion damage is a function of velocity, shape, material, water drop size and the distance the water drop must travel in the flow field aft of a shock. Thus, erosion damage obtained on an object with a 2-in. base diam will only simulate the damage to an equivalent nose portion of a corresponding larger specimen. The only method of obtaining quantitative results of rain damage to the total surface of a shape of much larger diameter is to test full scale. This was done by mounting fullscale radomes on a rocket sled and firing through a simulated rainfall.