Infrared-transmitting window survivability in hydrometeor environments

A very important consideration in determining the survival of infrared windows exposed to hydrometeor environments is the condition of the hydrometeors at the time of impact and the extent of the damage they may produce. The condition of the hydrometeors at the impact point is determined by the altitude and velocity of the space vehicle and the location of the infrared window. Computational procedures have been developed to evaluate the shape, orientation, and velocity of waterdrops (ice or sand particles) at the time of impact for the range of initial waterdrop diameters in a given rainfield. Specific examples are provided for the flowfields around supersonic vehicles with a front-mounted hemispherical dome and with a side-mounted window. The results from these analyses are used to establish the waterdrop impact test conditions which are unique to these specific flight conditions and vehicle configurations. It is then necessary to identify the waterdrop impact testing capabilities which can best satisfy the resulting requirements for damage assessment. This approach is in contrast to the typical approach which evaluates the survival of the infrared-transmitting window relative to the existing waterdrop impact testing capabilities which may not be at all representative of the vehicle flight conditions.