The High ALtitude Observatory (HALO) is an instrumented Gulfstream lib aircraft sponsored by the US Army Space andStrategic Defense Command as an optical data collection platform. Capable of operation at altitudes above 50,000 feet, theHALO's highly diverse and flexible sensor suite has provided infrared, visible and ultraviolet data on numerous BMDO,DOD and NASA test programs. This paper describes the design, fabrication and testing of two large (13" x 20" x 1") zincselenide (ZnSe) infrared windows employed by the aircraft's primary sensor system, the Infrared Instrumentation System(IRIS). The paper describes the design, fabrication and testing of the windows and coatings, the window proof testing forflight qualification, and summarizes the current operational experience with the windows.Keywords: infrared windows, infrared coatings, aircraft windows, proof testing, ZnSe, flight qualification. 1. INTRODUCTION The HALO aircraft, shown in Figure 1, contains three optical sensor platforms, the first of which is the Infrared

Design, fabrication, and testing of large airborne ZnSe windows

Many commonly used infrared window materials, such as zinc sulfide and zinc selenide, are subject to structural failure due to stress-corrosion induced cracking. This failure mechanism is of critical importance in applications in which the window experiences high static pressure loading for prolonged periods in humid atmospheres, conditions typical of airborne optical windows. The most effective means of screening windows against failure due to this mechanism is by use of overpressure proof testing. In this paper, the design of overpressure proof tests for large airborne infrared windows is discussed. The underlying physical phenomena and governing mathematical relationships are presented. A hypothetical proof test design for a large infrared window to be employed in a man-rated aircraft is developed to illustrate the application of the analytical methodology; Practical considerations in the execution of large infrared window overpressure proof tests are also discussed.

Charles J. Pruszynski

Papers

Overpressure proof testing of large infrared windows for aircraft applications