Foam-in-place dielectric materials for embedment of aircraft and space vehicle electronic networks operating at 3000 volts

1. Rigid polyurethane foams are feasible for encapsulation of intermediate voltage low-energy networks. From the knowledge of the dielectric system, design packaging voltages in 3 to S kv RMS range appear to be practical. This type of packaging would require careful attention to exposed electrode design so as to reduce dielectric stress levels to acceptable values adjacent to terminals. 2. Foam materials with an adequate closed cell structure will provide elevated insulation in a varying atmospheric environment. This includes changes caused by elevated temperatures and high altitudes as well as prolonged operation while passing through the atmosphere and in the vacuum of space. Thus the advantages of a solid encapsulant or a hermetic sealed container can be achieved by foams. 3. The length of time for which foams may be serviceable has not yet been determined quantitatively at room temperatures. At elevated temperatures in a vacuum environment gas diffusion will increase and a shorter effective life is expected. 4. The foam dielectric provides a design advantage over rigid and elastomeric insulating materials in that it has a unique combination of properties. It is light weight, mechanically and chemically stable, insulates in the intermediate voltage range and is suitable for use with many phases of network packaging for aerospace vehicles. 5. The ease of repairability further enhances the desirability of foam packaging for aerospace applications since complex network packages are seldom made in duplicate. In addition, with the advent of short-delivery schedules for most of today´s space vehicles and the lag of component reliability data, part or electronic component replacement in existing hardware is becoming more common. 6. The designer must also consider the need for dissipation of thermal energy when using foam. Thermal profiles should be established under operating conditions to determine if the energy dissipation will cause excessive heating of the components. High energy dissipation with foam materials will be a problem in both the atmosphere as well as in space. Contrary to this problem, foam will offer thermal insulation which quite often is desirable with spacecraft.