Stresses caused by differential gas pressures are reduced.
A technique for reducing in-flight loss of closed-cell foam insulation has been devised. In the original application, foam is used for thermal insulation on the external tank of the space shuttle. As the space shuttle ascends, aerodynamic effects cause an increase in surface temperature of the foam. This heating increases the internal cell gas pressure and reduces cell wall strength. The difference between the increasing pressure of the gases trapped in the foam cells and the decreasing pressure of the ambient air contribute to stresses that can break off pieces of foam during flight. Perforating the foam with small holes makes it possible for some trapped gases to escape, reducing the stresses sufficiently to keep the foam intact during ascent. This technique reduced in-flight foam loss by more than 95 percent. The vent holes could offer similar benefits in other applications where materials are subjected to thermal and pressure gradients.
A tool for making vent holes comprises a regular array (typically, a square pattern) of pins held in a backing plate. The shape of the array and the spacing, length, and diameter of the pins must be optimized for the particular material application, configuration, and environment(s). For the original space-shuttle application, the optimum dimensions were found to be those shown in the figure.
One needs at least two identical tools to ensure the regularity of the holes across a foam panel. To begin making the holes, one carefully places the first tool in the desired initial position with the pins in contact with the surface of the foam, then evenly and gently presses pins into the foam until the tips of the pins make contact with the substrate to which the foam is attached. The second tool is placed adjacent to the first tool, then pressed into the foam in the same way. Then the first tool is withdrawn and repositioned adjacent to (but on a different side of) the second tool, and so forth, until the pattern of holes extends over the desired panel area. The relative positions of the pins within each tool and the adjacency of the two tools ensure the proper positioning of the holes across the area.
This work was done by Hale Davidson of Lockheed Martin Corp. for Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing & Prototyping category.