White Sands Test Facility (WSTF) scientists have developed an energy-absorbing laminated shell that is a prototype of lightweight, inexpensive, and removable covers to shield and otherwise protect carbon-composite overwrapped pressure vessels (COPVs). Although COPVs are superior to metal vessels in strength and weight savings, shielding and other protection are needed because COPVs are highly susceptible to damage by impacts.
The present protective shell is not only an energy-absorbing laminate that can shield a COPV against damage by impacts but is also instrumented with a piezoresistive force sensor that can be configured as an alarm indicator to warn of impacts that exceed a threshold level. Shells like this one will thus prove invaluable not only to spacecraft and launch vehicles but also to manufacturers of composite structures (e.g., pressure vessels, support structures, aerospace surfaces, and life support vessels); moreover, utility may extend to the automotive industry, wherein protective shells for COPVs could protect fuel tanks against ruptures following severe impacts.
Spacecraft and launch-vehicle designers are increasingly storing high-pressure gases and liquids in COPVs. These vessels are fragile and can be easily damaged by being dropped, by rough handling, or by impacts of dropped tools. Ensolite foam has been used until now to provide some measure of protection. But WSTF studies have concluded that Ensolite foam alone affords scant protection against appreciable impacts. Indeed, the experiments have shown that when a COPV is shielded only by Ensolite foam, when an impact occurs at a speed of about 19 m/s, a maximum deflection of 0.156 in. (3.96 mm) takes place at about 7 ms later, and the impact leaves a permanent deformation of 0.015 in. (0.38 mm). In practice, such damage would necessitate replacement of a COPV — a costly proposition and a potentially hazardous practice in space flight. The WSTF test program has also revealed that COPV unprotected except for a layer of Ensolite foam can be severely degraded by an impact at an energy of 35 ft-lbf (47 J). Thus, the WSTF test program has revealed a need for protection better than that afforded by Ensolite foam.
The present energy-absorbing laminated protective shell has been developed to satisfy this need. This shell has several novel features: Aluminum mesh foam is used as an energy-absorbing material to reduce deflection damage; a composite hard shell material is used to absorb indentation energy; and the aforementioned piezoresistive force sensor is used to indicate impacts with energies above 15 ft-lbf (20 J) — the threshold energy level tested by WSTF. (In research and development to advance the design of the shell, the piezoresistive force sensor has also been used as an oscilloscope trigger.)
The laminate comprises four layers, each performing a specific function: (1) a 0.5-in. (12.7-mm)-thick Ensolite foam layer; (2) a 0.05-in. (1.3-mm)-thick composite inner shell; (3) the aluminum mesh foam layer, which is 0.5 in. (12.7 mm) thick; and (4) a 0.375-in. (9.5-mm)-thick composite hard shell. Ensolite foam gives protection against scuffing; the composite inner shell provides a flat, rigid surface on which the aluminum foam rests; the aluminum foam absorbs most of the impact force from compression; and the composite outer shell prevents penetration by sharp objects. The piezoresistive force sensor is embedded between the Ensolite foam and composite inner shell layers.
In a test performed on a COPV protected by this shell, an impact occurred at a speed of about 20 m/s and a maximum deflection of 0.05 in. (1.3 mm) took place at about 6 ms later. No permanent deformation remained after the test. These results demonstrate a significant improvement over a COPV shielded with Ensolite foam alone.
This work was done by Ralph M. Tapphorn and Tim E. Roth of Rockwell Space Operations for Johnson Space Center. MSC-22762