A leak-free valve has been proposed as a more reliable, less expensive, adaptable alternative to a one-time-opening, pyrotechnically actuated valve (pyrovalve). In the original intended application, the pyrovalve serves, before it is actuated, to isolate a pressurant gas (helium) from a propellant fluid aboard a spacecraft. Like a typical pyrovalve, the proposed valve would be triggered electrically, but unlike in the case of a pyrovalve, no pyrotechnic material would be present; all of the actuation energy would be supplied electrically and none pyrotechnically. Consequently, there would be no risk of premature detonation or of contamination of the pressurant gas by combustion products.

The Disks Would Burst in Rapid Succession (in the sequence 2,1,3) once the heating element was energized, allowing gas to flow from the higher-pressure to the lower-pressure side.

Because the proposed valve would not contain any pyrotechnic material, it would have an essentially unlimited storage life. The valve would be small, lightweight, and resistant to shock. It would be invulnerable to detonation by internal discharge of static electricity, even if its wires were not short-circuited. It would have no moving parts, and could be designed to operate for a wide range of pressures. Unlike a pyrovalve, the proposed valve could be triggered multiple times in the event it failed to open on the first try. The design of this valve could be adapted to non-aerospace uses in systems in which there are requirements for highly reliable, one-time-opening valves; examples include fire-suppression systems, emergency power units for aircraft, and emergency cooling systems.

The valve (see figure) would contain a cascade of three rupture disks welded in place. The use of industry-standard welded, domed rupture disks would ensure leak-free operation. During assembly, chambers A and B sealed by the disks would be filled with the pressurant gas (helium in the original application) at different pressures. Chamber A would be filled to the same pressure as that of the pressurant tank. Chamber B would be filled to a lower pressure equal to that in the propellant tank. Prior to actuation of the valve, there would be no difference in pressure across rupture disks 1 and 3, but there would be a pressure difference across rupture disk 2.

Chamber A would contain an electrical heating element mounted on a leak-free electrical feedthrough [the particular feedthrough would be the non-pyrotechnic part of a pyrotechnic device called a "NASA standard initiator" (NSI)]. Actuation would be effected by applying electric current from a bank of capacitors to the heating element, causing the helium in chamber A to become heated and thereby causing its pressure to increase. Chamber A would be made small enough that the available electrical energy would be sufficient to heat and thus pressurize the gas to a level sufficient to burst disk 2; this level would lie at a pressure difference between 20 and 50 percent above the storage pressure difference across disk 2.

The expansion of gas immediately following the bursting of disk 2 would cause the pressure in chamber A to fall rapidly to value almost as low as that of the propellant tank. The expansion would also cool the gas and thus the heating element. This action would give rise to a large difference of pressure across disk 1, thereby causing disk 1 to burst. The resulting inrush of pressurant gas into chambers A and B would further cool the heating element and would give rise to a large difference in pressure across disk 3, thereby causing disk 3 to burst. The bursting of disk 3 would remove the last barrier, allowing the pressurant gas to flow to the propellant tank. The flow of pressurant gas would also complete the cooling of the heating element.

This work was done by Douglas G. Dobbin of Rockwell Space Operations Co. and Larry J. Bamford of Allied Signal Technical Services Co. for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Mechanics category. MSC-22726


NASA Tech Briefs Magazine

This article first appeared in the March, 2000 issue of NASA Tech Briefs Magazine.

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