A hermetically sealed, normally closed (NC) zero-leak valve has been developed. Prior to actuation, the valve isolates the working fluid in the upstream volume from the downstream volume with a parent metal seal. The valve utilizes the magnetostrictive alloy Terfenol-D for actuation. This alloy experiences a phenomenon known as magnetostriction, i.e., a gross elongation, when exposed to a magnetic field. This elongation fractures the seal within the wetted volume of the valve, opening the valve permanently and establishing fluid flow. The required magnetic field is generated by redundant coils concentric to the Terfenol, but isolated from the working fluid. The response time for this phenomenon to occur and subsequently for actuation is on the order of milliseconds. The wetted volume consists of entirely parent-metal 6Al-4V titanium, compatible with all storable propellants, helium, nitrogen, argon, isopropyl alcohol, and argon. When coupled with the parent metal seal, this design gives the valve internal and external leak rates of zero.
The valve was designed as a drop-in replacement for the pyrovalve for both inspace and launch vehicle propulsion systems. Due to safety concerns of fluid leakage while personnel work around the vehicle or spacecraft prior to launch, these valves are used to isolate hazardous propellant or high-pressure gas from the downstream components. Once clear of personnel, the valves are opened, priming the system with the work fluids. The priming sequence usually occurs after launch, but this valve could be used for any normally closed permanent isolation application wherever a working fluid must be isolated from the downstream volume by a leak-free seal, and upon valve actuation, the valve is opened and fluid flow is established permanently.
The valve consists of two major subassemblies: the actuator and the flow cavity. The actuator is preloaded to 1,250 N by adjusting the preload bolt, pressing the Terfenol-D against the now-deflected belleville springs. When actuation is desired, either solenoid coil is charged in a pulsed mode, causing magnetostriction in the Terfenol-D. The elongation deflects the belleville spring stack, supplying an increasing load to the stem until the parent metal seal is fractured. Once fractured, the spring inside the bellows drives the bellows base downward, onto a raised boss at the top of the fracture plate. Once fracture has occurred, the stem and its spring stack is left, separated from the actuator column. The Terfenol-D is unloaded and returns to its original length. The spring inside the bellows ensures the valve remains open.