Cryogenic fluid control valves require actuation that controls the geometric position of the orifice in a thermally stable manner. Traditional actuator devices may have various materials used in their construction that have varying CTEs (coefficients of thermal expansion) and therefore may shift (expand or contract) relative to the reference mounting points on the valve body. This leads to a lack of valve orifice control and leakage in the valve. To provide a more thermally stable control valve for cryogenic fluids, Dynamic Structures and Materials LLC (DSM LLC) provided a piezoelectric ceramic-driven actuation system on a cryogenic thermodynamic vent system (TVS) valve.

Piezoelectric ceramic (piezoceramics) transducers are solid-state electro-active devices that convert electrical energy to precise direct linear expansion. Using metallic amplification frames that act like kinematic levers, DSM LLC actuators convert small piezoceramic expansion to levels sufficient for valve control. Since piezoceramics are often constructed using polymer dielectric coatings, DSM LLC investigated numerous types through experimental testing to identify a version that could withstand repeated thermal cycling from 580 to 139 R (322 to 77 K) and back. The cryogenic piezoelectric TVS vent valve was designed for use in a liquid oxygen (LOx)/liquid methane (LCH4) feed system developmental test. In the system, there is a desire to store sub-cooled cryogenic liquid at 325 psia (≈2.2 MPa) and 163 to 185 R (≈91 to 103 K). This raises the boiling point to ≈240 R (≈133 K) and allows propellants to absorb heat but remain sub-cooled at < 204 R (≈113 K). The fluid in the thruster usage will absorb and expel heat leak into the manifolds as useful enthalpy. A TVS vent valve is required in the event normal thruster usage is not sufficient to sub-cool reaction control system (RCS) line.

The process progresses to fully open the TVS vent valve in order to chill-in RCS feed line at startup and then control TVS valve positioning to simulate different orifice sizes (50 to 150 micron/0.002 to 0.006 inch) to maximize feed line conditioning while minimizing propellant losses. The valve system is opened (actuated) by the amplified piezoelectric actuator that controls the valve orifice in a thermally stable manner and does not geometrically shift during thermal excursions down to 80 K. Coupled with the appropriate structural metals, the actuator and valve are able to proportionally control the flow over the entire operating temperature range of 139 to 580 R.

DSM LLC selected a thermally stable flextensional piezoceramic actuator to drive the valve orifice control mechanism. The typical stainless metals used for the actuator frame were replaced with a suitable cryo-rated, low-expansion alloy. Flexure hinges were designed to operate properly at much lower stresses than typical to accommodate the alloy’s yield stress limits.

Significant experimental testing was applied to the problem to determine the CTE of the piezoceramic materials over the operating range. Appropriate expansive metal inserts were used in the actuator design to balance the thermal expansion characteristics of the metal actuator frame and the piezoceramics. New spring preload components were designed to keep the spring loading in the actuator at an appropriate level over the thermal operating environment. To withstand the temperature extremes, piezoceramic transducer elements with ceramic dielectric coating were comparatively tested against typical polymer coated options, and then successfully used in the actuator system.

This work was done by Jacob Collins of Johnson Space Center, and Patrick McGirt, John Kennedy, and Jeffrey Paine of Dynamic Structures and Materials LLC. MSC-25054-1

NASA Tech Briefs Magazine

This article first appeared in the February, 2016 issue of NASA Tech Briefs Magazine.

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