High-power electric propulsion systems have the potential to revolutionize space propulsion due to their extremely high performance. This can result in significant propellant savings on space vehicles, allowing the overall mass to shrink for launch on a less expensive vehicle, or to allow the space vehicle to carry more payload at the same weight. Many electrical propulsion systems operate in pulse mode, pulsing hundreds or thousands of times per second. Creating reliable valves that can operate in pulse mode for extremely long periods and at low power is critical in these applications. Current solenoid valves have difficulty achieving the life requirements. In addition, a valve with the ability to throttle has the potential to simplify the entire propulsion system by eliminating the need for pressure regulators or latching valves.
A normally closed piezo-crystal-actuated valve was designed with the capability to pulse at the frequencies required, and achieve the opening and closing times necessary for an electric thruster application. In the valve design, applying a voltage to the piezo crystal causes it to elongate. As the crystal elongates, it pulls a pintle off of the seat, opening the valve. The piezo crystal elongation is a function of applied voltage, resulting in a valve that can throttle.
The valve measures 1.44 in. (≈3.66 cm) long and 0.72 in. (≈1.83 cm) wide at the largest point of the base. The valve seat is very close to the base of the valve, ensuring minimal dribble volume. A bellows is used to contain the working fluid within the valve housing, and simultaneously act as the spring holding the valve closed. Critical features of the valve include the use of a two-piece pintle and a bolt-on seat. The two-piece shaft/pintle design was incorporated to ensure valve operation will not be affected by the valve temperature. By making the shaft the same length as the crystal, the lengths of both will change identically as the temperature changes.
The valve has two preloads that are controlled during fabrication and assembly. The seat preload is adjusted by grinding the seat face to the housing to match height of the pintle location. The piezo stack preload is controlled by a differential thread that attaches the shaft to the pintle, facilitating adjustments of less than 0.0002 in. (≈5 μm) by hand. The gas inlet line was selected to be a bolt-on unit to simplify the design and place the valve inlet at a location that is easily accessible once the valve is mounted in a thruster.
In the past few years, WASK Engineering has been working on a number of normally-open and normally-closed piezo valves (and cold gas thrusters) that expand their solenoid valve portfolio for specialized applications.