Custom Cryogenic Pneumatic Run Valve

Design Goals and Inspiration

The purpose of a run valve in a hybrid rocket is to hold the oxidizer in the tank before launch, and open on command to flow large amounts of oxidizer into the combustion chamber. Actuation time and pressure drop across the run valve are important performance metrics. As with all rocket components, reducing volume and mass are also paramount. The operating principle of this valve was inspired by the run valve built for the '23-'24 rocket, which used similar operating principles but was designed for nitrous oxide instead of liquid oxygen.  Cryogenic operation and oxygen compatibility were both significant challenges that required a complete redesign.

Design Challenges

• Cryogenic Operation: Creating a cryogenic valve with high pressures and dynamic sealing proved to be a significant challenge. All chosen materials must maintain good mechanical properties at cryogenic temperatures. For dynamic sealing, three reciprocating spring-assisted PTFE rod seals were used in the valve,  which are best practice for sealing at cryogenic temperatures. Accommodating for the operation and installation of these seals drove much of the design. 

• Liquid Oxygen Safety: Liquid oxygen is a very powerful oxidizer, and must be treated with caution. Any material with remotely fuel-like characteristics (hydrocarbons) must be avoided. In addition, it is best to keep velocities inside the valve low to minimize risk of high velocity foreign object debris (FOD) impacts, which could ignite.

• Tolerance Considerations: The spring-assisted seals used must be installed between two flanges, and have tight sliding tolerances with the piston to prevent gap extrusion. The first requirement drove the design of a three-part valve body with axial bolts. Careful geometric tolerances needed to be maintained during machining to ensure the piston could slide without interference.

Although there were some minor tweaks and changes after the critical design review, the presentation below encapsulates much of the design inspiration and process.

Manufacturing

The run valve consists of seven parts, all machined in house in the Columbia Makerspace. Six of the parts are stainless steel 303 (passivated in-house), and the valve seat is PTFE, or Teflon. Four of the seven components are designed such that they can be machined in one work holding setup on the  4-axis lathe in the Columbia Makerspace (Haas ST-20Y). The three outer body pieces are far more complex and require extensive milling on both sides after turning on the lathe.  Careful calibration, measurements, and constant attention to geometric tolerances was necessary to ensure proper integration and sealing.