Pneumatic Valve

[Old version, currently correcting and will upload soon] A major project I set for myself and the team was designing a new pneumatic valve for testing and flight. I decided to make this change for several reasons:

  • Overall system mass: 316 SS ball valve + 125 kg/cm servo, housing, and linkage added significant system mass to the fluids system. A custom aluminum valve with working gas provided by a QD would decrease overall system mass.
  • Flow Rate: The Swagelok 3/4″ ball valve we used in the 2022/23 rocketry design cycle had an orifice diameter of 0.4″. This had a calculated headloss of 49.21 psi, which significantly dampens blowdown performance.
  • Actuation Time: As discussed on my ball valve page, the actuation mechanism was a delicate dance between packing strength and actuation speed. The more torque applied to the packing nut, the higher the friction experienced by the ball and the slower the actuation. To minimize transients in the thrust chamber, the valve should ideally open as quickly as possible.

The pneumatic valve (p-valve) is a single-acting, PTFE-sealed valve actuated by air/carbon dioxide. It has a 1-inch diameter orifice.

The pneumatic valve is loosely inspired by the valve designed by the University of Waterloo rocketry team. I talked with their team extensively over Discord and they shared many of the lessons and tips for valve design and troubleshooting which has proven invaluable.

Before diving into the details, I just want to introduce the overall working principle of the pneumatic valve:

Step 1: The valve is sealed under static pressure during the fill sequence. The spring provides 12.6 lbf of pressure on the PTFE seat.

Step 2: Gas flows from the working gas inlet, which will be a quick disconnect on the flight vehicle, into a fitting block, which is just a component I designed for system simplicity in handling the in and outflow of gases. It flows through a check valve, which ensures the valve stays open after disconnect, and into the cavity. For dry testing, I plan to just use compressed air to actuate the valve. However, in static fires and flight, it requires significantly more pressure (see o-ring friction calcs) so I plan to use a carbon dioxide tank with a pressure regulator.

Step 3: The working gas (air/CO2) forces the piston upwards. The PTFE seal is broken and liquid nitrous oxide flows freely from the tank, around the valve seat, and into the injector. After the tank is drained fully, a small solenoid purges all high-pressure gas and the valve reseals.

I authored this document explaining all of the calculations used to size the pneumatic valve. Additional sections about pneumatic plumbing, CFD results, and conclusions from testing will be added for internal documentation soon.

This is a screenshot of the code I wrote to find the valve actuation time:

The second iteration of this code has more comprehensive models for the o-ring friction, actuation gas pressure, and valve motion. Here’s a copy of the new code:

Based on all of these parameters, I finalized the dimensions and allocated CAD of components to our subteam members. I instructed them in using the Paker ORD handbook for GD&T of o-ring grooves (dynamic & static), proper CAD modeling techniques in Fusion 360, and FEA of components to validate a suitable factor of safety.

I created this drawing packet summarizing each in-house component:

I plan to machine the valve with the team in February and pressure test hydrostatically. Then, we’ll do unpressurized actuation tests before using it in a cold flow and static fire.