Pneumatic Liquid Oxygen Fill/Abort Valve

Overview:

In 2024, the rocket team switched from a nitrous oxide/paraffin wax hybrid to a liquid oxygen/paraffin wax hybrid rocket. The jump to using LOX was huge; from dealing with cryogenic temps to maintaining oxygen safety, it was a large challenge that required a complete redesign of our fluids system. We had to design and manufacture three cryogenic liquid oxygen valves and design a gaseous nitrogen pressurant system. In my freshman year, I built a single-acting pneumatic valve to fill and drain our LOX tank. The valve is a passively open poppet-style valve with spring-energized seals. The valve has now been used on two rockets, supported 10+ cold flow tests, 3 static fires, and one launch. 

We launched our hybrid rocket at the FAR-OUT rocketry competition in the summer of 2025. FAR-OUT rules required us to have an abort valve that can drain the LOX tank within 30 minutes. Alongside this, the valve has to be passively open. We also had to run this abort/fill valve parallel to the LOX run valve, giving a max valve diameter of 1.54 inches to fit between the run valve and the wall of the airframe. I designed the valve to operate with a minimum pilot pressure of 250 psi. On the vehicle, we used a pilot pressure of 700 psi.

Given these constraints guiding my design, here is the single-acting pneumatic valve I designed:

The piston inside is held in the open position with a spring. Once the pilot line is pressurized (700psi), GN2 will push up on the bottom of the piston with about 283 lbf, closing the valve and allowing for LOX fill.

Operation: 

 In an abort scenario, we’ll open the abort solenoid valve on the GSE panel, and in the case of a failure of the abort solenoid, we have a ball valve attached to a long rope so we can perform a manual abort. Either method depressurizes our COPV and pilot line. The expected drain time of the 22L LOX tank (24-25 vehicle) is around 30 seconds.

Sealing:

Since the valve will be in contact with liquid oxygen, everything was machined from 303 stainless steel. Cryogenic temperatures prohibit us from using regular O-rings, so we elected to use PTFE spring-energized seals for the sealing between the two bodies of the valve. We also machined a PTFE seat that sits between the valve and the tank endcap; the poppet seals against this seat. Due to the space constraint, instead of creating flanges to fix the two halves of our valve together, I opted for six holes for 8-32 bolts to run through the walls of the valve.

Spring Selection:

I performed force balance calcs to determine what spring would work best. For poppet friction at cryo, I consulted the Parker handbook’s guide on spring-energized seal friction. This is a snippet of the force balance calcs I re-did in 2025 to pick a new spring for the 25-26 vehicle.

Valve diameter: 1.454 in 

Valve height: 1.419 in 

LOX inlet: .05 in^2

LOX outlet: .05 in^2

We ran a hydrostatic test to 1.5 times MEOP, and the valve passed:

Once the motor was fully integrated, we ran a successful abort test. The valve also performed as intended at cold flows, static fires, and our launch at FAR-OUT in summer 2025!

Fill/Abort Valve integrated and connected to LOX umbilical

Cold Flow

Static Fire!