After running some numbers, we have changed our minds and are now planning on using a directly actuated solenoid valve for the main run valve. Our relatively small flow rate at only 0.5 lb/s of Liquid Oxygen means that the orifice we need is fairly small, only 0.22″ diameter. At this size, a ball valve is pretty bulky and trying to make a self-piloting solenoid also ends up being pretty heavy. Due to this, we are choosing to use a custom direct acting solenoid valve which is the most simple valve, having only 1 moving part and only requiring dc power. It comes at a weight penalty, but at only 0.7 lbs per valve, we are still under our main run line budget of 1.5 lbs.
To size a solenoid valve, first you need to size the orifice using the same mdot = CdA*sqrt(2*DP*rho) equation as in a liquid flow injector. Our valve is sized for a 10 psi drop on the LOX side. Then you run a bunch of calculations as shown on NASA SP-125 (pg 305 of the old version) to size the wires and coil. I would recommend not going thinner than 28 gauge wire, or over 600 coils if you are winding your own, as it gets tricky. We are using 8 Amps, 24 gauge wire, and 200 coils for the first pass. It might be a bit high on the amperage (3.5 Amps are recommended for chassis wiring at 24 gauge), but it does push the weight and volume down. And we are pushing a lot of coolant through it. 🙂 This being said, the plan is nominally 4 cell LiPo batteries (14.8 V) for power, regulated to 12 V and 5 V for the electronics, but just used directly for the valves. We can also switch to 5 V power after the valve opens to minimize the power usage and heating.
We are planning on using neat PTFE for the seal with a Vespel and carbon filled backup plan. PEEK is also a good cryogenic options. PTFE will creep over time, but it is a good choice for the hobbyist. I also have ordered some Nitrous Valves in the hopes of reusing the plungers and coils, but we will see how they work at cryo.
Below is an initial sketch of the valve we are going with the standard commercial orifice below poppet, as opposed to the more common aerospace inline solenoid valve to save money on machining and for ease of assembly. That is about it for now; I want to get the valve working before I have anyone else follow me down the rabbit hole of making your own valves.