November 2023

LOX Prevalve for Test Stand

The test stand and engine need lots of different valves to control the flow of liquid propellants and other commodities. It is a requirement for many of these valves to be remotely controlled since I obviously will not be near the test stand during a hotfire or really any other test for that matter. There are lots of different types of valves and methods for actuating them. Test stands and other GSE (ground support equipment) usually heavily rely on ball valves with pneumatic rotary actuators, also frequently using various direct-acting solenoids for lower flow rates of gasses. For example I used direct acting solenoids for my igniter tests in May. There is a pretty big difference between “facility“ and “flight” valves though. Valves that actually fly on engines are much smaller, lighter, and their actuators usually operate at much higher pressures so the effective area can be less, making the overall actuator size smaller and lighter. Motorized valves are becoming more and more common on flight engines since they are somewhat simpler to throttle and extra batteries probably trades favorably against the complications of lots of gas bottles and plumbing.

For my purposes, weight is not an issue and basically all of my valves will be facility-style valves. Since the pumps can easily throttle the propellant flow, the burden of throttling does not lie on valves, furthermore, throttling is not necessarily a hard-goal for this project although it should be within reach. For this reason I am not pursuing motorized valves even though I have built some in the past. All valves for the system will essentially be on/off valves.

Most reputable valve distributors have very high prices in the $200-$800 range (valve + actuator) which is prohibitively expensive for me. There are air-actuated ball valves on websites like amazon, ebay, etc in the range of $75-$150 but even that is a bit high for me due to my severe budget constraints. At some point I picked up a 1/2” actuated valve for about $60 which I plan to use for one of the fuel valves. I might buy more of those to use also as purges and such depending on how some of my hand-calcs turn out.

I need to produce at least 3 valves for use with LOX but purpose-built cryogenic ball valves are definitely not available on common online shopping sites and are very unaffordable to me. The main requirements for use with cryo are, vent paths, compatible seals and metals, thermally isolated actuator. Thankfully, lots of commonly available manual ball valves are made of cast stainless steel and use PTFE seals making them compatible with LOX. Also a vent hole can be somewhat easily added to the ball. The main concern for me was developing a trustworthy actuator.

Pneumatic linear actuators also called air pistons or air cylinders are super cheap and come in a huge variety of bore and stroke sizes. I chose to pursue a design closest to the “scotch yoke“ as shown in figure 1 except I only used 1 piston. I found a ball valve on an industrial surplus website that satisfied all my requirements. One thing that really helped was that it had a small flange with threaded 10-32 holes on top. I crafted a simple aluminum bracket that attached to the flange and I slightly modified the original shaft-lever such that the full stroke of the piston would correspond to near 90deg turn of the ball. Upon testing, The piston was able to actuate the valve, but since the bracket was made of 1/8” thickness aluminum, it elastically deflected somewhat. The next steps to improve this valve are 1) design and build a stiffer bracket, 2) drill a vent hole, 3) cut a small PTFE shim to better thermally isolate the bracket from the valve body. I think this valve is very promising although my primary concern is that, when wetted with LOX, the valve will condense and freeze too much water (from the humid atmosphere) on the actuator linkage and make it hard to actuate. This style of valve is something I will probably continue to build iterations and varients of though for small, ambient-temp ball valves since it can be made cheaper than the cheapest actuated valves I can find.