Custom controls software and flight computer? Check.
3D-printed supersonic gas jet nozzles? Check.
So far it's been a fun ride and I've learned about a host of new topics. I've also managed to upgrade some of my practical engineering skills from 1990 to 2016. Now there's another new topic to learn: Solenoid valves to control the flow of gas to the nozzles. Well, new to me anyway - solenoid valves have been around since 1910.
Thinking about the HAPP, the design requirements for its valves look something like this:
- Must permit sufficient flow of gas at target pressure (100 PSI) to achieve nominal jet force. Valve flow is usually expressed in terms of a flow coefficient Cv. After doing the math, it looks like we need a Cv of 0.65 or higher for each of the 12 jets, or about 1.3 for a single valve powering 2 jets in tandem.
- Must permit high frequency actuation (on/off cycles) in the range of 10 per second or higher - remember the "machine gun" effect in the controls simulation?
- Actuation speed (time to open & time to close) must be as short as possible, in the range of 50 milliseconds or lower. This follows from the actuation frequency requirement.
- Lowest weight valve we can find that satisfies the first three requirements. Every extra gram is lost altitude when the HAPP flies.
- Packaging - how the valve is arranged and attached to the HAPP - is secondary. We can engineer around it. Of course, smaller is better.
After many hours browsing through vendor catalogs on the web, I came across a rather unique valve from Festo. The Festo MHJ series valves are lightning-fast with a switch-on time of 1.0ms and a switch-off time of just 0.5ms. That's more than 10 times faster than most solenoids. The MHJ10-S-2,5-QS-6-HF (high flow) version has a Cv of 0.66. The valves are also featherweight at just 47 grams. Typical 2-way, direct-acting soleniod valves can weigh two or three times more. A single 5-way, 3-position valve can have the same functionality as two of the Festos but may weigh 5 times more. Even more compelling, the Festos contain a built-in relay, so we could potentially eliminate the relay board shown in a previous post.
There's just one issue with these valves: Maximum operating pressure is listed as 87 PSI. We need at least 100. However, all the other specs seemed to blow away any other valves I could find, so I thought I'd take a chance that the datasheet is conservative and the valves might actually be able to take 100+. I ordered two for testing.
Since we eventually need to connect multiple solenoids with some sort of compressed gas supply, we also need a manifold. I found this little aluminum guy with one inlet port, six outlet ports, and an additional port we can use for a pressure transducer (the flight controller must know actual pressure to the jets in real time so it can estimate instantaneous jet force).
The last major component of the pneumatic system for static fire tests is some sort of compressed gas source. For the flight hardware we'll likely use a lightweight carbon-fiber tank and regulator, like this and this. For the static fire tests it will be inconvenient to repeatedly charge a small tank and we're better off using a common air compressor like the one I had in my garage. It will output 155 PSI, which should be more than enough.
Rounding out the pneumatic system for the static fire tests, I procured some standard 3/8"OD pneumatic tubing and a variety of quick connects.
Now it's time to set up the static fire stand, wire up the instrumentation, and make some noise!