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.
|The two Festo 2/2 valves at right (47g each) have|
the same functionality as the 5/3 monster at left (447g)
and are blazing fast
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).
|One tidy little manifold|
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!