Saturday, November 12, 2011

Student Pulsejet Project

TSTC welding student Justin Friend made the Waco paper Friday 11-11-11, with a pulsejet thruster he built from plans he found on the internet. He had already tested this device himself, but brought it out to the TSTC airport apron for a demonstration test Thursday morning. Most of the attending crowd were aviation maintenance and welding program students and faculty. This was a personal project for Justin, not a class project. I called Bill Whitaker, the editor at the Waco “Trib”, and he sent a reporter.

Justin is a college algebra student this semester with my colleague Otto Wilke, who attended the demo test, as did I. (Another math department colleague, Doyle Ware, also went with me to see the test.) Justin sought sheet metal geometry help from Otto, and operating and safety advice from me, once he found out that Otto and I are engineers. His welds were obviously very good, as the pulsejet tube runs very hot in places. No cracks or flaws of any kind have turned up to date.

This pulsejet tube is valveless, so there are no moving parts at all. It is a “folded pulsejet”, so that the back-spit from the short inlet contributes to its thrust. This one is a nominal 50 pound thrust device, big enough to push a go-kart around. It runs on propane. Justin cut the parts from flat stainless steel sheet, rolled them up, and welded them together, excepting the return-bend tubing. This return bend is on the exhaust side, and is the hottest part of the structure. It glows in broad daylight when running throttled up. I got copies of photos Justin made during his first tests. Two are here.


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This device has a spark plug on the side of its combustion chamber, and a propane injection manifold tube across the inlet right at the dump into the chamber. One starts the spark, some starting air from a leaf blower, and the propane, to light it off. Once running, starting air and spark are no longer necessary. It throttles up and down a wide range of thrust by simply raising and lowering the propane feed pressure.


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This thing is dangerously noisy: I estimate around 130-135 decibels, so ear protection is a necessity. You can feel the sound waves beating on your stomach. At the TSTC demo runs, you could feel the concrete airport apron shake beneath your feet. Unlike all other forms of jet engine, pulsejets “sing” at a definite frequency, the rate of the pulsed fuel-air explosions inside the tube. This size tube “sang” at about 80 Hertz, like an earth shakingly-loud operatic bass.

I haven’t heard noises that loud in decades. Being a part of this young man’s project was a huge amount of fun. I actually knew something about this engine and could help Justin with it, because decades ago I researched the military work done on them in the 1950’s and 1960’s. I always wanted to build one myself, but never actually did it. (That may change, this was just too much fun.)

Added 11-13-11:

Here is the first of two QuickTime Movie (.MOV) files I got from Justin. You can see the tube slowly warm up and quit spewing unburned fuel from the inlet (a thin whitish spray). You can also get a sense of the 80 Hertz "singing" tonal quality of the sound, but no real hint of how loud it was. Upon shutdown, the flames from the inlet are propane residuals from the fuel line venting into a very hot environment.

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Here is the second movie file from Justin. In this one, the tube is quite warm and operating very well at near-full thrust. You can see him disconnect the spark while it runs, with absolutely no effect upon the operation of the tube.

video

Added 12-7-11:

Since this article originally posted, Justin has mounted his 50-pound thruster to an old golf cart, and driven it at the Hearne, TX airport (an uncontrolled field). The video is of a pass he made after the tube was fully warmed up. The speed is close to the control limit for that cart.

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Justin has since begun procuring parts and materials for a much larger thruster.

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