Roaring Becomes Elektra Two, Too

[larry cottrill]

About an hour ago I got the Elektra II to sustain for the first time, using propane vapor and a ten-inch extension [total length: 36 inches], which is at least a little better than the model I. Though pretty damp, it did warm up a bit today, which got me testing again. A couple of days ago, with about 50 degF weather, I couldn't get anything to go except the old Elektra I at 39 inches. Even the Short Lady wasn't taking any encouragement from me under those conditions. Today it ended up being about 65 degF or so, when I went out there to set up. Unfortunately, I didn't get any photos today, though.

What may be of interest here is the intake treatment. I used 3/4-inch EMT, as you can see in the original prototype photo. However, along the way I pinched a venturi in the pipe out near the flare [this engine has one of my better wrought intake flares] which didn't seem to help at all. So, I went all the way and extended the pinch the whole rest of the intake length, just as tight as I dared and as close to the chamber as I could, to try to get separation of the intake flow at that point. The inside dimensions of the pipe are approximately 17/64 x 1-3/16 [6.75mm x 30mm]! Pretty snug.

The affect of this is that the engine now responds exactly as expected to changes in tailpipe length, unlike the prototype Elektra I, which stubbornly maintained frequency no matter what length tailpipe I tried to use [though it would only sustain after I hit 39 inches]. So, the E II is behaving more like a pulsejet should, perhaps. Getting just the right airflow in was pretty tricky, and I still had to let it warm up quite a while before I could tell it was ready to run on its own. The 10-inch extension has a pronounced flare at the end, just like the prototype tailpipe itself.

Once it was running, I was able to open up the gas a little for better power, and it really seemed to be running full grease, with the chamber and the first 4 or 5 inches of tailpipe getting good and red. I didn't play around with 'throttleability' much, since I wanted to make sure it was really going to sustain indefinitely, which I believe it would have. After a couple of minutes of good running, I tried easing off the extension, which killed the run instantly.

Another important detail is that I was using a severely shortened fuel pipe, which spouts at about 1 inch [25mm] inside the intake rim. This is as close as I've gotten to the nice setup Steve used on his later Short Lady runs.

L Cottrill

[Mark]

Congrats Larry. I as of yet have not ever run anything on propane. One day though ...
Seeing this jet and the other in my mind of Steve's self portrait propping himself or leaning on it as if it were a cane, had me wondering if one couldn't make a walking stick that could be push-buttoned started if you needed to fend off a charging dog or crazed robber, something to provide you added safety as you walk down your neighborhood street. It could be a James Bond-like protection device.
I mean how many people do you know that have a pulsejet walking stick? Crazy or not, I bet it could be done.
Mark

[Mark]

I was just saying propane is a tricky fuel for me, guess I just need to try using it more, there's nothing wrong with it other than the heavy fuel tank and somewhat more critical fuel to air ratio. There are some good things about propane. Every fuel has some plus or minus.
Mark

[larry cottrill]

Hey, congrats on the sucessful run. The L:D is a bit unfavorable, but it is always nice to verify that a design is sound. As I mentioned in e-mail, and as Mark implies, you may be doing yourself a disservice with propane. I take some responsibility for encouraging you to get a propane setup.

Ben -

Thanks. I knew it had to be sustainable, somehow, and the success with the Elektra I by lengthening it was the best indicator of what was likely to work. It's interesting how the smoothness of the design as a whole works out with the numbers. My first grab at the Reynst point at L/8 was apparently perfect for the Lady; on the Elektra I it turned out to be L/12; and on the Elektra II it is almost exactly L/10, right in the middle. You could make a schedule of Reynst engines on that basis, I guess, like a sort of Catalog of Design Geometries, for whatever good that would do you. That's one thing I love about this sport; you learn something new just about every time out. The bad thing about it is there is a lot of failure between successes. But, of course, that sweetens the success when you find it.

The only reason I decided to go with propane is it's a vapor fuel you can get refilled anywhere. The use of vapor fuel, of course, has one sterling advantage: You don't have to design a carburetor at the same time you're trying to work out the design for your jet (the remaining option, spraying pressurized fuel in through the wall of the chamber of a pulsejet, is too unsavory to me to merit further consideration ;-). Thus, you isolate two sets of variables so they can be worked on [more or less] independently. Of course, there are better vapor fuels, but none as accessible as propane, and none that are both better and cheaper, that I know of.

L Cottrill