Reply to a Question Re: Maggie Muggs

[larry cottrill]

Got a private email query on Maggie, and since I haven't shared much on this lately [didn't really know anyone was still interested], I'm putting my reply to him out here as well:

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J –

I am surprised that there is as much interest in this project as there seems to be. I just heard from my older son in Reno, NV, who says that this engine has a kind of ‘cult popularity on the net’, which I found an amusing and surprising observation. In the year or so since I finished building the Maggie Muggs engine, only two or three people have directly emailed me about progress with it, and your question is the latest of these. You are free to publish this answer to your query, as long as you use it unmodified and in its entirety and keep my name attached to it. I will also publish this response on Kenneth Moller’s Ramjet Forum, for those who might be interested.

At the moment, I have to chalk up Maggie Muggs as one of my many failures; but in this case, not because of technical or design problems, but rather because of my own hesitation. She is not a technical failure (yet), but rather, a personal one. The problem is, several things discouraged me from going ahead and firing Maggie. The first problem is that I had reason to lose faith in the adhesive material [which as you must know, is NOT specifically designed for high-temperature applications]. I used some of the J-B Weld in a couple of applications on my early Elektra I(TM) pulsejet design, and found that the material softens and yields at much less than red hot temps. This was not a disaster on that project, but would certainly be on Maggie. [Today, I would try something more like fireplace cement, though that might prove too brittle for the expected thermal expansion of the stainless parts. Maybe you just can’t win.] The other problem was some fair criticisms in private communications that sufficient pressure for reasonable thrust would never be obtained at the low speeds intended. In fact, my own measurements of cold air diffuser static pressure [using a large leaf blower] were quite disappointing: a small fraction of one PSI. Since the static pressure in the diffuser approximates what could be expected in the combustion chamber, that doesn’t bode well for an engine that has to at least overcome its own internal and external drag with some thrust left over. Though Maggie is light [less than 11 ounces], an engine of such large bulk would be absurd for trying to power an otherwise tiny model on only a few ounces of thrust.

But the main problem is that I had never built anything to accurately measure small values of thrust and drag. Remember that there were really two distinct things I was trying to simultaneously achieve with Maggie: To find out whether this design would: [a] render the bonding material reasonable to use for engine construction, by ensuring adequate cooling; and attain reasonable thrust at relatively low air speed (around 100 MPH). In line with those two ideas, I decided that, IF I could actually measure the thrust with a reasonable degree of accuracy, I WOULD fire Maggie long enough to do so, even if she self-destructs in the process! I have plenty of photos showing how beautiful she once was, so, why not? An engine is for running, not for sitting on the mantel somewhere to be admired.

In the meantime, I got off into other projects and set firing Maggie Muggs on the back burner. It happens that a few weeks ago, I managed to come up with my first reasonably successful valveless pulsejet design, possibly good enough for flight once it gets fully optimized. Suddenly, a way of measuring small values of thrust has become much more important, and I quickly devised an accurate and easy-to-build dynamometer: a double-trapeze device that can be suspended from my open garage door, with a laser pointer thrust indicator and linear scale. This mechanism is now under construction, and realistically can be expected to be finished within a couple of weeks of this writing (but of course, winter weather is coming soon, too).

This new tool will be built sturdy enough to hold both the leaf blower and Maggie. So, all I need to do is: [a] Measure the thrust of the leaf blower before the ramjet is mounted, so the thrust force from the blower alone will be known; Mount the ramjet and get a new reading, which will be the blower thrust MINUS the total cold engine drag, internal plus external; and [c] Fire Maggie, using propane vapor as fuel, and quickly establish the net thrust reading. This reading, MINUS the unfired net thrust, will of course be the thrust added by the action of the engine. All I need is a few seconds before she blows up to adjust for proper lean mixture, and I will finally know for sure just how good or poor this primitive design really is.

I should briefly comment on criticisms of this design, most of which come from people who’ve never tried to build anything like it. Any criticism that the cooling effect of the air going through can’t possibly keep the engine from failing are probably justified, and I’m prepared for that kind of failure, as explained above. However, criticism that the interior design just can’t work is totally unwarranted. This probably comes from the traditional view of a ramjet as a very low drag device with “classicâ€

[Stephen H]

well Larry, even if that thing never ever runs you still made one extreamly sexy looking jet there! its just so.... cool!