|| Larry Cottrill |
| Date :
|| 2003-08-07 15:46:26
| Subject :
|| Little Maggie Muggs - a ramjet design with NO WELDING
'LITTLE MAGGIE MUGGS' - Proposed Design WITH Dimensioned and Full-Size Drawings - Low-Speed Ramjet You Can Build Without Welding
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The basic idea behind this was posted by me a couple of weeks ago on the Off Topic Forum, since I had never spent any time here and didn't know any better:
Again, keep in mind that I haven't spent any time trying to determine whether something like this has already been done. The idea is to get a workable 'leaf blower' type jet out of [mostly] ordinary materials, with no welding required. Bonded construction using the popular 'J-B Weld' steel-filled epoxy material will be used exclusively. Here's the basic dimensioned SCALE drawing:
The various subsections of this drawing have been parsed out and blown up [with slight smoothing] so they will print as FULL SIZE plans. These are JPEG files and should be easy to download as a ZIP file from my site. Here are samples of the only two parts which require reasonably precise fabrication:
The engine mount rails [make from 1/2" x 3/4" x 1/16" metal angle - make one LH (shown) and one RH]:
The engine mount N-struts [make from 1/4" OD metal tubing (or 1/4" solid rod if you don't care about weight!) - make one LH (shown) and one RH]:
ALL the FULL SIZE dimensioned drawings can be had by downloading this ZIP file:
Microsoft Photo Editor will print these JPEGs as full size drawings if you use the '100%' setting rather than 'Fit to page'. IMPORTANT NOTE: The Left Side View drawing is too large for printing on 8.5 x 11 inch sheets; you'll need Legal size [8.5 x 14 inch] paper to print that one at the '100%' setting!
I don't know what will be more controversial about this engine, the design or the construction. According to the J-B Weld literature, it's supposed to withstand 600 degF without falling apart. My design tries to put the combustion zone well behind all the bonded seams, with all the bonded areas continuously bathed in cooling airflow from without and within. If any failure of this material happens, it will be due to conduction through the stainless sheet material from which the basic parts are stamped. Since SS is known to be a poor conductor of heat, I'm guessing that this will work as designed. We'll see ...
Fuel could be gasoline, methanol, ether or propane. I did not include a design for the fuel delivery pipe/nozzle, since this is probably one of the most important things that an experimenter would want to mess with. For propane, it could just be a simple tube out to the middle of the intake at the point shown on the side view. For liquid fuels, I'd build a needle valve and extension arrangement that would be just a bit more complicated. The main thing is you want fuel delivered right at the front so good mixing has a chance to get started ahead of the flameholder.
The flameholder itself is a 'Plumb Shop' brand basket-type sink strainer from Menards. Not just any one will do - I've looked for a good design to use for a long time, and the one I finally found is the best I've seen. It's the type that has a threaded stem that pulls a dish-shaped disk down into a seat in the drain - top-of-the line hardware. That central screw has to be removed to use the basic part for our flameholder. But what really makes this one work is that the whole flat bottom of the strainer is punched with SEVERAL DOZEN small holes, uniformly distributed over the entire surface - that's what you need to look for. There are also a number of small oval slots around the outside wall, so there should be PLENTY of air/fuel mix getting through this thing at moderate intake flow.
The mug shell I have for the diffuser section will fit the flameholder rim by simply butt-gluing it on with a small bead of the J-B Weld epoxy. The shell for the combustion chamber is, unfortunately, a different story - it's too big by 1/16 or 3/32 inch all around! This can be clearly seen at the point where it says '3.40 OD at rim' at the upper middle of the Left Side View drawing. My plan is to first bond the flameholder to the diffuser and let this set up, then build a beefy dam of J-B Weld all around the diffuser shell just ahead of the flameholder rim. After that cures, I'll file this down so the combustion chamber shell just fits. [I attempt to show this in the little 'cutaway' area of the Left Side View drawing.] Then, a small bead of the epoxy will be applied to the inside rim of the chamber shell and it will be pressed on to the tune of 1/4 inch or so and be allowed to cure. Final finish of that area will be a smooth fillet of epoxy all around the leading edge to smooth things up for external flow, but the mounting strut/rail assemblies will need to go on before that, of course.
Anyone who can build a model airplane should be able to duplicate Maggie Muggs, if they can get hold of these or similar parts. So-called 'stainless steel travel mugs' are available everywhere [mine came from Wal-Mart] - you just have to shop around a little to find good aerodynamic shapes [the plastic means nothing; the SS, everything]. Usually, these are easily disassembled; in many cases you can just break loose or unscrew the plastic bottom and then the whole thing just comes apart in your hands. Cheap, lightweight stainless shells of almost perfectly circular cross-section [exact composition unknown, though].
Wish me luck! The whole gruesome process will be photographed extensively and written up in a future issue of jetZILLA ezine [where else?].
All comments / criticisms welcome.