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Steel tubing forming shower of sparks from DynaJet exhaust [closeup] - photo (c) 
			2003 Cottrill Cyclodyne Corp.
From video by Ben Brockert - Photo Copyright 2003 Cottrill Cyclodyne Corp.
This unretouched photo shows what happens to 1/2-inch [12mm] OD mild steel tubing when subjected to the internal gas temperatures of a pulsejet. I believe that erosion of the steel results not just from melting, but also from actual combustion of the metal in the excess air present. The engine shown is a standard DynaJet running white gasoline as fuel. [Discussion of destructive testing of metals and more photos appear in this month's Feature Article, below.]

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    © 2003 Cottrill Cyclodyne Corporation
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Issue 2003-0918-0106-00                       September 18, 2003
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I N   T H I S   I S S U E . . .
   Metals in Pulsejet Interiors - 
   Destructive Tests Reveal the Truth!
   by Larry Cottrill

   Combustion Jam Jars
   by Henry E. Whittle

   Proposed design for Simple Low-Speed Ramjet Engine 
   by Larry Cottrill

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   > Eric Beck's Pulsejet Design Calculator
   > Building "Little Maggie Muggs" - an
     experimental ramjet built WITHOUT welding
     - Part I

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Metals in Pulsejet Interiors -
  Destructive Tests Reveal the Truth!
   by Larry Cottrill

Note from the Editor:

I have been interested for a long time in trying to get stream 
momentum measurements of pulsejet gases by using ordinary pressure
gauges. I initially assumed that a tubular stainless probe could 
go deep into a running pulsejet without difficulty, and 
communicate pressure to an external gauge. What I mainly proved in
trying this is that putting metal structures fully inside the 
exhaust stream of a pulsejet is more of a problem than most of us 
amateur builders imagine! Here's part of what I posted on Kenneth
Moller's Pulsejet Forum on May 6, 2003:

III. Internal metal structures are going to have problems 

Unless you are willing to use exotic materials [maybe Titanium 
would have held up OK?] or go to extremes of complexity [e.g. 
forced cooling of internal parts?], internal structures imposed in 
the hottest parts of a pulsejet are doomed to failure – in some 
locations, instantaneous failure! My hypothesis is that such parts 
are extremely limited in their ability to radiate excess heat, and 
easily reach welding temperature [yellow or white hot]. At such 
temperature, stainless is readily oxidized in air. I hypothesize 
that the DynaJet, in its stock configuration, handles a good deal 
of ‘excess air’ [air not absorbed in combustion], at least during 
the earliest parts of the combustion cycle, and this air is 
available for oxidation of the extremely hot material. Stainless 
does not oxidize appreciably at red heat, which is why the air-
cooled outer shell of the engine doesn’t suffer burn-through, even 
in static running without forced cooling. 

Here's what we learned in more careful subsequent testing of
metal samples. Take a look at what happens!  - Larry Cottrill

Why bother with these tests?

The main reason I wanted to test different metals deep inside
a running pulsejet is my desire to attempt pressure gauge 
measurements at different points in the tailpipe. I first 
tried measurements using an automotive fuel pressure/suction 
gauge connected to about a metre length of small stainless steel 
tubing, bent to act as a probe to be pushed into the tail of
the engine. This gave some very interesting results as far as
the gauge readings were concerned, but once the SS tubing was 
shoved fairly far forward, the tubing began to disappear in a
shower of white hot sparks from the tailpipe! Clearly 

My conclusion was that my basic method of measurement was 
sound, but that a much more stable metal would be needed for 
such punishing experimental conditions! Some recent posts
on the Pulsejet and Valveless Forums had expressed interest in
getting metal structures set up inside pulsejet exhaust streams.
So, I felt that we really needed to try some basic tests of The small business and nonprofit success tool ...   
a few different practical materials. 

A simple test methodology

Ben Brockert and I got together on Saturday, May 24, 2003,  
to do more investigation of the bahavior of structural 
metals immersed in pulsejet combustion gas streams. The 
general plan was to start out with each sample barely inside 
the pipe, and then incrementally go farther in with each test
run. Basically, we tested each sample 1 inch inside the pipe, 
6 inches inside the pipe, and 9 inches inside. [We started 
out with the idea of a smaller increment, but it would have 
taken more runs of the engine than even my neighbors could be 
expected to tolerate in a single afternoon!] In each case, I 
tried to position the tubing under test parallel to the 
tailpipe wall and in a "middle zone" between the center of the
pipe and the wall. Engine runs varied from under 15 to over 20 
seconds, mostly depending on how much difficulty was 
encountered [and hence, fuel wasted] in starting. 

Basic test setup - vise holding sample tube in DynaJet tailpipe - photo (c) 2003 Cottrill Cyclodyne Corp. VIEW LARGE
THIS IS A SETUP, I TELL YA! IT'S ALL A SETUP ... Our basic test setup is simple, almost to the point of being crude, but with care we can get the sample positioned and aligned in minutes. The small vise at the left supports the metal tube sample within the tailpipe of the DynaJet. At the far right, note the glass jar 'fuel cell', the air hose connected to the 'flowjector' and the high voltage leads still connected to the spark plug [energized only for starting]. The sample here is 1/2-inch [13mm] OD mild steel tubing, as described in the text, positioned only about an inch [25mm] inside the end of the pipe, in this case. Photo Copyright 2003 Cottrill Cyclodyne Corporation The first thing we tried, though, was to re-run the stainless tubing [1/4 inch OD] to duplicate our last effort, so I could try to get a still photo. This wasn't highly successful, because the outpouring of sparks is not a very steady phenomenon, but rather comes in fits and starts, so you can't predict when a really good fountain of sparks is going to be there so you can trigger the shutter. This was a continuing problem during the whole test session, though Ben did get some dramatic video shots of sparks blasting out and going all over the place. Here's a series of still shots my friend, avid R/C flyer Kevin Day, captured from the exciting video of one run using steel tubing - the stainless tubing run was very similar in behavior and appearance to what you see here:
Partial destruction of mild steel sample tube in DynaJet tailpipe - photo (c) 2003 Cottrill Cyclodyne Corp.  
Partial destruction of mild steel sample tube in DynaJet tailpipe - photo (c) 2003 Cottrill Cyclodyne Corp.  
Partial destruction of mild steel sample tube in DynaJet tailpipe - photo (c) 2003 Cottrill Cyclodyne Corp.  
Partial destruction of mild steel sample tube in DynaJet tailpipe - photo (c) 2003 Cottrill Cyclodyne Corp. VIEW LARGE
TAILPIPE FIREWORKS Some still shots made from the video of the first run where erosion of mild steel was observed. The tip of the tube was approx. 9 inches [230mm] into the tailpipe. After a large burst of molten metal shown in the top picture, there begins a brief phase of hot metal vapor emission with practically no sparks [second picture]. Soon, though, we see further explosions of molten droplets, along with some brightly colored vapor [third and fourth pictures]. Destruction of stainless steel was just as dramatic. The video was shot by Ben Brockert on a 'Hi-8' tape cassette; the stills were digitally processed from the video by my friend Kevin Day, working from a slow-motion copy he generated especially for this project. Video is almost ideal for analysis - while the engine is overexposed due to the camcorder's high sensitivity to infrared, this same quality ensures that even the tiniest sparks are made visible. Photos Copyright 2003 Cottrill Cyclodyne Corporation Steelyard blues For the first real test run, I set up a mild steel tube of 0.5 inch [13mm] OD and about .070 [1.8mm] thickness; this is many times more massive per inch than the stainless tube, but it was the smallest I could obtain easily. It held up fairly well until about 9 inches [230mm] into the tailpipe, where erosion began to occur. One side of the tube, the side closest to the tailpipe wall, remained almost unaffected, so the overall length of the tube didn't change. The innermost side was eroded back perhaps 0.5 inch [13mm], however, in a fairly smooth semi-elliptical pattern. A couple of inches behind this burned edge, there is a ridge of metal welded to the pipe, with the area in between staying fairly clean. Ben observed that there was a similar ridge of weld metal on the INSIDE surface as well. Advancing the tube farther in resulted in continuing erosion of the affected area, but still left the outermost edge of the tube more-or-less unaffected. The erosion observed was much less than we had experienced with the stainless tubing, but of course, this was much heavier material to start with, so it may not constitute a very fair comparison. The important thing is that severe erosion of fairly heavy walled tubing was observed, at a moderate distance into the tailpipe.
Erosion of mild steel sample tube in DynaJet tailpipe - photo (c) 2003 Cottrill Cyclodyne Corp. VIEW LARGE
A WEARING EXPERIENCE ... Results from the first run where erosion of mild steel was observed. The tip of the tube was approx. 9 inches [230mm] into the tailpipe. The near edge of the tube was reasonably close to the tailpipe wall, and suffered no noticeable deterioration; the far edge was close to the center, in the highest velocity part of the flow, and was seriously eroded away. Some of the metal 'slag' from the melting and burning of the steel can be seen welded on about 1.5 inches [38mm] behind the rim of the tubing; most of this material was simply ejected as white-hot sparks, however. Photo Copyright 2003 Cottrill Cyclodyne Corporation Getting exotic The thing we were aching to try, of course, was the Titanium alloy tubing kindly supplied by Mark 'Thixis'. When this was tried only an inch inside the pipe, the first thing I noticed was that this very thin, light tubing doesn't get up to red heat very quickly - not at all what I expected. The tubing does cool fairly quickly when extracted from the tailpipe, due to its very low mass [and hence, low heat content]. It was not possible to immerse the Ti tubing as far into the pipe as the steel tubing, because these pieces are scrap 'end cuttings', most less than a foot in overall length. The deepest we could get them in was about 10 or 11 inches [260 to 280mm], but we had already seen stainless completely destroyed at that depth, so we could at least consider this a fair comparison.
Heating a Titanium sample tube in DynaJet tailpipe (c) 2003 Larry Cottrill VIEW LARGE
REMEMBER THE TITANIUM! A test of 1/4-inch [6mm] OD Titanium tubing, at a moderate depth in the tailpipe. Note the 'hot spot' on the stainless shell of the DynaJet, showing clearly where the end of the sample tube is positioned. Also note the red heat of the tubing back in the vise jaws, where exhaust is running around and through it. A 20-second run of the jet showed absolutely no erosion of the sample metal whatsoever. The tubing is so light that a sample this size held in the hand feels no heavier than a plastic drinking straw, yet its bending strength when cold resembles high-strength steel! It is amazing stuff, but is one of the most expensive alloys you can buy. Small scraps of various sizes occasionally appear on e-Bay, making it affordable for the serious experimenter. Photo Copyright 2003 Cottrill Cyclodyne Corporation There was NEVER an indication of any material erosion of any kind or degree during testing of the Ti alloy tubing! I find that totally amazing, considering the thinness and lightness of the metal tested. When pulled from the deepest test we could do, the tubing was good and red hot. When it cooled, it was noted that it was covered with a THIN shell of lightly attached oxide [or something] which can be flaked off by gentle finger pressure. This is dark grey [almost Grab your slice of the Internet millions ... black], very thin and smooth, fairly strong but brittle. The metal underneath showed the usual 'rainbow sherbet' variegation of thin, tight oxide typical of heat-treated metal, but remained smooth and shiny. There WAS an important disappointing observation in regard to the Ti, however: Even though it showed no sign of erosion, it failed to hold its straightness! The tubing showed a severe 'warp' from the heating. At first, we naturally concluded that it had drooped from its own weight, but I now find myself seriously questioning this -- the weight per unit length of the material is so small that it would need to be as soft as cooked spaghetti to deform that much! I can now imagine a couple of other explanations: The stream forces of the tailpipe gases is one possibility [I am thinking of how difficult it was to try to get a piece of tubing into the tailpipe the first time I tried it, hand-held]; yet another is that there might be internal strains from the manufacture of the tubing that become relaxed at high temperature, causing warping due to the unbalanced internal stresses. Whatever the cause, the effect is quite pronounced after only 20 seconds, as the next photo shows:
Warping of a Titanium sample tube heated in DynaJet tailpipe (c) 2003 Larry Cottrill VIEW LARGE
BEND IT LIKE BECKHAM ... er, TITANIUM The 1/4-inch [6mm] OD Titanium tubing, after a 20-second run in the test shown in the preceding photo. Here the tested tubing is held against an unheated piece of identical size for comparison. This would be a serious problem for my pressure probe method, since we'd never be sure exactly where the end of the tubular probe was positioned in the tailpipe cross section at a particular moment. Photo Copyright 2003 Cottrill Cyclodyne Corporation I think my best hypothesis, though, is that the warp is simply the result of a temperature difference between one side of the tubing [nearest the tailpipe wall] and the other side [in or near the center of the combustion gas stream]. It is already well established that a significant difference in gas temperature exists between the outside and inside of the gas stream in a pulsejet tailpipe. Yes, the whole tube gets hot, but one side gets a LOT hotter than the other! This could easily account for some unbalanced relief of stresses in the tubing. Of course, alert readers may want to suggest other things I haven't thought of. The Ti tubing could certainly be used as a sensing tube for my pressure gauge, if I could get longer pieces, and if the warping tendency could be eliminated or carefully controlled. The warping is an important effect because to get meaningful pressure readings, we need to know almost exactly where in the pipe we're picking up the pressure to be sensed by the gauge, and we can never be sure if the end of the sensing probe is 'creeping' during the test period. Perhaps another time ... The sample of Inconel Mark sent me went untested. It is much heavier than any of the smaller pieces [5/8 inch (16mm) diameter x over 1/16 inch (1.6mm) thick] and is only 6 or 7 inches [152mm to 178mm] long, so I haven't figured out yet how to get it deep into the tailpipe. If I come up with a method, I'll test it. Also, I could probably borrow a lathe again and turn the 'leading edge' down to .020 inch [0.5mm] thickness, to try to get a fair comparison against the much smaller SS and Ti tubing we've tried so far. That's the only way I can imagine coming close to a fair comparison. Inconel alloys are supposed to be more durable than Titanium in prolonged high-temperature applications, and are preferred where weight is not a design issue [unlike Titanium, these alloys are about the same density as steel or stainless]. It is another fairly expensive material to use. Other lessons learned One thing Ben and I discovered fairly early on is how easy it is to foul up the natural cycle of the DynaJet by putting a hard, pressure reflecting surface behind it. As shown in the photos, the metal tube going into the tailpipe was held in a small 'bench vise'. To my surprise, When the end profile of the jaws 'covered' about half the exhaust path at a distance of about 1.5 inches [38mm] from the tailpipe exit, the DynaJet was impossible to start, even though good bangs and short loud pulsing blasts were obtained! With the vise at a distance of approximately 2.5 inches [63mm] out, the engine would start with a fair amount of difficulty, but would only run 2 or 3 seconds before fading away and dying. To me, astonishing results! The solution was to lower the vise as much as possible, so that the top edge of the jaws were no higher than a level approximately in line with the bottom of the tailpipe. With that configuration, it was possible to start the jet and obtain a normal run, even with the test material almost all the way inside the tailpipe and the vise really close in, as in the setup below [again, using Titanium]:
Heating a Titanium sample tube in DynaJet tailpipe (c) 2003 Larry Cottrill VIEW LARGE
HOW LOW CAN YOU GO? Another test of 1/4-inch [6mm] OD Titanium tubing, about as deep in the tailpipe as we could figure out how to place it. The bent end of the tube was turned down, so that the top of the vise could be dropped as low as possible [actually lower than it appears from this viewpoint]. With the vise this close in, it was impossible to start the engine with the jaws higher in relation to the rear opening of the tailpipe. Photo Copyright 2003 Cottrill Cyclodyne Corporation Lesson learned: Reflection of the exhaust 'pressure wave' back into the tailpipe does make a difference! Another interesting finding was that I was able to use my automotive pressure/vacuum gauge to measure the mean value of venturi suction at the intake throat of the DynaJet. The gauge showed almost exactly 1 inch Hg [25mm Hg] suction, holding rock steady throughout the run. To me, this seemed pathetic, not even getting close to the 'green range' for an automotive piston engine; however, this would represent several inches of fuel lift, so it is quite adequate for the job that needs to be done. Also, remember that this is the average over time of an intermittent process, so the peak suction could be twice this value, or possibly even more. The only difficulty in using the gauge was making sure the end of the sensing tube was at the right spot in the venturi -- this is not particularly easy with a handheld device, so there is some probability of positioning error; the result reported here should be taken as a rough measurement. So, another lesson learned: In my opinion, this does show once again that meaningful measurements of pulsejet action can be made with a standard pressure gauge of appropriate sensitivity and range [admittedly, with some interpretation required.] Conclusions and future directions Obviously, creating metal subassemblies that work well inside the hottest parts of a pulsejet will not be an easy undertaking. Stainless or even regular mild or high-strength steels would appear to work well in cooler parts, such as the region near the tailpipe exit. Titanium might work well for parts that are small, or any parts where some 'creep' of the More traffic for YOUR website ... Free exact shape is inconsequential to performance [e.g. a 'glow coil' or a small flameholder]. Something like Titanium can't be welded to dissimilar metals around it, so such parts would have to be mounted in an engine using bolts or other types of fasteners. I don't know when I'll get to do it, but sometime, I'll try one more test with the DynaJet: Back to the SS tubing, but coated inside and out with burned-on high temperature flux [designed for welding]. What you would do for a more complex internal jet structure is fabricate your part, then keep heating it dull red hot and plunging it into the powdered flux and re-heating it [or, you can just make a paste and coat the whole part first, then melt it on with the torch, by bringing one small region at a time up to red heat]. Eventually, the whole part would look like it's coated with a thin layer of black glass, but who cares, as long as this enables it to hold up inside your engine? I know from experience that this coating is VERY durable [even though it runs out quite thin] and forms a smooth and tight coating on the part, actually resembling black anodizing. And, while a black surface naturally absorbs heat very well, it also radiates it very well. So, this could be the answer for a practical pressure gauge probe [as I was after], or even a fairly complex internal engine structure. We'll see. _____________________________________________________ Photo Credits: All photos in this article were provided by, and are property of, the author. _____________________________________________________ Larry Cottrill is a pulsejet designer, builder and experimenter and Editor of jetZILLA ezine, living in the State of Iowa, USA. To contact him about this article, email: _____________________________________________________


 W E E K E N D   W O R K B E N C H . . .
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Combustion Jam Jars
by Henry E. Whittle

Note from the Editor:

Many people don't realize that you can have a lot of fun 
learning about the fascinating intricacies of pulsating
combustion WITHOUT the expense and bother of building your
own full-blown pulsejet engine! 

Here, Hank Whittle shows how to put together one form of the 
simplest "pulsejet" of them all - the "jam jar" combustor.
Jam jars, while not exactly 'permanent' engines [they break
after running a while], have some advantages for experimenters.
They are unbelievably quick and easy to build, and cost you
nothing for materials [just collect jars you would normally
throw away]. When run in semi-darkness they offer you a look 
INSIDE your "engine" so you can observe how the air flow and 
combustion really work. They can be run on a variety of 
cheap fuels.

Hank also talks about how to enjoy these fascinating devices
safely. We hope you'll enjoy trying this unique experimental
learning tool!

- Larry Cottrill, Editor


With just the expended effort of cleaning a glass jar with a metal lid of
the remnants of its contents and perhaps ten minutes of hand labor you
can create a burner that will sustain combustion for as long as its fuel
holds out. I mention here at the outset that we are dealing with a device
which creates enough heat to burn flesh. Adjust your brain accordingly:
Think Safety.

Materials and Construction

Find one jar of about 6 fluid ounce capacity; a metal lid is an
imperative as other materials used in packaging may not stand up 
to the heat. Remove the lid and clean both jar and lid with water 
and detergent. Allow both to dry completely. Now you are ready to 
modify the lid into your Intake/Exhaust Assembly:

   Jam Jar Combustor described in the article text - drawing (c) 2003 Henry E Whittle

Step one is to drill a 1 inch hole in the center of the lid using 
either a hole saw or by 'walking' the point of a nail around the 
circumference of the hole you wish to create with a hammer. Next, 
drill or punch six 9/32 inch holes radially (about 60 degrees) 
between the central exhaust hole and the perimeter (outer edge) 
of the lid. This completes the basic Jam Jar. If you want to get 
fancy and tune the operating characteristics of your engine (flow
rate) you can add an exhaust duct to your engine by cutting a 
length of 1 inch [25mm] diameter metal tubing and fitting it into
the 1 inch diameter hole. Twisting a bit of wire around the tube 
on both sides of the lid will secure it. Marmon Clamps (hose 
clamps) can also be used to secure the exhaust duct. The volume of
this duct provides a velocity increase (and thus, flow rate 
increase) as the gases flow through the engine. The longer the 
column of gases moving through the duct, the higher the flow rate
through the intake ports.

Fueling the Jam Jar is done by pouring enough of the combustible
substance you wish to use as fuel to fill the Jar to 1/8 inch 
(around 3mm) depth. I recommend Acetone as a suitable fuel. [Dry
methanol, such as auto 'gas dryer', or lacquer thinner can also be
used - Ed.] Ensure that the lid is tight on the jar and set the
assembly out of doors on a flat, nonflammable surface. The 
safest, most rapid form of ignition to get the unit combusting is
a hand-held Propane torch. Hold the flame of the torch against 
one of the intake holes, taking care to stay out of the way of 
the initial burn. Keep your head out of the way of the exhaust 
port. This engine can shatter when run for a while and will most
certainly crack. The sheets of flame inside the cracked engine 
provide a bit more of a show than the symmetrical combustion of 
the engine uncracked. [If a glass jam jar combustor is allowed to 
run for more than a few seconds, extensive cracking WILL occur, 
resulting in a small but potentially hazardous fuel spill. One good
safety measure that I use is to set your jar in a shallow pan or 
Pyrex dish before igniting. Breakage CAN BE violent, with small bits
of glass flying off in unpredictable directions. ALWAYS wear 
protective goggles - they are available at any home improvement 
store, and are very cheap "insurance". Be prepared to clean up
some broken glass at the end of your experiments. -Ed.]

Safety and Disclaimer

It is up to you to run your "jam jar engine" safely. Never use it
in the vicinity of combustible materials. Get yourself a small CO2
fire extinguisher and have it nearby. Wear protective goggles.
While jam jar combustors built exactly as detailed here can be 
experimented with quite safely, the author disclaims any 
responsibility for ill effects from the use of this engine as 

Safety First - Have Fun! 

Hank Whittle is a pulsejet experimenter and an active denizen of
Kenneth Moller's Pulsejet and Valveless Pulsejet Forums. He is a
resident of Central Florida, USA.


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Post-publication Note from the Editor:
After publication of this issue, a 'Work In Progress' page for
the 'Maggie Muggs' engine project was created and put online.
You can view this page here:  Maggie Muggs WIP Page
_____________________________________________________ Note from the Editor: In this feature of each issue, we'll feature one of my own designs from our Gallery of Hopeful Monsters. These will generally be jet engines or related equipment that are PROPOSED designs that are as yet untested and unproven -- BUILD AT YOUR OWN RISK !!! No full-size plans, scale prints or detail drawings, other than what we show here, are available. Also, since these are usually just proposed designs that we haven't even built ourselves, we offer almost no technical information -- these are definitely for advanced experimenters who are used to working out the fine points on their own! So, these designs are mostly presented to give you something to think about, although advanced hobbyists can try to build them and get them to run. Let us know if you have any amazing successes to report! One of my latest designs, from July of this year, was this ultra-simple design for a low-speed ramjet. Aside from the fact that no one was quite sure what demand there might be for a low- speed ramjet, several denizens of Kenneth's forums found it interesting. It was originally presented in two forms, with the second geometry proposed as a viable way of getting it built using chemically bonded construction, rather than welding the parts together. While this was somewhat controversial, there was a fair degree of support for a test of the basic idea and this construction method. Later, on August 7, 2003, I posted actual scale working drawings of such a design and proposed to go ahead and build it. This engine is now under construction, and I plan to do a complete series of articles on building and testing this engine. It is mainly constructed from the stainless shells of two 'travel mugs' and a stainless kitchen sink strainer, bonded together with the well-known 'J-B Weld' epoxy repair product. Hope you'll enjoy seeing this! - Larry Cottrill _____________________________________________________ Proposed design for Simple Low-Speed Ramjet Engine by Larry Cottrill Copyright 2003 Larry Cottrill
Basic concept drawing for proposed Low-Speed Ramjet engine [version I] (c) 2003 Larry Cottrill
LOW-SPEED RAMJET ENGINE - Version I The original concept drawing, as disclosed on 10 July 2003. The construction of such an engine would be welding of easily available stainless parts. Drawing Copyright 2003 Larry Cottrill
Basic concept drawing for proposed Low-Speed Ramjet engine [version II] (c) 2003 Larry Cottrill
LOW-SPEED RAMJET ENGINE - Version II The original concept drawing, as disclosed on 10 July 2003. The construction of this version was proposed to be 100% chemically bonded. Drawing Copyright 2003 Larry Cottrill Original disclosure 07/10/2003 on Kenneth Moller's Off Topic Forum [forum discontinued - original text not available]: This is actually a somewhat old idea of mine, but for many years I never found a suitable piece to use for the 'perforated plate' type 'flameholder'. Recently, though, I ran across a stainless sink strainer part that appeared to me to be the perfect solution. Since I have collected many different stainless partsOneMinutePoll - Easy-to-build polls & surveys that would work for the remainder of the engine, I decided I could disclose such a 'design' [really just an idea for a design] with some hope of describing something that could actually be built and run! This was disclosed as a somewhat whimsical attempt to develop a working ramjet made of cheap, universally available parts needing little or no modification, minimum effort and minimum expense. The idea was to use a couple of stainless shells from different styles of the now popular, so-called 'stainless travel mugs' [really just plastic mugs with nicely formed thin stainless steel outer shells] and a stainless kitchen sink strainer 'basket' as the main parts. Version I was a welded design - however, amateur welding of thin stainless is pretty 'iffy', so I proposed turning the strainer basket around for Version II, which I hypothesized could push the flame so far back that there would be sufficiently good cooling flow [and sufficiently poor heat conduction through the metal] to allow the use of 'J-B Weld' epoxy material as the sole means of holding the engine together. This met with some skepticism, and some enthusiasm, with the enthusiasm coming most often from those who had actually used J-B Weld for repair work on metals. And, there seemed to be little criticism of the basic engine design, although the efficiency of a 'low speed ramjet' would obviously be very poor, and the uses for such a device practically unknowable [since ramjets will not run at all standing still, unless ram air is provided artificially with something like a 'leaf blower', and using them in a vehicle requires some sort of high-speed 'boosted launch']. Since the response was surprisingly encouraging, I proceeded to work out the dimensions of such an engine, using mug parts I had already collected for just such an opportunity, and the newly discovered sink strainer basket. In an unusual move for me, I actually laid this out as a scale working drawing, which I posted on Kenneth's Ramjet Forum on 07 August 2003:
Scale drawing for 'Maggie Muggs', an experimental low-speed ramjet engine (c) 2003 Larry Cottrill VIEW LARGE
LOW-SPEED RAMJET ENGINE - 'Little Maggie Muggs' The scale working drawing, as disclosed on 10 July 2003. The engine is designed to be fabricated entirely from easily available parts and cheap materials, with 100% chemically bonded construction - absolutely no welding required. This engine is now under construction by the author, and is nearing completion. Drawing Copyright 2003 Larry Cottrill Theory of Operation I recently summarized the theory of operation in a post on the Ramjet Forum, thus: Air enters at moderate speed [probably at least 60 MPH], picks up fuel vapor and becomes pressurized by a combination of flow along the diffuser and the drag of the grill. The small perforations of the grill provide multiple high-speed jets of air/fuel mixture which keep the 'flame wall' isolated a short distance aft, but still well within the intended combustion zone [largest diameter of the tail shell]. Large airflow around the outside of the grill through the multiple oval slots bathes the front section of the shell wall with cooling air. Poor conduction of the stainless and this constant cooling flow protect the bonding agent from destruction, in its forward location. Slowing of the mixture speed and good combustion conditions are maintained by the high degree of shear and small-scale vortexing aft of the grill, between all these airflows. Expansion and rearward acceleration is handled by the smoothly curved nozzle shape, and drag is minimized by the micro-corrugations encircling almost the entire nozzle area. Highest static pressure in the engine is in the diffuser section, just ahead of the grill, with combustion maintained at a slightly lower pressure behind. Relatively low chamber static pressure is needed to provide significant acceleration, and hence, usable thrust. What Makes This Engine Unusual? Most everybody interested in jet propulsion knows that a classic ramjet is built with a seemingly 'low drag' interior profile, using some transmogrification of those famous conical flameholders. Everybody knows that this is how an aeronautical engineer would design a simple ramjet. But why exactly is that, and what leads me to think the guts of Maggie should be somehow "better"? The only reasonable answer is, you have to consider exactly what the machine is meant to do. And this is where we do well to remember that "form follows function". What the engineer is after, generally speaking, is high efficiency - not just high thermal efficiency, but the highest propulsive efficiency he/she can get, which is something different entirely. On the other hand, what I am after is operability at absolutely minimal forward speeds. There are NO efficiency concerns at all! All I need [for model aircraft use] is something that will run reliably, producing significantly more thrust than the drag of the air going through it, at an EASILY attainable model airplane speed. That's it. This is a problem I have thought about off-and- on for years. What design difference does that make, exactly? Here is my own idea of the essential difference between a conventional ramjet and what I choose to call a 'low-speed ramjet': - The main design problem in a conventional ramjet is DIMINISHING the [relative] flow speed of the air/fuel mixture enough so that combustion and expansion will remain in the combustion section of the engine where it can do useful work. - The main design problem in a low-speed ramjet is INCREASING the [relative] flow speed of the air/fuel mixture enough so that combustion and expansion will remain in the combustion section of the engine where it can do useful work. The problem with actually accomplishing this in the low-speed ramjet is that you STILL need elevated pressure [which means slowing the intake air down!] to provide the pressure difference for the jet [i.e. the nozzle] to accelerate the exhaust gases. So, you still need a 'diffuser' section to pressurize the intake air. Now, there is already a combustion chamber design that faces this problem and resolves it with great simplicity: the ordinary design of the combustor of any modern gas turbine or turbojet, with its many-holed cylindrical grill. So I decided that what I needed was a highly simplified form of this grill as a substitute for the usual ramjet 'flameholder'. Such a grill would represent huge drag at really high speeds, but at the low speeds I'm interested in, it is no problem - in fact, it has a design advantage: Not only does it provide LOCAL high speed 'jetting' of air/fuel mix through the little holes [effectively holding BACK the combustion 'wall' of flame in the chamber], but it helps create 'back pressure' in the rear end of the diffuser, where increased static pressure is desired. I had obtained a stainless mug shell that was the perfect pre- formed shape for the kind of low-speed diffuser I had roughly envisioned, and a pair of mugs that seemed perfect for the combustor/nozzle section. Next, I discovered that my 'diffuser mug' would almost exactly fit a sink strainer basket rim -- yet, I had never seen a strainer basket that would let enough air through. Finally, the old drain in our kitchen sink started leaking, and my son Jonathan said he'd fix it right away if I got the new parts - so, I went to Menards and found a nicer set than I'd ever seen before. And when I saw how it was made, and how easily the strainer could be torn apart, I KNEW I had the answer, even though my engine would be about twice as big as what I'd really like]. Soon after that, I posted the original design on the forum, and before long the design drawing was produced, without resorting to a single algebraic calculation [all the parts just "looked right" together].
Materials for 'Maggie Muggs', an experimental low-speed ramjet to be built without welding (c) 2003 Larry Cottrill
PARTS IS PARTS Basic materials for the construction of 'Little Maggie Muggs', an experimental low-speed ramjet that can be built without welding. Photo Copyright 2003 Larry Cottrill I'll be detailing how Little Maggie Muggs is built and tested, starting in the next regular issue. This is a highly experimental project, so before you get excited and "jump in with both feet", you'd better wait and see! If you decide to try to build a working model of this experimental design, make sure you get plenty of photos and email them to us in GIF or JPG format to display in a future issue [we will make the final decision as to the best ones to use, and we'll show your name as a photo credit.] And, why not try writing an article about building and firing it? We don't pay for articles, but we'd be glad to help you "get your name in print". Also, any author whose article we accept gets a free ad for your e-business or Web site! Naturally, we will provide editing as needed for publication in readable US English. - Larry Cottrill _____________________________________________________
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  Larry Cottrill with Reynstodyne(TM) Shark(TM) engine prototype after early test firing - Photo Copyright 2003 Cottrill Cyclodyne Corporation
  Larry Cottrill with ReynstodyneTM SharkTM engine prototype after early test firing
Photo Copyright 2003 Cottrill Cyclodyne Corporation
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