Hi there, new... and experimental capped design ?

[steve mac]

Hello all, from down under. I'm new to pulse jets, but I have the time and enthusiasm to slowly learn and stick with discovering the ropes of this fascinating hobby by trial and error. I've spent the past week researching the net on the subject and realised the benefit of participating in your knowlegable online community.

Basically, a pal and I have a background in tinkering/inventing/ small model rocketry and a fair bit of aeromodelling and want to wake up the neighborhood with something more impressive. - actually we are quite rural.
I have the capacity to learn and work on the technical or theory side, and my friend is a tradesman welder and panel beater.

We would love to start of with one of Larry Cotrill's starter Jets, a Beck-Tech jet or something reliably similar but money is very tight right now. However in the meantime we have a bit of steel scrap and an old 23" long 75mm cannon shell at hand that might suffice to experiment with a capped tube design to start with, fueled by LPG. Not sure about the capped concept though...

Now, my first innocent thought with a capped tube was that it may be somewhat tunable by positioning the cap forwards or backwards to the open combuster face by very small degrees, but after having done a search here I read that they may present other problems in starting and running, so i'm not sure what to think of trying to tune one, being inexperienced, let alone even getting a hybrid started, if possible. Anyway, we are keen to have a go and its no skin off our nose if it doesn't work, we will move on to better things if so without a complaint when money allows.

Does anyone have any advice on a capped system, or even if not to do it and experiment instead with a more chinese style intake pipe layout? The jet exhaust outlet diameter is 100mm given by the base of the 23" long cannon shell, I haven't yet determined the length of the 75mm joiner pipe between the shell and the combuster, its taper if at all, or the exact combuster dimensions. So its still a clean slate.

Any thoughts or advice would be great!
Many thanks from steve.

[Axt]

G'day Steve, one of my first posts here went much the same as yours is with the capped design sparking my interest for its elegant simplicity. I called in the "coaxial comet" for obvious reasons, a glowing red ball with annular flames out the rear sure sounded sweet. You should be able to find that topic under its name is you try a search (~'bout 5yr ago). Coincidentaly I found the parts in the shed a few days ago and were sitting right beside me as I read your post.

As one can see I never did finish it, however I'll post a pic of the setup with the domed front end and stand that was to allow for a variable sliding intake may give some ideas.

I've never known a capped design to be shown working, many concepts but never a running engine (some of the regulars here may know better?) so that alone says its not near as easy as it looks, and not a good first jet as working dimensions are likely to stray widely from what other designs use. You'll likely get frustrated and lose interest like I did when my first beautiful full scale spec lockwood failed to function

Coaxial Comet design

Coaxial Comet intake

Stand for fine intake adjustment while running, (5yr of rust buildup)

Coaxial Comet concept.jpg (5.63 KiB) Viewed 9577 times

[PyroJoe]

Design by Dave Gibbel:

viewtopic.php?f=3&t=4519

hope this helps!
Joe

[PyroJoe]

This one is bass ackwards, in the cap goes into the CC. There are little spacer rods around the cap, and a main rod that holds the cap firmly in place. Hasn't locked in for more than a few seconds with a Rosscojector. Testing multiple (3) injectors this weekend. Success hinges on proper fueling.
Joe

In my opinion these designs are a bit advanced for beginners. Probably best to run a Muffler Shop Special as a beginner and work up to stuff like this, due to fueling issues. A central intake pipe is much easier to work with.

[steve mac]

Thanks Axt and Joe for your replies. Yep I am tempted to steer clear of the cap system for a beginner attempt, but I tell you what, for some reason I feel really captivated to try that style, maybe later- I was thinling of a triple gas pipe inlet fuel system but i just don't know enough yet about where to locate them for best effect, there appears to be a hundred variables to consider.

I'm taking a trip to a local auto exhaust shop and scrap yard today to look for a potential combuster, it'll be a mix and match affair I 'spose in the slim hope the unit might find its Hz to go bang to. We will try for clean internal joints and all the obvious stuff, and a needle valve gas feed.

Joe, all the best with your triple fed capped Jet tests this weekend, and thanks for your advice, I will work my way up to the capped jet to save me some frustration.

I'll report back when things are starting to come together.

steve

[Ghrey]

My current build is coaxial, but, unlike any thing I have ever done before it is a test unit. Never meant to fly. Just answer questions.

Also I am using pipe. More on that L8r


See also ::: viewtopic.php?f=3&t=2641&st=0&sk=t&sd=a&start=90


Good read.


Cheers

[Axt]

Ar well i stand corrected Joe, any other specs on that engine other then the pics?

Quite an oddball jet you have there Joe , let us know how the three injectors go and how you did it. I drew 4 injectors in the picture above and I'll attach how i was planning to do it. Just a brass compression fitting into a stainless socket, copper tube bent in that way to fan the fuel out hopefully covering the intake area.

Theres another coaxial like that one on youtube Ghrey, http://www.youtube.com/watch?v=WHDTxfiZ ... annel_page . Though these aren't capped jets where intake is at front of chamber.

comet injector

injector.jpg (5.83 KiB) Viewed 9399 times

[Mike Everman]

Theres another coaxial like that one on youtube Ghrey, http://www.youtube.com/watch?v=WHDTxfiZ ... annel_page . Though these aren't capped jets where intake is at front of chamber.

Ha ha, that one was built by Bill Hinote, and the video is of it running in my back yard!

BTW, the most famous and extensively tested capped annular design was the Saunders Roe. I've been looking all over for the photos, but just found some low res from the Tharratt "Propulsive Duct" paper.

[Ghrey]

This is a basic lay out of what I am working on ( see attached... )

I call it crash dummy

The actual engine will consist of plumbing parts, set up to be totally modular. this will allow me to swap in and out all kinds of harebrained ideas, quickly
and easily ( or relatively so ). I'll be using an old but serviceable support frame that has served may times before.

Crash dummy

Edit: There are some pics of some of the parts of this project posted on the " The Great 1.5L Max Valveless Contest! " Thread. Yes I should post a build thread at some point...


Cheers.

[PyroJoe]

Containment has been poor in the inserted cap engine. The flame easily climbs outside the intake. At specific tail lengths it displays a dual cycle internal-external combustion.

The three injectors helped, reduced intake area to 70% of exhaust area. Shortening the tail allowed the engine to lock in somewhat normal fashion internally. Fuel consumption is relatively high. Starting to tread deeper into pressure jet territory. Where in the Land of Roaring Pipes do we find the divide line between pressure jets and pulse jets?


Noticed the Saunders Roe engine is fairly short in the tail. Does anybody have info. on the fuel & fuel pressure for that design?


Saunders Roe
found here:
viewtopic.php?f=3&t=371

[Ghrey]

Here is a little more:


http://v3.espacenet.com/publicationDeta ... &NR=793239


Wish I had more time interesting search...

[Viv]

I also like the Foa designs for this type of capped engine, I should add thats from the book the propulsive duct

Viv

[PyroJoe]

The text reads that it is no pressure jet. Just needs compressed air for starting. The intakes are more central than the one I built, location somewhat close to GRIMs six-shooter engine. Interesting stuff.
Joe