Ducted Ramjet that is meant to fly.

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kent
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Ducted Ramjet that is meant to fly.

Post by kent » Fri Jul 27, 2007 11:38 am

I am new here, i have been fooling around with rockets for a while, but i have been thinking about building an ducted version or ram rocket for some time.

The problem is (as it seams to be for many) to get the thing up to speed to get useful performance out of it. Thanks to Glen Olson and his site http://www.alt-accel.com/, that made me decide to give it a try.

This forum has a lot of fantastic looking engines and theory that in many instances are beyond my understanding for the moment. what i miss is actual flying models.

So if anyone has any practical experience of flying ramjets let me know.

I have only set aside about $3000 for the project so it wont be to fancy, target speed is around mach 3,5, and the fuel will be methanol.

The basic idea is to use the ramjet duct, as a rocket engine to get it up to speed before going to ramjet mode, fuel in ramjet mode will be from a gas generator rocket so i dont have to worry about flame holders or ignition.

Due to the variable nozzle requirements i was thinking about using an aerospike type nozzle with a movable spike and a movable diffuser that is depressed by air pressure. I hope the drawing properly illustrates what i am planing to do.

Has anyone tried or thought about anything like this before? if so please let me know if you have any ideas or tips.

I am a bit worried that shock waves in the offset diffuser in ramjet mode will ruin its compression, i guess i will just have to try.

I will also have some serious challenges with temperature control at these speeds, i have bought some alumina for my combustion chamber and leading edges, Anyone have any experience with this?
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kent
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Ceramic alumina

Post by kent » Tue Aug 21, 2007 10:49 am

I have been working on making the engine tube for my experiment, but Ceramic alumina is a horrible material to work with, before its fired its brittle and cutting/grinding isnt really working, it always ends up like a broken plate.

Does anyone have any experience with using ceramic alumina? if so i would love to get any suggestion on manufacturing methods.

kent
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Still cant manufacture in this material

Post by kent » Wed Sep 05, 2007 7:55 am

Failure! yet another attempt wasted. I am starting to think that i wont be able to manufacture the body tube from alumina.

I tried cutting it with a CO2 laser but the temperature gradient created enough internal stress in the material to blow itself apart.

The pictures are the biggest surviving pieces. I will have to re consider making the body tube in a different material. It seams thin walled alumina is hell to work with!

I dont know how to solve this issue since the temperature in the combustor will reach about 2000F, and steel just wont cut it, and active cooling is out of the question since it will be to heavy and complicated.

Back to the drawing board i guess. "sich"
Any suggestions ANYONE?
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Broken alumina tube
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pezman
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A suggestion for alumina

Post by pezman » Wed Sep 05, 2007 1:09 pm

You can probably make a tube with a refractory coating. There are some high-temperature adhesives available through the McMaster-Carr catalog -- gluing tiles to the inside of a tube shouldn't be too hard. Whether they would hold up or not is an open question -- a lot of people have talked about doing ceramic pjs or ram-jets, but I don't know of anyone that has had real success.

One related fabrication technique that I have mulled over is the notion of winding structures on armatures. For example, you could make a wax or foam body, coat it in high-temp adhesive and then wind fiber on that. You can get alumina thread or you could use alumina gasket material. Even something like rock-wool might work. At any rate, the idea would be to build layers of fiber stabilized with high-temp adhesive. You could even wind a stainless shell using fine welding wire. Once the structure is done, you heat it to remove the armature (i.e. it's a lost wax or lost foam technique).

Wax would have the advantage that you could cast the armatures. Foam armatures can be made easily using a lathe and a profile jig (but it is likely to be more work and a little on the messy side). There is also the possibility of making a solid armature that can be assembled/disassembled like a puzzle, to allow it to be removed from the structure, but this would require some serious pattern making skill.

At any rate, if you can perfect a relatively simple, weldless fabrication technique, you'll be a hero.

kent
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Good Idea!

Post by kent » Wed Sep 05, 2007 6:49 pm

Thank you for the input, i did try the refractory idea early on. I coated the inside of a steel tube with a high temperature coating.
The problem with this approach is that the metal and the refractory expanded at different rates when heated.

The refractory simply fell out since the dimeter of the steel pipe expanded much more then the refractory coating.

I do however love your filament winding idea! i cant think of any immediate reason that it woudnt work! A friend of mine has a filament winding machine at work that i can probably nag him into allowing me to use. At least to test the concept.

I will need to source a suitable adhesive as well as wire its probably best to go for alumina or something similar or perhaps low expansion carbon fibre?

Anyone have any suggestions?

The foam shape schould be a piece of cake! i have spent weeks grinding solid alumina, and i cant think of anything that could be messier than that.

I am sure my friend can do the winding pattern on the machine i have seen it in action once.

I guess i have some work ahead of me sourcing materials to test the concept. Thank you for this idea i think this might work

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Refractory linings

Post by pezman » Wed Sep 05, 2007 10:30 pm

Reading between the lines, it sounds like your refractory lining was one-piece? As you pointed out, the differing coefficients of expansion are bound to lead to problems However, glued-on tiles might work (kind of like an inside-out space shuttle). You probably would need to cover the voids between the tiles by doing them in two layers and you'd probably need to be careful that adhesion points are small and that adhesive on "inner" tiles doesn't bridge the "outer" tiles -- basically to allow the three layers to expand and contract more-or-less independently. An alternative to floating layers might be to pack alumina rovings between the tiles.

Mike Everman was using alumina rovings for something a while back -- based on his description, it sounded like it had all the worst attributes of glass insulation, only more so.

I did search for alumina fiber a while back, but I didn't find any sources that were suited to amateur experimenters. I think that the winding idea would work with rovings. The winding machine might not like it, but it should be easy to use a lathe to lay on high-temp adhesive and then glue random fibers on a spinning, sticky armature. A few layers of that should form a relatively strong shell. It is probably possible to do a low-temp version of this using furnace adhesive and rock-wool just to prove it out. The resulting structure would probably even hold up to actual operating conditions for a while.

I looked into carbon fiber a while back. I was unable to find out how tolerant it was of high temperatures. Most literature that I found on the temperature sensitivity of carbon fiber actually focused on the characteristics of the binders.

I think that Mcmaster Carr sells small alumina plates that might be usable as refractory tiles. They also sell "green" alumina that is supposedly machinable. I think that this is partially fired alimina in an organic binder, so it's much softer than real alumina. Once it is machined, you fire it to its final strength. I'm not sure how much it shrinks in the process of final firing...

They also have castable alumina and zirconia. All kinds of ceramic goodies, in fact.

If these materials were a little cheaper, I'd probably have done some experiments with them by now. Although they are cheap enough to be a good alternative to metal (assuming that you can be reasonably sure of a successful outcome), they are expensive enough to take some of the fun out of the amateur experimentation process ...

You're the first guy that I've seen on here that looks to be openly pursuing ceramic fabrication with the kind of earnest effort that could actually lead to success.


Dang -- now I'm all chatty and in an experimental mood ...

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Post by Mike Everman » Thu Sep 06, 2007 5:50 am

This idea always sucks me in, too. Dang, Kent, winding machines and CO2 lasers! No shortage of tools there.

Carbon fiber is not all that hard to get a hold of. The trick is getting it turned into a carbon-carbon state for this.
I did some structures out of it once. Rectangular tubes for a spacecraft solar array. As I recall, the epoxy or polyester matrix of a normally processed layup gets burnt to carbon in a furnace, then they do some process that hardens (infuses?) the now carbon matrix. I believe it was delicate until that was done, but oh man, was it strong after.

I can't imagine a better material for a pulse jet. I'd love to be the first kid on the block with one!
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kent
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This schould work.

Post by kent » Thu Sep 06, 2007 7:18 am

Thank you for all the ideas and innput.

Pezman

The refractory i tested with the steel pipe was actually alumina potting compound that i spun cast into the pipe, i then stuck it in the firing oven and it came out as 2 separate pieces.

Your tile idea might work but it will complicate fabrication and i am not sure what it will do to the aerial resistance through the engine. Since this is a ducted rocket, the air from the intake will not actually be slowed down to subsonic speeds prior to combustion and i have no idea what multiple internal shock waves would do to the engine?

I tested green alumina but its to weak even after firing. I could easily press my finger right through it like it was sun baked clay.

The temperature requirements at the speed i am aiming kind of rules out all other materials apart from ceramics and titanium. And ceramics are dirt cheap compared to that stuff.

I will probably have to expand my budget a bit though, so far i have used a third and still i have nothing solid to show for it.

But i cant let it go, unfortunately i get obsessed about these things. Its my biggest weakness i guess i just cant let it go.

Mike Everman

I am just really good at begging :) The equipment i borrow is far beyond any means i could ever dream of the laser for example cost over a mill according to the owner. Its normally used to cut thick steel plates for ships.

I will read up on carbon carbon, its probably beyond my abilities and means but who knows maybe there is a possibility to manufacture it at home.

Anyway i have talked to my friend with the filament winder, he says i can use it but he has set very specific requirements to the bonding fluid.

Apparently it has to be of a viscosity between water and thin flowing honey, and its curing time must be at least 4 hours.

The viscosity influences the winding speed apparently? and the fiber is so tense when winding that it needs to rest before the binder sets to equalize internal stress. (who knew this stuff was so damned complicated)

Alumina wire is out of the question since the machine actually tightens the thread with a minimum of 10 kilos of tension while winding it on. Carbon fiber schould be no problem, mechanical values etc but its surface doesn't bond with all types of adhesives.

Your idea Mike of using a binder that can be fired to carbon would be the best solution, but i have no knowledge about this process so i will have to study.

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Ok, now I'm really impressed

Post by pezman » Thu Sep 06, 2007 5:28 pm

That's quite an impressive list of technology trials.

kent
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Carbon carbon is out of the question

Post by kent » Fri Sep 07, 2007 2:30 pm

Well so far i haven't made much progress, carbon carbon composites are out of the question, a rough calculation gave me a manufacturing cost roughly 53 000 over my remaining budget. The materials specilised equiptment and energy needs is way beyond me.

I also have a problem finding a suitable bonder for filament winding the part, most high temp binders are cement like and they dont bind well with carbon at all. and their viscosity is not even close to what the winding machine demands.

i will keep searching for a suitable binder, another suggestion that i received was to spin cast alumina potting compound into woven carbon fiber fabric, but i don't know about that? if it is one thing i ave learned from trying to manufacture with alumina its that its a horrible material to work with, potting compound chinks with about 2% when fired so im afraid it will break apart again from the internal stress.

i will look for a suitable binder first.

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Ducted rammy

Post by Irvine.J » Fri Sep 07, 2007 3:10 pm

Well, this is kinda shooting from left field as its stupidly late...
Though I know the Russians have remarkable heat shielding success with nothing more then cork. Yep cork! You can buy it in flat sheets and literally roll it to make shapes you want, some cork is more ductile then the other, anyway its just an idea for pezmans sacrificial coating.

Otherwise, it might be time for jimbocrete. Over the years I had some crazy obsession with Pikecrete. Simply Ice and sawdust, and when mixed would literally stop a bullet...I'm not kidding. Anyway, I used a various number of glues instead of ice in the past to make lots of crazy shapes some I could belt with a hammer and basically do nothing to them, and when its a paste you can litterally spread it out over something that will make a coating or shape that burns away later. As for expansion, I have no idea. Anyway, just the ramblings of an excessively tired individual. I really don't like the box...
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Post by PyroJoe » Fri Sep 07, 2007 5:53 pm

I read an article years ago about the Chinese using Oak panels on one of their satelites. The Oak was to directly take the brunt of the thermal load during re-entry.


If memory serves me it was treated with boric acid, or the like.

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Post by LINZ » Fri Sep 07, 2007 8:35 pm

Kent
"The refractory i tested with the steel pipe was actually alumina potting compound that i spun cast into the pipe, i then stuck it in the firing oven and it came out as 2 separate pieces."
Are you sure your refractory lining fell out because of differential thermal expansion? The CTE of Alumina and steel are both around 5 ppm per degree C I think. I would think it's more likely that the Alumina shrinks during firing and that's why it fell out. If that's so, you may be able to build some small features - bumps or ridges - inside the steel tube, even the inherent geometry of the jet, to hold it in place without causing it to crack. It may then be possible to shrink the metal tube, using local heating like a shrinking tool used for car bodywork, back down onto the refractory lining. It sure would be tricky because you'd have to shrink it fairly evenly all over without overdoing it and crushing the lining.

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Post by Jim Berquist » Fri Sep 07, 2007 10:32 pm

I like the cork Idea!!! Cork would Carbonize into a spongy high temperature thermal insulator. To simple! Eaaaa!
WHAT TO FRAP, IT WORKED![url=callto://james.a.berquist]Image[/url]

kent
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Cork !

Post by kent » Sat Sep 08, 2007 7:15 am

Irvine.J

Cork what a great idea! i will buy a cheat and burn test it today!

Jimbocrete? with heat resistant binder? might work as well actually? another experiment i schould do.

I dont think i will try the boric acid though.

LINZ
Are you sure your refractory lining fell out because of differential thermal expansion?
No i am not sure, im not exactly a material scientist, but as i see it you dont need to understand how a car works to drive it :)

The potting compound shrinks when cured and it might be the reason it separated? i guess it could be a combination of the to. I just don't know. The thin coating of alumina separated from the tube was very brittle it cracked easily.

I dont think it would survive in a combustion environment without being bonded to a solid structure.

The next test is definitely the cork

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