Cheating at pulse-detonation

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metiz
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Cheating at pulse-detonation

Post by metiz » Thu Jun 19, 2008 3:08 pm

Pulse-detonation. It's a bitch. There are some working prototypes but you need a complete laboratory and pressurised oxygen and the likes to let them run. Could there not be a way to cheat at detonation?

most of those engines use propane/ oxygen/ acetyleen to operate but how about using some sort of liquid or powdered explosive as a fuell?
Some sort of explosive that still require air to explode (to not make it a rocket) , you wouldn't even need a fancy design - just a (realy) reinforced working pulse-jet design. How about it, would such a thing work? (I know there are still a lot of things to consider with something like this but bear with me)
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Re: Cheating at pulse-detonation

Post by Viv » Thu Jun 19, 2008 5:14 pm

There are different modes of operation to consider, probably the simplest and lowest energy requirement is deflagration to detonation, with this method you need

1/ a tube long enough for deflagration to accelerate to detonation speeds and pressures
2/ a tube of large enough diameter to sustain the triple point shock cell size of the fuel used, (propane 75 mm plus)
3/ a tube strong enough to handle the pressure of detonation up to 6000 PSI, I have seen 12.5 mm steel quoted.

So if you have a 3.5 meter long tube 75 mm internal diameter and 12.5 mm wall thickness you are good to go ;-)

Pulse jet designs wont work as PDEs are not acoustic engines and the pressure is up to 6000 PSI were as all pulse jet designs can only get a few pounds over ambient if they are lucky, valveless designs being the lowest while valved are the highest.

A simple closed end tube with inlet valves for the fuel and air or oxygen is all that is needed along with a really good high energy igniter circuit, you could try a different design but this is were a lot of people start, GALCIT has plenty of info on their web site.

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Re: Cheating at pulse-detonation

Post by metiz » Thu Jun 19, 2008 9:31 pm

I mean, with using a explosive as fuel you don't need to worry about DDT or high pressure because when the explosives ignite, they, as there name implies, explode. but DDT might be a good alternative opposed to instant detonation. I can't by the life of me imagine a full on detonation occuring without any sort of valve system (WITH valves I could - to bad there's a pattent describing that EXACT system.....*sigh* story of my life :P)
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Re: Cheating at pulse-detonation

Post by Viv » Thu Jun 19, 2008 10:12 pm

You have a mixture of definitions there that make it a bit confusing to work out were to start.

Detonation in this frame of reference defines a speed of a reaction, combustion for example happens at around 30 meters a second under compression in the cylinder of a car engine, detonation happens at about 300 meters a second or better in TNT (or any middle east country of your choosing).

Explosives are simply substances that detonation (reaction) occurs in, and at the above speed of reaction, petrol burns, at a very different and much slower reaction speed.

You can burn a lot of explosives with out an explosion! it takes a detonator to start the detonation, a match just sets light to it so it burns ;-)

In a PDE, the detonation reaction, takes place in the thickness of the high pressure shock wave, as it travels along the tube, in a pulse jet the fuel burns over the length of the combustion chamber but in a pulse detonation engine the reaction zone may be less than a millimeter thick.

Oxidation and detonation, not some thing you want to get mixed up at the design stage of your project I think ;-)

Viv

PS post a link for the patent please so we can all enjoy the fun
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Re: Cheating at pulse-detonation

Post by metiz » Fri Jun 20, 2008 12:51 am

Sure but explosives vary you'd have to choose the right one - maybe a stabalised form of nitroglycerine or something (no, not dynamite:P)
Also, I'm not planning anything atm. Clearly, as you point out in your comments, I've got shitloads to learn still - I'm just speculating/ brainstorming

here's the link to the engine pattent
http://www.freepatentsonline.com/6978616.html

and attached (bottom one) is my 5 minute paint sketch without any previous knowledge about the pattent. Look familiar? *bigger sigh*
Attachments
2 stroke pde.JPG
dual piston
pulse detonation.JPG
look familiar?
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Re: Cheating at pulse-detonation

Post by Viv » Fri Jun 20, 2008 1:07 am

There is no criticism at all implied as I am just trying to answer your questions in a constructive way, I did try and put in some key peaces of information for you to help with that process as PDEs are a very different subject to Pulse jets.

I have seen a lot of these attempts to include piston power take off in PD type engines but I do have to wonder if thats at all sensible in the long run and to what overall advantage it will give.

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Re: Cheating at pulse-detonation

Post by metiz » Fri Jun 20, 2008 11:12 am

No, don't get me wrong, I'm not critisising you for critisising me, I'm just saying that the terms you use and how you state your replies are more technicall then I can realy follow. This means that I still got loads to learn. And about the piston, I don't see why it couldn't work; it's more or less a heavily modified internal combustion engine.
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Re: Cheating at pulse-detonation

Post by Viv » Sat Jun 21, 2008 2:10 pm

Nice little video may help, the guy has some more worth looking at too

Viv

http://au.youtube.com/watch?v=D-dLjHJuF ... re=related
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ssabot25
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Re: Cheating at pulse-detonation

Post by ssabot25 » Tue Jul 01, 2008 5:34 pm

There is a way to cheat a PDE pretty crudely...

For example by injecting pellets of desired explosive into microwave-flooded chamber, or into a wake of a microwave laser. :D

Makes me think paintball guns... :wink:

Seriously, though, you could optimize the pellet for engine operation, for instance using different kinds of explosive materials inside one pellet, using coatings, varying the cycle by injecting a pellet of different explosive material at the right time, detonating the pellets in a desired place inside the chamber for the wave to rebound in a desired manner, detonating them in oxygen-free chamber, or whatever-you-feel-like-at-the-moment filled chamber...

The possibilities are endlless. At least to the point where the pellet delivering mechanism chokes.

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Re: Cheating at pulse-detonation

Post by Latrocinium » Thu Aug 20, 2009 5:55 am

Hey, I'm new to the forum, but have some input on possibly cheating or actually achieving DDT.

We all know that the amount of thrust you get from pulse jets/deflaggeration, is not very viable. Or that is my opinion, considering net thrust a lot of times is like 5lbs or less for a moderately sized one. I would like to achieve 250lbs of thrust out of a moderately sized jet, not an incredibly huge jet. lol.

However if we used some of the DDT principles we might be able to achieve higher speed in the exhaust AKA more thrust.
I've included an image to describe what I was thinking.
Image

Stage 1 would partially fill the cavity in both ends with a pressurized air fuel mixture.
Stage 2 would ignite the mixture causing a wavefront to travel up and down the cavity.
Stage 3 & 4 would be the percussion of the wave coming in contact with the secondary air fuel mixture, further compressing and igniting it.
Stage 5 would be a highspeed wave traveling back down the cavity to the exhaust.

Lengths and diameters of the tubes would be negotiable according to fuel, etc. Also I would need some help figuring the right measurements. Considering that I would like to use gas.

But if this only increases the velocity of the exhaust by a factor of 3 or 4, then that should translate into an appreciable amount of thrust.

If I'm wrong, :( please say so and point me in the right direction.
:wink:

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Re: Cheating at pulse-detonation

Post by Mike Everman » Thu Aug 20, 2009 7:23 pm

Nice illustration, but IMO not a way to achieve detonations. The point is that a small detonation sends a supersonic pressure wave through a fuel air mix to detonate IT.
If you believe that the Aurora plane existed, and I have not researched that in a long time, so maybe it's old news one way or the other, then it's been around for more than 30 years, and the skunk works boys almost certainly cheated, for lack of all the more modern research going on. It's kind of a case where they did it, and have been funding research ever since to make it practical.

Nice pull, and nice idea for a thread, Metiz.
I've thought often about, if I were to be faced with the problem, and I were at the skunk works 30-35 years ago, how would I approach the problem? The idea is as you posed it with explosive charges, and I'd further add that the quickest way to the party would be to use a machine gun with blanks at the front of the chamber.
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Re: Cheating at pulse-detonation

Post by Viv » Thu Aug 20, 2009 7:54 pm

Beginning to sound a bit like a poor mans project Orion ;-)

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Re: Cheating at pulse-detonation

Post by Latrocinium » Thu Aug 20, 2009 11:12 pm

Well atleast it was a thought pattern. lol.
Figured I should post an idea, instead of continually trolling the board, leeching ideas. :lol:

I've heard of the doughnut shaped trails left by the aurora. If it exist, but alas my security clearance to area 51 has expired. :( :wink:

But the question still remains, would this help add thrust to a standard valveless pulse jet?
Or should that be another thread?
Also what might be some more viable ways to achieve a detonation? Pelets, and other solid fuels(such as machine gun rounds) are to hard to continually feed or to expensive.

Let me have a poor man's aurora. :twisted:

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Re: Cheating at pulse-detonation

Post by ace_fedde » Fri Aug 21, 2009 5:35 pm

Hi L.,

To learn about detonations, read this one:
viewtopic.php?f=12&t=1418

I like your idea, and it looks like an idea that I have. I haven't got a clue if it might work, but I'm afraid you'll have to do more to build up pressure. I think that pressure shortens the deflagration to detonation lenght. (the structure of a solid explosive causes, after ignition, the pressure inside the explosive to get extreme, thus the DTD lenght is there the same as the size of the explosive: imidiate detonation.)
If you have the possibilities: build and try!

If forum members write that they think that and why something won't work: Don't take it automaticly for granted, but also don't ignore it.
Try to understand why people think it won't work and then make your own conclusions.

Back to the subject: there are a few ways to get to detonation. The one that Mike mentioned is one of them, and I agree with Mike that that one won't work for your case. But hey, Mike has got tons of more experience than I have, so don't take my opinion for granted. :D

Once again, read the tutorial and if possible for you: Try!

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Re: Cheating at pulse-detonation

Post by Latrocinium » Fri Aug 21, 2009 8:30 pm

Hey Fedde, I really appreciate the inspirational talk.

Unfortunately I'm to stubborn to walk away after one no. :oops: I think the idea has some merit, but I'll have to school myself more before I make any solid conclusions as to what merit that might be. I welcome the criticism and ideas. There's something to be gained from every point of view. But as my dad told me, take everything with a grain of salt. :wink:

I have to take mike at his word on some of it. His name is all over this board, and this board and it's info is the only reason I was able to put together a small pulse jet. I'm currently working on building pulse (maggie) ramjet, if it is a succesful build. I'll post vids and pics, so I can get some cred on the board. Instead of being the complete noob. :lol: Although I guess the cred would really go to the designers of the maggie(cottrill), and beck for the pulse jet. Hey I'm a good copy cat though. :lol:

Thanks for the link. However, I found that gexcon is down. So I googled it and found a copy on rapidshare. http://www.filestube.com/515d544ff6ffe1 ... tails.html For anyone else that might be interested in learning about detonations, and gas combustion.

If anyone has a link breaking pulse jets down, like gexcon. I would appreciate it. I've read the article by beck-technologies. Which is excellent, but I was hoping to find out more about the angles for exhaust, combustion chambers, etc. I'm currently trying to find a good copy of Reynst book. Hoping that will fill in the blanks.

Thanks again to everyone. If you need any help with working on an idea, graphics, etc. Just ask. If I can I'll be happy to lend a hand.

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