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high mix hybrid ideas and questions.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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high mix hybrid ideas and questions.

Unread postAuthor: Jeeperforlife » Tue Jan 26, 2010 11:36 pm

I have been trying to come up with a good way to build a hybrid that could take high mixes but not require a lathe. My hopes are 200X mixes. Here is the Idea I have; I found some steel tubing for the chamber 4” OD, 2” ID, 1” wall and 24” long, It is seamless DOM. For end caps I am thinking 1.5” thick steel plates. I don’t have the ignition or fueling down yet but do have some ideas. The end caps will be held by 8 5/8” alloy studs. The studs will be threaded in to tubes that run and are stitch welded the full length of the chamber. This way the end caps are basically pulling on each other and I don’t have to worry about the tubes pulling away from the chamber. The barrel will be 2” ID ¼” wall with 2 more tubes each one ¼” thick over it coming out of the chamber (see picture). I will use a gasket in the back made of the same stuff as a head gasket in a car. The Burst disk will be clamped between the front end cap and the chamber.

What mixes could this take as you see here? I am still working on the ignition and fueling, I want both in the same hole in the back to keep it stronger. I included some pictures from Sketchup. Keep in mind this is the first thing I have ever drawn. It is very close to scale. If anybody would like the Sketchup file to play with or show ideas let me know and I will E-mail it to you. Keep in mind this is over 8’ long as you see it.

Image
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Image
I know what many of you are thinking. This will never be built, All I need to do is convince the wife to let me use some of the tax refund and I can build it this spring.
I would like any comments, suggestions, or questions. This is one project I need to do right the first time.
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Unread postAuthor: DYI » Wed Jan 27, 2010 12:42 am

Well, structurally speaking, there are no obvious, glaring problems. Assuming a decent quality steel with 60KSI tensile yield strength, you'll have a safety factor of at least 2 on the chamber, 1.5 on the barrel, and 5 on the rear cap (if I've remembered how to calculate failure pressure of a flat plate correctly).

Unfortunately, it's not as simple as this - holes in pressure vessels can cause much less predictable structural behaviour. This varies with the size and geometry of the hole, and can be worse if the hole is threaded. Your safety factor of six may drop to less than two. It may do to increase plate thickness to 50mm or so on the rear cap. As for the chamber-barrel coupling; I don't know how you plan on attaching your barrel, but it will also be a concern. If you're going to weld it...
Well, it had better be a hell of a good weld :lol:

There will need to be some HGDT work done here to determine just how long the thicker barrel sections need to be, but that shouldn't be much of an issue.

The bolts, assuming they are of at least moderate quality and evenly tensioned, will be in no danger of simple tensile failure, and will have a safety factor of 3 - 6 in that regard, dependent upon their grade. Because of the potential for uneven loading, you'll want the highest grade you can get, probably grade 8 (there are bolts available that far exceed grade 8 specs, but they're uncommon and expensive, generally).

More of a concern may be thread stripping on the bolts. If you use high grade nuts as well, the safety factor will be lower than that for tensile failure of the bolts, roughly three.

So there you have it. Ragnarok will be able to tell you more about the front plate issue, and how to solve it. You'll find that fueling and ignition design are more of a problem for high mix hybrids than is the structure itself. Also, I assure you that aligning everything and sealing the burst disk will be fun :wink:
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Unread postAuthor: inonickname » Wed Jan 27, 2010 1:30 am

DYI's mathe is correct. Any holes in the chamber or plugs will drop the safety margin considerably; keep them as small as possible (while still making sure the threads will hold. You can thread things in from the inside (with a increased neck to be forced against the wall) so smaller, finer threads can be used safely.

I recommend you outsource anything you aren't confident in doing to a machine shop (such as welding the barrel).

It would probably be better to make a burst disk adapter which attaches to the barrel instead of the chamber.

I'd have one end of the bolts fixed, and torque the others down with a drill (or similar) preset to the right amount of tension to help distribute stress evenly.

Good luck fueling it :)
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Unread postAuthor: psycix » Wed Jan 27, 2010 6:20 am

Any holes in the chamber or plugs will drop the safety margin considerably; keep them as small as possible

Or avoid them as much as possible.
However, you WILL need to make a hole for fueling and ignition.
I'd think it would be the easiest to ignite with a glow-wire like Larda did.
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Unread postAuthor: Jeeperforlife » Wed Jan 27, 2010 9:40 am

Thanks for the fast replies.
Let’s see if I can put this in order.
DYI-
The weld on the barrel does not concern me to much. My plan was to drill the hole large enough to have the barrel slip into it. That way I could weld it on both sides of the steel plate. I have butt welded thinner plates to pipe on several occasions, plus I was toying with the idea of reinforcing the barrel by cutting flat gussets and welding them to the front plate and the barrel.

The only hole in this will be in the back for both fueling and ignition. It does pose the question though, would 2 smaller holes be better? I can make the back end cap thicker if needed it just means more work though.

The studs I have picked are stronger than a grade 8. I realize I will need to use my toque wrench so I can the nuts tightened down properly, also I already planned to double nut the studs to help prevent the nuts from stripping out.

inonickname-
unfortunately the only thing threaded is the ignition and fueling. This means I need to remove with every shot so I can't thread from the inside. Would I be better off with 2 small holes or 1 large hole?

I'm not sure of the issue with the burst disk between the barrel end cap and the chamber. This will give me 1" all the way around for a good seal. The only problem that I can see is I will have to take the front 8 nuts and barrel lose every time. But on this gun I really don't care about reloading times.

I am not using bolts, they will be 6" long alloy studs with Locktite in the pipes they thread into around the chamber.


psycix-

For ignition I want 2 or 3 ignition points. I am going to use a wire system similar to Larda's. Speaking of witch does any body know how he built the "spark plug" I am trying to figure out the best way to keep the center pole insulated but I have not come up with any thing I trust yet. To get all the wires to ignite at the same time I was thinking of using a small capacitor bank. That way they would pop at the same time.
Thanks I still have along way to go before I can start the process.
Here is the steel I have in mind.

Chamber
https://www.speedymetals.com/p-3588-4-od-x-1-wall-dom-steel-tube.aspx

End caps
https://www.speedymetals.com/ps-598-39-1-12-x-6-cold-finished-1018.aspx

Studs
https://www.speedymetals.com/ps-2838-186-0-b7-alloy-threaded-rod.aspx

Tubes arround the chamber for the studs.
https://www.speedymetals.com/ps-3499-204-1-od-x-250-wall-dom-steel-tube.aspx

Barrel
https://www.speedymetals.com/ps-3509-204-2-12-od-x-250-wall-dom-steel-tube.aspx

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Unread postAuthor: jeepkahn » Wed Jan 27, 2010 9:51 am

What and WHERE are you going to shoot this thing??? You will seriously want to consider the backstop/range... depending on the projectile, you will be looking at lethal force at several miles downrange... based on the scale and mix ratios you're going after, power will rival that of light tank guns... I hope you've got a carriage plan as well...



On another note, what jeeps do you own???
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Unread postAuthor: Moonbogg » Wed Jan 27, 2010 10:47 am

Looks good. Why do you have tubes around the studs? If you aren't careful, the tubes will absorb all your torque, and then the end caps might not be tight enough, or evenly tightened.

EDIT: I guess what i'm saying is, just make sure they float and aren't compressed.
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Unread postAuthor: inonickname » Wed Jan 27, 2010 11:07 am

Larda used a glow wire, not a spark plug. You only need one ignition point, any more is definitely pushing it further. Steel wool can be used although nichrome wire would be more permanent/reusable.

Two smaller holes would probably be better, cleaner and easier. To insulate it use an engineering plastic like delrin, or insulate it with epoxy. Try always to place to load on steel parts rather than composite ones.

Jeepkhan is correct. Simply firing a 16g piece of plastic larda's hybrid produced around 7 kilojoules of muzzle energy, which could be substantially higher for a heavier round (and that was a 100x mix or below, not the full 200). This however, looks bigger. If you hit someone with it, you will kill them.

Jesus, 2" bore. For the love of god, make it smaller. Or drop the mix a bit. You can be exerting over 62,000 lbs of force on the projectile. That's over four times the force exerted on the projectile by this, which can penetrate 40mm of steel plate.

Downsizing it would be a might good idea.
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Unread postAuthor: jimmy101 » Wed Jan 27, 2010 11:26 am

Won't most fuels autoignite (diesel) if pressurized to 200X? Certainly gasoline and diesel can't be pressurized in air to that ratio without autoignition.
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Unread postAuthor: inonickname » Wed Jan 27, 2010 11:31 am

jimmy101 wrote:Won't most fuels autoignite (diesel) if pressurized to 200X? Certainly gasoline and diesel can't be pressurized in air to that ratio without autoignition.


Not really. If you compress them quickly, yes. You're more likely to run into problems with the mixture liquefying, not detonating. Anyway, when this gun has a burst disk seated and is being charged, nothing valuable should be downrange and nothing should be near it.
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Unread postAuthor: DYI » Wed Jan 27, 2010 12:28 pm

Considering the cost of materials for this, it would be a good idea to build a scaled down test model first, to make sure that all the systems will work properly. Larda had a gauge worth almost $1k on his fueling system. Unless you want to pay a huge sum of money for the gauge, it'll need to be suited very closely to the pressure range you're working in, and you'll need separate gauges for fuel and air.

Try to avoid threaded plastic in the ignition system, it has a tendency to fare poorly at these pressures. It is quite possible to create insulated electrodes where there is nothing but a slight compressive load placed on the non-metallic parts. Due to the necessity of insulation, there will always be some shear loading present as well, but that can be countered effectively by making the area exposed to chamber pressure as small as possible, and increasing the length of the insulator. Glowing wire ignition is almost a given here. The voltage required to create even a 1mm arc in the chamber would be on the order of 200kV.

The muzzle energy will not be that difficult to deal with. I had a 2" bore hybrid with roughly 2/3 the muzzle energy that this one will have, and while not suited for firing within a few kilometres of residential areas due to noise issues, dirt berms and changes in elevation stop everything just fine. This will be at least 10kg heavier than mine, so a good stand is a definite priority.

Also of interest is the shutoff valve. There are no affordable commercially available valves which are rated for such pressure. Larda used a needle valve oriented such that the face of the needle (about 1mm<sup>2</sup>) was on the chamber side. I don't think that ever failed.

I also share Moonbogg's opinion that the tubes to house the studs are, at best, unnecessary.
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Unread postAuthor: POLAND_SPUD » Wed Jan 27, 2010 1:56 pm

uhmm I have to agree with what others have said... 2" and 200X mix won't be very practical...

do you think you can find ammo that can survive such pressures ? or do you have enough land to fire this thing safely ?
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Unread postAuthor: psycix » Wed Jan 27, 2010 2:03 pm

One glow wire will be enough.
Multiple ignition is unnecessary unless you are having very intense problems with DDT.
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Unread postAuthor: DYI » Wed Jan 27, 2010 2:10 pm

Spudguns in general aren't very practical, and they're not supposed to be. Generally, they're for entertainment, not practical usefulness. Having operated, for a few months, a cannon of similar mass, size, and muzzle energy as this one, I can say that it won't be *easy* to use or set up. It will also be tremendously entertaining when it fires. Whether or not that is enough to make up for the difficulty of use is up the builder.

One does not require a vast swath of land to operate such a thing, especially considering that the useful range will only be a few hundred metres at the best. A change in elevation or a dirt berm is obviously required unless there is a vast swath of uninhabited land or water downrange.

200X will generate a bit over 20000 psi. Most rifle rounds are chiefly composed of copper and lead, neither of which are noted for phenomenal compressive strength, and they are routinely fired at pressures in excess of 60,000 psi. Even a low grade steel or aluminum alloy would be more than sufficient. Some composites would be suitable as well. The M829A2 round used a carbon fiber sabot, with a breech pressure in excess of 80,000psi. Even with basic manual machining capabilities, there will be a wide range of possibilities for projectiles.
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Unread postAuthor: POLAND_SPUD » Wed Jan 27, 2010 3:00 pm

@DYI
or a dirt berm
or a small shed, right ? :D

Even with basic manual machining capabilities, there will be a wide range of possibilities for projectiles
well yea but that's doesn't sound very practical, right ?

this thing is going to be expensive, loud and powerful so it's best to make him aware of all this... also this thing is going to attract a lot of attention if fired close to habituated areas
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