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50KJ and up...

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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50KJ and up...

Unread postAuthor: mobile chernobyl » Tue Nov 08, 2011 12:26 pm

I've searched a bit and have found a few, but I'd like to get a general idea of how many people have played around with creations capable of putting out 50KJ muzzle energy and up. Besides VERA and DYI's creation stolen by the OPPosition, has anyone else really "dabbled" in this class of launcher?

Basically I'd like to know whether the launchers in this class primarily go for added mass, or extreme velocities to achieve such high energy output.

I've got designs laid out for both avenues, but would like to have a thread devoted to the "upper up" of the muzzle energy's.
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Re: 50KJ and up...

Unread postAuthor: jackssmirkingrevenge » Tue Nov 08, 2011 12:45 pm

mobile chernobyl wrote:Basically I'd like to know whether the launchers in this class primarily go for added mass, or extreme velocities to achieve such high energy output


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Double the mass, double the energy.

Double the velocity, quadruple the energy.

Higher velocity also has the advantage of flatter trajectory and better penetration.
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Unread postAuthor: mobile chernobyl » Tue Nov 08, 2011 12:48 pm

Thank you JSR. Unfortunately I already know that all too well. :lol:

I'm looking for examples and thoughts on designing the very large monsters and any unique problems people have run into on such a large scale. I will eventually find out myself, but some personal experience or critiques by those who have ventured into this realm are very welcome!
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Unread postAuthor: paaiyan » Tue Nov 08, 2011 2:11 pm

Well, I'm mostly echoing him, but FWIW if you half the mass of your projectile for a relatively modest increase in velocity, you still get higher kinetic energy. For example: 10 kilos at 10m/s is 500 Joules. 5 kilos at 15m/s is 562ish Joules. Half the mass, 50% more velocity, ~12.5% more energy.

So, if you want to push a projectile the same size faster, you might look at what's exiting your cannon, just as much as you look at the cannon itself. Boattail projectiles are your friends.

What kind of "monster" are you planning on building?

EDIT: And what kind of projectile? You're going to have to push a 1 kilo projectile at almost the speed of sound for 50KJ
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Unread postAuthor: DYI » Tue Nov 08, 2011 2:19 pm

Killjoy's FEAR is readily capable of >50kJ, and certainly the most advanced launcher in that class. I also must point out that the pumpkin cannons are all capable of this (they are, of course, horribly backward in design, and likely not at all what you're looking for).

FEAR is big on the "added mass" side of things. Killjoy was talking about 50lb projectiles at one point (VERA is also, of course, for low speed shooting). Recoil was also a major consideration in this case.

Looking at dollars per joule, I think you'll find that after a certain point (which is based on your propulsion method), heavier will be cheaper. The only real limitations are, of course, heat loss into the projectile and barrel walls caused by longer transit times for extremely heavy rounds. It will, of course, be higher for hybrids than for pneumatics, and higher for high pressure guns which can accomplish heat transfer faster.

But your probably knew that too, didn't you?

SB15 and I were looking at a hypothetical 10MJ hybrid at one point, and found that anything but relatively low (<100X) mixes would be prohibitively expensive, mostly due to the lack of available pipe and fittings - relying on custom parts in very large sizes is not advisable if you have no goal other than high energy. It is also, of course, a good choice to rely solely on liquefied gases, or what can be provided by a compressor. Owing to excessive fueling times for anything but very low pressures with available and affordable compressor tech., liquid oxygen would be the most affordable oxidiser for the purpose. Simply using the fuel gas for a buffer would be another cost-reducing measure. Fuel cost could be brought well below projectile cost in this way.
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Unread postAuthor: mobile chernobyl » Tue Nov 08, 2011 2:42 pm

OK, so now that the kinetic energy has been explained.... :o

I suppose I should have thought this thread out a little bit better before I posted it - Where it's going and where I wanted it to go in my head are not very close.

Topics I was interested in discussing were more likely suitable for the hybrid thread (if this can be moved, I would appreciate that! sorry for the brain fart).

So for starters - let's deal with the extreme ends to accomplish 50KJ and up. I'm not interested in the projectile so much as the means to accomplish the acceleration of it. We can assume that a larger lower pressure cannon will use a heavy projectile, and a small higher pressure design will use a lighter weight projectile. Again, this is dealing with hybrid style combustion gas gun design.

For the low volume high initial pressure designs:
Items such as fueling for higher mixes where we should no longer assume "ideal gas" conditions exist. Breakdown voltage (Paschen's law, and others to consider...) for ignition of such higher mixes. Chamber considerations in geometry and material for higher pressures. Unique designs to supplement or replace the standard burst disc for higher pressures. And on...

For the large volume low initial pressure designs:

Items again such as fueling - static mixing vs active of the fuel air mixture. Ignition point locations, and specialized timing vs synchronous ignition. Chamber geometry - diameter vs. length for achieving volume (with consideration given to cost relationship of diameter vs length in materials). Breach design - Burst disc, Shear, or piston design...

So there are a few things to consider. At this point I plan on eventually constructing both designs, but I would like just to hear anyone's creative input on these concepts.

I use the LVHP vs HVLP comparison because I don't think a compromise of both would be financially feasible for most people. A LVHP design allows you to use easily attainable round stock, and dom tubing, and concentrate on a realistic sized creation using reasonably sized machinery. A HVLP design allows you to use easily attainable sch40 or 80 steel piping, and fittings if need be for chamber and barrel.

A "MVMP" design would require a relatively heavy walled, relatively large diameter chamber utilizing raw material that would not be easily attainable by most people, and the barrel would be a challenge in it's own right as well.

Sorry if this is more of a rambling's and thoughts thread...

EDIT - Thanks DYI, we seemed to of posted at the same time, but your thinking along the same lines as what I was!
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Unread postAuthor: RJB INDUSTRIES » Tue Nov 08, 2011 4:16 pm

Hi.
Post here some quimical or physics equations to calculate the energy we apply on the projectile.
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Unread postAuthor: D_Hall » Tue Nov 08, 2011 10:06 pm

Based on my experiences with VERA...

Items again such as fueling - static mixing vs active of the fuel air mixture.
Definitely go active. Not so much because fuel may or may not get mixed well. Honestly, that's a variable I never studied as VERA's fuel injection scheme forces significant circulation due to fuel injection; there's no way around it given her design.

So why do I advocate active? For one of the same reasons the advanced combustion folks do: Artificially induced turbulence does wonderful things for your combustion process. When I fired VERA with her mixing fans turned off, it took 250 ms for the pressure to build up enough to burst the disk. With the fans turned on? 90 ms. 'Nuff said.

Ignition point locations
I stuck with 1 ignition point for roughly every diameter of chamber. I did so to minimize the time it took for the flame to propagate and I feel it works well for speeding things up AND avoiding DDT.

and specialized timing vs synchronous ignition.
This is another one that I didn't experiment with. I did spend a lot of time thinking about it and I just couldn't think of a reason why I wouldn't want it all to burn as fast as possible. ASAP means synchronous so that's what I did.

Chamber geometry - diameter vs. length for achieving volume (with consideration given to cost relationship of diameter vs length in materials).
Every person is going to have to make that decision on his own. I used 30" pipe but I had access to 20, 24, and 48 inch pipe. The 30" just seemed like a good balance between volume, overall system design, and buildability (yes, I just made that word up).

Breach design - Burst disc, Shear, or piston design...
For a gun on the scale of VERA, pistons are right out.

Burst vs. shear?

I wanted to go shear. I really did. But the more I thought about it the more I realized that going shear meant more complicated debugging in the event of an in-barrel failure. Simple burst disks allow for a clean separation of phenomena and that is gold when it comes to debugging. Based on my observations, I believe it was a good call. There were some definite lessons for me to learn on projectile design and without the separation of disk and projectile, I'm not sure the lessons would have been as clear as they were.
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Unread postAuthor: Fnord » Tue Nov 08, 2011 11:37 pm

The most realistic way I can think of to get super high energies on a realistic budget would be to use a mid-range mix with a mid-volume tank. 300 psi is probably the practical limit to high-volume compressors(The punkin chunkin crowd uses stuff in this range). For a tank I'd have to say a pre-built compressor tank surrounded by a concrete/rebar composite layer would be the only low-budget option.

-The barrel will be expensive. There's really no way around this one.

-A flange type burst disk holder is pretty much the only option here.

-If ignition voltages present a problem then I'd go with an 'exploding wire' running the length of the chamber. It would be neither expensive nor hard to replace (when you can reach right into the chamber it kinda simplifies things).

-Mixing... I think D_hall made the point well enough.
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Unread postAuthor: mobile chernobyl » Wed Nov 09, 2011 12:40 am

Fnord - Interesting idea with the concrete reinforced tank - I know I've heard this idea batted around before, but has anyone actually gone to such extents to make it a reality?

An extremely cheap variant isn't really what I'm looking for - I'm aiming for 6" Sch80 steel "tanks" and a 3" sch40 steel barrel - for the HVLP variant.

All said and done I'd like to hit appx 100KJ with a 1Kg projectile, therefore a speed of appx 447.2m/s is needed... again for the HVLP variant

I'll try and post a rendering of what I'm thinking of later in the week.

I'd really prefer to use a static mixing design, but as Dave has mentioned there are outright benefits to turbulence in the mixture during deflagration.

Ignition and static mixing (gas injection) element will likely be a coaxial combined design pulling from my current hybrid's success's and failures.

The appeal of a low mix large volume hybrid right now is that I can simply design it to fit on a trailer with a 20lb LP tank for fuel and on-board compressor stolen from an existing one - controlled by a digital pressure switch for oxidizer.
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Unread postAuthor: Technician1002 » Wed Nov 09, 2011 12:41 am

-If ignition voltages present a problem then I'd go with an 'exploding wire' running the length of the chamber. It would be neither expensive nor hard to replace (when you can reach right into the chamber it kinda simplifies things).


By limiting your Joule energy and using a high voltage for high current, the exploding wire can be kept well below the melting point and won't need changed each shot. Use a wire with high melting temp such as Nichrome, Tungsten, etc instead of copper, iron, or aluminum.

The wire can be flashed to incandecense for ignition. Use a capacitor discharge ignition source to light the wire. Use high voltage and limit the energy by capacitor bank size. Secure all primary wires to prevent wire whip.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 09, 2011 12:45 am

Going back to this Paris Gun article, the relevant wisdom is in the last line:

Simply, von Eberhard speculated that if the unmodified L52.5 35.5cm gun, which fired a conventional shell of 535kgs, were to use a 90kg 21cm shell slotted into a 120kg 35.5cm calibre short cylindrical ‘carrier’, the combination weighing 210kg, a muzzle velocity of 1500m/s was easily attainable. As the assembly left the muzzle of the barrel, aerodynamic drag on the flat-fronted cylinder would rapidly decelerate it, causing drag separation of the slender 21cm shell and allowing it to continue at the same velocity and reach 100km range.

Von Eberhard had essentially invented the sub-calibre discarding-sabot shell. Rausenberger’s objections to it appear odd, in retrospect. First, he decried the light weight of the shell, only 90kgs. Yet the Paris Gun as built fired shells weighing, on average, only around 100kgs. Further, because von Eberhard’s solution offered a gun firing the sabotted ammunition at normal barrel pressures, his 90kg shell could have relatively thin walls and contain around 18kgs of explosive, as opposed to the Paris Guns’ projectiles with their thick walls (to resist the enormous barrel pressures) and consequent light explosive payload of only around 7kgs. Even better, with the guns operating at normal pressures, barrel lives of 800 rounds could have been confidently expected, as opposed to the Paris Guns’ ephemeral barrel lives of around 65 rounds.
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Unread postAuthor: mobile chernobyl » Wed Nov 09, 2011 12:47 am

Technician1002 wrote:
-If ignition voltages present a problem then I'd go with an 'exploding wire' running the length of the chamber. It would be neither expensive nor hard to replace (when you can reach right into the chamber it kinda simplifies things).


By limiting your Joule energy and using a high voltage for high current, the exploding wire can be kept well below the melting point and won't need changed each shot. Use a wire with high melting temp such as Nichrome, Tungsten, etc instead of copper, iron, or aluminum.

The wire can be flashed to incandecense for ignition. Use a capacitor discharge ignition source to light the wire. Use high voltage and limit the energy by capacitor bank size. Secure all primary wires to prevent wire whip.


This is all and well, but I prefer a non-consumable ignition source. Call me crazy since just about everything else on the gun is consumable in one way or another lol.

Haven't we proved that at higher mixes this hot wire ignition is almost always jeopardized? I forget the thread but I thought someone had tested this. The momentum of the gasses leaving the chamber can be expected to destroy the thin wire I would think.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 09, 2011 1:26 am

By using relatively large proportions, with a big bore for saboted projectiles and as long a barrel as you can manage, there's no need for silly pressures.
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Unread postAuthor: D_Hall » Wed Nov 09, 2011 11:41 am

mobile chernobyl wrote:I'd really prefer to use a static mixing design, but as Dave has mentioned there are outright benefits to turbulence in the mixture during deflagration.

Keep in mind I was strictly discussing HVLP. For an HPLV design, I suspect it's not nearly as important.
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