Propylene Fueled Thermobaric Mass Accelerator

[SB15]

Evening all, I'm new around here, but not new to the world of pyrotechnics, kinetics, and other forms of energetic materials.

 

Over the past couple months, I've been working on a project that's loosely based on a traditional combustion spudgun design. The goal of this project was to create a large bore cannon that was capable of being safely shoulder fired, while producing significantly higher muzzle energy values than any normal spudgun could muster.

 

The operating principle is quite simple, though more similar to an ICE than what most people would envision a spudgun to be. It uses multiple atmospheres of stoichiometric air/fuel mixture to achieve much greater energy density, with a pressure differential triggered valve to seal the mixture in the chamber prior to ignition. Typically, it is fired at a 5x fuel charge/5 atmosphere air mixture (~62PSI gauge pressure prior to ignition), which produces a peak chamber pressure of around 600PSIG, propelling the projectile to a relatively high velocity.

 

The chamber is constructed from 2" nominal diameter SCH40 steel pipe and malleable iron fittings, which based on a formula derived from the distortion energy theory of pressure vessel failure, has a burst pressure well in excess of 3KPSI, giving me a safety factor of ~5, which is highly important for a handheld device. It has a volume of 85in³, as determined by fluid volume measurement. The valve housing is built from identical materials, and is designed to easily withstand the operating pressure.

 

The valve itself is a pressure differential triggered piston valve, which is actuated by the rise in chamber pressure accompanied by the combustion of the fuel mixture. It works by storing a pressurized volume of air directly behind the piston, which acts on the full cross sectional area (~3.4in³), and is sealed in by an o-ring around the circumference of the sliding valve component. The opposite end of the piston seats against a port which extends into the housing, and permits airflow into the barrel when the valve is open. A rubber face creates a seal between the piston and this port, and by varying the outer diameter of the seat, a surface area differential is created, allowing the net force to remain shifted in the forward (sealed) direction, even with a much greater pressure on the front (chamber) side of the valve. When the chamber reaches sufficient pressure to unbalance the force and shift it to the backward direction, the piston slides back in its housing, forming a flow passage into the barrel, while simultaneously opening a port in the rear air chamber, allowing the pressure in the back of the valve to drop to atmospheric. The precise travel distance is a calculated value, based on simple flow equations and area calculations.

 

The fuel/air meter is the backbone of this design, and controls several functions at once. The fuel metering component is a trapped volume design, used to isolate and inject a specific quantity of gaseous propylene. With a measured volume and variable pressure capability, it allows you to easily and precisely control the molar quantity of fuel being injected into the chamber. The calculation for this is largely based on the ideal gas law and Boyle's law.

In order to balance the excess of fuel in the chamber and supply a stoichiometric quantity of oxidizer to the reaction, air is pumped in through a port, where it flows through the fuel metering pipe, the turbulent flow assisting entropy in mixing the gases, while simultaneously filling a secondary tank on the side of the launcher. This secondary volume is then used to re-seat the piston after the shot, and fill the valve's air chamber to the correct pressure.

 

In order to vent the combustion products (Primarily CO² and H₂O) from the chamber after the shot, a brushless fan in the front barrel support is used to force air through the system volume, venting it in seconds.

 

The ignition and fan systems are operated using a control box on the side of the launcher, which houses the high voltage ignition circuit, power supplies for both the fan and ignition system, a power switch for the fan, an ignition safety switch, a momentary firing switch, 2 LED indicators, and the assorted resistors that go along with them.

 

Currently, the barrel is a 6' length of 1.5" nominal diameter SCH40 ABS pipe, which although not rated for high operating pressures, has passed extensive testing in similar test situations, and has thus been deemed suitable for this application. It is sleeved near the breech area, which is where the pressure is expected to be highest. Loading the projectile into this barrel is accomplished by means of aluminum cam-lever fittings.

 

Here are some pictures of the cannon in its current state. It is reasonably comfortable to shoulder fire, and capable of achieving nearly 4000ft-lbs of muzzle energy with a 100g projectile.

 

http://i859.photobucket.com/albums/ab160/Canadian_Pyro/IMG_2436.jpg

 

The integral valve component that makes it all tick:

 

http://i859.photobucket.com/albums/ab160/Canadian_Pyro/IMG_2433.jpg

 

Control box:

 

http://i859.photobucket.com/albums/ab160/Canadian_Pyro/IMG_2439.jpg

 

Here's a couple videos of the cannon (In its prototype stage) firing 75g marble buckshot rounds:

 

 

 

On a rather unrelated note, does anyone know how I would go about gaining access to the HE section?

 

EDIT: Severe Engrish

Edited August 28, 2010 by SB15

[inonickname]

Have a talk with Mumbles about getting into the HE forum spud. Mumbles is very reasonable. I know you know your stuff, so if you chat with him for a while you'll be right.

 

For anyone who's interested in this kind of cannon, have a look at www.spudfiles.com - this is where we normally reside.

 

Very nice cannon SB. For anyone who isn't well versed with stuff like this, here it is in laymans terms:

 

It's a normal spudgun, but you're adding more fuel and extra air. While these can achieve very high velocities, they won't match a powderburner volume for volume.

 

Me and spudblaster are also designing small bore high energy cannons with more energetic propellants. I synthed some NC for the low energy testing of mine today..

 

Welcome back SB

[Cookieman]

Wow! talk about taking it to the next level, with almost 4000ft-lbs of muzzle velocity I'm surprised the glass marbles survived coming out of the barrel. If it's shoulder fired how heavy is it and have you tested it with lead shots or chrome balls?

[SB15]

While these can achieve very high velocities, they won't match a powderburner volume for volume.

 

Yep, it's still a long way off from the energy density of solid fuels, but from a legality and cost-per-shot standpoint, gaseous fueled launchers are an appealing concept.

 

I synthed some NC for the low energy testing of mine today..

 

What you really mean is that you tried to throw a few nitro groups onto a random hunk of plastic.

 

...with almost 4000ft-lbs of muzzle velocity I'm surprised the glass marbles survived coming out of the barrel.

 

Although the muzzle energy is high, the average acceleration in the barrel is much less than what even the lowest pressure firearm cartridges can provide, so the marbles do survive if a good sabot is used. Muzzle energy is the integral of force over distance, so I use a long barrel (6') to achieve the high performance.

 

If it's shoulder fired how heavy is it...

 

Just over 30lbs. The weight is quite high, though manageable, and it really helps to cut down the recoil energy produced by heavy round.

 

...have you tested it with lead shots or chrome balls?

 

Yes, with steel ball bearings. They are much more effective at penetrating hard targets, but pretty expensive to use as shotgun type ammo.

[Ventsi]

WOW that is one heck of a spud gun! Really great job, you deserve a bag full of kudos.

 

I believe I am speaking for all of here when I say, you are more than encouraged to post more videos of this beast in action. Have you tried tougher targets with the marbles? Have you shot solid projectiles{one big slug}?

[inonickname]

"What you really mean is that you tried to throw a few nitro groups onto a random hunk of plastic."

 

I only did that once yesterday. Second time was a success

[Dr Boom]

SB,

 

That IS an impressive setup you have- but please keep in mind that when you change the projectiles to something that could be classed as "anti-personnel" you have a Destructive Device and that means you are playing in the BATF's backyard.

 

I have asked a couple field agents concerning spud cannons before and both of them agreed on potatoes being ok... something about deliverable energy and elasticity of a projectile. The caption is asking for trouble as it is not a shotgun nor contains no parts of such.

 

I'm not telling you to stop experimenting with your launcher- just be careful who knows you are shooting marbles from a spud gun.

[SB15]

Have you tried tougher targets with the marbles? Have you shot solid projectiles{one big slug}?

 

Yes, I have fired solid projectiles, though I haven't done much testing yet. More videos will come when I have time to head out into the middle of nowhere and do some shooting.

 

That IS an impressive setup you have- but please keep in mind that when you change the projectiles to something that could be classed as "anti-personnel" you have a Destructive Device and that means you are playing in the BATF's backyard.

 

As I do not live in the USA, BATFE regulations do not apply. The laws here are much more vague, and from various conversations with law enforcement officials regarding launchers such as this, I've gathered that this cannon is indeed legal to own and operate, so long as it is not used in a manner that could potentially cause harm to persons or property.

 

The caption is asking for trouble as it is not a shotgun nor contains no parts of such.

 

Agreed. I'll probably change it to "spudgun" instead, though the current title does a fair job of emphasizing what exactly is being fired.

 

I'm not telling you to stop experimenting with your launcher- just be careful who knows you are shooting marbles from a spud gun.

 

I'm also in agreement here. As with all hobbies related to energetic materials, keeping a low profile is key to avoiding unwanted confrontation with authorities. Not long ago, a friend of mine and fellow energetics hobbyist from across the country had his residence searched and all of his property confiscated, simply because too many people around him knew a little bit about what he was doing.

 

As far as anyone in my area knows, it's a spudgun.

[mike_au]

keep in mind that when you change the projectiles to something that could be classed as "anti-personnel" you have a Destructive Device and that means you are playing in the BATF's backyard.

 

Since the projectile is loaded separately to the propellant it would be considered an antique mechanism and therefore it isn't classed as a firearm by the BATFE. State laws may still consider it a firearm but it would be fine at a federal level.

 

(Obviously not an issue for SB15, but might be relevant if someone wants to make a copy)

[Dr Boom]

Yep, SB15 is good, just wanted to make it more clear..."careful who you tell..."

[asilentbob]

Cool overall!

Love the idea of doing significantly higher internal pressure than ambient, have always wondered about how often that was used.

 

Wondering with the pressure rating of the steel you found, is that a static pressure failure point? Or a shocked pressure failure point?

 

The thing looks very sturdy, don't get me wrong, just wondering if that factor of 5 is really so with a shocked pressure increase.

 

I guess an example would be like how a spherical glass ball could be pressurized slowly and hold quite a good pressure, but if it went from the low pressure to the high pressure in fractions of a second it would likely fail.

[SB15]

The calculation I used for that approximation is based on yield strength and hoop stress, which means it is optimized for static internal pressure. Modeling shock loading is much more complex, requiring the inclusion of such factors as the rate of pressure change, the material's ability to absorb energy (Toughness), the inertia of the pressure vessel walls, etc., and is not really relevant to this application.

 

Static loading is a better model here, because the rate of pressure change isn't all that high. My best estimates put the duration of combustion in the range of 20 - 40ms, depending on how quickly the flame front transitions to a turbulent flow. This is very slow by conventional explosives standards.

 

I don't think the hollow glass sphere analogy is applicable to the proposed scenario, because glass is a very brittle material with little toughness, and it responds very poorly to rapid changes in loading. Steel/malleable iron on the other hand is a ductile metal, and would be capable of absorbing large amounts of rapidly applied stress before undergoing tensile failure/plastic deformation.

[GalFisk]

Nice cannon! I'm happy to see another person using the same valve design as I did, most valved hybrids seem to be using a closed pilot chamber. Unfortunately my cannot has yet to be posted to Spudfiles in its entirety, because of fueling woes, and other interests taking precedence. I hope your cannon will lead to more people using this valve design, it's reliable and elegant (and the cannon can be easily used as a pneumatic, which is what I've been doing mostly).

[DYI]

It's nice to see "advanced" gas gun tech spreading out from Spudfiles, SB15. Once again, great job on the build. Your dedication to making the TBMA work just right probably rivals my current ETG project on sheer time investment.

You're surely one of the most important contributors to the field for inventing a handheld "spudgun" capable of equaling modern firearms in muzzle energy.

 

It doesn't look like you've mentioned the later developments in this thread, but would you mind commenting on your particular choice of buffer gas to oxy/fuel ratio in the finished product? Having fiddled with GasEq far more than is reasonable looking at the performance of various gas mixes, I'm left curious as to the rationale behind your 60:40 mix. Was it picked out of convenience due to the pressure capabilities of your buffer gas source (and thus the valve pilot pressure as well)? I ask because generally, if available pressure wasn't an issue, you'd be using more nitrogen in the mix, which would likely help the operational lifetime of the seals while increasing the average effective propelling pressure (says the fool who was using 50:50 30X mixes two years ago, and has the impressively ablated butyl rubber to show for it...).

 

This, of course, raises a second issue; how far can we take this particular design? The seals are not positioned nearly so favourably in the firing process as those in a traditional rupture diaphragm valve. Do you have any opinion on what mixes this concept might be capable of, barring the use of unreasonably advanced manufacturing processes that most hobbyists don't have access to, or can't afford? Sure, Utron's CLGGs may use over 1000X, but their breech mechanism looks a lot more like a rupture diaphragm held closed with hydraulic/pneumatic rams where the projectile incorporates the disk and seals.

[psyco_1322]

Thats an awesome piece of work there. It looks like it would mount in the should well, but it looks like it would be awfully heavy.

 

What is that line that runs to the back of the upper chamber for? It looks like it might be for using stored air to clean out the exhaust gases after a shot.

 

Also, does it really use propylene as a fuel?