Self contained 40mm propane grenade

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
Wyrlok
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Thu Nov 14, 2019 5:57 pm

Hi all :)

I plan on making propane combustion 40mm grenades. As a first step I want to make the a simple M79 style launcher, breach action and all, to focus on the shell itself. I'll later come to blowback if possible but I want to start easier.

I've read a lot of topics on this forum, and came across this particular one.
The project is awesome but it apparently lacks power.

I designed my idea, it's very primitive. I thought about a normal version, and a hollow projectile to increase a bit the combustion chamber, not sure if it would improve performance or not.
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Image

the ignition mechanism will be a piezo, connected through the butt of the grenade, via some metal bits in the back of the barrel, in the part not moving when breached. I used a simple circular pattern to allow one to place the grenade in any position. It could be spring-loaded to ensure it touches the contactor at the back of the grenade when the barrel is closed, but it will be loose of only ~2mm, more of a security than a real spring mechanism.

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The goal is to keep operation simple so i like the idea of just using propane, with proper quantities via syringe injection as i saw on this forum. I only will shift to hydrogen or other combustibles as a last resort.

So that's for the ideas i had before browsing this forum. The problem is that from what i've read, my idea would certainly lack power. I'd appreciate if someone had any advice or experience :bounce: the shell dimensions are 40mm diameter and about 46mm length to match 40x46mm, if it's feasible. The projectile size/weight/shape are not determined.
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jackssmirkingrevenge
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Fri Nov 15, 2019 8:08 am

Welcome to the forum!
I designed my idea, it's very primitive. I thought about a normal version, and a hollow projectile to increase a bit the combustion chamber, not sure if it would improve performance or not.
The issue with an atmospheric pressure combustion is that peak pressure is relatively low, so even if you increase the chamber volume, power is not going to be that impressive. It really depends on what you want to achieve.

If you want to increase pre-ignition pressure, then you're going to have to find a way to contain it before firing, and that's where things get as little tricky.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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hectmarr
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Fri Nov 15, 2019 8:20 am

Hello! Welcome !
Interesting what you are trying to do. It is something different from the common and threshing combustion launchers that are seen everywhere. Do you plan to include the stoichiometric mixture of butane and air inside the grenade?
If you can use ammunition as a compression piston, you can compress the mixture at about 3X before firing. I mean charging at atmospheric pressure and then, once inside the weapon, bringing the ammo to the side of the ignition, decreasing the volume of the cartridge to 1/3 of its original volume, and locking there with some system, which retains the ammunition . When exploding, the blockage should yield to pressure and expel the ammunition. The locking system can be simple, a 1 "steel ball, which is partially inserted into the firing barrel, and is compressed by an outer spring, which you can vary the force. To compress everything, you could use a multiplier lever of force, which the weapon should have. For your dimensions and to compress 3X you should apply a force of approximately 38 kg. With an appropriate lever, you could reduce the force you should do to no more than about 10 kg. It is only A thought out loud what I am telling you. :roll:
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jackssmirkingrevenge
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Fri Nov 15, 2019 9:27 am

Here's how I would make a hybrid cartridge using the epoxy casting construction method, assuming you do not have access to anything more complex than a drill and hacksaw:

Image

It's doable, but naturally more complex to build and also to fill as well. You have to inject the fuel, place the burst disk then pressurize the air to the desired level.

The advantage though is that using reasonable mixes, you can achieve energy levels literally 5-10 times greater.
hectmarr wrote:You have to make many weapons, because this field is long and short life
Wyrlok
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Fri Nov 15, 2019 11:01 am

jackssmirkingrevenge wrote:Welcome to the forum!
Thanks :D
jackssmirkingrevenge wrote:The issue with an atmospheric pressure combustion is that peak pressure is relatively low, so even if you increase the chamber volume, power is not going to be that impressive. It really depends on what you want to achieve.
Honestly I don't really know what i want to achieve, but like many members of this forum i love explosions, the "thump" sound and things that fly fast through the air :bom: maybe a 15 meters range, firing from a 45 degree angle should be nice (the more the better, but i don't even know if 15m is possible). So with what you said i can right now forget the simple not-pressurized air+propane combustion, it would be too weak to propell a little projectile with a short 40mm diameter combustion chamber (35mm though, cause 40mm is the ext diameter but nvm).
jackssmirkingrevenge wrote:If you want to increase pre-ignition pressure, then you're going to have to find a way to contain it before firing, and that's where things get as little tricky.
This seems like a good solution but as you said, somehow tricky. The only viable solution that comes to my mind, if I stick to 46x40mm shells, would be to add a valve to the shell. That makes the refill of the shells a little more complicated really but more powerful. I'll list all possibilities and weigh the pros and cons of each one after reading all your kind replies :)
Wyrlok
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Fri Nov 15, 2019 11:16 am

hectmarr wrote:Hello! Welcome !
Thank you :D
hectmarr wrote: Interesting what you are trying to do. It is something different from the common and threshing combustion launchers that are seen everywhere.
Thanks, I really like the idea of shells but I only saw a few projects using it.
hectmarr wrote: Do you plan to include the stoichiometric mixture of butane and air inside the grenade?
Yes, I thought about using a syringe to simply drop the proper quantity of butane according to the shell volume. However I'll certainly have to change either my combustible or pressure. In both cases i'll try to include the stoichiometric mixture of whatever fuel I'll use, and take in account the pressure.
hectmarr wrote:If you can use ammunition as a compression piston, you can compress the mixture at about 3X before firing. I mean charging at atmospheric pressure and then, once inside the weapon, bringing the ammo to the side of the ignition, decreasing the volume of the cartridge to 1/3 of its original volume, and locking there with some system, which retains the ammunition . When exploding, the blockage should yield to pressure and expel the ammunition. The locking system can be simple, a 1 "steel ball, which is partially inserted into the firing barrel, and is compressed by an outer spring, which you can vary the force. To compress everything, you could use a multiplier lever of force, which the weapon should have. For your dimensions and to compress 3X you should apply a force of approximately 38 kg. With an appropriate lever, you could reduce the force you should do to no more than about 10 kg. It is only A thought out loud what I am telling you. :roll:
Your idea of using the ammo to compress air sounds good :o and the lever mechanism shouldn't be too complicated to incorporate in a breach action gun. However I wonder if the potato (or more likely a 3d printed bullet) wouldn't need to be somehow heavy to be strong enough not to crack under pressure ? Maybe not, 3x air pressure doesn't seem to be really high.
Wyrlok
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Fri Nov 15, 2019 11:44 am

jackssmirkingrevenge wrote:Here's how I would make a hybrid cartridge using the epoxy casting construction method, assuming you do not have access to anything more complex than a drill and hacksaw:

Image

It's doable, but naturally more complex to build and also to fill as well. You have to inject the fuel, place the burst disk then pressurize the air to the desired level.

The advantage though is that using reasonable mixes, you can achieve energy levels literally 5-10 times greater.
Very nice design, and the epoxy is a good idea to insulate the shell. What would you use as a burst disk ? I guess it has to be strong enough to contain pressure but weak to break when it explodes.
What a coincidence I have a set of 10 schrader valves coming from china in a week or 2. I didn't need them but I knew it could be useful some day. Wouldn't epoxy "unglue" from the shell in case of explosion ?
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Fri Nov 15, 2019 12:29 pm

Ok so we now have several options:

normal pressure butane or propane : not so much power so it's a no from me
normal pressure air without explosion : needs more pressure for the same power, not viable

Hectmarr's pressurized air/butane or propane, compressed when inserting the projectile in the shell/in the barrel
- retention mechanism may be a little difficult to make
+ is relatively easy to reload
Jackssmirkingrevenge's pressurized air/butane or propane, with valve to compress air (and gas that is already inside)
- shells may be a little difficult to make
- seems long to reload
+ seems somehow more reliable than the previous solution
i'll add normal-pressure hydrogen to the list
- need to produce hydrogen to fill the shells
- i could die
+ simple mechanism makes it easy to reload and reliable (except if the tube explodes)

Thank you for you suggestions, I'll try to see how I could make them to see which one is better/possible :wink:
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Fri Nov 15, 2019 3:58 pm

Good design what you propose. I imagine that the larger size of the cartridge is an advantage.
A simpler way, although not as elegant as the one you propose, is to add an o'ring to the ammunition to seal it in the cartridge. At atmospheric pressure the cartridge with the ammunition is long. Insert the already loaded cartridge from the rear, it moves forward until it is retained by a large o'ring, which is housed in the launch tube. It is necessary to close by the rear part to thread, to compress everything. The thread should test a good multiplication of the force one makes.
In fact, the only "delicate" job you have is to give the approximate adjustment so that the retention is adequate. Depending on how many turns you turn to the threaded back cover, it is the pressure that triggers. The back cover does not differ much from the combustion guns that are loaded by unscrewing the back cover, spraying alcohol and then closing again. :bom:
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Fri Nov 15, 2019 5:44 pm

Very interesting concept Hector, I like it!

I wonder if it would work like this:

Image

The o-ring on the projectile compresses the atmospheric mix, and then is forced into a cartridge that has a narrower diameter and then doubles as a friction seal.

That being said, the path of the "valveless" hybrid cartridge is a long and frustrating one, I should know :roll:
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Fri Nov 15, 2019 6:09 pm

I imagine that if the force of the oring against the adjustment tube is adequate, it should work correctly. What I see dangerous is the method to compress the "piston-ammunition" from the launch tube. Having my hands close, and the whole body while you are compressing, to remove the rod that enters the launch tube, from the left, which would be necessary to compress, is a bit dangerous, I do not see another way to carry the piston-ammunition to the right. Probably a person of 75 kg like me, should use the weight of the body to achieve it, I do not know what you think of this aspect. Otherwise it would work well :roll:
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Fri Nov 15, 2019 6:28 pm

I'm not sure I would trust it! That's the thing with friction seals...

However, you can imagine holding the projectile in place in another way, like for example a shearable screw or rod. The issue with all these methods however is that the projectile needs to be a specific design and you lose the "shoot whatever fits in the barrel" aspect that is what makes spud guns more fun for certain things compared to airguns and firearms.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Fri Nov 15, 2019 6:43 pm

It is very difficult to know if a friction seal can retain the projectile, or not. The only way is to perform some tests because if I am absolutely certain of something, it is that the mechanical, physical, chemical reality etc, teaches us something in each test. Apparently, it costs a lot of work and a little sweat, to acquire "real" knowledge.
On the subject of retaining the projectile safely, so that it does not fire unexpectedly, I think that a steel ball that looms in the lamella tube, with a variable force to regulate the system, should work. Any projectile that enters reasonably tight in the barrel and is solid can be fired. This is what I am thinking of doing to retain the ammunition and achieve maximum peak pressure in my next hybrids.
The concept is approximately like the drawing, which is not to scale, it is only to make it clearer. The first straight screw, where it is threaded, and then internal conical, (black), is the one that moves by adjusting the ball bearings more or less. With the angle of the cone, the displacement of the screw is related depending on how much the three springs are compressed, which tighten the three balls.
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Sat Nov 16, 2019 7:17 am

An adjustable detent is definitely the best way to get the burst disk effect without the burst disk, however for cartridges it is a little complex as a mechanism for cartridges.

I had some success with a lead projectile simply jammed into the mouth of the cartridge here, perhaps that is worth revisiting at some point.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Sat Nov 16, 2019 8:27 am

Hi Jack, good morning!
Oh yeah! I have seen that test and it seems a success, no doubt. Missing, perhaps, optimize the design in general and make about five cartridges, to shoot them from a launcher. On the other hand, it has an apparatus to electrocute the target if the ammunition fails !! :)
"...sin embargo, para los cartuchos es un poco complejo como mecanismo para los cartuchos.
Yes I agree. This is for a hybrid weapon without cartridges, I bring it here to show my thoughts, but it would ruin a cartridge that should be first and foremost simple.
The system of tightening an o'ring to close the center, (with the condition that it cannot expand outwards), is a good method to regulate the retention force. I am using this and you can regulate fine retention. Remember?
Very simple, reliable system, but ... You have to change the o'ring periodically because it deteriorates with the gases at high temperatures that have contact with it in each shot. I calculate that 15 or 20 shots and must be replaced. In mechanics miracles do not exist jajajajaja!! :)
 
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