New to the Forum, New Project

[charre]

Hi all, I'm breaking into the forums with a new projects over the course of the next few weeks. I'm designing a not-quite-mini combustion.

I'm thinking of an over and under 1.5" barrel with an d=3" by 8" combustion. However, Id like to shrink that even more. I'd like something around the size of the miniL1 built by the burnt latke guys, but I cant find any specifications for that one. So I guess the question is how small can I go in terms of combustion chambers before I really start to lose the bang. I mean in theory as long as you use a scalable projective a mini should fire at the same speed as any other gun (propane burns at the same rate no matter how much you have of it.) I'm looking for portability without losing the power and sound of the big boys.

What say ye?

[Labtecpower]

If you scale your gun down, it will be less powerfull.

Why?

With a scaled down gun, the barrel length & diameter will be less.
This means that the pressure in the barrel has less area to push on, and a shorter time to do so than with a longer barrel

This is why you will find your power to be less with a smaller gun, unless it is a hybrid.

However, it doesn't mean you can't have fun with it !

[saefroch]

I'm looking for portability without losing the power and sound of the big boys.

The only thing I can think of a massive cannon on a trailer. I think you'd need to re-design how you're going to make it portable, because scaling down will lose you energy output unless you re-engineer some aspect of the device.

So I guess the question is how small can I go in terms of combustion chambers before I really start to lose the bang.

Your question is highly ambiguous. Some people will be really impressed by a muzzle report that doesn't faze others. More's the point, the louder your cannon is, the less efficient it is.

So are you trying to make a noisemaker or a projectile launcher? Anything in the middle is a judgement call on your part, but it's certainly possible to just make something that goes "BANG" or shoots a projectile efficiently.

[charre]

ok by bang I mean power, but relative to its size. I feel that I should be able to launch a small projectile as fast as a larger gun launches a larger projectile. I'm not trying to "set the world on fire" with this gun. I want good power for its size.

To really lay out what I'm looking for:

How small can I go in terms of the diameter of the combustion chamber?

I am using HGDT as a design tool. Is that a good basis to design a gun?

If there are some good mini combustion guns, please leave a link I have been looking for some good examples

If someone knows more about the miniL1 on burnt latke's website, please let me know.

[saefroch]

I should be able to launch a small projectile as fast as a larger gun launches a larger projectile.

Still too subjective to be able to use as a design, but I do understand what you're trying to get at.

I think you might be able to use HDGT as a design tool, but keep in mind that it is designed to model burst-disc hybrids, not a valveless atmospheric combustion. A 1x hybrid is slightly more powerful than an atmospheric combustion, and especially so if you set the burst disc to rupture near peak chamber pressure.

You can get the combustion chamber pretty darn small. Like this one which I think is the smallest on the forum.

If you haven't already, spend some time looking through the combustion cannon showcase for ideas.

[charre]

That is small, but too small.

Looking to build basically this: http://www.burntlatke.com/mini.html
except in an over/under configuration with on board propane.

I just cant really tell what the dimensions are.

Edit: the article mentions that the chamber uses a cpu fan, which are typically 70mm. That fan is awful tight in the chamber so I'll assume that its a 3" diameter chamber.

Edit 2: Using a rough scale from one of the photographs I determined that the miniL1 is approximately 3" in diameter and 8.5" long so my initial approximations was very close.

[saefroch]

You could approximate the dimensions by comparing it to the propane bottle in the picture, or just contact Burnt Latke. There is an e-mail on the homepage, after all.

[charre]

Yes, but the page hasn't been active since '04. I doubt that those emails are used by them anymore.

[saefroch]

Did you try it?

[charre]

absolutely, I just don't have high hopes.

Also, HGDT is supposed to be able to model simple combustion guns as long as you set the burst disk pressure to 0 so that's helpful for seeing the potential of different barrels. I found a website that sells a pressure rated schedule 40 U bend pipe (the style that is usually waste or vent only) in 1.5" diameter so I'm looking into that for my barrel size. They have it in 1" also, but I had some ideas for making sabot's out of 2-part polyurethane foam and the 1.5" would be more room to play with.

[charre]

Ah another question:

When calculating the volume of the combustion chamber on an over/under configuration, do you include the 180 degree portion of the barrel (the length of the barrel between the chamber and the projectile.

[saefroch]

I'm beginning to regret posting on this thread.

[charre]

alright.

[Gaderelguitarist]

When calculating the volume of the combustion chamber on an over/under configuration, do you include the 180 degree portion of the barrel (the length of the barrel between the chamber and the projectile.

Technically, thats part of your chamber. And yes, you do factor it in.

Saefroch, cut the guy some slack, he at least has some basics down.

[charre]

Thank you that is very helpful. This project would be my first over/under so I was unsure of how to account for that portion of the gun.