Breaking the Sound Barrier: I'm going for it

[starman]

- rather than just the rather "American" approach of bigger is better.

Well sure it is...what else is there..?.. It's kind of part of our DNA by now and we really can't help it...for better or worse I suppose.

[clide]

A 20 - 30' barrel has a considerable amount of air in front of the projectile, right? As it travels down the barrel, will there be a significant pressure buildup in front of the projectile? I say signifcant in that will it hurt performance?

I'm not positive, but I believe GGDT accounts for the air in front of the projectile in the model.

For a slight performance increase you could put a thin sheet of something over the muzzle and draw a vacuum in the barrel.

[potatoflinger]

Why not try to use some other type of material for the chambers, such as steel or some other type of metal. I have a feeling that to actually be able to break the sound barrier, you will need to use pressures of more than 200 PSI (possibly even more).

If I were you, I would try to make this launcher all metal, except for the barrel. I'm not trying to rain all over your parade, but it would be a shame for someone to spend so much money on a launcher, and then not achieve their goal. (Even though it still wouldn't be a waste of money, since this would still be a beast of a cannon even if it wasn't supersonic).

I say go for it.

[koolaidman]

maybe im missing something and this is a noob suggestion, but, why not make it burst disk. I have no idea where youd get a 4" union or disks, but still itd be alot more efficient and cheaper.

Edit: woot! post #69!

[willarddaniels]

maybe im missing something and this is a noob suggestion, but, why not make it burst disk. I have no idea where youd get a 4" union or disks, but still itd be alot more efficient and cheaper.

It is my understanding that with a pneumatic, there will be no practical way of bursting the disk at a particular moment. You will have to continually push more gas into the chamber until the psi exceeds the strength of the burst disk. Hence, there is no traditional trigger. Folks have used mortars with a nail in the end of the projectile to puncture the disk, but that would be impractical given his need to have a tightly sealed projectile.
4" union? justa bout any plumbing supply house (not hardware store) in the US has them or can get them in... I priced a sch 80 for $64 and then I got 2 on ebay for $20 each.

@ bigsause: you are talking some serious bucks for a pvc cannon. I would suggest either changing to metal and making a hybrid or build your own piston valves. The hybrid would probably be about the same cost but smaller and with more power. If you are set on the impressive look of your gun, build your own valves.

[Ragnarok]

It is my understanding that with a pneumatic, there will be no practical way of bursting the disk at a particular moment. You will have to continually push more gas into the chamber until the psi exceeds the strength of the burst disk. Hence, there is no traditional trigger.

More recent builds like DYI's SCTBDC (Slow Change, Triggered Burst Disc Cannon) do have triggers. Normally you have two sets of burst disks, with roughly half the intended chamber pressure between them, then you pressurize the chamber.

Letting out the air from between the pair of burst disks will trigger the cannon, by increasing the pressure differential across the first disk until it breaks, which will then allow the full pressure to reach the second disk to burst that one too.

[Brian the brain]

http://www.spudfiles.com/forums/overkil ... t8855.html

Read all about OVERKILL..

[D_Hall]

Letting out the air from between the pair of burst disks will trigger the cannon, by increasing the pressure differential across the first disk until it breaks, which will then allow the full pressure to reach the second disk to burst that one too.

Alternatively, you can impulsively ADD air to the space between the gaps. This way you're adding to the overall power when you trigger rather than subtract. A 12 gm CO2 cartridge should work very well as a trigger.

[paaiyan]

Letting out the air from between the pair of burst disks will trigger the cannon, by increasing the pressure differential across the first disk until it breaks, which will then allow the full pressure to reach the second disk to burst that one too.

Alternatively, you can impulsively ADD air to the space between the gaps. This way you're adding to the overall power when you trigger rather than subtract. A 12 gm CO2 cartridge should work very well as a trigger.

Now I like his idea, that's a good one. You'll just have to make sure that the inner seal breaks first, else the whole shot goes to heck.

[biggsauce]

Alright I do appreciate the feedback, I'm now realizing this is a much more daunting task than what I had originally imagined.

I was suspicious about the numbers ggdt gave me, in that this particular design couldn't be a very efficient design. With the type of valve I want to use, and the radial chamber configuration, there would be a lot of dead space. Now I see that at sub 200psi pressures, even with the lightest pistons, small pilot volume, a fast set of pilot valves, this has become a increasingly pricey, and somewhat unrealistic goal.

If it is common knowledge that the sound barrier can't be broken using air, then I have much learning to do. At first I thought this was a fairly reasonable goal, given the time and proper funding. (which is always the problem)

For the sound barrier, I'm now looking at a relatively small coaxial steel pneumatic. Something I could take to 300psi and above hopefully. Long barrel, light piston, and a quick pilot seem to be much more manageable for the mach 1 goal. Then theres the issue of a chronograph, which I may have found somebody that will let me borrow one, so we'll see.

I've never been a fan of a burst disk pneumatic, but since I've only played with one, that doesn't make me much of an expert does it?

As for the "undecided-name-cannon" it will still happen, just no hope of supersonic performance. A supersonic cannon on that scale, regardless of materials used, will be a sure way to drain my bank account and take far too much time away from more important things. This bad boy will still fire the coke bottle projectiles, and will still hopefully feature the trailer. Pressures will most likely top out at 120, 150 on a brave day. It will still be a fairly ridiculous endeavor, but will be FUN FUN FUN!

Thanks again for the positive encouragement and realistic advice. I look forward to showing yall these two significant cannons

[hyldgaard]

.........
For the sound barrier, I'm now looking at a relatively small coaxial steel pneumatic. Something I could take to 300psi and above hopefully. Long barrel, light piston, and a quick pilot seem to be much more manageable for the mach 1 goal. Then theres the issue of a chronograph, which I may have found somebody that will let me borrow one, so we'll see.
.......

I have just today finished a 6mm coaxial with a ½" x 100cm chamber and a piston weighing >1g. At 450psi i truly believe that the soundbarrier was broken, but i have no way to tell for sure as i have no chronograph. The muzzleblast was a whole lot louder than at just at bit lower pressure, so i might have something, who knows
Anyways, what i wanted so say was: small steel coaxial - yay

[jackssmirkingrevenge]

food for thought?

[Ragnarok]

Now I like his idea, that's a good one.

So do I... blast, why did I not think of that?

You'll just have to make sure that the inner seal breaks first, else the whole shot goes to heck.

That's not actually possible - there is no combination where you add pressure to that void space and have the internal disk burst first - at least, where the outer disk would still break.

[dongfang]

Hi biggsauce,

I have broken the sound barrier once, with a 15' barrel (46 mm ID) and a big plain-combustion chamber enriched with 40% oxygen. The projectile was very light.

Joel of Spudtech claims to have broken the sound barrier with a pneumatic. There is (was?) even a picture of the gun on his site.

But try it, try ... you can always start out with a lower design target, and then trying out different ideas for improving the gun. See which of them work, and which don't. You will need to be able to measure projectile velocity, of course.

Regards
Soren

[biggsauce]

Well Soren, I appreciate the encouragement. I do remember seeing Joel's cannon, and that was what sparked this whole idea of a large scale supersonic cannon.

That must be one hell of a combustion you got there! 8) Do you happen to have it posted here?

As for accuracy of my claims, I will be able to borrow a chrono, but it will only be for a day or two, so I'll have a few projectiles assembled for testing day.

Tanks again,

Kelly