Yet Another Vacuum Cannon and materials selection

[nik282000]

Hi guys, I am on the quest to put a Ping Pong ball past mach 2 and this forum has a lot of great info.

To start, I am already getting speeds right around 500m/s which is satisfying/terrifying from little celluloid ball. The first GIF below was filmed at 1200fps with a 1/2000s shutter speed and only shows the ball for 2 damned frames!

I was surprised to see "cartoon physics" as the back of the can exploded like a banana peel.


I layout is nothing surprising, I have a 10'x1-1/2" ABS barrel with a 2'x4" vacuum accumulator on muzzle end. For the pressure chamber I erroneously chose ABS having read that PVC shatters and that DWV ABS is good up to 150psi, I've read through a number of threads and will be switching to PVC before I fire again, thanks guys. The front burst disk is made of space blanket and the rear disk is mylar strengthened with "tuck tape" and with a small wire between the layers that is heated for triggering.


I have 2 questions of any one who has the time, is there an ideal ratio for barrel volume to air reservoir volume? And the same goes for the vacuum accumulator. I plan on turning my (old) pressure chamber into a new, larger, vacuum reservoir because I am still seeing the muzzle membrane inflate just before the ball exits.

Thanks and I look forward to a long career in the recreational abuse of plumbing.

[Ragnarok]

I have 2 questions of any one who has the time, is there an ideal ratio for barrel volume to air reservoir volume? And the same goes for the vacuum accumulator.

It depends how you define ideal, but the likely answer is no.

Two possible ideals are:
- A chamber volume where acceleration falls to zero at the muzzle, meaning all energy is extracted from the air.
- A chamber volume with the greatest possible velocity at the muzzle.

In the first case, such ratios exist (although depend on specific pressure and flow, meaning they're difficult to calculate), but are not usually particularly productive. Most of the barrel will have a very low acceleration, meaning that similar results could be achieved out of a barrel half the length.

In the second case, it's diminishing returns. Increasing chamber size will increase velocity (as the pressure will stay higher for longer), but with no limit. That said, in practice, the velocity usually plateaus fairly fast after a C:B ratio of about 2:1 (and even less for valves with poor flow).

a 2'x4" vacuum accumulator on muzzle end.

I was always disappointed that the Mythbusters missed this trick with their ping-pong cannon, but then their air cannons were never the most sophisticated.

(The other thing I wanted to see them do was use a lighter-than-air propellant, such as hydrogen or helium. With their kind of resources, their velocities of ~500 m/s were really quite lacklustre. With even half of their kind of budget, I'm sure some of our members could have pushed that past 1000 m/s, possibly even twice that).

[nik282000]

I finally got my hands on pressure rated pvc pipe and fittings. Apparently it's a hard to find item in Canada, the 4 inch pipe was $10 a foot and had to be special ordered.

I went with a 3:1 chamber to barrel volume ratio and nearly 1:1 pressure chamber to vacuum reservoir. Unfortunately the centering device I used to keep the barrel coaxial inside the vacuum reservoir was dislodged for my last firing so the ball came out as a shattered mass but it still managed to cave in the side of a steel mini-keg. Having re-centered the barrel I need to drain a new mini-keg and try again.

In the mean time I built a simple hot wire trigger to melt the mylar and Tuck Tape membrane that I use for a burst disk. Oddly the wire doesn't break and can be re-used 3-4 times despite being subject to supersonic winds.

https://www.youtube.com/watch?v=LoGrVxyyQfU

This spring I'll be receiving a 20k frame per second camera which should let me get some really wild footage.

[Anatine Duo]

Looking forward to seeing more!

BTW I wonder about the front disk bulging before launch... is this an indication of blow-by?

[nik282000]

Good eye, that is from blow-by, I didn't get a hard enough vacuum. I have been working on repeatability but a couple cold days have mean that I can't get much faster than 400m/s.

I did see something wild this week though, the energy dissipated during impact is enough to make either ball or contamination on the ball burn! I didn't get a full streak in this footage so I can only calculate a minimum speed but it was still going at least 440m/s.

http://i.imgur.com/BwWbRC1.jpg

[Anatine Duo]

I was thinking a better ball to barrel fit rather than more vacuum, not sure what that involves though.

[nik282000]

I'm using 38mm balls and 1-1/2 pipe is a pretty close fit. I could try some wadding but I don't know if the added friction would counteract the better seal. Once my bag of 50 balls and mylar sheet shows up from ebay firing cost will drop off significantly, at the moment it's ~70cents a shot with consumables.

[nik282000]

I was thinking a better ball to barrel fit rather than more vacuum, not sure what that involves though.

Four months later and I have an answer for you! There was a lot of blow-by and I'm not sure if it can be eliminated but I found out that it's effects can be almost completely canceled out. My first barrel had a very small vacuum reservoir, only about 2' of 4" pipe sleeved around the 1-1/2". My new version had about 4' of 4" pipe acting as a vacuum reservoir and I no longer see any bulge before the ball hits the mylar. I even managed to catch the breaching event at 37Kfps and it can be seen that when the ball hits the mylar the pressure inside is below 1 atmosphere.

There is still air getting by the ball but it looks like the extra volume gives it somewhere to go instead of getting stuck between the ball and the mylar. Keep in mind that there was no high pressure air used for this video, just atmospheric pressure to push the ball, but even after the ball is several cm out of the barrel air continues to rush INTO the now open muzzle. The extra vacuum volume also makes the whole thing slightly quieter as well, which is nice for a garage-use-only device.

Animated gif linked for hugeness: http://i.imgur.com/1UU4XD3.gifv

[jackssmirkingrevenge]

I had missed this, very nice work! Surprised that you're only using relatively low pressure, surely metal fittings and 850 psi from a paintball bottle would push the performance up significantly.

[nik282000]

Compared to most of the pneumatic cannons here I am using no pressure at all, to get a repeatable 450m/s I am only using 80psi in my pressure chamber, any more than that and I have found that the balls shatter before making it out of the barrel. Given the barrel is only 11" long the ball is feeling more than 3000Gs of acceleration so I may be nearing the physical limit for those balls. But, I can always make the vacuum section longer with rubber couplings and more pipe as long as they are lined up really well.

The world record shot (over 800m/s) was done on a 20" barrel as far as I know but he was also using 80-90psi of helium for his propellant because it can expand quite a bit faster.

[jackssmirkingrevenge]

Perhaps - if hypervelocity is the goal - it would be a good idea to switch to an airsoft barrel, you can get lightweight highly polished projectiles that won't crush for very cheap and a smaller diameter means exponentially less stress on the system for a given pressure allowing you to up the parameters without needing hardcore materials and fittings beyond what is used in paintball.

For more destructive impacts with denser projectiles you will also find spheres in silica, aluminum and steel in 6mm.

Also, posting this paddle shot here because I like it so much

[nik282000]

Do not aim at eyes or face.

I have always been curious about how strong those full face shields really are, after today I have decided that they should be re-labeled as splash guards. The face shield was made of polycarbonate and was about 1/8" in thickness, I has -27in/Hg of vacuum in the barrel and 80psi in the reservoir, standard 38mm ping pong ball.

Gif linked due to bigness: http://i.imgur.com/8nSCG31.gifv

[jackssmirkingrevenge]

Impressive. A reminder if one was needed that when it comes to penetration, velocity is king.

There is a rather lovely study done by a W.H. Noble of the Ordnance Select Committee in 1863 (referenced here but sadly it's poorly scanned text with no illustrations and jumbled up tables) which is quoted in Ian Hogg's "A history of artillery":

A 68 pounder smoothbore and a 7 inch Armstrong gun firing 200 lb shot had been fired at 4.5 inch plate backed by 18 inches of teak. The 68 pounders had penetrated the target, while the 200 pounder had made hardly any impression on it. Noble showed that the answer lay in the relative velocity of the two projectiles. The 68 punder had been moving at 1,425 feet per second when it struck, while the 200 pounder was loafing along at a mere 780 feet per second. On the face of it, the 200 pounder with 156,000 foot-pounds of energy should have out performed the 68 pounder with 96,900, but the low velocity of the heavier projectile allowed the plate to deform and resist the blow, whereas the higher velocity of the 68 lb shot tore through the plate before it could begin to absorb the blow.

As Noble said in his paper, "What is wanted is velocity; if you sacrifice it to weight you will only be able to keep knocking at the door without entering."

Are you considering using more solid projectiles and upping the pressure?

[nik282000]

I had suspected that there is a limit for "deforming speed" (is there a real name for this?), polycarbonate is usually super tough and will distort heavily but at such high speeds it just shattered.

I'm not sure about going up much in pressure until I lengthen the barrel, like I mentioned before I think I have reached the G limit for a ping pong ball. And as the cooler weather gets closer I am going to have to keep my pressure chamber indoors or switch out for steel pipe, I don't really want to pump up my PVC to any pressure when it gets to be -20 around here.

As for harder projectiles, I saw some video showing solar roof tiles being tested with a hail cannon, 38mm hail stones might be a good projectile. They also wouldn't have the potential to perforate the garage after hitting a target, all of my shooting is done indoors (noise and living in the burbs) so I don't want to fire something that I can't safely catch.

Interesting link, I always forget to check archive.org when doing research.

Edit: and just because, splitting the ball in flight http://i.imgur.com/u0yA34t.gifv

[jackssmirkingrevenge]

They also wouldn't have the potential to perforate the garage after hitting a target, all of my shooting is done indoors (noise and living in the burbs) so I don't want to fire something that I can't safely catch.

I always forget that most people have more reasonable standards when it comes to testing

and just because, splitting the ball in flight

Haha excellent! Seeing the two halves deform then shatter really is amazing. Keep up the great work!