large vacuum cannon ?

[matti]

I got this idea today.. I have seen many "ping pong ball vacuum launchers" posted at youtube and many other sites. They are quite powerful, launching light weight ping pong balls to high velocity. And what I understand about vacuum cannons is that they are best suitable for this type of ammo (large and light weight).. So.. how about a basketball/football one ?
I now mean those vacuum cannons that only uses the vacuum pressure difference for launching the projectile, not the ones with valves and pressure chambers.

Would this be possible ? I wonder if the basketball (or similar large ball) is going to be crushed/break under the pressure difference ? What fps would be possible for large projectile like this in vacuum setup ?.. anyway fun this would be.

[jackssmirkingrevenge]

They are quite powerful, launching light weight ping pong balls to high velocity.

... but not really compared to a conventional pneumatic launcher. I would think you're better off going with a burst disk pneumatic.

Is this related to your need for a large autonomous compressor

[matti]

They are quite powerful, launching light weight ping pong balls to high velocity.

... but not really compared to a conventional pneumatic launcher. I would think you're better off going with a burst disk pneumatic.

Is this related to your need for a large autonomous compressor

You might be right about the pneumatic launcher.. And no this is not related to the compressor thing, this was just a thing that came to my mind

The compressor is actually for my large hybrid that would need a fast reloading. I'm building a super fast and very accurate fueling system with static gas mixing for hybrids and hope to post topic about it when i get it tested
Now I use 200bar/20litres pneumatic cylinder for air supply, but filling it is not fun or cheap..

[Boomer58cal]

I've pondered the vacuum Canon myself from time to time. One problem you would have with any inflated ball( foot ball) would be that as the vacuum increases the inflated ball would have a tendency to expand inside your barrel adding to friction drag or even causing the ball to get stuck in the barrel. By using the proper diameter barrel and adjusting the pressure you inflate the ball to most of this problem could be alleviated.

With a purely vacuum cannon I'm not sure the pressure wave behind your projectile would be dense enough for heavy objects. There's simply no substitute for high pressure.

For lunching heavier projectiles I think the best idea would be to add a vacuum barrel to an existing pneumatic, to increase its performance. My triple valve canon is already burst disk capable so I have considered adding an additional burst disk/union to the end of the barrel so I could draw a vacuum inside the barrel and see how far it increases my range.

I have nowhere near enough know how to definitively say it wouldn't work, I'm just don't think vacuum alone would give you performance equal to a traditional cannon with heavy projectiles. I have never tested a vacuum Canon myself so this is solely based on the research I've done over the last few months and should be taken with a grain of salt.

[Anatine Duo]

wouldn't the vacuum elongate the ball and reduce friction?

This sounds like a fun project too... if you could get -10psi pressure in the barrel that's around 700 lbs force on a basketball and atmosphere makes a heck of a reservoir/chamber... you would need a large pump to suck the air out.

[jackssmirkingrevenge]

There are certainly benefits to having an evacuated barrel since the projectile is accelerating in very little air drag, this is commonly done on light gas guns:

[Boomer58cal]

wouldn't the vacuum elongate the ball and reduce friction?

it would expand in all directions. Take a helium balloon for example... if you release it into the air, the higher in altitude it goes the larger it will expand as the atmospheric pressure decreases, until it pops that is.

Friction... you still want a proper fit. Too tight of a fit and you have too much drag to lose a fit and the air will leak around your projectile. Same as any other cannon.

[Anatine Duo]

Ah, after I posted I started thinking (terrible I know, not only thinking, but thinking after posting;)

I get that in a vacuum the ball will approach spherical. I was thinking during launch, when there will be vac on one side stretching the ball and reducing projectile caliber. Something tells me a lot will depend on the relative pressure and strength of the ball skin...as you said.

For fairly rigid balls like a ping pong ball it seems perfect.

[Boomer58cal]

Ping pong balls work great but... because their very low mass and horrific drag coefficient they lose over 50 percent of their energy within 5-10 yrds of exiting the muzzle.
Take pellet guns for example. My magnum .177 cal will send a 4 grain alloy pellet at 1250-1350fps. It Produces about 15 foot pounds of muzzle energy and retains 9.1 foot pounds of energy at 10 yards. With a 8.3 grain pullet it produces 16.5 foot pounds at 950 fps and retains 15.2 foot pounds at 10 yards.
Well the 1.5 foot pounds of energy difference at the muzzle isn't that big, 6.1 foot pounds at 10 yards is a big difference.

In just ten yards the lighter pellet loses 300 feet per second, compared to the heavier pellet losing only 100 fps.

Lightweight projectiles are good for only two things... cracking the sound barrier ( which has advantages and disadvantages )and putting big numbers up on your chronograph. Some would add the third advantage of a flatter trajectory, but it depends what you're using it for.

About the vacuum stretching the ball, upon firing there will be so much pressure behind the ball that the end of the ball will become almost flat, expanding the balls outside diameter. The ball will become ege shaped, slightly pointier( is that a word ) at one end then at the other. So yes you're both right and wrong

[PracticalProjects]

Should be an interesting project! I recently watched the mythbusters episode where they built one to fire a ping pong ball over 1000mph. Pretty cool