7 replies to this topic

[udsaxman]

In the never ending quest for more FPS, I've been toying with the idea of a vacuum cannon for firing nerf darts (possibly megas). I haven't been able to find anything solid about anyone having tried this so I'm looking for any experiences you guys may have.

I'm shooting for Mach 0.5 if I can pull a good enough vacuum and fit it in my Jeep so I'm aware that 500 fps darts are WAY beyond anything I could use during a war so this will be an exhibition peice.

Thoughts/comments/horror stories?

[Aeromech]

No. Just some physics.

Kinetic Energy = (1/2) * M * (V^2)

 

Standard dart out of a well made homemade blaster = 0.001 kg @ 250 FPS (76 m/s)

KE = (1/2) * 0.001 * 76 * 76

KE = 2.88 Joules.

- OK so thats right around where paintball is in terms of muzzle energy. There are other factors at play, like decelleration on contact, surface area that the forces act upon, but we won't worry about that right now.

 

Mega dart out of vacuum cannon = 0.0025 kg @ mach 0.5 (171 m/s)

KE = (1/2) * 0.0025 * 171 * 171

KE = 36.55 Joules.

-In other words, that's nearly 13 times the energy of the "standard" control blaster setup.

 

But all this math really doesn't mean anything because the Mega dart would be vaporized by that kind of energy behind it.

 

That being said, please be careful. I would ask potato gunners / punkin chunkin guys more about the stuff you are proposing, as the pressures and stresses you are going to experience are more akin to those kind of projects.

Edited by Aeromech, 01 August 2016 - 10:57 PM.

[udsaxman]

I've been a spud gunner for 15 years. Those are pressures though, the goal of this is to evacuate the air from the chamber to avoid having to deal with air resistance. The pressures actually shouldn't be as dangerous since schedule 40 < 2" should be fine from what I've seen at a 27" vacuum which is where I'm going to limit it. I look forwatd to hearing what the other tato pumpkin guys have to say.

 

This is drawing inspiration from and from the Purdue University supersonic project https://youtu.be/YYNCGZCul1Q
 

[WarMachine]

You will need some way to seal the front of the barrel. Most people use packing tape for that purpose but if you have the time for it i would either find or make a rubber cup that you could put on a cord that would go over the end. Now for the barrel it self i would advise schedule 80 3/4 inch pvc as i have found that to be a nice snug fit for mega darts but be careful with that as not all batches of mega darts will fit. The next thing that should be of note is that the first 4 to 5 inches of the barrel where the dart would sit should be fluted and the point where the vacuum is applied should be between the fluted section and the valve for your high pressure supply with some form of post to stop the dart from covering that point to prevent the negative pressure from destroying the darts. Aside from all those things i would suggest that you run some vacuum test so that you know how low you can take the barrel pressure before it destroys the dart. Also please note that at the point where your dart has reached the front of the barrel you need to be at atmospheric pressure to prevent the dart from rupturing.

[CaptainSlug]

The vacuum is going to expand the closed cell foam to the point that the dart is likely to lock itself into the barrel prior to firing. I don't think it will be enough to rupture the dart, but the pressure scaling will make the dart-to-barrel interface somewhat unpredictable so you'll need a much looser fitting barrel than you would otherwise normally use for a regular air-powered blaster.

 

You might want to first run some tests using a dart in a vacuum jar. If you get the fit wrong then you're likely to initially be blowing foam confetti out the end of your barrel.

[udsaxman]

Good point about the closed cell foam, I hadn't even considered that. The plan was to just use the PVC pipe as the barrel with mylar over the ends. I will need to test this and might have to move to boomco straws if the closed cell foam is a problem. Someone mentioned that rival balls might hold up better so I will be testing those too once I finish hacking together a vacuum pump.
On a related note, turning a dart into foam confetti would definitely be interesting to watch...

[Montymarks]

That wouldn't really be practical in a war, if it was mostly to proportion (the clear pvc to your pvc and the ping pong ball to the rival round) and also a rival round wouldn't really have that kind of air resistance like a ping pong ball, so that would be a bit pointless, because if it would be to scale, the force of the air would probably just be like a [k26] at full compression. It's still cool, but take that into consideration. I'm going to do some math on this, and post it later.

UPDATE:
Ping pong balls are 40mm and rival rounds are more or less 23mm. I hope this helps you in making a vacuum cannon with rival. Or you could make a COUPLERED vacuum cannon (couplering solves all of My problems. I'm the guy who wants to coupler a doublestrike). It still might work. It would probaply still kill the darts though.

UPDATE:
The pvc would have to be a good 23.5 mm inner diameter. This is not the most achievable project, but it could work, if given enough time and effort.

-Montymarks

Edited by Montymarks, 03 August 2016 - 09:49 AM.

[Majestic]

When I suggested the Rival round, it was thinking about the foam of a Mega dart expanding and plugging the barrel or creating too much friction to get the initial movement that you need. There might be nothing to it, but in my experience spheres roll easier than cylinders.

 

Montymarks, no one is trying to use this in a war. Udsaxman just wants to put up some big numbers and maybe destroy a soda can or two.