The supersonic ping pong gun

[stuffbuilder]

Found this article on Gizmag (good website) and immediately thought of Spudfiles. Judging from the description in the article and what I've seen on this site, this doesn't seem impossible to build at home.

http://www.gizmag.com/how-to-build-a-su ... dium=email

[ThisGuy]

Hey SB, i was just browsing Reddit when i stumbled upon the video link for this gun.



I would love to try the vacuum concept on an existing gun. I think a pneumatic co-ax would be a great starting point. Also, in the video he fails to mention what pressure he is charging his chamber to. That would be interesting information to have as well.

We might just have the "next generation" of launchers come from the vacuum concept. We all love to squeeze a few more fps out of our launchers and he is propelling a ping pong ball with vacuum alone to 600-700fps. It's awesome!

[MrCrowley]

We've been discussing this a bit in the Off-topic Thread (not because it's off-topic, we usually just post whatever in that thread). Seems that the nozzel concept isn't new to SF.

We might just have the "next generation" of launchers come from the vacuum concept.

This has definitely been discussed here before, you may have to search "vacuum barrel" (without the quotation marks) and select the option to search for all terms and then sift through the topics. Most of the time it seems to come up in discussion, rather than as a dedicated topic but here is one such example:
http://www.spudfiles.com/forums/pressur ... 22886.html

I think someone may have even tried it on one of their cannons before.

I don't think the ping pong cannon will change much around here unless you plan to replicate what they've built.

[mobile chernobyl]

I couldn't resist - you may or may not be seeing a supersonic spud gun video posted as a comment

As far as the implications of incorporating a CD nozzle on a spud gun... it's been a topic of conversation that has popped up once a year as long as I can remember. Paintball guns briefly attempted to use it as well.

The problem lies (and this goes without me putting pencil to paper and quantitatively proving this) in the fact that we are shooting objects that typically have masses at least a magnitude greater then that of a ping pong ball, often two magnitudes greater (i.e. 250g vs 2.5g). My only guess and this is very unscientifical - is that the inertia of the projectile as it approaches a certain fraction of that of the gas propelling it will take over and inevitably slow down the gas speed - therefore transferring the primary method of acceleration back to pressure on exposed surface area.

It's something worth trying on some of the more "legit" gas guns on here. A CD nozzle on a Hy-Bird, now that's what I'm talking about!

But oh wait, they have 8 degrees, a whiteboard with largely empirical science written on it, special equipment, and a cardboard backstop with a can of acetone down range of the barrel - all we have on Spudfiles.com is about a hundred or so man-years of experience.

It would have been nice to at least see a force balance equation to predict the muzzle velocity

The vacuum barrel has been discussed at random as well - if you look in some of the threads aimed towards higher velocity it's often discussed. If you do some more research into hyper velocity methods - it's a regular requirement.

[MrCrowley]

But oh wait, they have 8 degrees, a whiteboard with largely empirical science written on it, special equipment, and a cardboard backstop with a can of acetone down range of the barrel - all we have on Spudfiles.com is about a hundred of so man-years of experience.

I love the bit when it said not to try it at home because they have loads of college degrees and special equipment

I'm sure we could pull together a few dozen college degrees on this forum: IT, business, science, engineering, arts... you name it, we've (probably) got it

[mobile chernobyl]

They may have special equipment, but that's usually only compensating for their actual "equipment".

And thread de-rail in 3... 2.. 1.

sorry lol

[btrettel]

As I said in the off-topic thread, I am skeptical the converging-diverging nozzle helped here. They didn't do a test without it best I can tell, so there's no solid way to know if it helped. These nozzles are mainly used when the inlet air is extremely hot to convert the heat into gas kinetic energy. Adiabatic compression will make the gas hot, but not nearly as hot as most converging-diverging nozzles. The nozzle might have just acted as a flow restriction here.

But mobile chernobyl is right. If a converging-diverging nozzle can help, it's unfortunately limited primarily to very light projectiles.

I'll look into incorporating converging-diverging nozzles and vacuum barrels into the simulator I'm developing this year. Both should be interesting to investigate at the very least.

Edit: The vacuum barrel should help increase muzzle velocity greatly for supersonic shots. The reason why is that with air in the barrel pressure would build up just in front of the projectile as it approaches Mach 1. This excess pressure will slow the acceleration of the projectile. Without significant air in the barrel, the compression wave can't exist or is significantly weakened if it does exist.

[sagthegreat]

For the nozzle thing. Just make 2 identical pen guns and bend the barrel in near the sealing point, this should give us a crude nozzle. Then do tests with both guns same pressure and ammo

[POLAND_SPUD]

As I said in the off-topic thread, I am skeptical the converging-diverging nozzle helped here. They didn't do a test without it best I can tell,

they did - watch the video

[mobile chernobyl]

they did - watch the video

It did not appear that they tested the pressurized version against itself with and without the nozzle tough is what brettel was getting at. From what I saw they only tested an atmospheric pressure vs vacuum version without a nozzle using a constant diameter tube (i.e. a single pipe) with both ends taped off.

If they did test the pressurized version while also pulling a vacuum on the barrel without the CD nozzle - I must have missed that part.

[btrettel]

I didn't watch the video before. I watched most of it now, and I only saw one test. If anyone could point out specifically where they did an otherwise equivalent test (e.g., with the vacuum) without the nozzle, I'd be interested in knowing.

It's also worth noting that the professor in the video says (at about 4:15) that he's not even sure that the convergent-divergent nozzle is doing what he hopes it does. I'm not sure there's a good empirical way to test that, though it'd be somewhat easy with a good CFD code (like the simulator I intend to write).

Another reason I'm skeptical of this is that in the one book on high speed guns (The Theory of High Speed Guns by Arnold Seigel) best I can tell converging-diverging nozzles are not even mentioned. The book has an entire chapter on ideas they tried that did not work. Maybe they didn't think of it, though I'd be surprised if they did not think of it because it's a well known concept to those working in compressible fluid dynamics. Though maybe it doesn't work if you want Mach 30 but does work if you want Mach 1.2.

[POLAND_SPUD]

hah I watched it again and you're right
it didn't occur to me someone can first test a typical vacuum gun and then modify it into a high pressure burst disk pneumatic
...and conclude - oh we get higher MV but it's becuase of the nozzle (we think but we're not sure)

[btrettel]

I emailed the professor. I'll post any response I get. My email is below.

Hello Prof. French,

I am a graduate student working in computational fluid dynamics who is also interested in the internal ballistics of compressed gas guns. I recently saw you and your students' supersonic ping-pong ball gun and I have a question about the tests performed on it.

Did you all do otherwise equivalent tests with and without the nozzle? I am curious about the difference in performance without the nozzle.

I am somewhat skeptical that the nozzle would help much, though it has a good chance of working for lighter projectiles. A supersonic flow is necessary for a supersonic projectile muzzle velocity, but generally convergent-divergent nozzles reduce the temperature and pressure at the outlet greatly. So the net effect is not immediately clear. (The pressure will, of course, increase as the flow slows down to match the velocity of the projectile.)

I am reasonably sure that the supersonic velocity was achieved primarily by the combination of the vacuum barrel and higher inlet pressures. The projectile is very light and could provide little to no resistance to the flow. Without a significant compression wave building up in front of the projectile, the projectile can go supersonic much more easily.

Thanks,

Ben

[jakethebeast]

Hmm, the nozzle idea seems pretty nice, i wonder if it could be applied to blowgun with power increase..

[mobile chernobyl]

Hmm, the nozzle idea seems pretty nice, i wonder if it could be applied to blowgun with power increase..

It has been implemented in paintball before - and I believe the reason you don't see it anymore is that it did not help overall efficiency of the system (efficiency is what you'd be going for in paintball due to the velocity restriction)