The Ultimate Potato Cannon

[cannon freak]

Has any one ever built anything like this

http://ares.jsc.nasa.gov/Education/webs ... rs/lgg.htm

Now I don't mean anything near as big, but has anyone built something based on the way that it works.

[drac]

Nope, the potato would most likely be destroyed by the pressure and the velocity. Besides, hyrdrogen is dangerous, and definitely not safe for use in PVC.

If i ever get a job I may machine a small one out of aluminum, but i don't expect it to work well.

[Benny]

i might attempt it with solid propelent (no more info) and galvanised pipe, probadly not tho.

[boilingleadbath]

Nope... to my knolage, no 2-stage gas guns have been built.

[SpudStuff]

im confused on the two stage part. isn't it just like a super hybrid or is there something im missing

[)DEMON(]

you must be crazy to build this gun

[boilingleadbath]

No, it's not comparable to a hybrid. Here's what <i>they</i> do:

(~.5-1 lb of gunpowder [piston] hydrogen or helium gas at ~60 psi | ======

Point being that the light gas is compressed to enormous pressures - a million psi or so, I believe - and during the compression gets quite hot.

And of course, a hot light gas has a very high speed of sound, allowing typical muzzle velocities of ~5-7 km/s (~15,000-20,000 fps).
I'm not certain about this, but I think there are some 2-stage light gas guns which push 15km/s (45,000 fps)!

Now, it would be quite possible for <i>us</i> to have a system like this:
(chamber with air-fuel mix [piston] helium gas at atmospheric | ===

Muzzle velocities of more than 3000 (4500 with hydrogen) fps would be possible, and if tuned properly the piston would probably be reusable. I'd make in steel (or at least the part near the burst disk), so that you can take advantage of the potential to generate enormous pressures.
And the higher the pressure gain, the more the gas is heated, and thus the higher the potential muzzle velocity.

If you are going for enormous velocities, I heavily reccomend filling the barrel with a light gass to. You can hold it in with a peice of tape over it - just remove it before you shoot. (or let it stay there, it'll be removed just fine...)

[FiveseveN]

I had an idea a while back: what if you replace the light gas with vacuum? You could use tape, Al foil or celophane to seal it at the muzzle, as blb suggested, and suck out the air in the barrel with a vacuum pump. It won't be perfect vacuum, but should yield a significant increase in performance. Think I'll try it on my next gun.

[dgr-c]

I had an idea a while back: what if you replace the light gas with vacuum?

It wouldn't work as a light gas gun if you could pull a perfect vacuum. You can find further explanation in wikipedia here:

http://en.wikipedia.org/wiki/Speed_of_sound

and here

http://en.wikipedia.org/wiki/Light_gas_gun


Basically the equation for an ideal gas is:

c = sqrt(k*p/r)

where
c is speed of sound
k is a constant
p is pressure
r is density.


The goal is to build the highest pressure possible in a gas with the greatest speed of sound. That gas would be hydrogen but the probability of getting yourself blowed up with compressed hydrogen is vanishingly close to 1. Don't go there.

Air is close enough to an ideal gas for the equation above to be valid. That said:

Pulling a vacuum drops density toward zero and drives c toward infinity. A good thing.

But pulling a vacuum drops pressure toward zero and drives c toward zero at the same rate.

In the world of Newtonian physics, zero beats infinity every time. With a perfect vacuum, there no gas in the chamber, therefore nothing to compress and/or heat up to provide energy to push the round down the barrel. In conditions other than a perfect vacuum, pressure and density cancel each other out and speed of sound is pressure independent.

But wait:

If you can control gases well enough to get even partial vacuum, you can probably also fill the chamber with a light gas like helium. You could then do a helium pneumatic or use the light gas gun concept to have an explosion drive a piston to compress the working gas. At sea level, a good number of the speed of sound in air is 761 mph. For helium, the number is more like 2170. Whatever you can get out an air-driven gun, you can get 3x out of a helium-driven gun.

Teamsushi got a potato up to Mach .9 with a helium pneumatic at 300 psi. Here's the thread:

http://www.spudfiles.com/forums/viewtop ... &lighter=&

good luck, don't get blowed up,
dgr-c

[sgort87]

I hate to break it to you because you made a good argument but he said only create a vacuum in the barrel which would, in fact, create more power because there is nothing to put force on the projectile in the opposing direction except the friction between it and the barrel.

[huse_spud]

man that sucker would be NASTY if it were sitting on a trailer bed!!!

looks like a team of genius engineers made that thing.

how much time and money would you need to make something like that.
geesh!

[FiveseveN]

Of course my idea wouldn't work like a light gas gun, it's a totally different thing. My thought was: the projectile is not limited to the speed of sound in vacuum. Plus, the pressure difference betwen the back of the projectile and the front of it is increased by one atmosphere (around 14,5 PSI). It's not a LGG since it doesn't use a piston (it wouldn't work, for the reasons dgr-c posted above). It would work in any type of cannon (combustion/pneumatic/hybrid), requires minor modification and doesn't have the cost of hydrogen/helium.
Perhaps a member with some free time could experiment? I'm in the middle of exams.

Later edit: I stand corrected, it wouldn't work in a normal pneumatic unless you heat the air some way or use a lighter gas. I think it's still a good idea for cumbustions, though.

[Shrimphead]

I'm considering doing a small bore, simple version. Of course I would have to get some experience with combustions, then hybrids, then this. K, how about a length of 1.5" steel, reduced to .5". Build a piston out of steel to make sure it holds up. Then just set up a regular propane injected hybrid set-up behing the piston. I don't think that I would worry about the burst disk, because it would be powerful enough without it. Get some large-bore bullets (can you buy 50 cal bullets? I don't know much about real guns) Go over to the other side of the lake and shoot it with a chrony to see how well it works. Does this sound reasonable? Do you think I need a steel piston? And I don't think that I need to worry too much about high pressures behind the piston, so a normal shrader or quick disconnect will work, right? Also, are steel threads strong enough, or would I have to weld it?

Edit-I just realized that a burst disk will be required for high performance. So add a steel union in there and maybe thin sheet metal as a burst disk. I'll probably be using a high x mix after a few shots. So how I just realized that there will be high pressures behind the piston from the combustion, so my question is how much pressure can a typical quick disconnect handle?

[boilingleadbath]

A steel piston is probably over kill and dangerous to the pipe. No experience there, though.
I'd make it out of PVC type 2 or other reasonbly cheap, impact resistant material. (UHMWPE could quite possibly be suppreme, and definatly the cheapest.)

The threads should be enough. No calculations on that, however.

Yeah, .50" bullets are available. Personaly, however, I wouldn't use them.
You are building this for really high muzzle velocities. If you use a heavy bullet, you might as well sabot it and shoot it from a normal spudgun.
I'd reccomend using a hard wax. Machinabel wax is probably fine... and I could collaberate with buster to make you some molds.

[dgr-c]

... but he said only create a vacuum in the barrel which would, in fact, create more power...

Agreed. I wasn't thinking out of the box enough. I noticed "replace light gas..." but not sealed barrel.

I agree, sealing the barrel and dropping the pressure will create a higher muzzle velocity. A quick look at GGDT, accepting all the defaults and adding only 15 psi to the pressure (approximately the additional pressure differential associated with having the barrel at 0 instead of 1 atmosphere) muzzle velocity goes from 411 ft/sec to 437 ft/sec. I'm guessing you would lose most of the velocity and energy gain from busting the sealing disk, perhaps not. You'd surely have some interesting non-linearities when the projectile left the barrel and encounted 1 atmosphere at it's leading face.

I bet filling the barrel with a light gas at ~1 atmosphere would be easier, allow a weaker disk, and give overall better performance.

But hey, I'm an armchair theoretical type. It would be fun to see some results from someone who does the experiment.