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Unread postPosted: Fri Jan 09, 2009 12:30 am
Author: willarddaniels
This fits with chemistry, but it is more for a physics class:
Make a simple vacuum gun. It is cheap to make (if the school already has the vacuum), you can use it indoors, it is quite impressive with calculated muzzle velocities of nearly mach 2 (yes, that's right) and fires harmless pingpong balls.

I believe 1.5" sch 40 pvc is the right size pipe you need, about 2 m long.

Thread in a fitting for the vacuum hose toward one end. Tape over that same end of the pipe with a few layers of packing tape, get a good seal. Insert ping pong ball (low mass, can accelerate quickly - both positively and negatively) start vacuum pump and place tennis ball can lid over other opening, or other similar lightweight sealer. When you have removed "all" air from chamber/barrel, take a pin or small knife and punch a whole in the tape. In comes air and blows out the ping pong ball at amazing velocities. Use the cheapest ping pong balls you can find, they have the thinnest walls and therefore the lightest mass.

The teacher may prefer this safer method- it is a great teaching experiment. But- I would offer a myriad of choices to the teacher and try to get several different guns there. Locate a chrony so that you can measure the results of each gun (learn about the various laws of fluids and stoichiometric mixes, blah blah blah) and so that the teacher can feel justified in allowing even more time for the whole class to participate.

Unread postPosted: Fri Jan 09, 2009 11:11 am
Author: john bunsenburner
You get to build guns and i have to prove ohm's law with a dim witted russian who still doesnt understand the concept of toilet paper :cry: ...Bah life is so unfair...

Unread postPosted: Fri Jan 09, 2009 12:26 pm
Author: psycix
This fits with chemistry, but it is more for a physics class:
Make a simple vacuum gun. It is cheap to make (if the school already has the vacuum), you can use it indoors, it is quite impressive with calculated muzzle velocities of nearly mach 2 (yes, that's right) and fires harmless pingpong balls.

I believe 1.5" sch 40 pvc is the right size pipe you need, about 2 m long.

Nearly mach two with a 2 meter long barrel and only 1 bar pressure differential?!
Any reference?

Unread postPosted: Fri Jan 09, 2009 12:39 pm
Author: john bunsenburner
Yes that sounds pretty high...why arent people using it? Oh wait is mor ethan one bar, because the air is rpidly sucked in and that spike pushes the projectile... :o

Unread postPosted: Fri Jan 09, 2009 1:32 pm
Author: jimmy101
Depending on how anal the school gets, you might consider a mini.

A long handled fireplace lighter for ~$5 can be used for both the fuel source and ignition.

You can geta 60cc plastic syringe at a good farm supply store for about $1.

Pill bottle mini. The screws are both the mount and spark gap. The small stick is the ramrod for loading. LIttle pieces of spud or a BB wrpared in tissue make a good ammo.
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Fuel supply + ignition from a long handled lighter. (Ignore the white 35mm film canisiter)
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Gun, fuel, ignition (the fuel hose is the light colored tube and is just setting underneath the unscrewed cap). A syringe would be a more accurate way to measure the butane.
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A digital camera with a movie mode would let you get crude timing data on the combustion process. A laptop with a microphone will let you measure muzzle velocities.

Chemistry, Physics, Ballistics all in one!

Unread postPosted: Mon Jan 12, 2009 10:58 am
Author: willarddaniels
[quote="psycix']Nearly mach two with a 2 meter long barrel and only 1 bar pressure differential?!
Any reference?[/quote]
A fair question that I anticipated.
Without details, the references and numbers can be seen and derived by using both Bernoulli's equation and Pascal's principle.

Unread postPosted: Mon Jan 12, 2009 2:26 pm
Author: Daltonultra
How about a burst-disk cannon powered by a gas-producing chemical reaction?
Even good old vinegar and baking soda should produce enough volume to pop a light burst disk.

Unread postPosted: Mon Jan 12, 2009 2:32 pm
Author: john bunsenburner
What he has is chemistry class and not fun in mums kitchen. If he does do what you recomend it should be a proper reaction, with a big boom and a ploom of colored smoke...Why not just a plain old combustion?

Unread postPosted: Mon Jan 12, 2009 5:25 pm
Author: strangers
I think I'll try and get the to let us build a full-sized combustionwith the following dimensions:
1.5"x6' barrel sch40
4"x9" chamber sch40
over under construction
stungun ignition
pc fan chamber fan
or if they'll let me I'll bring in my latest cannon, a 10.5' long over under cannon with a 1.5" barrel and a 1.5' 4" chamber. I just need to pick up a stun gun wednesday and it should be finished.

Unread postPosted: Mon Jan 12, 2009 8:34 pm
Author: jimmy101
willarddaniels wrote:A fair question that I anticipated.
Without details, the references and numbers can be seen and derived by using both Bernoulli's equation and Pascal's principle.

Hmmm, I think I'll have to call Shenanigans on this one.

Not going to get supersonic, certainly not mach 2 outside the barrel.

Best I can find puts the velocity at roughly mach 0.8.
http://www.physics.ubc.ca/~outreach/phy ... Lesson.ppt

http://www.youtube.com/watch?v=qHbcsLp8dDI, using a balistic pendulum they measure ~260m/s (Mach 0.78 ).

Not anywhere near Mach 1, let alone Mach 2.

Unread postPosted: Mon Jan 12, 2009 10:26 pm
Author: starman
Here's that YouTube without the comma at the end....

[youtube]http://www.youtube.com/watch?v=qHbcsLp8dDI[/youtube]

Good info Jimmy!

Unread postPosted: Tue Jan 13, 2009 7:12 am
Author: psycix
Only downside of a vacuum gun is the low projectile weight.
Though it is impressive to see that a pingpong ball can penetrate soda cans.

Unread postPosted: Tue Jan 13, 2009 6:40 pm
Author: Daltonultra
john bunsenburner wrote:What he has is chemistry class and not fun in mums kitchen. If he does do what you recomend it should be a proper reaction, with a big boom and a ploom of colored smoke...Why not just a plain old combustion?


Trust me, do a little research and you can come up with a combo of acid and alkali that will give you a pretty spectacular reaction. In fact, some chemical reactions that would be safe in PVC pipe give off fumes that are pretty flammable. In fact, you could even do a hybrid. Dump in your chemicals, wait for the pressure to build, then hit the ignition...
And it would be great for a CHEMISTRY class, because you'd be demonstrating a multiple-stage process, along with the electrical and mechanical aspects of the gun.

Unread postPosted: Wed Jan 14, 2009 3:03 pm
Author: john bunsenburner
It would be chmistry if he made a balaced reaction of all the things that happen once you dump it in and if he calculated how much energy all of it would but out, find out what velocity you will get, how far the projectile will go, what pressure will be achieved and what temperature and all the rest of it. On the othe rhand i have the feling this should be a fun project so a simple propane powered gun with pure o2(if available) made fom what eve with a piezo igniter would be great also i would love to wright a eport about a spud gun, imagin all the variables and you could make a wonder full graph of the divverent fuel to air or o2 ratios or something, lots to do and find out, wondeful opertunity! :twisted:

Unread postPosted: Thu Jan 15, 2009 9:09 am
Author: psycix
Using pure O2 is a bit dangerous. Not only is the amount of pressure increased because you can add more fuel (as much as in a 5x mix hybrid), but also the shockwave will become significant.
DDT happens WAY faster with pure O2 then with air due to the increased burn speed.
You will need a metal chamber to work with pure O2 and I think it is a bad idea to use it in a school project.