PNEUMATIC SPUD SHOOTER
PURPOSE
Ever seen those silly little hairspray powered spud guns that are
all the rage? How’d you like to
show up those guys with a cannon capable of launching spuds
nearly 5 times farther? Read on…
Testing of hairspray spud guns revealed they can
develop no more than 10-20 PSI chamber
pressure. Most are built from schedule 40 PVC pipe.
3" PVC pipe has a burst pressure of 260
PSI. People have suggested using other fuels to
boost pressures higher, however this is much to
dangerous. More volatile fuels can be very
unpredictable and create pressures that would shatter
the PVC pipe. Compressed air can be
regulated to exact pressures making it an excellent
propellant. The trick is releasing a large
volume of air at once to launch the projectile. Here’s how
it’s done…
DRAWINGS
You may need a drawing to fully understand the concepts of operation and
assembly. For those of
you that have a CAD program that can read DXF files, download the
zipped version.
PARTS
All pipe and fittings are schedule 40 PVC
3" pipe 36" long
1 1/2" to 3"
bell reducer
3" female adapter
3" cleanout plug
diaphragm (see below)
a couple old trouser legs
source of compressed air
blow gun with tapered nozzle
(What’s a blow gun? It’s a valve/nozzle combination that
attaches to a compressed air hose.
They’re commonly used in machine shops and
garages to blow dust away. You can usually find one
at most autoparts stores.)
ASSEMBLY
Cut the pipes to length. The barrel
pipe must have a perfectly square and smooth surface on the
breach end. You may have to flat
file and sand it to make sure a good seal with the diaphragm is
achieved.
Cut a
3 3/16 diameter disk of 3/32 thick polypropylene or polyethylene. A cheap dishpan from
Kmart
may be a good source of diaphragm material.
The front bell reducer has a small ridge
that would prevent barrel from being pushed through it
during assembly. Use a half round file
to remove the ridge. Make sure you can push the barrel
though the reducer. Remove the barrel
and set it aside.
Drill a hole through the center of the cleanout plug. Size this hole
to achieve a tight fit with the
tapered nozzle of the blow gun.
Assemble the
outer chamber first. Glue the bell reducer and the female adapter to each end of the
3"
pipe. allow the glue to dry before proceeding to install the barrel.
Place the
diaphragm in the female adapter and screw the cleanout plug in firmly. Push the barrel
through
the reducer until it sets firmly against the diaphragm. Pull it back out 3 inches to allow glue
position
against the diaphragm. Hold the barrel in place until the glue dries.
After the gun is assembled, remove the cleanout plug and check to see if the barrel is properly
hose about
1" long and force them between the barrel and the outer chamber.
Slip the old trouser legs over the chamber area of the cannon and tape them in place. This simple
go
wrong.
FIRING
To fire the cannon, unscrew the cleanout plug and
remove the diaphragm. Push a pre-cut spud into
the breach of the barrel, making sure it is an
inch or two below flush. Replace the diaphragm and
screw the cleanout plug in firmly. Point
the cannon down range. Place the blowgun in the charging
hole drilled centered in the cleanout
plug. Start squirting air into the cannon with the blowgun. When
the sound of hissing air
quiets down or stops, the cannon is charged and ready to fire. Keep the
blowgun valve pressed
down. Aim carefully and pull the blowgun from the charging hole.
TIPS
A
tapered nozzle on the blowgun is a must. Size the hole in the cleanout plug to fit the nozzle
snugly.
Sharpen a short piece of 1 1/2 pipe on the outside to precut your potato. Portability
can be had
with a 5 or 10 Lb. bottle of CO2 and a regulator. feel free to experiment with
different barrel and
chamber lengths. This plan was only a starting point. Get a couple of
friends to help spot your shots.
The spud leaves the barrel *REAL* fast and can be hard to
see. I’ve got one shot to go 755 yards!
THEORY
Because the diaphragm is
somewhat flexible it will bend it’s outer edges forward from the pressure
and allow the air to
fill the outer chamber. At the same time this forward pressure is sealing off the
end of the
barrel preventing air from escaping up the barrel. Pulling the blowgun out of the charging
hole allows the air behind the diaphragm to escape to the atmosphere. At this point all the
pressure
stored in the outer chamber bends the diaphragm in the opposite direction sealing off
on the outer
edge of the cleanout plug. The diaphragm continues to bend under the pressure and
unseals the
barrel end allowing the stored air to escape up the barrel behind the spud. This
in turn, propels the
potato distances before unknown to mortal spud marksmen.
In
the pneumatics industry, this device is called a quick exhaust valve. It’s commonly used on truck
to
apply the brakes fast.
SAFETY
DO NOT FIRE THE CANNON AT ANYONE
OR ANYTHING. At nearly 500 feet per second
muzzle velocity, a potato would be lethal if it
were to hit someone.
The pipe should be clearly marked SCH-40 260 PSI. ACCEPT NO
SUBSTITUTIONS or you
will be reinventing the pneumatic hand grenade.
When
assembling the gun BE SURE TO USE THE PROPER TYPE OF PIPE CEMENT! Do not
use cement made for
ABS pipe. If you have any doubts about your gluing technique, GET
PLUMBER TO HELP YOU!
USE ONLY 100 PSI MAX! This gives the plastic pipe a safety factor of more than 2.
the pipe to
become dangerously brittle.
As pipe ages it may be a good idea to build a new cannon
and discard your old one. All plastics
have a limited life span and it would be very difficult
to estimate the safe life of the cannon.
ALWAYS KEEP THE TROUSER LEGS IN PLACE ON THE CHAMBER
WHEN FIRING!
BE CAREFUL and have fun!
Return to Backyard
Ballistics
Copyright © 1994 Bob Simon
Potato Bazooka
Plans
You may want a couple of buddies to go in buying the material with
you, or make 3 at the same
time, as the pipe comes in 10 foot chunks. I bought all material at
a local "Home Depot" for about
$15.
Note on choice of material: I have
seen and heard of plans for spud guns using PVC pipe. In fact,
an article in the February
issue of "Modern Gun" uses PVC. I chose to use schedule 40 ABS
plastic. The black
pipe usually used for sewerage. If you want to know why I chose ABS, take a
chunk of PVC pipe.
Hit it with a 25 lb sledge hammer. It fragments into many *sharp* pieces. Try
this with ABS.
The sledge hammer bounces off the pipe and smashes into your foot. But it didn’t
break! (The
pipe, that is, I don’t know about your foot). PVC also gets brittle with exposure to
sunlight.
ABS just gets hot. Only ever use schedule 40! That’s the thick stuff. It costs a little more,
but not that much more. The bill of material says 10 foot lengths, only because that’s as small a
piece
as is normally sold.
Web master’s note: There does seem to be some
confusion about what pipe is safest to use.
Finding pressure rated ABS pipe is nearly
impossible in many areas. Most spud shooters are
therefore constructed of PVC pipe. If you
stick to conventional propellants and do not use
oxidizers, PVC should preform with adequate
safety.
Bill of material
1 10 foot piece of 2 inch diameter schedule 40
ABS pipe
1 10 foot piece of 3 inch diameter schedule 40 ABS pipe
1 3 inch to 2 inch
reducing bushing
1 3 inch coupling
1 3 inch threaded (one side) coupling
1 3
inch threaded end-cap
1 can ABS solvent-weld pipe glue. NEVER USE PVC GLUE on ABS!!!
1
Coleman sparker - these are easily found in any sporting goods store that has a decent
camping
section. They are normally made for putting inside a Coleman lantern or stove so
you don’t
need matches.
Construction
Step 1 - Cut the combustion chamber to size.
Cut off a 14 inch section of the 3 inch diameter pipe.
You don’t need the rest of the 10 foot
length, so save it for future bazookas, or make one with a
couple of buddies splitting the
cost.
Step 2 - glue the 3inch to 2 inch bushing into one side of the 3 inch coupling,
glue the other side of
the coupling to one end of the 14 inch combustion chamber. Make sure
the joints are clean first and
be liberal with the glue.
Step 3 - glue the
threaded coupling to the other end of the combustion chamber (using the slip-joint
side,
obviously) make sure the glue doesn’t run into the threads.
Step 4 - Cut the
"barrel" to size. Cut off a 36 inch (3 foot) length of the 2 inch pipe. Glue this into
Step 5 - Using a file, taper
the "muzzle" for the last half an inch on the outside. This will serve to cut
the
potato as it’s rammed in.
Step 6 - You’ll need to mount the sparker inside the end
cap. If you got the Coleman one, it is
threaded and has two nuts with it. There is also an
angled piece of metal meant to hold the ignitor
inside a lantern. Take the knurled knob off
the end of the shaft. Be careful - there’s an extra flint
inside the knob. Unscrew the nut and
discard the angled bit of metal. Drill a hole dead center in the
ABS end cap of a diameter to
take the shaft of the ignitor. Mount the ignitor inside the end cap, put
the nut on the
outside of the shaft and tighten until the ignitor is held in place. The shaft will slide
back
and forth, but won’t come out. Put the end knob back on and tighten the lock screw.
Step 7 - Make sure the glue has "cured". I left mine overnight before firing.
etc.
will do. Measure and make a mark about 2 feet 8 inches down the ram rod.
To
fire: remove end cap. Ram a potato from the muzzle end. The tapered end will cut the potato to
size. Make sure it has a good seal as you ram it down with the ramrod. Ram to the mark you made.
gasses can escape. Spray 2 - 5 seconds worth of cheap hair spray (white rain, aqua net) I’d use
an
"unscented" one if you can, or the gun stinks after a few shots! Start at 2
seconds and build up!
After the hair spray, quickly screw in the end cap. One twist of the
ignitor knob sends the spud
skyward!
Safety
Once you shoot this,
you’ll see the potato comes out with enough force, you wouldn’t want to be on
the wrong side!
Usual safety about pointing the muzzle etc. still apply. This is for fun only. I don’t
make
any guarantee you won’t blow your arse off. (You may laugh it off, however). Personally, I’d
never use acetylene, starting fluid (ether), black powder, lighter fluid, gasoline etc. as a
propellant,
but you may not value your body parts as much .
You can get 3 shots
off a big spud. Partially baked ones are fun - they seal in better and shoot
farther, but they
do break up and the barrel is a mess to clean up afterwards.
Clean up
Soap and water. Push a small towel through (here’s a case where it’s OK to clean from the
muzzle).
I’ve been shooting mine since 12/94 and have been having a barrel of laughs. The
spuds will go
nearly 200 yards! I plan to make the "220 swift" variety by coupling a
one inch barrel to the three
inch combustion chamber. I wonder if you put the barrels on
threaded couplings you could have
interchangeable barrels. Sort of an "Idaho
Contender".
Other tricks
Ram in a cardboard container from McDonalds
from an order of large fries. Leave the fries in the
container. The cardboard serves as a
sabot and voila - a shotgun!
Warning:
These things have a tendency to
attract every 8-12 year old kid in the neighborhood.
Have fun!
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