Intro: Howitzer Potato Cannon
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This is a two part Instructable designed to demonstrate how to construct 1) A hybrid potato cannon capable of accelerating a projectile to supersonic speeds and 2) A cannon mount that resembles a WWII Howitzer field piece.
This is about is powerful as it gets with this size of potato gun. The boom is like a thunder and if you aren't careful you will quickly lose sight of the projectile. It definitely packs a punch and is "just a blast" to fire.
I found most of the parts for the cannon at a plumbing supply store in town and the rest at Home Depot. Then I pieced together the mount from parts I had laying around my home.
The whole cannon can be made with no welding experience as all the parts can either be screwed together are epoxied together. The only difficulty that comes with this is that you have to fix a lot of leaks before you can operate the cannon, but this is not hard to do.
Step 1: Principles / Concepts of Cannon
The regular potato gun that every serious DIYer has built utilizes just a regular combustion of gases at atmospheric pressure. This is the most basic of potato guns and usually the least powerful. The next most powerful type of gun is a pneumatic cannon that uses pressurized air to launch a projectile. After that comes the hybrid potato cannon which is a combination of the other two (hence "hybrid").
A hybrid is powered by the combustion of pressurized gases. This allows for much more fuel and oxidizer to react inside the combustion chamber and it will produce much higher pressures than the other two types of potato guns. More information can be found at http://www.spudfiles.com/spud_wiki/index.php?title=Hybrid_launcher
There are 4 major components to a hybrid potato cannon:
- Combustion Chamber
- Metering System
- Ignition System
Operation: The potato gun is first fueled via the metering system which injects just the right mixtures of fuel and air into the combustion chamber. The fuel mixture is then ignited by two spark plugs connected to a stun gun. The resulting combustion produces enough pressure to break the burst disk holding the gases inside of the combustion chamber and to shoot the projectile out at high speeds
Design Considerations :
- One of the most important things to consider when building any potato gun is the chamber to barrel volume ratio. Experiments have shown that a chamber to barrel ratio of 1.5 : 1 will efficiently utilize all of the guns power.
- The next thing to consider is the barrel length. The longer the barrel is the more time a projectile has to accelerate. However, the larger the barrel, the larger the chamber. So you have to strike a compromise and just decide on a practical and economical barrel length.
- For the meter pipe I got the largest pipe I could at the longest length that would fit. A meter pipe with a larger volume means that you don't have to pressure it up as high when measuring fuel. This is what I would suggest.
- When you incorporate multiple ignition sources you increase the power of the cannon. Igniting the fuel mixture at multiple points will create multiple pressure waves which will increase the velocity of the gases. I have built two ignition sources for now, but I have left room for two more.
My main resources when researching this project were:
- The next step will demonstrate how to build the barrel.
Step 2: Barrel
The barrel is made out of a 2 1/2" SCH 40 steel pipe that is 63 inches long and that encloses approximately 300 cubic inches.
This was threaded on one end and attached to one end of a 2 1/2 " steel union pictured below.
(Most threads were successfully sealed using teflon tape. Any remaining leaks were sealed using epoxy)
Step 3: Combustion Chamber
The combustion chamber was made out of 5 pieces:
- A 34 inch long piece of 4 inch, SCH 40 steel pipe which was threaded on both ends.
- One 4 inch pipe cap
- A 4 inch to 2 1/2 inch pipe reducer
- A 2 1/2 inch steel nipple
- The other half of the union
It is essential that the chamber is sealed and able to be pressurized within a range of 60 to 100 PSI.
Note : During hybrid combustion, the steel pipes will be experiences pressures in excess of 300 PSI. This poses a possible danger to the operator which is addressed in step 13.
Step 4: Metering System
The purpose of the metering system is to inject the correct ratio of fuel to oxidizer into the combustion chamber. The simplest way to do this is to pressurize the gases inside of a pipe of a known volume and then inject them into the chamber.
Note : Step 12 will explain how to operate the metering system
The metering system is perhaps the most complicated part of the cannon, and it will require quite of bit of work to get it working perfectly.
When using threaded parts, it is useful to know that there are many different classifications of thread types. There are compression threads (FIP) and standard pipe threads (MIP). This design of a meter uses both, and so it will switch between the two once or twice. This is just my particular design, so if you can think of something better, go for it.
The parts list is as follows:
- A 4 inch to 3/4 inch brass saddle
- 3/4" 90 degree elbow
- 3/4" 2 inch nipple
- 3/4" Ball valves (2)
- 3/4" 24 inch pipe threaded on both ends
- 3/4" brass nipple
- 3/4" to 1/2" Pipe reducing coupling
- 1/2" brass nipple
- 1/2" to 1/4" Pipe reducing coupling
- 1/4" brass nipple (3)
- 1/4" brass tee (2)
- 1/4" Pressure gauge (MIP)
- 1/4" Schrader valve (FIP)
- 1/4" FIP to MIP adapter (to connect the Schrader to the brass tee)
- 1/4" Pressure regulator with gauge
- 1/4" Compression connector with MIP adapter (to attach the end of the meter to the vinyl hose)
- 5 foot 1/4" vinyl tube
- 1/4" Compression connector (to attach to end of torch head)
- Propane pencil torch head
The regulator isn't absolutely necessary since the torch head already does regulates the flow, but it is handy and allows for more a precise measurement.
- Assemble the parts as shown in the pictures below.
Step 5: Ignition System
Multiple ignition sources:
The simplest way to create multiple sparks inside the chamber is to use a high voltage source such as the one found in a stun gun.
The stun gun is connected to two spark plugs, which are practical since they are already insulated. The spark plugs are screwed and epoxied into two holes which I had to drill and tap into the chamber.
Drill the holes for the spark plugs using a 7/16 bit and tap the holes using a 1/2 inch, course thread tap.
Next, connect a wire from one electrode of the stun gun to the outside of one of the spark plugs. Then wire the two spark plugs together on the inside of the chamber. Lastly, wire the outside of the second spark plug to the second electrode of the stun gun.
When the stun gun is activated, there should be two large sparks inside of the chamber.
I just taped all the wires and the stun gun to the outside of the chamber for now, but I do plan on making a m
Step 6: Final Construction
Since all of the parts are threaded, you can screw them all together using either teflon tape or a yellow gas line tape as a sealant.
Once all the parts are screwed together (except the end cap), you will need to connect the metering system to the combustion chamber via the brass saddle. To do this you will need to drill a 3/4 inch hole where you want to connect the saddle. Then epoxy and tighten the saddle around the hole using a liquid gasket silicon to seal around the hole.
A wooden block is recommended to support the other end of the meter.
Next you will have to drill and tap two holes for the spark plugs. Drill the holes using a 7/16 inch bit and tap using a 1/2 inch, course thread tap. Then epoxy and screw the spark plugs in.
Wire the two spark plugs together on the inside and to the stun gun on the outside.
Now, once everything on the inside of the chamber is taken care of, you can screw the end cap on.
When the whole cannon is constructed, you can check for leaks by pressurizing the system and spraying some sort of cleaner on all of the joints. If a part is leaking, bubbles should start to form.
A burst disk can be between the two halves of the union to seal of the chamber. The easiest and cheapest way to make a burst disk is to just fold a sheet of tin foil into 8-10 layers.
Step 7: Howitzer Design
The howitzer mount is both designed with practicality and style in mind. Really, for any cannon this big you will need some sort of mount to support it and take the stress off of the join connecting the chamber to the barrel.
The mount is able to do 4 principle things:
- Support the cannon for firing
- Supply a method for controlling the elevation of the gun
- Afford a means of transportation
- Keep the cannon from moving too much from recoil
I made the mount from spare wood and parts I found laying around my home, so it cost me little to no money.
Step 8: Wheels/ Axle + Base
Wheels: The wheels were just two old spare tires I found laying around my yard. Old wheel barrow wheels will probably work as well.
I had to screw a piece of wood on both sides of the wheel and then drill a hole into the center of it (Figure 2). This allowed a 1 1/2 inch pipe to fit snugly through the tire.
Axle: The axle was made of a 1 1/2 inch steel pipe that was capped on the ends by two pipe tighteners.
Spacers : Two spacers were made of 3" long sections ABS pipe to separate the wheel from the base.
Base: The base is the wooden frame that fits on the axle and supports the cannon. This part is important to make strong as it will be taking a lot of weight. The base was made from 2 by 8 boards and screwed together to make a base two feet wide and two feet tall as shown in Figure 1.
Step 9: Mount Trail
The trails will balance and support the mount when apart, and allow the mount to be towed when closed together. The trails are made of boards 6 1/2 feet long attached to a door hinge.
Step 10: Cannon Cradle
The cradle is critical to make accurately since it will be directly holding the cannon. The main section is two 6 1/2 foot sections of board screwed together to form a 'V'. Then spacers for the different section of the cannon are screwed in at different intervals.
The spacers must be thick enough to support the different sized barrel and chamber. It will take some time to form them to the right thickness.
Step 11: Final Construction
First, screw the trails onto either side of the base. Then construct a support for the cradle that can fit into a pipe running along the top of the base. Finally, get either two lengths of rope or two cargo straps and put one on either side of the base connecting the main axle to the cradle. this will provide a way of controlling elevation. You just tighten one and loosen the other to change the elevation.
Then, put some sort of cap on the cradle to prevent the cannon from sliding out and bunjee cord the cannon into the cradle
Step 12: Operation
First, set up the mount and fix an elevation. Then attach the cannon to the mount and make sure it is secure. Insert a burst disk made of tin foil into the steel union.
Charge Fuel Pressure in Meter (PSI) Air Pressure in Chamber(PSI)
x1 30 15
x2 60 or 2 charges of 30 30
x3 90, or 2 charges of 45 45
x4 2 charges of 60 60
To operate the cannon, first open the propane valve and measure a certain pressure into the meter. Release this into the chamber and use an air compressor to add a certain pressure of air into the chamber. For more information see http://www.spudfiles.com/spud_wiki/index.php?title=Stoichiometric
Then stand aside, turn the stun gun on, press the button, and listen to the boom.