EXPERIMENTAL COMBUSTION CANNON
(XCC)
Updated: 2/2/2006
Following the Bolt-action rifle I caught the bug for ultra long ranges. A friend and I kicked around the idea of scaling down a potatoe gun to shoot Nerf darts. The project started with small demonstrators pieced together in 10 minutes. The idea evolved into the XCC which still uses combustion but also incorporates some other unique features. In an attempt to increase ROF for a combustion weapon two tanks were setup. The main tank is situated just behind the barrel as you would see in any spud gun. I also included a reservoir that resided just below the main tank. This tank would be fueled and pressurized. A valve is depressed that lets compressed fuel/air into the main chamber. A standard quartz lighter was used for ignition. The cannon had a simple breach that accepted the same shells as the bolt-action rifle. The breach was located just ahead of the main chamber and was held closed with rubber bands. This cannon was never intended for active duty but rather just an experiment in the physics of high speed dart propulsion. It was a lot of fun and worked quite well.
Initially the XCC was intended to be just a scale down boring old potatoe gun. It was to have a tank comprised of 3" PVC and the appropriate endcaps and a barrel of about 30" of 1/2" PVC. That was the initial design. A standard aim-n-flame type lighter was to be used to ignite the gasses. Initial construction yielded this very design and it was tested with typical results. Unfortunately there are no images of this configuration as it did not stay intact for very long.
The rate of fire was extremely low. This was due to the fact that you had to remove the barrel and refuel the tank between each shot. Down time between shows was around 20 seconds. A long time. I came up with a concept of having a reservoir of fuel/air that could be quickly shot into the combustion chamber for a another shot. A reservoir of about twice the volume of the combustion chamber was added below the combustion chamber.
The reservoir was fitted with a ball valve on one end. This allowed easy access to the chamber to inject fuel. the ball valve was then closed and the tank was pressurized with a pump cannibalized from an RF20. The high pressure fuel mixture was then injected into the combustion chamber through a vale operated by the operator. A dose of fuel was injected and then ignited. For a full tank of fuel and high pressurization the cannon was good for 4 or 5 shots before needing fuel or repressurization.
The above figure shows the layout and sizes of the combustion chamber (top) and fuel reservoir (bottom). The two hoses entering the reservoir are for input from the pump and output to the valve and combustion chamber. The combustion lighter is inserted through the rear of the combustion chamber and is gripped with the right hand. The fueling valve is placed in the stock just above the lighter trigger. The valve is actuated with the operators thumb as he holds the lighter. It is a very ergonomic setup. The pump was fitted in the front of the stock on top of the tank. The pump handle extends out over the breach. it became comfortable to tilt the weapon up to fuel and pump it up because both the filling valve and the pump were located in the same area. The figure below shows a better view of the plumbing and controls.
Rate of fire had been increased by eliminating the need to refuel between each shot but there was still the need to reload the barrel each time. This process itself as very time consuming and negated the purpose of the tank/reservoir system. The solution involved the use of shells again. I already had a supply of shells from the bolt action rifle so it made sense to make them usable in this cannon. A simple reciprocating breach was fabricated to house the shells. it would be held shut with elastic bands to ensure good seals on the shells and the barrel.
To load the gun, the barrel is pulled forward and a shell is dropped into the breach. The barrel is released and the elastic bands pull the barrel back and seat the shell at the end of the barrel and provide a strong sealing force. This addition brought the rate of fire way up. Reloading the cannon took only a few seconds now and it was ready to be fired again.
Ordinary PVC piping is the main material used in the construction of the XCC. The sizes used are as follows:
3" PVC (Tanks)
1" 200 psi PVC (Breach)
1/2" PVC Couplers (Bolt, Barrel spacers, Shell Rim, etc)
1/2" PVC (Barrel, Shells, etc)
Some other special PVC fittings were used in certain situations. A 3/4" Ball valve was used to charge the main reservoir and allow for pressurization. For situations where a flat surface was required balsa sheeting was used. I use 1/8" sheeting and hand picked the hardest pieces for durability. Other materials could be substituted in place of the balsa, like lexan or another type of wood. I like balsa because it is easy to work worth, can be hard, and I have an abundance of it. To space the PVC pieces inside one another, I used electrical tape. This works extremely well and can produce extremely tight fits. I personally use CA glue (aka superglue) to due all the bonding involved. It adheres extremely well to both PVC and balsa. The advantage is the near instant drying time. CA will hold just as good as PVC cement.
A variety of propellants were tried. Some of the more exotic ones were black powder, Bacardi 151, and a few other random combustibles. In reality the fuels used were aquanet hairspray and WD40. Through trial and error we found that a 50/50 mixture of WD40 and hairspray performed the best. I have no proof of this, but it is just the way it seemed.
Ignition was sometimes sketchy. Getting the right fuel air mixture depended on the pressurization initially and it was never exact or the same each time. through more trial and error a good protocol of time to fuel and pumps was come up with that gave quasi-reliable ignition.
Firing produced a healthy *whooomp* noise and about a 2 foot blue muzzle flash. Contrary to popular belief the dart receives no mal effects from the launch. It does not come out on fire. It does not get melted or burnt. In fact the dart doesn`t even get warm. It is not in contact with the flame long enough.
The first rendition of the cannon had only the combustion chamber, barrel, and lighter. It looked just like your average spud gun. The fuel reservoir, stock, and breach were quickly added and led to the version shown below. In this version the fueling valve was located in a pistol grip attached to the underside of the reservoir.
this configuration proved uncomfortable and was the filling valve was quickly relocated to thumb operation in the stock. This put all the controls in one hand so that the other could deal with reloading. A coat of black paint finished the beast off and the version below is the current configuration. The ball valve has been removed in the picture, but it is still needed.
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All Image and Content © Evan Neblett 2006