12/12/06 Big Update: Got the new 8' barrel! Using Oxygen/MAPP at atmospheric pressure yielded a 25% increase in maximum range, now reaching almost 1000 feet.
There are a few new cool effects:
- You can hear a distinct "ssssss" whistle of the spud splitting the air. Not a crack (probably nowhere near supersonic yet) but it is a distinctive sound I never hear before.
- Interesting sound effect shooting at a brick building 200' away. Very distinct "POW pow-smack." That is the initial blast (POW), the echo of the blast (pow), then the spud hitting the building (smack). The amount of time between each sound was similar though closer together on the pow-smack, which tells me the spud averaged more than 1/2 the speed of sound. I know we can look at a digitized recording to measure the ms between spikes. Will do this some day.
- At least in the cold, makes a cool momentary white blast fog at the end of the barrel about 3' around and 4' long. Fog disappears instantly.
- A hard-packed snowball exited the barrel as a cloud of snow.
- It is no longer possible to track the path of the fired spud. It vanishes into the distance too quickly to follow. I measured range by watching lake surface for landing spot.
Several minor problems crept up:
- Loud enough now to need hearing protection. Really makes your ears ring.
- Low-flow valve was shooting a small 4" flame before snapping shut. I suspect that oxy/fuel mix was filling the general area of the valve, so for a split second combustion outside the valve was balancing the combustion inside the valve, thus preventing it from snapping shut faster. Was probably losing some potential chamber pressure.
- Weather was cold, around 28 degrees F. I suspect that this robbed huge amounts of combustion pressure by limiting the theoretical adiabatic temperature.
- Cold also caused reduced maximum fuel tank pressure below what is needed to use the fuel meter pipe correctly. Using oxygen greatly increases the amount of fuel that can be used. Ended up using a lower pressure but charging twice.
- Cold makes the heavy clear vinyl fresh air supply hose inflexible. Very difficult to keep it on the blower nipple.
- Plumbing configuration was making the MAPP tank point downhill when the gun was elevated for trajectory. This allowed liquid MAPP into the regulator which caused the regulator to freeze and freely vent raw MAPP gas. Not a nice odor.
- The pulley & counter weight used to support the 8' barrel is a pain in the ascii. It is very effective for supporting the barrel, but you end up constantly trying to reach through the wire. The wire hanging straight down at the back of the gun also interferes with the panning action of the gun mount.
- Starting to get minor spud shredding before exiting the barrel.
What's next: Extreme hybrid mode. Built, but not tested yet. I will fill you in as soon as the weather allows. This just may become the most powerful combustion spud gun on the planet. Sure wish I could find a competition in my area (Northeast US).
- Sealed off low-flow vent with solid hunk of aluminum
- Replaced fresh air check valve with ball valve for perfect seal
- Re-sealed ignition wire hole (was leaking at high chamber pressure)
- Added silicone gasket glop to chamber ends to make perfect seal
- Closing the breech on a piece of very heavy polyethylene makes a nice burst disc that easily holds 30PSI, and double layer should hold even more. Couldn't be simpler! Would like to find a non-hydrocarbon material because polyethylene becomes very flammable in pure oxygen.
- Built much longer remote trigger for added safety during extreme testing.
- Charging will be much different than the old meter pipe method. First fuel will pressurize the whole chamber to say 15PSI, reaching the setpoint of the regulator. Then oxygen will continue to raise the pressure to say 30PSI. This will take a lot of experimentation.
- I will test the anti-materiel destructive potential on various materials with various projectiles.
- Still trying to find a Shooting Chrony to borrow/rent.
- Built a jig for freezing 3" long ice slugs, very uniform in size and shape. Makes 7 at a time. Should be very destructive.
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12/02/06 Update: new interior photos
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Link to huge photo
All aluminum, breech loading, car ignition. Fuel with any combination of metered air, oxygen, acetylene, MAPP, or propane. Extremely destructive. Extremely loud with acetylene (and safer than you might think). Detailed web page here. Massive pan & tilt tripod is collapsible for transport. These photos are a bit out of date. The fuel system has evolved quite a bit over the past year and the gun is much cooler looking now. Built 8' barrel but have not tried it out yet.
My friend Jason demolishing a picnic table using air/MAPP.
Link to animated breech action
