Spud Gun Fuel Experiments

These are a few attempts to quantify and scientificify some aspects of the noble art of spudding. Thanks to Bob Simon for providing the inspiration and pointing out that the world was lacking in hard facts.

Methyl Hydrate / White Gas

I tried methanol or methyl hydrate as it has a broad range for flammability in air. This would help by making the amount of spray less critical. As it turns out it also makes it possible to regulate the power to some degree. It is also cheap and easy to get. The first tests provided hopeful but not satisfactory results. I used a pump sprayer set to fine mist (trigger type) to spray the fuel into the chamber. Mine sprays 9ml in 16 pumps or 0.5625 ml per squeeze and my chamber is 3" by 12.5" inside. It worked at about four pumps. There was a lot of residue after the shot. It worked better if I pre heated the gun by lighting a load and letting the barrel act as a chimney. As the gun cooled (air temp. about 40 F, three or four shots) the gun misfired (flooped) or would not fire. Power on the good shots was better than hair spray.

• To provide better ignition I added 20% white gas (camping fuel) to the alcohol. The amount could be varied for different temperatures? I learned this trick from many cold days flying model aircraft. Those little engines need a perk in cold weather to get them started. This did the trick! I have done ignition tests and am very happy with the results. These were done with a wad of newspaper instead of a spud. I will do power comparisons next.
• Test gun 12" x 3" chamber, 3' x 1.5" barrel, approx. 2" spud length. Air temp 60 - 70 deg. F Pump at fine spray. 16 pumps = 9 ml.

IGNITION TEST 20% white gas, 80% methyl hydrate

• 1 pump ignites , weak blast.
• 2 pumps ignites , better blast. weaker than hair spray.
• 3 pumps ignites , very loud blast , some unburned fuel left in chamber.
• 4 pumps no ignition.

IGNITION TEST 100% white gas

• 1/2 pump no ignition.
• 1 pump ignites , good blast.
• 2 pumps no ignition.
• As you can see, the range is broadened considerably with the alcohol fuel and the white gas solves the ignition and cold air troubles that were experienced with the straight alky.

Here is some flammability data from a book. The optimum mixture is somewhere between these limits (more experiments? Or do you have a better book?)

In Air By Volume

• hydrogen 4% - 74% (wow!)
• butane 2% - 8.6% (lighter fuel)
• propane 2% - 9%
• methane 5% - 15% (natural gas should be close)
• methanol 7% - 37% (methyl hydrate should be close)
• gasoline ? - ? (I need more books.)

Power Test of Alcohol / White Gas Propellant

Today I tested more of the 20% alcohol/white gas mix. The results were confusing. After the exciting results of yesterday, I was a bit disappointed. I could not get the same results at all.

Test gun 12" x 3" chamber, 3' x 1.5" barrel, approx. 2" spud length. Air temp 60 - 70 deg. F Pump at fine spray. 16 pumps = 9 ml.

• 4 pumps no ignition
• 3 pumps no ignition
• 2 pumps no ignition
• 1 pump ignites, good power, similar to hair spray
• 1/2 pump ignites, low power

• Recheck the pump, still 16 pumps = 9 ml
• Mix new batch of propellant, no change.
• These results look the same as 100% white gas.
• Consistency is poor.
• Better consistency was achieved with 2 pumps then let the chamber stay open for a few seconds.

Why?

I think air temperature may be the reason. Today was much warmer (near 70 F) than last time. In the colder conditions the alcohol probably was not evaporating, so the mixture actually involved in the ignition was not changing in proportion to the amount of pumps. The excess liquid shot out with the load, so I didn't notice a residue until it was excessive. The white gas probably works nearly the same at the lower temperature as at the higher. I guess we need to keep track of conditions to make the data more useful. I am not too bummed out though, I got 50 lb. of great spuds for free today from the local market! They have a few sprouts on them. Terms like projectile, spud gun, etc. seemed to confuse them, "feed potatoes" did the trick.

Propellant test - Propane

Today I made some modifications to the test gun for testing propane. -The chamber was accurately measured by filling it with water. -A 5/32" hole was drilled in the plug to inject the fuel into. Don't worry about loosing power through this hole, I measured the loss with the seringe in place during a shot. The leakage was 32cc of exhaust gas through a 5/64" hole. That should work out to about 128cc for the 5/32" hole. This hole makes the loading more convenient, and eliminates variations in the mix caused by air and fuel moving in and out of the gun while you fumble with the plug.

I purchased a large syringe from the vet. It's volume is 60 cc and it is graduated 1 cc increments. It is also marked in ounces. 2 oz = 59 cc. To fill the syringe, I unscrewed the head of a propane torch. Turn on the valve, and push the syringe into the end of the pipe. The seringe fills by itself from the pressure. A short piece of surgical tubing would make it easier to get a seal. Then the gun is loaded normally (with the plug off to let air in) and the fuel is injected into the hole in the plug. The syringe can be filled again for adding more than 60 cc. No significant amount of gasses move through the fill hole.

Test gun - 3"x12" chamber, 1.5" x 3' barrel

Measured chamber volume - 1650 cc

approx. 2" spud length

Air temp - 45 deg. F

Humidity - low

Results

• 60 cc no ignition
• 65 cc good power, loud bang
• 70 cc great power, loud bang, 220 yards, 40 yards past hair spray
• 80 cc good power, slightly less sharp bang and feel
• 90 cc good power, some smoke or steam
• 100 cc less power,
• 110 cc foof! 60 yard punt, very quiet but lots of laughing.
• 120 cc no ignition

This propellant is easy to use, has lots of power, and it is cheap. It would be useful to make a slightly smaller chamber that fires well at 60 cc. I am told that 60 cc is the largest syringe available. The test gun is 1650 cc chamber volume and works best at 70 cc (when warm), so it would be about 86% or 1414 cc. The actual length of the 3" pipe is 11".

1650-1414=236 cc, 3" pipe is about 7 cubic inches per inch or 116 cc, 236cc / 116 cc = 2 3/8 inches

The custom propane gun should have a 9" pipe in the chamber. I don't know if all fittings are the same so it would be good to measure the volume before gluing.

I have found some more data on limits of flammability in air. Here is an updated chart.

In Air By Volume

• natural gas 4.7% - 15%
• propane 2.2% - 10% acetylene 2.5% - 81% (I will fire this remotely!)
• water gas 7% - 72% (where do I get this?)

Velocity test

Propane

For the purpose of gathering more detailed data to compare propellants, gun configurations, and other good stuff, I have set up a simple chronograph. It is capable of reading the muzzle velocity within 10 fps.

The setup consists of a wood frame to hold two strips of aluminum foil four feet apart.(not critical, but measure actual distance to use in calculations, mine turned out to be 49"). A nine volt battery provides a signal through the strips. The oscilloscope is triggered by the first strip being broken by the spud. The display is at nine volts and sweeps at 5mS/div. When the second strip breaks the trace jumps to zero volts. A pull down resistor of 10k or so (on both signals ) would make for a more accurate reading. A storage scope would be nice, but if you turn up the intensity and wear ear muffs so as not to close your eyes from the muzzle blast, a normal scope will do.

I am thinking of using my sound blaster and two microphones for a crono. This would make it easy to check down range velocity too. It would also allow easy exchange of test results on the net.

Test conditions

gun - 1650 cc chamber, 3' x 1.5" barrel

propellant - 85cc propane

air temp - 40 deg. F

projectile 2" length spud, 3oz

Data

Calculations

formula used - distance (49 inches) / 12 x time (seconds) = Velocity (fps)

Conclusions

The average velocity for this group was 207.4 fps.

The average of the best shots (clear reading, good tight spud.) 3,4,and 5 was 208 fps. I calculated the acceleration to be 230 G's on the poor little projectile. This is 43 lb. of thrust for a 3oz spud. I calculate an average pressure of 25 psi. Ignoring the effects of recoil, cooling due to expansion and contact, the peak pressure would be 30.7 psi. for a 1650cc chamber and 1044cc 36" barrel. Recoil would reduce effective barrel length by a couple of inches and would make this figure a bit low, the cooling would also make this figure low. If you don't have a scope there are lots of other ways to obtain muzzle velocity. A computer (even a vic 20) with a little machine language programming works very well. It can even give a direct readout of everything you can think of. This could make a nice science project. ( Might take the parents minds off of the usual concerns in the schools.)My sound card idea should work. Record the "whizz" of a near fly by at two locations with two microphones and use an audio editor to make measurements of the time interval.

New Gun Configuration

Today I tested a new gun. We made it with a smaller chamber so that a 60cc syringe could give it a charge of propane in one shot. Its chamber measured 1350cc. It shot just as far as the gun with the 1650cc chamber. Both guns have a 36" x 1.5" barrel. I tested the velocity and got the same speed on both guns - 210 fps.

Whatsit mean?

My guess is that the extra volume on the bigger gun does not increase the initial pressure. You could take a chamber any size you want and it would still generate only 30 lbs pressure. The extra gasses generated are just blown out with the projectile. To test this theory we need a longer barrel to try to take advantage of the extra gas available. If this holds true then there is an optimum chamber size for a given barrel.

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