20,000 FPS Potato Gun Slow-Mo & Cartridge Combustion

Capture

Ever wonder why a primer is at the rear of a cartridge and not the center? Would it not make more sense to have the ignition in the middle of the case to get a better powder burn? At face value the questions seem like a logical hypothesis.

Testing the question is Dustin with Smarter Every Day, a fantastic YouTube channel that utilizes graphics, slow-motion video capture and graphics to break down often complex phenomena like how cats always land on their feet and burning astronaut pee. Or interesting topics like being strapped into a powerless helicopter and how to escape from a car window. 

Using a clear potato gun (which was used to demolish a watermelon to spectacular slow-mo effect), Destin tests two permutations of ignition, at the rear of the cannon and in the middle. Surprising to me, the ignition in the middle is actually the less efficient of the two methods due to the fuel consuming oxygen prior to reaching the edges of the combustion chamber.

By his experiment, the rear-igniting cartridge is actually the most efficient capture of energy as the oxygen needed for the explosion is consumed at a steady rate, not outpacing the rate of combustion.

Enjoy the video below, and you will at least be smarter today.



Nathan S.

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

Nathan can be reached at Nathan.S@TheFirearmBlog.com

The above post is my opinion and does not reflect the views of any company or organization.


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  • SignalFromTheRim

    Beautiful!

  • BillC

    It’s also better to spray first and then push the potato down the barrel because it compresses the air which increases the efficiency.

  • Darkpr0

    This analysis does not hold very well for firearms as potato cannons use a mixture of fuel and air (which contains the oxygen). It requires the fuel and air to mix properly to work. Firearms (black powder or smokless) have oxidizer in the powder; they don’t need air, and they don’t need to mix. His hypothesis of the pressure wave compressing leftovers at the back of the chamber is neat, but not directly usable for firearms using powder. The pressure wave may indeed affect burn rate, but you won’t be able to demonstrate it in this way. This experiment is not an effective piece of evidence to argue that a rear-seated primer is the most efficient method, regardless of whether it ends up being true or not.

    • Yeah, center ignition is actually better for small arms ammunition, it’s just tricky to execute properly in a high pressure cartridge.

      • Paladin

        Would it not work to simply put a “straw” on the primer so that the sparks would travel into the middle of the cartridge before igniting the powder charge?

        • Yes, that’s how some artillery rounds do it. They are called “flash tubes”.

  • gunsandrockets

    Okay, that clear spud-gun is pretty cool. And observing the burn was fun. But what I’d like to know is, did the burn efficiency make any difference in effectiveness? Was there any difference in projectile MV?

    Isn’t it possible that the center ignition system would have delivered the same MV with a smaller amount of fuel?

    That experiment also makes me wonder about a combustion chamber with different proportions, perhaps a shorter and wider chamber is better.

    • Roy G Bunting

      The problem i see with short wide chambers is the pressure on the shoulder as it is funnelled down the barrel. With a firearm, not a big deal, with a potato gun it would have a higher risk of the combustion chamber separating at the shoulder. But I could be wrong.

      • Gary Kirk

        RDP.. Rapid Disassembly of Parts

  • Mack

    I read that title totally wrong. I was thinking there was no way they could get a potato to 20,000 feet per second but it had me very interested. When i found out it was frames per second, i still wasn’t disappointed, but when ahhh you dummy.

    Awesome video.

    • Cymond

      Glad I’m not the only one!

      • dmh

        me too

        • BigFED

          dildo oops ditto!

          • mbrd

            yep…

  • smartacus

    i was thinking of it from an automotive perspective of gas expansion moving in every direction outward of the flame kernel.
    If it is heading in one direction, it can accelerate metal down the cylinder at very high rates of speed.

    • Gary Kirk

      Even in the ICE aspect.. A centrally mounted ignition point at one end of the cylinder works better, i.e.. The hemi, although the most famous, not the only engine family to use said tech.. Molar just got it right

      • smartacus

        exactly. At one end of the cylinder works better.

        • Gary Kirk

          And in the center

          • smartacus

            like primers

          • Gary Kirk

            Ding! As long as both of these seemingly not, but completely intertwined industries have been in existence, they’ve pretty much figured things out.. There are still oddballs that come along (rotary engines.. Uuuggh), and who remembers pin fired cartridges?

          • smartacus

            oh no! not pin fire. keep it away.
            *holding two fried-chicken bones into a cross now 😉

  • Southpaw89

    Ok, that is some awesome footage.

  • Kivaari

    Look at the US recoil-less rifles. Alpha 66, the anti-Castro group in the ’60’s gave instructions on reloading the rounds. There is a central tube that lights the charge full length. First drill out the spent primer. Insert a .303 British case (live primer no bullet). Second put a paper tube inside the central tube. Third fill that with black powder. Fourth, insert butcher paper to cover the case wall holes. Fill completely with 4831, leaving only enough room for the projectile. Fit the slug. You are ready to go. From memory they then tied it across the bow of a fast small boat, raced into Havana Harbor, went perpendicular to the Soviet merchant ship Baka, and tourched off a round. It was a Limpet mine that sent it to the harbor floor. Now, gun magazines did have some interesting articles. IIRC it was Gun World.

    • It wouldn’t surprise me that the article was written by Robert K. Brown during his pre-SOF Magazine career. If it wasn’t Gun World, it might have been published in Guns.

      • Kivaari

        Very likely.

  • IndyToddrick

    I’m not too familiar with spud guns. What was the propellant? Ether, hair spray?

    • Sianmink

      Traditionally it’s hair spray, but some alternate fuels have been used.

      • mbrd

        aquanet… nothing else will work. i am assured of this.

  • Pete Sheppard

    WHOA! That was flat-out AWESOME!!!

  • Bob

    way back in 1974 or so, a Champion Spark Plug man came to the tech school and did a demo similar to this to demonstrate the SMOOTH burn rate of premium fuel vs regular fuel and what a “ping” (detonation) does to a piston.
    It was quite interesting, and the temperature increase by advancing the timing on your automobile (muscle car) back then was about 600 degree increase for only 2 degrees advance (under the right conditions of course), which can burn a hole in the pistons.
    He had a thermocouple hooked up also.

  • jon spencer

    Inside 3″50’s and 5″54’s shells the primer has a tube to push the flame nearer to the center of the propellent.
    At least they did many years ago.

  • Karl Vanhooten

    Back in the ’60s we used to duct tape opened steel (NOT aluminum) beer cans together to shoot worn tennis balls using Ideal lighter fluid as a propellant. It was a blast. The modern potato guns are far superior, and probably more lethal when one considers we were shooting from one end of a dorm hall toward a friend on the other end.

    • mbrd

      the 60’s??? godd@#$%it, i thought we invented that in the 70’s!

      we were smart enough to use tennis ball cans, though.

      • throwedoff

        We did the same thing and called them “Polish goose guns”. By accident we found out you could scare the snot out of your neighbors and little kids by dribbling a little lighter fluid on the tennis ball before dropping in down the muzzle. I’m surprised we didn’t burn anything down.

  • Archie Montgomery

    Probably the biggest reason for having the primer at the base of the cartridge is cost in manufacturing. The Dreyse “Needle Gun” comes to mind. A more modern solution is to attach a hollow tube to the interior of the case above the firing pin hole. The primer then flames through the tube and ignites the propellent charge at the end of the tube – which can be varied in manufacturing.

    However, either solution runs up the price of ammunition making; or makes the ammunition quite fragile in handling.

    By the way, the older style 106mm recoilless rifle ammunition uses a ‘long primer’. The ignitor compound is located essentially as in a primer, but it has a long – half the length of the cartridge or so – perforated tube attached to funnel the ignition flame through the propellant charge.

    One criticism of the testing and conclusion. As Darkpro already mentioned, smokeless powder contains an oxidizer and does not need ‘free oxygen’ in order to burn.

    I would recommend a book called “Gibbs Cartridges and Front Ignition Loading Technique” from Wolfe Publishing (1991 – not sure if still published). It is actually a composite of Rocky Gibbs wildcats AND the front ignition idea. The front ignition section is credited to Rocky Gibbs.

    • BigFED

      The “long primer” rod was not unique to the 106mm RR or 3″x50, 5″x38/54 Navy guns or many of those self contained, large caliber rounds. They were also used in 75mm, 76mm, 90mm and 105mm tank rounds. It was done to initiate the powder burn in an better manner. Without those rod, the powder would start to burn and that would cause the compression and initiate the projectile moving down the barrel and the powder charger would never be completed before the projectile reached the muzzle. With the rods, powder burn was initiated from the center of the round and the burn rate of the propellant would be completed from the CENTER of the round and build pressure fast enough to provide the velocity needed. There was alway an OVER burn since the blast/muzzle flash was tremendous for their size!

      BTW, We used to take the primer rods out of the cases and make “swagger”/sticks for the officers to use as pointers. Braze a .50 cal BMG bullet on one end (the front) and an EMPTY .50 cal BMG on the other end, tada, a cool pointer for all those map and sand table presentations! 90mm tank rounds were used in may day!

      • Archie Montgomery

        Thank you. The 106mm RR was the only one I knew of for sure. The others don’t surprise me much; it’s the same problem and solution in all cases. (No pun intended.)

        My repurposing plan was to remove the primer from the fired case, then use a roll of opaque plastic within the case; insert a light (cord from the bottom) and a wooden ‘bullet’ and use them as table or mantle lamps for the man cave. Probably have to sand down and paint the cases.

        Again. Thanks for the additional information.

  • Secundius

    At ~3.78-miles/second, Potato would VAPORIZE Long Before EVER Reaching its Target…