SpudFiles

Joined: 01 Apr 2006
Posts: 35
I don't know if this is or would be considered a new topic; probably not, but I have a question concerning the "assumed" power of combustion powered cannons. When I first logged in to this site today as a 38 year- old newbie to this site, I saw where a fella who had been out of the combustion scene for awhile and had gone on to pneumatics said that thunder guns only produce @ 30-60 pounds of thrust. -I've yet to see a chart anywhere to calculate that, outside of propane metered systems.How does one come up with that figure? Do we set up a bathroom scale to shoot and assume that the number shown in the little window is the amount of pressure that was pushing the spud along? AND, I question its voracity due to the distances I've achieved with heptane, using fist sized spuds down to spud/carrot, etc. plugs @ thumb sized. Can any one clarify this for me?
I am aware of the ballistics involved in carrying a heavy projectile down range, but 30 to 60 pounds of thrust ? There must be more than that involved if I'm shooting LARGE russet potato's through two sheets of 3/4" subflooring married together with sub-floor adhesive from 160' (taped) away.-- That from a 42" x 4" chamber, double sparked off of a BBQ ignitor, and a 2"x 6' barrel. Propellant : Heptane.
With this same setup I can shoot a raquet ball through slightly less than taught chicken wire from @ 80' away, and not even cut up the ball; but leaving the tell-tale appearance in the wire where it stretched to the give point like wrapping foil around a ball.
Now, come on people, don't just come in here and view this, shrug your shoulders and leave. Let's have some real think- tank action here; after-all, isn't this sight touted as the "Greatest Spud-Gun Sight" on the web ?
Put your money where your mouth is and walk the walk. Give me info or give me ignorance or give me spud-bux compensatory to back when I first posted this query.

if you shot a bathroom scale it would break in half

voracity?

Russ, I think he means putting the launcher of the scale when firing. In order to determine the veracity of the 30-60 lbs of thrust assertion, it's a simple question of applying Newton's Third Law - for every action there is an equal and opposite reaction. basically, the momentum of the spud flying forwards is identical to the momentum of the spudgun flying backwards.

A simple experiment therefore would be to use a chronograph to determine the velocity of a spud of a known weight, multiplying the weight by the velocity will give you the spud's momentum. Dividing this figure by the weight of the spudgun will give you the velocity with which the launcher itself moves back, allowing you to calculate the actual thrust produced.

As a slight deviation, how exactly do you use heptane as propellant?

Wouldn't putting a pressure gauge in the chamber give you a fairly accurit number? I ask because i don't know. Seems to me it would, if you trust the gauge.

jrrdw wrote:Wouldn't putting a pressure gauge in the chamber give you a fairly accurit number? I ask because i don't know. Seems to me it would, if you trust the gauge.

That would give you the pressure generated in the chamber, but there are a lot more variables you need to know to calculate the thrust (recoil, essentially) produced.

Two handles, 16" apart. The first 12" from the butt-end, the second 16" forward of the firng handle, shaped much like a Black and Decker Powershot stapler, but actually a converted, junked metal detector handle. The second handle, a piece of 1/4" aluminum sheet stock cut to 1 1/4"w and bent with bending irons into the outline of a mushroom and mounted to two wood blocks that 1, conjtour the barrel & 2, provide a mount surface for the forward handle that looks much like the forward handle on a weed-wacker. The base piece actually steps down (or up) to fit both the chamber OD and the reducer to barrel OD. When it fires, it REALLY likes to LURCH backwards; kind of knocking you off balance even if you're expecting it...and I weigh 260 lbs.. The weight of the canon is just shy of 17 pounds, as it is also doubled in two places with milled out couplers.
It is big enough to literally club someone to death if you had to, in on or two whacks; carrying it in one hand w/ out a barrel attached is even awkward.
I about roflmao about shooting the scale in half... that was kinda the idea, sticking the scale meter in place with the impact mushrooming inwards the foot-pad. I suppose we could butt the monster up against it and see what that produces,although I think that'd probably destroy the scale,too.
Say, did you know it'll shoot a 3" limb off a tree with no problem?, yeah, off up in a tall Ponderosa pine! That was truly thrilling!
I don't have a chronograph,maybe I could take it to a gunsmith or a rifle range... or I could have my wife race the potato to some point parallel
On the sidenote of how to use the Heptane as a propellant, I attach a modified freon can hose(it has a one-way valve) to the top of the tank and use an old airless sprayer gun to pffft some( after the surgial tubing expands a bit) into the chamber through the doubled side wall about 16" from the back. Incidently, the back is a screw-in clean-out. And the chamber is gray sch40 and the sleeves (couplers) are ABS.
It can shoot a golf ball through a car door, 3/4" ship-lap siding, 4 pieces of 15/32" OSB, twelve feet from my table saw- through the old three-panel man-door of my shop- 26' further to through a 5/8"x6" pine fence board 4 years old on acroos the next yard 42' to leave @ a 1/3 the size of the golf ball impression (showing signs of the dimples) in the neighbors steel-pole barn siding and the bouncing back up and over both of our yards and coming down in the intersection to the northwest of me-on the otherside of the block,3 houses over and back. My kid saw it go back over that way and heard it "chuck" off the pavement( at least we hope it was the pavement!) and presumably bounce again,as we never did find it; not that we went running after it right away,with all the noise it made! , it does a nice job on street lights,too, as well as snow-men (wow, never thought exploding snowmen could ever be fun to watch!)... the kids were so impressed by it that they built several in the back yard for the sole purpose of shooting!

Theres a video of a snow man bitting the dust some where around here. It's really funny to watch the 1st time, i think it had some fruit colored water in a gallon milk jug, of course the water was red. Are you going to try the math that Jack put out for you?

Yeah... but I don't have a chronograph; though I could probably have somebody run a stopwatch while I fire the canon, and try to see if we can work up an average time from the time of bawooom to kurthud or thwaaack
There's an old Grampa living next door who had a chance to fire it and his immediate reaction was that we should go out to the woods and get a deer!! I don't know about that, but I bet it'd sure hurt like heck or at least break a bone, who knows maybe if I shot the deer in the head?
Edit: I've received a note in my box from a fellow member who has expressed the very best intention of advising me about my droning on about Grampa next door and the deer.... that perhaps it sounded a bit harsh and perhaps I may get banned over it if PC see's it as it is written. The thing to remember is that if you've EVER gone hunting big game( deer or better) you know that unless you hit the breadbasket or the very core of the brain, the latter of which can be done via one of two ways--which I've done, the critter will keep moving until it is either bled out or exhausted. I would NEVER attempt to take out a deer with a spud-gun ---unless it was a matter of survival and starvation...but what are the chances of ending up in a scenario like that with a spud gun on hand?
Couple this with the point that Pc knows of me from elsewhere; and though I may be an outspoken dufus at times, I am certainly not one to act on conjecture. Especially since even the old man knew that though the canon held a certain amount of power, it surely would be fool-hardy at best to REALLY go after a deer with one.
Your concern, FRIEND, is well taken and treasured. Your anonymity is safe with me. You are amongst the coolest 8)

At one point, using the <a href="http://www.burntlatke.com/cb.html">burnt latke data</a>, I calculated the pressure in the barrel.

The burnt latke data was generated by taking a velocity reading with a chronograph, cutting the barrel shorter, taking another reading... and so on, with the goal of finding the ratio at which the velocity was maximized.

The math I used was as thus:
1) figure out the volume of each pipe segment between each tested ratio. (PiR²L)
2) figure out the energy required to accelerate the projectile from the speed at the first ratio to the speed at the second ratio. ((1/2MV₂²) - (1/2MV₁²))
3) Find the amount of force that must have been exerted per area. (Energy/Volume)

Now, those formulas are rough, in that most of 'em involved some constants so that'd we get the units we wanted, but you get the idea.

...now, as I was saying, I applied this analysis, and we got this curve:
http://img.photobucket.com/albums/v611/ ... svspos.png

I generally read this as meaning we have peak pressures of about 55 psi.
(I suspect the 70 psi data point is wrong.)

The peak chamber pressure in "typical" combustion spud guns (propane + air) has been measured a couple of times. IIRC nobody has ever measured a peak pressure greater than about 60 PSIG. Unfortunately, measuring the peak pressure isn't as easy as it sounds. The pressure peak is so short that many types of peak-recording gauges may not respond fast enough.

Like Boilingleadbath said, it is fairly easy to calculate a rough estimate of the pressure in the gun based on the mass of the projectile and the muzzle velocity. The calculation isn't perfect but it should be close enough. I've calculated it, as have others, and it typically falls around 50 to 60 PSIG.

Also, remember that we are not talking about 50~60 pounds of thrust. We are talking about pounds of pressure. In a 2" diameter barrel the force (thrust) on the spud's ass is 150 to 180 pounds. (Area of a 2" D barrel is about 3in², force=pressure x area).

I've been working (off and on) on a simulator for combustion guns similar to GGDT for compressed air guns. I posted perhaps 30 pages of background, calculations, thoughts etc. on the SpudTech forum earlier this year. Unfortunately, those post all appear to be history now.

The peak pressure in the chamber really is in the vicinity of 50~60 PSIG. The theoretical peak pressure for the combustion of propane in air is 120 PSIG. But, combustion is an amazingly slow process in these guns (see http://home.earthlink.net/~jimsluka/Piezo_Spud.html). Once the pressure in the chamber gets high enough to overcome the static friction between the spud and barrel the spud starts to move, the pressure rise slows (or even drops), and the spud exits the barrel before combustion is complete.

A non-intuitive result of the slow speed of combustion is that the more static friction there is the higher the muzzle velocity is. This is true up to the point where the static friction is greater than the peak pressure in the chamber, at which point the spud doesn't move at all. This is why combustion guns perform better if the spud cutter muzzle of the barrel is beveled on both the outside and inside of the barrel.

Feral_patriot, you sound like you have enough physics / math / balllistics background to do the basic calculation. If you don't have a chrony to measure muzzle velocities then figure you get 100 FPS for every 12" of barrel (spuds from a 2"D barrel, hydrocarbon+air combustion mix). Calculate the kinetic energy of a 100g spud moving at say 600 FPS. Compare that number to a 0.22 or 0.30-06 round and see what it tells you about the relative energies of the various weapons.

Then calculate how fast a 100g spud is moving at the end of a 5" barrel assuming a constant pressure behind the spud of 50 PSIG (ignore dynamic friction).

BTW, any thoughts you have on modeling spud guns would be of interest to most hard-core spudders. The world of spudgunning is long on theories (most wrong), short on reliable models (particularly for combustion guns) and almost completely lacking in real data.