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Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: Ragnarok » Tue Feb 05, 2008 7:03 pm

POLAND_SPUD wrote:rag what is wrong with using metric units?

I never said that. What I said was, and if you look at the order I of what I originally said (which I can't have changed, because _Fnord has quoted it, and I can't fiddle with his post) - There is no problem with being from a country that isn't the US, a lot of members aren't either, me included - and the reason I had suspected he wasn't was that he had used metric units.

Don't get me wrong, I would never use anything but metric units for a proper calculation, and I would normally if I needed to, I would sooner convert to metric, then convert back again when I was done. There are no real errors from doing that, I know all of my metric to imperial figures more than accurately enough.
1" = 2.54cm (exactly)
1 lb = 0.45359237 kg (exactly)
1 ft lb = 1.3558179483314004 J (exactly, but usually 1.3558 is more than good enough)
14.5 psi = 1 bar (More than accurate enough)

Imperial is fine for a rough estimate. I do tend to think "well, it's about 10 inches across/long/wide" (the important word being about), rather than "25 centimetres".
But for accurate work, a system of 12 inches to the foot, and so on and so forth isn't half as good as a decimal system, and I can't really understand why it remains popular.

Basically Imperial is what my mind does when it's doing rough estimates, metric is what it does for proper maths. And the thing is, the Imperial system is still commonly known as English units, despite the fact that it's used a lot less in England these days.
I can only really think that it's commonly used in pubs (pints of beer), street markets (pound of bananas, although the EU says it should be done in kilos now) and speed limits (and I suppose fuel consumption, which is a little annoying, as fuel is bought in litres so you have to convert to gallons first).
There aren't many cases of something still being marked in Imperial. It might still be a round Imperial weight (for example, many tins are 454g, essentially one pound), but it's marked in metric.

It's the US that uses it more these days really. It's outdated, and I don't suppose it can ever fully die, but it is a bit of a nuisance.
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Unread postAuthor: frogy » Tue Feb 05, 2008 7:25 pm

I do the same thing as you Rag. I measure nearly everything roughly in my mind as inches or something else imperial, but I'd much rather work with metric units...

Coming from someone born and raised in the US, I say that imperial units are far outdated, yet I can't stop using them, due to most civilians (like myself) that use them regularly. As soon as I step into a lab though, I'd never think about using anything other than grams\kilograms, milli\centi\kilo\meter. I rarely every use the term liter, though I often use milliliters.

Some imperial units I don't think I'll ever stop using:
Pint
Quart
Cup
Tea\Tablespoon
Gallon
Inches

I rarely guesstimate weights in grams, but I also rarely need weights (other than chemicals) in grams... I'll never forgot 453.59237, 2.54, 1.3558. 14.5, etc.... I like using Joules much more than ft-lb...

Wow I was rambling quite a bit in this post...
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Unread postAuthor: Ragnarok » Tue Feb 05, 2008 7:37 pm

frogy wrote:Some imperial units I don't think I'll ever stop using:
Pint, Quart, Cup, Tea\Tablespoon, Gallon, Inches

Inches and pints I might occasionally use, but I tend to think of volumes more in litres - and when someone starts talking about cubic inches, I tend to have to convert that to cubic centimetres. I know HEAL's chamber is around 40 ci, but that's about it - but ask me in millilitres, and I know it's 652.

The other units I rarely use (but I do use a metric form of tea and tablespoons in cooking). It's irony in motion that I don't have to think about converting metric to imperial - I can do that in the blink of an eye, but I often have to think for a moment about converting ounces to pounds, fluid ounces to pints, pints to gallons, etc - and often, I will get it wrong.
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Unread postAuthor: psycix » Wed Feb 06, 2008 6:34 am

Im happy im grown all up with metric :)
No need to convert for a calculation.
On the other hand it was a bit of getting used to imperial when I encountered spudding on the internet.

Btw, do you guys convert your numbers by hand?
Google can do it for you!
Just type in something like "8,45 bar -> psi"
and google will tell you:
"8,45 bar = 122,556888 pounds per square inch"
This is also possible for "inch -> cm", "feet/s -> m/s" or "feet pound -> joule or anything else.

note: google cant recognize 2" as 2 inch, write down inch as a whole word.
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Unread postAuthor: Ragnarok » Wed Feb 06, 2008 7:54 am

psycix wrote:Btw, do you guys convert your numbers by hand?

Yup, because it's quicker doing it with a calculator - you don't need to mess around with typing in the names of the units like you do on Google. About the only thing I use Google converter for is finding out a conversion the first time I need it, after that, it won't see use again. And often, as you're converting to use in another calculation, it's easier if the number is already in your calculator.
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Unread postAuthor: psycix » Wed Feb 06, 2008 11:17 am

Good reason :)
But Im always too lazy to write down the conversion factor and always have to look it up, so then why not also calculate it at once then.
But your method is faster then.
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Unread postAuthor: jimmy101 » Wed Feb 06, 2008 4:41 pm

Ragnarok wrote:
jimmy101 wrote:Your approach is a very clever way to get an estimate of the pressure in a combustion gun.

My greatest concern is that the ball valve will greatly limit the performance of the pneumatic, so he's going to need a higher pressure than the combustion is actually developing to equal it.

That's a fair point. But, like I said, it is only an estimate.

Ragnarok wrote:The other slight concern is the higher speed of sound in a combustion means also that it's more "pressure efficent" if you can see what I mean - for the same average pressure, you'll get slightly more performance. Much like a helium pneumatic will beat an air one, which will beat a CO2 one, even if they all use the same pressure, at just about any velocity.

On the other hand, the combustion loses a LOT of energy as heat lost to the chamber walls. Pneumatics don't have this problem. (Indeed, a pneumatic probably gains a bit of energy as heat transfer from the gun walls.) I would say that a combustion is much much less "pressure efficient" than a typical pneumatic gun because of the heat loss issues. A combustion gun really should hit an average pressure near 60 PSIG (half the theoretical max pressure for a CB 1 gun), I believe it is the heat loss that keeps the average pressure down near 30 PSIG.

Ragnarok wrote:I normally reckon on the average pressure in a combustion of a sensible ratio being around 2 bar, or 30 psi, if I'm doing a rough mental calculation.


Yep, that is about what I calculated a couple years ago for typical combustion guns. Average pressure ~30 PSIG, peak pressure ~60 PSIG. That is roughly equivalent to a pneumatic charged to 60 PSIG with a CB of 1. The pneumatics average pressure would also be 30 PSIG if charged to 60 PSIG. Of course, really should calculate using absolute pressures then subtract out 1 ATM but it is close enough. Besides, the combustion gun looses a lot of energy as heat loss, the pneumatic actually probably gains a bit of performance from heat gain as it is fired.
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Unread postAuthor: Ragnarok » Wed Feb 06, 2008 7:10 pm

Ah, I wondered if I'd been a bit ambiguous.

When I was talking about pressure efficiency, I'm just talking about in two cannons with the same static pressure curve in the barrel over time, the one with higher temperature gas will perform better, because the particles are moving faster.

In air, the particles do around 500 m/s, so while it may exert a unit of force on a stationary object in the barrel, if that same pressure is trying to push something doing 250 m/s, because the pressure is proportional to the square of the relative particle speed, it will only give 25% of the force.

But with a gas with particles doing twice that speed, even if it's at the same pressure, it can will exert over twice the force in the same 250 m/s situation because the gas particles have a much higher relative velocity.

Combustions may lose a lot of energy in heat loss, but that wasn't what I was trying to talk about. I was simply saying that a gas with a higher particle speed (in this case, a heated gas) will yield better launcher performance, even for the same gas pressure. Same reason that using helium in a launcher will increase velocity, even if you're not getting close to the sound barrier.
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Unread postAuthor: McFear » Thu Feb 07, 2008 3:16 am

i just found out that to find the pressures and forces the school has some digital force plates. so i will set them up and just shoot them. lol awesome i get to shoot school property with a cannon.

Your approach is a very clever way to get an estimate of the pressure in a combustion gun.

Some thoughts;
1. The guns should have the same chamber volumes
2. The guns should have the same barrel dimensions (length and diameter)
3. The guns should be firing the same ammo
4. The guns should be fired with the same inclination (angle from horizontal) if you are using range as your measurment of performance.
5. You might consider building your own chronometer instead of relying on distance as your performance measurement.


The guns are exactly the same essentially. the barrel and chamber volumes are the same. there is a slight difference in the guns because as you may have noticed there is screw caps on the combustion and a single screw cap on the pneumatics chambers. the screw caps add half an inch to the volume. but that i can add to the error in the working. the guns are both shooting a standard tennis bass and are both going to be set up on a mount which will let me shoot at same angles. (only if i cant get the force plates to work). i also think that the combustion is running at 20PSI and the pneumatic i run at 30PSI and they seem to have similar power. but thats just guess work. the pneumatic at 30PSI though did blow the bottom out of a bucket and continue a flat flight for 20 - 30m though then it disapeared in the scrub and i couldnt find it.

This is because when i was making the endcap with the tire valve in the end of it for the pneumatic i put a broken one in to start off with because the pin had been snapped off. pretty much, dad has a tire checker that you put on a valve and when it has a pressure on it, it shows what pressure is in the chamber/tire but it doesnt go back to 0. you have to zero it yourself. so we just held it on the broken valve and shot without a projectile in the barrel. it showed 20PSI.

umm i think i covered all the points so far. let me know if i missed anything.

Mcfear.

ps the force plates are accurate to 0.05% of a newton i think it was. and they show the impulse and pressure ratings and graph it for me and all that crap that is annoying to get accurate.

pps. i am aussie :P
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Unread postAuthor: Carlman » Thu Feb 07, 2008 3:56 am

McFear wrote:pps. i am aussie :P


O yea another aussie :D

that is all.
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Unread postAuthor: McFear » Fri Feb 08, 2008 8:20 pm

hey all

i got a video of the spud gun shooting. i also have readouts of its velocity and acceleration. i was using one of my schools sensors. i had the sensor set on a sample rate of 250Hz.

when i did a second shot it shot just as far but i forgot to set the sensor sample to 250 and it was only on 10 and didnt catch the ball.

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The Velocity of the ball. Approx 516m/s


Image
The acceleration of the ball, i read to be just over 250000m/s/s unless i misread it

Click on the images for a more detailed view

i will add a video to the net sometime and give a link to the shot. the tennis ball goes over the sensor and into a 1/2inch thick lockbox that i made and punches a hole straight through. i will add when i have downloaded the video from the camera.

Thanks for all your replies. will add more data when i have it available.

McFear
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