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Where did the 100 psi assumption come from?

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 » Wed Nov 26, 2008 9:17 pm

@D_Hall: Curses, forgot that they'd be big bolts. :roll:

Moonbogg wrote:They have shot pneumatics and just judge what the PSI is likely to be from a combustion.

The average pressure in an advanced combustion (with the fan and metering) reasonably close to the 1:1 ratio is about 30 psi.

This makes it quite easy to work out an approximate ft-lb figure for any advanced combustion of about that sort of ratio - just multiply the barrel's volume in cubic inches by 2.5.

<SCIENCE>
Because Pressure * Area = Force, Force * Distance = Energy, and Area * Distance = Volume, therefore:
Pressure * Volume = Energy.

The reason it's 2.5, rather than 30, is because multiplying by 30 gives inch-lbs of energy, which needs to be divided by 12 to get ft-lbs.
</SCIENCE>

Obviously, this is just a rough figure, but it points out the fact that the end energy is a direct result of pressure (which is essentially fairly constant in a combustion) and barrel volume.
So basically, a combustion's muzzle energy is just a result of how big it is.

Pneumatics are more complex because they have a major choke point at the valve and wildly varying ratios which makes it much harder to apply a simple back of the envelope calculation to them, although I can usually guestimate these things these days.

An interesting thing came up when I was larking around one day. Although most spudders are aware that as projectile mass increases, pneumatic spudguns tend to be more efficent at putting energy into a projectile (in other words, heavier projectiles will tend have more energy than a light one if fired from the same launcher).
I started looking at M*V<sup>3</sup>, which is loosely proportional to the power (using the term power properly) required to do that - assuming a constant acceleration, which is obviously not the case, but meh. This falls with projectile mass - that is to say, heavier masses take less power in their acceleration (but this is applied for longer

Curiously, the result of the calculation for M*V<sup>2.5</sup> is pretty constant across wide mass ranges...

Unholy buggery, you have got to be kidding me. That did NOT just happen. :shock:

...I'm going to have to excuse myself to go and check this.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: man_o_brass » Wed Dec 03, 2008 10:22 pm

Someone with a mechanic buddy needs to ask to borrow his cylinder compression tester and just hook it up to a gun. They're designed to do this exact thing. My brother's got one, but I really don't have the time or inclination to mess with it.

(edit) Here's an example:

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200329225_200329225
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Unread postAuthor: jimmy101 » Thu Dec 04, 2008 1:13 pm

man_o_brass wrote:Someone with a mechanic buddy needs to ask to borrow his cylinder compression tester and just hook it up to a gun. They're designed to do this exact thing. My brother's got one, but I really don't have the time or inclination to mess with it.

(edit) Here's an example:

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200329225_200329225

Yep, that type of gauge has been discussed many times and actually used one or twice. They have a couple problems, (1) lots of temperature drop in the tubing to the gauge, hence pressure drop. (2) The small volume of gas that is trapped in the gauge is hot going in. As the temp drops the pressure drops. (Not a problem for the gauges designed use since on a car engine it is "pumped up" by tens of cylinders firing before it is read.) (3) The needle has inertia and wasn't designed for a single pressure transient that is as fast as it is in a combustion spudgun.

A generic tire pressure "pen" gauge is basically the same, and also has accuracy problems. The pen gauge doesn't have the temperature drop problem.

The pressure in a combustion spudgun is really pretty well known. ~120 PSIG if the round jams, ~60 PSIG peak for most ammo, ~30 PSIG average for most ammo. (Assuming say a 1:1 CB, stoichiometric fueling, fan, etc.)
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