Achieving consistent muzzle velocities

[Ragnarok]

Gas in a small container will rapidly adjust to the container temperature.

You might be surprised. Gases aren't very thermally conductive.

Take that hair dryer. Hold your hand in front of it for a minute. Perhaps unpleasant, but you'll be fine. Now touch the hot element for a second. You now have burnt fingers.
There's a big difference between the temperature of the air and the temperature of the element because of poor thermal conductivity.

The fog vapour dispersing after a cannon shot is due to air currents and natural diffusion. Take a clear chambered cannon with a ball valve - shut the valve after firing, and the vapour will persist for some considerable time.

And in the UK, we get hot and cold winds from all over the planet - even over trips of hundreds of miles and many many hours, the air still carries quite a bit of its original temperature. It's a well documented phenomenon if you should decide you don't believe me.

Air is not thermally conductive. There's a reason any serious cooling system uses liquid coolants rather than air - much higher thermal conductivity and heat capacity.

There's a reason why I'm using the Daemon's Kettle for the job.

[Technician1002]

Model gas temperature in a barrel and chamber in GGDT and watch the temperature. It lingers, but the sharp differential drops rather quickly in a smaller space with the turbulence of the shot. Low differential takes longer to finish in a diminishing rate..

[Ragnarok]

If you've never actually done such a thing with a clear chamber, bear in mind that I have. For that reason I welcome to you either try and see for yourself or save yourself the trouble and just take my word on it. I'm not in the habit of making up stuff for a lark.

Even if you want to ignore that, the hairdryer air/element temperature difference and the point on global winds still stand.

[Technician1002]

If you've never actually done such a thing with a clear chamber, bear in mind that I have. For that reason I welcome to you either try and see for yourself or save yourself the trouble and just take my word on it. I'm not in the habit of making up stuff for a lark.

Even if you want to ignore that, the hairdryer air/element temperature difference and the point on global winds still stand.

The fog can exist for quite some time when the dew point is high. It exists at temperatures below the dew point. I will take your word for the fact the fog can exist for a very long time. It vanishes when the temperature rises above the dew point. Dew in the camber can exist with chamber gas temperatures of 65 degrees F.

Global winds are a long distance away from a wall of a drastically differing temperature. Thousands of feet verses a few inches does make a thermal path several orders of magnitude different. Large air masses in millions of tons of mass do hold temperature as they move. No argument there. Mega Tons of air mass vs a few cubic inches away from a wall. you betcha the thermal path is not the same.

[Ragnarok]

You betcha the thermal path is not the same.

You betcha the time scale is different too. We're talking times which make up significant portions of days, not mere minutes as we're talking about in a chamber. The fact that air currents can resist temperature changes over trips of thousands of miles and oceans of notably different temperatures is testament to the poor thermal conductivity.

I notice you're still not talking about the hairdryer point.

[Technician1002]

You betcha the thermal path is not the same.

You betcha the time scale is different too. We're talking times which make up significant portions of days, not mere minutes as we're talking about in a chamber. The fact that air currents can resist temperature changes over trips of thousands of miles and oceans of notably different temperatures is testament to the poor thermal conductivity.

I notice you're still not talking about the hairdryer point.

Thermal path? Length? Even by those standards copper and aluminum are very poor thermal conductors. Melting the end of a copper wire in a torch with the other end a foot away remains cold for a very long time and does not require thermal gloves to hold in your bare hand. A thermal path of a few hundred feet is much less conductive to heat. A copper rod a mile long caries very little heat to the other end. What do you consider a good thermal conductor?

Hair dryer.. OK, just how long does the air hang around the hot coils to do this heat transfer and how much air actually contacts the heating coil and how much passes by closely with no contact on the way through? It's a pretty quick heat transfer considering the transfer time and considering very little of the air traveling through actually contacts anything hot.

[jeepkahn]

Ok ,I've been watching this for 2 days and now I'll step in with my lil bit of knowledge.... Thermal conductivity in my line of work(internal combustions) when related to cooling of components, is the ability of a medium(air/liquid/solid) to REMOVE heat, not CARRY heat.... Air cooled engines at motion are the most effecient(not necaserily effective) method of cooling.... Thermal conductivity in this instance is relative to temperature differance between the object and the medium, moving air will pull heat out of metal faster for a longer period because the temp differance remains very large... water does a good job of transferring heat, but not removing it, water will draw heat from an object but it does a poor job of dissipating it, hence the need for radiators(to remove the heat from the water....

Unless you have a constant fresh cool water flow, air is a better heat remover... and even with a constant flow of water,it is such a poor conducter, you have to restrict the flow to give the water time to absorb the heat...

If you're looking for consistency, look to maintain a higher air temp, or build you a copper coil that you can run your air through a cooler full ice/salt/water to act as an intercooler....

[jimmy101]

The easiest practical way to deal with the temperature affects is to simply let the chamber sit a couple minutes after pressurization and then do a final tweak on the pressure. Either by adding a small amount of additional air or by venting a small amount of air from the chamber.

The initial charge of air will give a significant temperature swing. Let the air cool/heat to the ambient temperature. The final tweak in pressure is a much smaller change in the system pressure and will generate much smaller changes in temperature.

[Ragnarok]

Either by adding a small amount of additional air or by venting a small amount of air from the chamber.

I would point out here that a simple solution would appear be the use of a auto-venting regulator. But this is not however recommended for negative acting pilot filled valves (i.e. QEVs, unfortunately).

What do you consider a good thermal conductor?

To be honest, almost nothing carries heat particularly efficiently. However, on a relative scale, one of the best I can think of off the top of my head is silver. Copper comes in a close second.

@Jeepkahn: Actually, the Daemon's Kettle is a trick that combines the beneficial effects of the two. Temperature differential kept at near the maximum, thermally conductive (relatively) coolant, heat easily dissipated from the coolant.

Not that it doesn't have it's own downsides, but for the job it will be doing, I believe them heavily outweighed by the positives.

On a completely unrelated note, human blood tastes unbelievably odd.

[jimmy101]

I would point out here that a simple solution would appear be the use of a auto-venting regulator. But this is not however recommended for negative acting pilot filled valves (i.e. QEVs, unfortunately).

Though you need to know which way you need to adjust the pressure. It should be possible to set it up one way for filling from a resevoir and another way when filling directly from the pump head. In one case the pressure will rise over time, in the other it'll fall.

To be honest, almost nothing carries heat particularly efficiently. However, on a relative scale, one of the best I can think of off the top of my head is silver. Copper comes in a close second.

Diamond and gold are both excellent heat conductors. Copper is probably the best taking into account cost and practicality.

Air (and gases in general) are kind of odd balls when it comes to heat conduction. Air itself is an excellent thermal insulator (almost as good as a vacuum) if the air is stagnant. Stagnant air is the basic principle behind most thermal insulation. If the air isn't stagnant it is a pretty good heat conductor. Typical house insulation, like fiberglass or blown cellulose, is basically stagnant air, the solids (glass, cellulose ...) are just there to keep the air from moving and setting up convection currents.

In a spud gun, pnuematic or combustion, the gases are seldomly stagnant. Even if nothing is happening in the chamber any temperature gradient will setup convection currents and result in increased heat transfer.

In a closed combustion chamber the rate of heat transfer from the combustion gases to the chamber is very fast. The thermal half life is down in the vicinity of a couple tenths of a second. Combustion gases that start at >2000C will be down to basically ambient temperature within a second or so.

[Ragnarok]

Though you need to know which way you need to adjust the pressure.

A venting regulator should do either way automatically. But like I say, not an option for QEVs, because it would just set them off if it vented...

Diamond and gold are both excellent heat conductors. Copper is probably the best taking into account cost and practicality.

Copper actually bests gold (380 Wm/K to 320 Wm/K), and diamond is uniquely high - indeed, its thermal conductivity is used to identify fakes.

Not exactly cheap though!

[btrettel]

After further thought, I realized my procedure for finding the drag coefficient is convoluted. If theory is correct, a small wind tunnel should return the same result without the major inconsistency issues associated with range tests.

The confidence interval for the mean was between 0.17 and 0.51... I'll just use 0.5 for now until I make a mini wind tunnel. The specifics don't matter much at this stage and I can always change the configuration of the gun in the future.

Also, from the books I've read the drag coefficient of a cylinder should have been about 0.8, so I'm pleasantly surprised to learn that Nerf darts are lower. I do not know why their drag coefficients are lower; perhaps the wind tunnel tests that determined them are unrealistic (and if so, then I'll end up right back where I started). Perhaps lift plays an important effect (I was told by a former Nerf engineer that it does not in Nerf darts). I'll figure it out.

[THUNDERLORD]

Skimmed over thread...
with velocity, everything has "lift" (upward movement).

The reason is that in a normaly miniscule way, air pressure above a projectile is always lower than air pressure below it.(edit:pretty simple)

So as it moves rapidly through the air, it takes the path of least resistance, little by little (the lower pressure air is above it)...
Makes a lot of sense...The velocity increases it's path upwards(faster). when you think about it. (lower velocity drops off quicker, but still rises first).
With enough velocity anything has lift.

Off topic, but with enough magnetism anything is magnetic too.

God loves trajectory, that's why it happens this way probably...
Praise the Lord! (for ballistics and life) 8)

[btrettel]

The question is whether or not the lift is negligible, not whether lift exists. I was told by a former Nerf engineer that it is negligible for Nerf darts. Given the discrepancy in drag coefficients, I'm thinking that it's not.

I might make a full 6DOF external ballistic model now just to see whether the other effects could explain this inconsistency... luckily I understand most of what this involves already.

[jimmy101]

Measuring Cd is pretty tricky. Cd's are not constant and are very dependent on the aspect between the shell and the air flow. For bullets fired from a rifled barrel the Cd increases with distance from the barrel since the angle between the round and the air flow increases the longer the round is in flight. For a fin stabilized round the Cd is more constant throughout the flight since the aspect is basically constant.

The comment about lift generated by different pressures above and below the round, presumably caused by the difference in altitude is an affect that is in the range of "ya, it's true, but there isn't any way you could detect an affect that small so just ignore it".

A much bigger contibuting affect is just the direction the round is pointing. If the nose is a bit higher than the tail then the round acts like a wing and will generate significant lift. I suspect that is what Nerf darts are doing, they fly slightly nose high and act like a wing.

(Contrary to popular belief a wing does not require unequal cord lengths between the top and bottom of the wing. All a wing requires is that the leading edge is a bit higher than the trailing edge.)

EDIT: removed incorrect statements about pressure vs. altitude.