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Dead Space

Sorry for noob question

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#1 Mas Ketchup

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Posted 17 March 2009 - 10:22 PM

I was watching angels BBB mod and I saw that he was putting hot glue in the front barrel to eliminate dead space. So my question is, what is the point of eliminating dead space?

Yes I did search.

Edited by Mas Ketchup, 17 March 2009 - 10:25 PM.

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#2 Soothsayer

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Posted 17 March 2009 - 10:27 PM

Delivers as much air/pressure to the dart as possible upon firing.
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yeah I'm that guy who made that cool thing with the cool paint.


#3 umpshaplapa

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Posted 17 March 2009 - 10:29 PM

The more dead space there is, the more places the air can go instead of the barrel.
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#4 cheesypiza001

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Posted 17 March 2009 - 10:34 PM

If there is very little dead space, then the air travels faster, (because it doesn't have as many places to go) and more efficiently. Basically, removing dead space is just creating a more efficient travel for the air from the point of exiting the valve/plunger tube, to where the expanding force of the air forces the dart out the barrel. Though, I'm sure others can explain it better than I can.

EDIT: (Empty space where my example used to be)

Edited by cheesypiza001, 18 March 2009 - 12:46 AM.

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#5 Doom

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Posted 17 March 2009 - 10:53 PM

To reiterate what others said more explicitly, a large amount of dead space reduces the peak pressure acting on the dart, which can reduce performance.

With that being said, "dead space" isn't necessarily bad. In fact, I noticed as I wrote a simulation of a pneumatic gun that a certain amount of dead space is beneficial. Having a little cushion of air acts like a miniature air reservoir in the barrel (the valve separates the barrel and the gas chamber). The peak pressure matters less because the pressure drops in the barrel less slowly so an elevated pressure acts on the dart for a longer portion of the barrel, increasing the total energy transfered to the dart. Sorry if that explanation is inadequate--it's not the easiest thing to explain in simple terms.

There are no significant flow restrictions that cause a separation of pressures in spring guns so as little dead space as possible without affecting flow is ideal.

Taking what I said about pneumatics into account too, note that the difference between no dead space and optimal dead space often is negligible. Have as little dead space as possible without affecting flow unless you want slightly more optimal performance and are willing to do testing and simulation.

Edited by Doom, 17 March 2009 - 10:55 PM.

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#6 Echnalaid

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Posted 17 March 2009 - 11:20 PM

An example would be NF coupler styes. TantumBull's coupler style has almost no dead space, and is 99% air tight. On the other hand, doing a simple barrel change then putting the coupler on has a lot of dead space, and it doesn't provide maximum power to the dart. It would lose pressure in the barrel holding the coupler.
(I hope I'm right... I could be thinking of something else)
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#7 Daniel Beaver

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Posted 17 March 2009 - 11:42 PM

I have seen the effects of removing dead space, but I'm still a bit unsure of what is "actually happening". Removing dead space eliminates some head loss, but that can't be the whole story, can it? Also, air ought to behave as an incompressible fluid at the velocities and pressures you see in a nerf gun, but the simple fact that tight barrels give you better performance in stringers seems to debunk that. Anyone know what I'm missing?
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#8 KingBouyah

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Posted 18 March 2009 - 12:08 AM

I'm no engineer, and this isn't exactly the answer, but the engineering toolbox should help a little. This explains a little bit what minor loss coefficients are, specifically look at those for inlets, and you'll kind of get an idea of what restricts air flow and what doesn't.

For example, If you are trying to broil water in order to cook pasta, you would not want to use a 4 gallon pot to broil 6 cups of water. There would be lots empty space that would time for the heat to fill. However, if you were to use a smaller pot, the water would broil faster due to the lesser volume of the pot which would allow it to get hotter faster. I am pretty sure that this example is true. If it is not a good example to use here, please notify me and I will remove it.


Yeah, bad example. Because specific heat is still the same and if anything it would boil faster in a bigger pot because it's shallow, meaning there's less pressure to overcome as the vapor pressure of the water at the bottom of the pot increases at the heat source. In short, I see no correlation to Nerf.

And as far as broiling water is concerned, you've been browsing Engrish.com for far too long.

Edited by KingBouyah, 18 March 2009 - 12:25 AM.

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#9 Draconis

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Posted 18 March 2009 - 12:25 AM

The information has all been stated by reliable sources, but I wanted to give you an example with numbers.

TTGs have a plunger displacement volume of 22 cubic centimeters, and have little to no dead space between the plunger chamber and the back or the dart.

Recons have a plunger displacement of 25ccs, but even with very strong springs (which eventually break the tube), they cannot reach the average distance of a mildly modded TTG. This is due to the over 10ccs of dead space between the front of the plunger and the back of the dart.
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#10 cheesypiza001

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Posted 18 March 2009 - 12:44 AM

For example, If you are trying to broil water in order to cook pasta, you would not want to use a 4 gallon pot to broil 6 cups of water. There would be lots empty space that would time for the heat to fill. However, if you were to use a smaller pot, the water would broil faster due to the lesser volume of the pot which would allow it to get hotter faster. I am pretty sure that this example is true. If it is not a good example to use here, please notify me and I will remove it.


More or less true, but in practical terms almost completely irrelevant, because the difference would amount to at most a few seconds out of several minutes. And the weight (technically, the thermal mass) of a pot -which is what matters- is not all that directly related to the capacity. But it is hilarious, not the least because of your consistent use of the entirely wrong word. Broiling is the application of radiant heat from above, pretty much the worst possible choice for boiling water in a pot.



For example, If you are trying to broil water in order to cook pasta, you would not want to use a 4 gallon pot to broil 6 cups of water. There would be lots empty space that would time for the heat to fill. However, if you were to use a smaller pot, the water would broil faster due to the lesser volume of the pot which would allow it to get hotter faster. I am pretty sure that this example is true. If it is not a good example to use here, please notify me and I will remove it.


Yeah, bad example. Because specific heat is still the same and if anything it would boil faster in a bigger pot because it's shallow, meaning there's less pressure to overcome as the vapor pressure of the water at the bottom of the pot increases at the heat source. In short, I see no correlation to Nerf.

And as far as broiling water is concerned, you've been browsing Engrish.com for far too long.


Yeah......I did not get that much sleep last night (stupid excuse, yet very true). Also, for Daniel Beaver, click the following link. Mod 401 - Darts and Barrels by Cxwq Hopefully that helps you understand. If this is not in any way relevant to what you were saying, I apologize.

Edited by cheesypiza001, 18 March 2009 - 12:45 AM.

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#11 Hipponater

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Posted 18 March 2009 - 12:45 AM

For example, If you are trying to broil water in order to cook pasta, you would not want to use a 4 gallon pot to broil 6 cups of water. There would be lots empty space that would time for the heat to fill. However, if you were to use a smaller pot, the water would broil faster due to the lesser volume of the pot which would allow it to get hotter faster. I am pretty sure that this example is true. If it is not a good example to use here, please notify me and I will remove it.


Yeah, bad example. Because specific heat is still the same and if anything it would boil faster in a bigger pot because it's shallow, meaning there's less pressure to overcome as the vapor pressure of the water at the bottom of the pot increases at the heat source. In short, I see no correlation to Nerf.


Depth or surface area of the water has no impact on vapor pressure. It would boil faster in that large pot if it increased contact with the heat source, so less energy was wasted. (Maybe over an open fire, or an electric stove that doesn't shrink the heat down to the size of the pot).

For dead space in a nerf gun, the air you're moving (via built up pressure from a pump gun, or from the plunger of a spring gun) will fill all the dead space first, then try to push the dart out of the barrel. You're "wasting" air almost, Draconis's point about the recon vs the TTG is a good example.
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#12 Daniel Beaver

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Posted 18 March 2009 - 10:50 AM

What you're missing there is a premise with any validity whatsoever. Air is compressible, and the "velocities and pressures" of nerf don't even come close to providing any reason why anything else should be imagined.

I'm thinking of something else, then. I really ought to run something like this through CFX and see if I can't come up with a quantifiable answer. Everyone's just figuring things out by trial and error right now.

cheesypiza001: I've read that article before, but it is a good recommendation nonetheless.


Now, what about air guns? Dead space has a demonstrable effect on the performance of springers, but I've noticed very little range decrease from dead space when using air guns. RCSB clips, in particular, will only drop the range on my AT2k by about 10ft, but totally destroy the ranges on my 3B.

Doom: I see your post now, not sure how I missed that. It answers the questions I had.

Edited by Daniel Beaver, 18 March 2009 - 02:34 PM.

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#13 Doom

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Posted 18 March 2009 - 11:37 AM

Daniel Beaver, you seem to have skipped my post where I detail precisely what's going on here. My statements come from computer simulation, not trial and error. Nothing as complicated at CFD is required for an accurate simulation (I assume CFX is a CFD package). The system of equations that governs spring and pneumatic guns are very well defined. See the link in my signature for more information.

The problem with springers is that their pressure builds up slower and the dart starts moving down the barrel, which further compounds the pressure problem. This reduces your "effective barrel length." Having higher friciton (especially static friction) can allow higher pressures to build up and more energy to be transfered to the dart.

For our purposes air is compressible. I suppose if you liquify the air that'll be different, but I see no point in that. Other gases could run into phase change problems as energy is extracted in the form of work, but no one uses CO2 or methane, so it's a non-issue.

As for dead space in pneumatics, I already stated that a certain amount actually is beneficial to performance but too much reduces performance. The benefit can be on the order of 5% higher performance compared against no dead space, so it's a very real benefit, but you often can't get any benefit that large. Keep dead space to a minimum if you want a simple rule.

I'm no engineer, and this isn't exactly the answer, but the engineering toolbox should help a little. This explains a little bit what minor loss coefficients are, specifically look at those for inlets, and you'll kind of get an idea of what restricts air flow and what doesn't.


Minor loss is irrelevant to the discussion and in practical terms is not even calculated. A flow coefficient based on empirical data will describe most nuances of the flow including the mentioned effects.

Edited by Doom, 18 March 2009 - 11:40 AM.

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#14 Mehku

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Posted 19 March 2009 - 09:28 PM

Dead space lets air from the accelerating plunger head to go places other than the barrel. Dead space is bad, and you should try to fill as much up with whatever glue you have on hand (hot glue works the best). I filled up the space behind the old AR in my NF on the part that the plunger head hits (around the inner circle, over the three prongs) and I can hit 59' without a barrel replacement. That's pretty damn good considering it has no real barrel.

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