14 replies to this topic

[Jpec07]

Okay, so this is a concept that I'm not sure if it has been discussed before here, or even if it's been thought up, but it seems to me like something needs to be said about barrel length and the relation it has to fluid dynamics in an ideal closed system. I feel I need to preface this by saying that I don't mean to step on anyone's toes, and with the complete understanding that I am a newbie on these forums, at least by post count. And if the concept is already known to the community at large, then please someone inform the staff so they can close/delete this. Also, if this belongs in the Articles Section, I wasn't sure if I was actually allowed to post it there.


Introduction
The problem I see that exists with barrel modifications is that the overwhelming majority of these modifications tend to make the barrels entirely too long, and that they place the darts in locations that do not provide for optimum performance. Mind, this theory takes some explaining, so if you don't feel like reading about my physics, take my word and skip down to my conclusion.

The Basics
First, it is important to establish how Nerf guns work. And I'm talking more in-depth than "plunger pushes air, which pushes dart," because while this is a functional and conceivably accurate description of the process, there's a lot more at play here than simple indirect push-shove of airflow. Now, it may be shocking to some of you, but the darts we love to shoot aren't actually "pushed" out of the barrel, they are actually "pulled." Here's how it works. When the piston (i.e. plunger) shoots forward, the pressure within the chamber (the space between the piston and the dart) rapidly climbs. This difference of pressure creates a vacuum on the other side of the dart. Now, I don't know how many of you have taken high school physics, but it doesn't take a physicist to say that vacuums suck, or rather that in fluids (like air), systems seek equilibrium. The air behind the dart is all trying to be at an equal pressure with the air on the other side. And since the dart can move down the chamber, expanding the volume and thereby decreasing the pressure, the dart tends to very quickly move towards the end of the chamber. As long as the pressure within the chamber is greater than the air pressure outside, the dart will accelerate. In order to maximize the dart's velocity (and thereby its range), the idea is to maximize the amount of time it spends accelerating, which means maximizing the amount of time that the air pressure behind it is greater than the air pressure in front of it.

Barrel Length
Now herein lies the difficulty. In an airtight system, any barrel where the dart loses the vacuum in front of it before leaving it is actually incredibly inefficient; meaning any barrel where there is a lack of air pressure pushing on the dart before it leaves the gun and equilibrium can be restored is actually slowing the dart down. This is because once the pressure inside the chamber is equal to the pressure outside, there is no more vacuum for the fluid to flow towards. What happens, then, is as the dart's built up momentum carries it further down the barrel, the pressure in the chamber behind it actually decreases (because the volume is increasing). This means that there is a vacuum behind the dart, and the difference in air pressure is actually pulling back on it, causing it to accelerate negatively ("decelerate"). Practically, this means that the dart's momentum is actually fighting against air pressure if the barrel reaches this length.

To solve this problem, the barrel from where the rear of the dart is placed to the end of the barrel should be equal in volume to the air compression chamber (where the piston and spring are). Here's why. When the trigger is pulled, the compression chamber takes all of its air and rams it into that small area between the chamber's end and the dart. Thus, the volume is the variable that has decreased in the pressure equation, and the easiest way for the system to equalize is to make up that lost volume. As I've said, the dart will continue to accelerate until the pressure is equalized, and if that occurs when the lost volume is made up, then it stands to reason that that would be when the volume from the bottom of the dart (the reverse piston, if you will) and the end of the compression chamber to the end of the barrel is equal to the volume of the compression chamber.

Dart Placement
A second difficulty I have seen is that the darts and "breeches" are placed relatively far away from the end of the compression chamber. The problem with this is that the initial volume of the system is larger, and so the change in volume (and thereby the change in pressure) provided by the compression chamber are reduced. Think of it this way: if the compression chamber adds 30ccm' (cubic centimeters) worth of air to the chamber, if there's already 100ccm of air in there, the difference will only be 130ccm:100ccm. If, though, there's only 5ccm of air in the chamber, the difference will be 35ccm:5ccm. For any of you who are good at math, it's fairly easy to point out that 35ccm:5ccm>130ccm:100ccm. For those of you non-math oriented, the 100ccm model only puts 30% more air into the system, where the 5ccm model puts 600% more air. And where there's more air, there's more pressure, and where there's more pressure, the darts will accelerate faster.

So presuming the goal of trying to maximize acceleration, it makes greater sense to place the dart as close to the compression chamber as possible. This minimizes the initial volume, thereby maximizing the pressure difference upon compression and maximizing acceleration. By the same token, it also makes sense that when constructing Stefan darts, a hole should not be burned into the bottom unless it is absolutely necessary. This is because that hole increases the initial volume of the system when it's primed, thereby reducing the pressure difference on firing.

Conclusion
The goal of power modification should be to maximize the acceleration of the darts through the manipulation of the fluid dynamics in the system of the gun. This can be best accomplished by: (1) limiting the barrel--from the bottom of the dart to the end--to match the volume of the compression chamber (a good length for most guns is usually just a little bit longer than the compression chamber), (2) placing the dart's initial spot as close to the end of the compression chamber as possible, and (3) if you're using Stefan darts, to not burn a hole/dent in the bottom of them.

Please also note that this is from a purely theoretical standpoint. As I believe I've said, I have yet to perform a barrel mod to my guns, namely due to a lack of time and resources. I also want to acknowledge that this assumes ideal circumstances, namely an airtight system, which is very rarely the case. It also needs to be said that this does nothing for the accuracy of the gun, which a longer barrel may correct, even if it is at the cost of firepower. I don't expect anyone's guns to be perfect on account of this, but it should help people come close (the closest I've seen is the Fast Action Rifle by Boltsniper, because it exhibits all of these principles).

*bows*

Edited by Jpec07, 03 October 2009 - 02:55 AM.

[Homestarune]

I got a limit approaching infinity when I calculated the ratio of time spent researching [Tr] to the amount of time actually modifying guns [Tm]. For those who don't like word problems, lim Tr/Tm as Tm -> 0+ = ∞

Edited by Homestarune, 03 October 2009 - 05:44 AM.

[Split]

Meh. This is basic stuff that's known to anyone who really cares, but doesn't actually elaborate nearly enough. Boltsniper took the same concept and actually derived equations and ratios for our inefficient blasters several years ago. A 1:1 ratio of barrel to volume to plunger tube volume would NEVER be the best solution in the real world, and would lead to (literally) 12' long barrels on +bows, and would be absurdly impractical anyway.

Even fundamentally, the reason for making the volumes the same is just so that the pressures are the same. A more accurate way of finding the ideal barrel length would be based around finding pressure equilibrium, which inherently accounts for air losses from the dart seal to the barrel, plunger head to plunger tube, and all other sources. It also takes into account work done by non-conserved forces - namely friction. Undoubtedly, these have to be experimentally found - probably and expensive and daunting process as well.

In short terms, not too many people care about this stuff. I find myself to be an exception, as this is basically all I do in nerf nowadays (and finding dart/barrel ideals is just the tip of what I'm working on). And they shouldn't really have to care about it. If they want to do the science, the science isn't too difficult to figure out for the most part. Otherwise you're over-complicating a cheap, simple and silly hobby. The reasons against this are the same reasons that we use blanket bans on blasters over case-by-case basis and using a simple ruleset like 3:15 over tactical simulations and other things (not that they don't happen successfully time to time).

*anti-bows*

Homestar was hitting on the same point, by the way. The hobby is about nerfing, not so much about physics.

Edit: Just another note I wanted to quickly harp on. Pressure is not everything! Often, it can be completely ignored. The perfect example is a blowgun. Darts tag upwards of 80+' at lung pressure, somewhere at a max of 3psi initially, then spread out over several feet of tubing.

Edited by Split, 03 October 2009 - 09:13 AM.

[Darth Freyr]

I see a few problems with your concept. First of all, vacuums don't "pull", but the accompanying high pressure pushes. Furthermore, your estimation of barrel length does not take into account friction or the possibly insignificant momentum of air. Not including friction leads to barrels that are entirely to long. Burning a hole in the back of a stefan would most likely increase the dart-barrel seal. Some people have actually had a increase in their real ranges using stefans with holes. Also, breaches can be very effective in placing stefans very close to the air source, also called reducing "dead space". Instead of making estimations with physics concepts, try making some actual calculations, using fluid dynamics modeling software, or getting some barrel material (you can use empty Crayola or Rose-Art marker cases) and doing a mod instead taking the time to write a huge post.

Beaten to it, thought I'd post anyway. Would an "anti-bow" be bending over backward?

[Blacksunshine]

That's a lot of words. Myself I just find what works and stick to it. I find myself having to agree with Split.There's really no sense in applying such specific calculations to a hobby that is inherently inconsistent and inaccurate.

[imaseoulman]

I wrote this up earlier today and then lost my internet signal. I decided not to post it, but since there is a discussion going, I thought I may as well include it:

Well, I wasn't going to respond to this, but it will probably benefit you and perhaps others.

First point: Nothing in a modified nerf blaster even remotely resembles an "ideal system." So don't try to theorize optimal characteristics based on "ideal" equations and expect to have decent results. An understanding of what barrel length to use comes from experimentation with different barrel lengths. For example, when I was getting started with my SM1500's, I tried about a dozen different barrel lengths with six different types of foam before FINALLY settling on an optimal combination. I spent a couple hours on a few different days test firing a few different SM1500's to get enough data.

It's good to experiment with new ideas or we wouldn't have options like the RSCB or "automatic blowgun" or a plethora of other innovations. The most important thing, however, is to actually NERF this will give you the understanding necessary to create an effective blaster. While NERFing, think about what it is that you need your blaster to be capable of. Don't try to solve problems that don't need fixing. If ROF is really a problem, try to solve it, if it's not, leave it alone. If your range is good enough, then leave it the way it is. The only reason I tend to make monster integrations is because I LOVE playing capture the flag and I often assault the opponent's base by myself. I need several shots immediately available in order to clear out a base defended by 3-5 people and make it out with the flag. The Arachnophobia Pistols, however, were designed for standard team slayer, specifically to be able to take out a single problem opponent.

And finally, your theories don't even support your conclusions. You say that we should have barrel length equal to the volume of the plunger tube. You then say the barrel should be a little longer than the plunger tube...that's a little odd. Most plunger tubes have an ID significantly greater than the fairly standard 1/2" barrel. Let's just "theorize" and say that we have a plunger tube with 1" ID and that it is four inches long. That would give it a volume of 3.14 cubic inches. To get the same amount of volume out of a half inch barrel, the barrel would have to be SIXTEEN INCHES long! That's not "a little bit longer" than the plunger tube, it's four times longer.

If you are still convinced that a more advanced understanding of physics will help with the modification of NERF blasters, never neglect friction and gain a better understanding of ENTRAINMENT. If more people in the community understood entrainment better, we might have some very interesting designs.

Split, I'm VERY interested to hear more about what you are doing. If you are doing the kinds of things it sounds like you are, you're probably going to come up with some interesting ideas. When I was experimenting with barrel types, dart material, flow rate, etc., I came up with some conclusions that allowed me to achieve significantly greater ranges. I now focus mostly on flow rate (eliminating restricting orifices) and having the right dart/barrel fit (I now actually use two different types of darts because I can't get the barrels just right for one kind of dart). I've not yet found my favorite springer barrel fit... perhaps that's because I don't have a favorite springer.

Also, I'm glad somebody corrected the whole "push/pull" thing as it is true that it can't be "pulled" (in order to "pull" the dart, the air would need to some how hook on to the dart, and push on that hook...which again would be "pushing" and not "pulling"). It was, however, hinted at that the minimal momentum of air was irrelevant or so minute as to not affect anything. I'm not sure this is the case, especially for blasters that release the air more slowly. This is why I mentioned entrainment. Entrainment is a very important principle to understand when propelling solid objects with a fluid. I was first exposed to this when studying steam systems, but it has good application in NERF. I suggest for all those interested in the "physics" involved in NERF to do some basic research of entrainment, as for many people it is counter-intuitive, though it has been hinted at in discussions about the RSCB and the "automatic blowgun."

Edited by imaseoulman, 03 October 2009 - 11:26 AM.

[CA13]

Really, efficiency in nerf is basically how fast you can dodge/sneak up on people. If we used math, there would be guided nerf emplacement mortars that we fired from our own houses.

[Jpec07]

Wow, way to welcome a newbie with, guys. I did a bit of research, trying to find if anyone had considered these things before, and it makes good sense that Boltsniper would have actually figure out the math behind it--something I was wont to do, but didn't have the time.

It's good to see that this is something that the community at least recognizes, which was my ultimate goal. I'll admit that I should've done a bit more research before posting my THEORY, perhaps deriving the same formulas that Boltsniper had before posting, and perhaps trying to figure out the semantics of the forum (what with dead space and all). In either case, in an ideal system (which, if you all had read, I said was incredibly rare to find in a Nerf gun), my theory stands.

But let's all continue to play "Attack the n00b," shall we?

Oh, and as regards the marker barrel mod, that would be an utterly fantastic idea if any of my friends were still using such rudimentary tools in their art. I'm in college, so money is incredibly thin (and my artsy friends have moved on to pastels and paints--I have no ties with anywhere that would have a plethora of markers to borrow from, either). As it stands, trying to track these things down is proving incredibly difficult and almost not worth the time. I did just manage to get my hands on a BBB, though, so there is a trip to Lowe's in the future for some PVC and/or brass so it'll shoot darts (whenever I get the cash).

And you all are right when you say that the best way to figure these things out is through experimentation. What I'm doing is not creating math to make a new phenomenon; rather, I'm taking the phenomenon that I can imagine, figuring out the math behind it, and using that math to try and figure out (in my then very sleepy mind) how to optimize the system. I'll admit that I got a couple things wrong when writing that up in the limbo between Zombieland and sleep, but those have been addressed for the most part (with the exception of friction: I intentionally left that out, because mine was a discussion on air pressure, and I figured friction was something the community was already very much aware of--which you all are).

Oh, and while I was talking about an ideal system with no leaks or anything of the sort, I'm not so foolish as to think that such a system is actually attainable. Even in attempting to seal up the leaks in my newly acquired BBB, I realized how much of a fool's errand it was. The principal does still apply, but there is a lot of compensation that needs to be made for it (to imaseoulman, while the volume of the two tubes does greatly depend on the internal diameter [ID, I presume], by my estimate, the leaks in an unsealed gun would typically make up for that difference).

And again, I don't mean to step on anyone's toes. I just had an idea that I wanted to see if people were already aware of: which it seems most of you are.

(nice back bend, though, Split)

[imaseoulman]

Wow! If you thought that was bashing, you're in for quite a shock around here. I thought everything was handled very maturely. We were simply saying that equations are almost useless here but we aren't ruling out the possibility. Just so you know, your whole "snob appeal" of using superior artistic supplies does you no favors over the internet.

The point of a forum is to collectively learn, usually from more experienced to less experienced, but we all do learn from each other. So, when a new member posts something, it is understood that they are open to criticism. Why else would they post? To show off? To look smart? The only reason to post information is so others can learn from it and you can get feedback on it from others. Your post had errors and we corrected them so others wouldn't just accept them as truth and we offered advice that will save you time and money.

Have and get your learn on. Most of the guys that posted in this thread really know their stuff.

[SonReeceSonJensen]

Really, efficiency in nerf is basically how fast you can dodge/sneak up on people.

A topic I would not mind seeing started at all: physical prowess vs. blaster quality. I have racked up lots of kills because I still retain my “high school athleticism”.

BlackSun: if you have data from your dart/barrel tests (big if here, I would probably not have written it down) I would love to see it posted. I’m sure it could be applicable to many blasters.

Of course I also wish there was a Barrel Length Directory pinned in the mod section, but I don’t have the time to build one so I can’t really complain.

[J cobbers]

Jspec07 is actually showing some thought, which is more than a lot of new guys you see on here. That and he admits he doesn't have all the variables down. So Jspec, don't worry about getting ripped on, think of it as constructive criticism.

Here are my 2 cents (more like a buck 50 in this economy).

Way back when I first started modding in 2000, I tried to make a barrel that had the exact same volume as the plunger tube in question. What I found is that it sucked. I cut an inch or two off and viola it kicked ass. Usually that's all you got to do to get close enough to an ideal barrel length. Start too long on purpose and cut down until it works great. It tends to be faster than doing the math for which you need data that is hard to calculate anyway. But if you can figure it all out, more power to you.

On the push pull thing, folks it's 2 sides of the same coin The energy to move is imparted to the dart from the higher pressure air behind it, but at the same time what allows that energy to move is the fact that on the other side is a lower pressure atmosphere which is in essence pulling that energy out of the barrel to equalize the system. So really the air behind the dart is what is getting pulled, the dart is just along for the ride.

Other factors that play into barrel physics, as mentioned by someone earlier, friction, which can be both a friend and enemy to optimization of barrel lenght. In a spring powered blaster you want high inital friction, followed by low friction further down the barrel. High fiction allows pressure to build up as the plunger compresses the air behind the dart before it starts to move. Once the dart starts moving you lose that compression and therefore energy. Once the dart starts moving however, you want as little friction as possible while still maintaining a seal in the barrel to minimise the energy lost to drag inside the barrel.

On a valve based gun all the pressure is built up before pulling the trigger so a barrel that is just tight enough to keep a good seal around the dart is ideal for the entire length of the barrel.

Which brings me to my last point, maintaining a seal with your dart in the barrel prevents the higher pressure air from escaping around the sides of the dart, and therefore maximizes the energy imparted to your projectile. Your darts should never be loose in your barrel, and should be just tight enough to maintain a seal, but not so tight as to cause a lot of friction.

Which as stated by Darth Freyr putting a hole in the back of the dart helps maintain a seal. Foam is stretchy and what a hole does is create a pocket that the high pressure can push against to seal the back of the dart against the walls of the barrel, and creates a better seal. The loss of inital pressure should be minimal if the hole isn't very large, that is, it doesn't go more than say a quarter inch into the back of the dart.

You are however quite right that dead space matters, putting a dart closer to the source of pressure maximizes the energy that is transfered to it. Well designed breaches do this, though some sacrifice ideal placement for increased rate of fire, especially if they have a clip attached. RSCV clips have similar issues, but again you get more shots between reloads so it just depends on what you need as a nerfer.

What this boils down to is that barrel length will never equal the point at which the would pressures equalise in an ideal system. Rather it will be shorter than that, due to friction, dead space and imperfect seals.

[imaseoulman]

I just want to interject that your generalization don't always apply. For air tank blasters, different tanks need different barrel tightness. For example, a smaller tank, such as an SM1500/AT2K, needs a relatively loose fit because all the air escapes very quickly and the seal really isn't all that important. The dart is brought up to speed by the air moving it even if the air is moving around it (like small objects in high winds gaining great velocity). For that type of blaster you want a dart with the least amount of friction, so as loose as you can get it without it falling out when running with the blaster. If you doubt this, experiment with different fits.

On the other hand, a larger tank that releases the air more slowly (like a Blast Bazooka, etc.) requires a tighter fit for maximum range. You don't want it so tight that you have to twist it in, but just snug enough that it goes in and firmly. For this sort of application, I prefer to use CTD's. Perhaps it works a bit like burning a hole in the back of the dart (which I've never seen make a noticeable difference when collecting empirical data), but the tighter fit and longer barrel with higher tank volume allows for much higher ranges.

So yes, I really use two different types of darts depending on what blaster I'm using because I've never been able to get two different barrel materials to fit the same size dart just how I want it to.

Just thought I'd throw in my observations.

P.S. JPec, thanks for posting about this topic which has sparked a conversation that is helping everyone to analyze their dart/barrel choices. Welcome to the Haven.

[Jpec07]

Aye, thanks for the welcome. And I apologize for assuming that you were all trying to shun my ideas and play some kind of decrepit honor-shame game. Bad past experiences as the newbie and all.

Interestingly enough, while the search before posting was fruitless to anything relating to this topic, after I looked again, the concept was sure enough included in one of the stickied "basics" threads.

And J_cobbers settled the whole push-pull thing pretty well, I think (and added to my conception with the dart-seal thing: I hadn't even considered the flexibility of the dart material and how the pressure effects it). I just suck at articulating my knowledge sometimes.

Now here's a question I have for you folks (as I prepare to begin adding barrels to some of my guns): would some form of dry lubricant (like graphite) in the barrel be a good idea?

Also, wouldn't it also be best to experiment with the different masses of darts with different guns--especially one such as an SM1500/AT2K, as you referenced? I imagine it's similar to different bowlers using different weights of bowling balls.

[Draconis]

Now here's a question I have for you folks (as I prepare to begin adding barrels to some of my guns): would some form of dry lubricant (like graphite) in the barrel be a good idea?

Not graphite, definitely. If you are using stock Taggers, Suckers, or Sonics, then a dry silicone inside your 1/2" PVC might be okay, but only because of the rubber heads. If you are using Streamlines or stefans, with CPVC, then don't bother with lubricant.

Also, wouldn't it also be best to experiment with the different masses of darts with different guns--especially one such as an SM1500/AT2K, as you referenced? I imagine it's similar to different bowlers using different weights of bowling balls.

If you look around, we have done LOTS of testing with dart mass.

[Blacksunshine]

Really, efficiency in nerf is basically how fast you can dodge/sneak up on people.

A topic I would not mind seeing started at all: physical prowess vs. blaster quality. I have racked up lots of kills because I still retain my “high school athleticism”.

BlackSun: if you have data from your dart/barrel tests (big if here, I would probably not have written it down) I would love to see it posted. I’m sure it could be applicable to many blasters.

Of course I also wish there was a Barrel Length Directory pinned in the mod section, but I don’t have the time to build one so I can’t really complain.

Sorry. All my modding I do on the fly and work out issues in my head while I am building. Trial and error kinda stuff.