9 replies to this topic

[quiet asian man]

Hello all.

I did a physics project on nerf guns. I tested the effect of RSCB barrels on dart velocity using normal barrels as a control.
big SNAP data
photogates

Dart velocity did not gradually decrease as I fired more darts. In fact, velocity seemed go randomly go up and down after the first shot. However the gun also had inconsistent results using normal barrels, so I can't draw a direct conclusion other than "the plunger is so big that the deadspace filled by darts means nothing." But it still got me thinking about energy conservation.

My theory is that when you prime and maneuver the gun, darts shuffle around randomly inside the ammo tube. When shot level to the ground, the gun wastes random amounts of energy moving these darts to the back of the ammo tube before firing the dart. The gun would waste more energy if the darts were clumped at the base of the barrel than if they were at the back of the ammo tube.

This explains the problem Carbon had with his ball SNAP with an endcap follower here:
http://nerfhaven.com...ndpost&p=128410
The endcap is like a very heavy ball that the gun moves in addition to his first ball. The gun wasted more energy moving the endcap up than it did going through deadspace. In addition the last ball fires the hardest because the gun wastes more energy moving the extra balls in the ammo tube.

I unfortunately do not have time to take more data for my project, but I will test whether the gun really wastes energy moving loose darts by pointing the gun at a 45 degree angle up and down to clump the darts and compare the velocities.

Testing Conditions
-Big SNAP with CS-866 spring in addition to cut down SNAP spring (full spring is too long and warps when fully compressed)
-all barrels are 17/32 brass
-plunger volume ~17.7 cubic inches (2.125" ID and plunger travel 5")
-deadspace caused by 11 shot RSCB ~5 cubic inches
-darts are 2 inches long and weighted with 1/4" slingshot balls and a flat hotglue tip (pressed against a wet plate to harden)

Data was collected by mounting a photogate on the end of my barrel. The beam is about 1~ infront of the the barrel. The end of the gun was stabilized on a bar because the mass of the photogate causes the barrel to bend. The photogate had to be mounted on the barrel because recoil causes darts to go high and barely scrape or miss the beam entirely.

Can anyone confirm my findings? And would anyone else be willing to get solid velocity data using similar equipment? I'm especially curious how my snap compares to the +bow, which has less plunger volume but likely a much lighter plunger. I also suggest photogates as an alternative to range testing if your school has access to these tools.

[Ubermensch]

I don't think enough people understand all of this stuff. We're not phosisticated enough...

[Split]

Do you really think that the wasted energy is full random? (between certain parameters of course). I mean, there are dozens of different factors. Did you add and extra bullet every time you fired one? Perhaps the weights and balances and empty space made a difference. Did you motorize the trigger pull? Incomplete or inconsistent pulls can do a lot of weird things to the performance of the gun. Did you reinforce the internals or make a new gun with each shot? Perhaps bending and warping played some role. Did you measure heat and humidity? Did you find the theoretical muzzle velocity (both with and without friction of barrel plunger?) Was there a solid brace to hold the gun in position?

It just feels not controlled enough for my tastes. I'm wondering how your professors (assuming it was for a college course) took in your results. Almost always, nothing is completely random, even within parameters.

[Peter]

Making a new gun for each shot in full on inconsistent.
As long as the trigger was pulled swiftly and with relatively the same amount of force, there should be no problem, and if for some reason it did matter, all he needs to do is go with a short swift trigger pull with more force, and a longer less forced trigger pull and retest..

Heat and humidity? This only matters if you tested in different locations.

Theoretical muzzle velocity? Why? So there can be more error within findings of the spring constant and the friction coefficients?




This sounds like it would be the problem, of course there is going to be lost pressure, the gas wants to fill the area with the least resistance first, that being the looser fit of the darts in the clip.

Edited by Peter, 03 June 2008 - 02:00 PM.

[Split]

Peter, on Jun 3 2008, 10:59 AM, said:

Making a new gun for each shot in full on inconsistent.

How is making a new gun inconsistent?

Peter, on Jun 3 2008, 10:59 AM, said:

As long as the trigger was pulled swiftly and with relatively the same amount of force, there should be no problem, and if for some reason it did matter, all he needs to do is go with a short swift trigger pull with more force, and a longer less forced trigger pull and retest..

Heat and humidity? This only matters if you tested in different locations.

You're talking relatives, I'm talking exact. You're also mentioning starting over. In laboratory reports, the goal is to make the experiment repeatable. He asked for someone else to confirm his findings. We need to know the conditions they are in in order to do that. That includes heat and humidity.

Peter, on Jun 3 2008, 10:59 AM, said:

Theoretical muzzle velocity? Why? So there can be more error within findings of the spring constant and the friction coefficients?

Uh, bud. You can't find the muzzle velocity without knowing the spring constant, and, if factoring in friction, the coefficient of friction. Just another line of details left out. What kind of caulk saver, glue, mass of gun. Tons of things could be measured. Just for accuracy and repeatability's sake.
The reason for finding muzzle velocity is that having two constants to measure results against is better than having one, just as having one is better than having none.

Peter, on Jun 3 2008, 10:59 AM, said:

This sounds like it would be the problem, of course there is going to be lost pressure, the gas wants to fill the area with the least resistance first, that being the looser fit of the darts in the clip.

What's "This"? A brace(last thing I mentioned)? Refilling the ammo tube(makes more sense)? Gas doesn't go to areas depending on resistance; it's all about pressure.

[AlaskaRecon]

Ubermensch, on Jun 2 2008, 03:09 AM, said:

I don't fire crap for nothing

what are you talking about? Anyhow, that is really complicated of course im only in middle school

[Ubermensch]

AlaskaRecon, on Jun 3 2008, 09:07 PM, said:

Ubermensch, on Jun 2 2008, 03:09 AM, said:

I don't fire crap for nothing

what are you talking about? Anyhow, that is really complicated of course im only in middle school

What the hell?!

[quiet asian man]

Splitlip, I appreciate your input, but I think you are overcomplicating things.

Allow me to restate my question and theory:


Does this really happen and waste energy? I do not have clear piping so I cannot be completely sure if the darts actually get moved.

The secondary question was, do people with RSCB barrels notice range decrease as they fire more darts, or do they stay concistent? As far as I've been lurking, I've seen people say the former, but my experiment showed the latter.

I would never expect anyone to copy my entire gun and environment. When I asked if anyone could confirm my findings I refered to the above questions.

If I gave that impression by listing gun details, I appologize. That was to give something to go against incase anyone else actually gets velocity data. I am using a big SNAP with probably double the normal spring power. If anyone could match my velocities with say, a nightfinder, I'd be extremely doubtful and ask how it was done.

[VACC]

AlaskaRecon, on Jun 4 2008, 01:07 AM, said:

of course im only in middle school :gay:

aaaaannnnnnddddd, end scene.

Nerfhaven: Pubes required.

VACC
~we don't have a gay emote? I need to fix that~

[Carbon]

Interesting work!

I’ll add some input based on my understanding of gas law and the physics of these guns.

Quote

Dart velocity did not gradually decrease as I fired more darts. In fact, velocity seemed go randomly go up and down after the first shot. However the gun also had inconsistent results using normal barrels, so I can't draw a direct conclusion other than "the plunger is so big that the deadspace filled by darts means nothing." But it still got me thinking about energy conservation.

Part of the problem is that the amount a dart seats in the barrel is somewhat random: one time it may seat in a quarter inch, the next time there may be a gap. Since the quality of the initial seal is determined randomly, it’s hard to get truly consistent results. The only way you could try to get more consistent results would be to manually seat a dart, and then replace the barrel. Even then, that doesn’t take into account dart inconsistencies.

Quote

My theory is that when you prime and maneuver the gun, darts shuffle around randomly inside the ammo tube. When shot level to the ground, the gun wastes random amounts of energy moving these darts to the back of the ammo tube before firing the dart. The gun would waste more energy if the darts were clumped at the base of the barrel than if they were at the back of the ammo tube.

It would explain it, but it isn’t an accurate representation of how pressure behaves. Air pressure doesn’t release its energy in a linear fashion like a spring (needing to expend energy moving other objects before the dart). Our pressure is created near instantly, and it seeks to equalize, pressing outward equally in all directions. (The pressure against the base of the dart is what causes it to fire). Since the rear of the RSCB with all the darts is closed, the pressure simply equalizes around the darts. With the darts present, the pressure would be equalizing in a smaller volume, and would mean a higher pressure at the base of the dart.

Clumping of the darts wouldn’t have any effect, since it’s the volume the darts represent which effect pressure. Five darts end to end have the same volume as five spaced out darts.

Quote

This explains the problem Carbon had with his ball SNAP with an endcap follower here:
http://nerfhaven.com...ndpost&p=128410
The endcap is like a very heavy ball that the gun moves in addition to his first ball. The gun wasted more energy moving the endcap up than it did going through deadspace. In addition the last ball fires the hardest because the gun wastes more energy moving the extra balls in the ammo tube.

The idea behind the endcap was to hinder the entrance of air into dead areas. Since equalization of pressure seeks the path of least resistance, more air would be channeled against the base of the ball. That was the theory, anyway. The lower ranges weren’t due to air pressure energy being wasted moving the endcap, but because a ball fires better with high volumes of low pressure air. Notice that ranges were also worse with a small dead space (muzzle-loading with no inline clip area), and greatly improved with a large inline clip.

In the case of the ball inline clip, no energy is being “wasted” moving previous balls, because up until the lead ball is fired, the air pressure is equalizing in a closed system: air pressure is created and presses outward equally in all directions. The balls simply take up volume, causing air pressure to equalize in a smaller area.

Edited by Carbon, 04 June 2008 - 10:24 AM.