# Gas station without pumps

## 2012 July 12

### Nerf gun barrels

Filed under: Robotics — gasstationwithoutpumps @ 19:08
Tags: , , , , ,

In Nerf gun progress, I discussed possible approaches for handling the problem of Nerf-brand darts not fitting in the barrel:

• Find a source of (probably non-Nerf) foam darts that are 1.45cm (9/16″) diameter with heads that are no wider than the body. I think that they came with an NXT generation crossbow, so replacement foam darts for that may be what we need.
I’ve ordered a couple dozen NXT generation darts.
• Buy Nerf  (or other) darts with the right size bodies but oversize heads, remove the heads, and make new ones (out of what?). This would be cheap, but tedious, and the darts would probably fly poorly, unless we made the new heads have a decent weight.
I’m not willing to try this yet.
• Use clip-system darts for compatibility with the popular Nerf guns, but find a smaller diameter tube than the ½” PVC pipe (where? and how would it be connected to the solenoid valve?) It looks like Schedule 40 3/8″ steel pipe has a inside diameter of 0.49″, which is just right, but steel pipe is rather heavy.
I’m still looking, but I’ve not found any lightweight tubing that I think will work.
• Use clip-system darts, but convert to the Nerf-standard tube-inside-the-dart launching system.  This limits the effective barrel length to the inside length of the dart (about 4.5cm) and the barrel diameter to the inside diameter of ¼”, which will limit the top speed of the darts (OK for safety, but probably not as much fun).
I tried this out today, as the outside diameter of ¼” copper pipe seems to be just the right size.  We already had a piece of copper pipe stuck into a 3/4″ threaded end cap (it was part of the vacuum bottle for the ROV), so I could test this easily.

Here are the two barrels. The long barrel of 1/2″ PVC has an NXT Generation dart poking out the end. It can be loaded all the way into the barrel with a ramrod. An old-fashioned NERF-style dart with the same body size, but a head too big to fit into the barrel is shown below the barrel. The NERF Clip-system dart is shown mounted as far as it goes (only 4.3 cm) onto the 1/4″ copper tube.

The Nerf method of firing darts (with the dart surrounding the launching tube) is similar to our firing of paper “rockets” from the outside of the 1/2″ PVC barrel.

Loading the darts onto the tube is a bit finicky, as the foam is a tight fit over the copper. It might help to smooth the end of the tube, and perhaps use a dry lubricant (soap?). Rapid loading might be a problem.  I can also see why so many Nerf enthusiasts modify clip-system darts—only 4.3 cm fit onto the tube, but one would expect closer to 6cm to fit.  The depth the tube goes into the foam corresponds to the length of the barrel in the bullet-and-barrel system, and the longer the barrel the higher the muzzle velocity (to first approximation).

The darts fire fine though and seem to go fast enough.

Firing the darts this way makes a much higher pitched “pop” rather than the deep “thump” of the long barrel, because the shorter tube has a much higher resonance frequency.  Actually, I don’t know whether this arrangement is better modeled as a closed tube as I did for the long barrel in Nerf gun on the oscilloscope or as a Helmholtz resonator, which would have a resonant frequency of $\frac{v}{2\pi} \sqrt{\frac{A}{V_0L}}$, where v is the speed of sound (about 34320 cm/sec), A is the area of the neck (about 0.217 cm^2), V0 is the volume of the pipe between the valve and the neck (about 22 cm^3), and L is the length of the neck (about 9cm).  Hmm, the Helmholtz resonator is at about 180 Hz, and the pitch is definitely much higher than that, so perhaps an open-pipe model [Wikipedia’s article on acoustic resonance] is called for: $\frac{v}{2(L+0.3d)}$, which gives 1750 Hz.

1. Sch80 1/2″ PVC is has a nominal ID of .526″, which is a decent low friction fit for the “streamline” nerf clip system darts, and will fit the same fittings as the sch40 PVC that you already have.

Comment by KaneTheMediocre — 2012 August 8 @ 20:17

2. I’ve seen pipes with both IDs (Not much in between, oddly enough), and either one should work fine with a system like yours. I’ve even used a nylon tube with a 9/16″ bore. Since the projectile is so lightweight, leakage around the edges will be negligible during it’s acceleration through the barrel. On the tight side, 1/2″ CPVC usually has an ID of around .480, which provides a great deal of friction. This is actually helpful if you have a slow-opening valve or spring/bow powered system in which there is some delay before full pressure is reached behind the dart. Sometimes, when a spring or bow powered homemade nerf gun is too leaky or weak to use a full CPVC barrel, I’ll couple a short length of CPVC nested in sch40 PVC with a longer length of sch80 PVC, which gives the needed delay with less lost energy.

Comment by KaneTheMediocre — 2012 August 9 @ 22:16

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