Failed attempt at a silent TB launcher

[Technician1002]

Just for grins I built a tennis ball barrel for my marshmallow cannon out of 160 PSI 2.5 inch PVC 36 inches long for a compact attempt at a silent antenna launcher. The theory was if I take the 54 cu in tank and use about 3 bar, it would expand to the barrel volume of 140 cu in without a loud report. On the contrary, it is very noisy. Am I missing something? In testing a 30 PSI launch fails to exit the barrel and a 40 PSI shot is good for 75-150 feet. 70-100 PSI shots all land in about the same area at about 300 feet with a range difference of only 30-50 feet. 60 PSI shots are not too bad, but 100 PSI shots sound like gunfire with a sharp report.

The small chamber makes a very good use of the air for easy hand pumped launches.

I used Google maps for range checking. The longer shots were oriented off the 3 tall trees casting long shadows in the lower right of the screenshot.

My larger t shirt cannon is very quiet when the expansion is very close to the barrel volume.

Any thoughts why this isn't quiet like a larger cannon?

Edit*** for those looking at the photo to see the parts used, there is a 1 inch PVC female adapter screwed on the tank, a 1 to 1.5 reducer, and 2 inch to 1.5 inch reducer in a male 2 inch threaded adapter make the white PVC parts in between.

The barrel has a 2.5 inch coupler with a 2 inch ABS female ABS adapter inside a modified coupler. This permitted recessing the cleanout for additional strength. The 2 inch threaded is the standard used for the larger launcher for it's 2 inch threaded outlet. A cement made for ABS/PVC was used to join them.

Added a photo of the ABS female adapter in the barrel. It was part of the effort to keep the distance from the valve to the shirt very short in the competition. It worked well.

[jackssmirkingrevenge]

If it's quiet at 50 psi and you double the pressure, you're naturally going to get more noise if you don't change any other parameter like barrel length.

[Technician1002]

If it's quiet at 50 psi and you double the pressure, you're naturally going to get more noise if you don't change any other parameter like barrel length.

It's only quieter, but still plenty loud. It has a pow instead of a bang. I was hoping for almost silent operation like I can get with my much larger launcher at low pressure. The large 700 cu inch launcher with the 3 inch barrel is almost silent at 20 PSI. Stealth shots in a campground is possible to rain stuff down 3 or 4 campsites away.

[jackssmirkingrevenge]

Using GGDT you can model roughly what the gas pressure will be at the muzzle and this will give you an indication of the noise. Technically a big chamber at low pressure will give the same value as a small chamber at high pressure, though the latter gives a bigger impulse to the projectile and therefore more power for a given volume of air.

[Lentamentalisk]

Right. 20psi is only 1.3 bar (ish), meaning it will expand to a barrel with volume 1.3 times that of the chamber, and then it will start forming a vacuum. 100psi on the otherhand, will expand down a barrel 7 times the volume, before it starts forming a vacuum. If you don't have it 7 times the volume or more, then it will be very loud. There is nothing you can do about that, other than get your pressure to volume-ratio ratio perfect.

[Technician1002]

Right. 20psi is only 1.3 bar (ish), meaning it will expand to a barrel with volume 1.3 times that of the chamber, and then it will start forming a vacuum. 100psi on the otherhand, will expand down a barrel 7 times the volume, before it starts forming a vacuum. If you don't have it 7 times the volume or more, then it will be very loud. There is nothing you can do about that, other than get your pressure to volume-ratio ratio perfect.

My problem with the small cannon is even though the volume ratio is about 3:1, (54 cu in chamber : 140 cu in barrel) at about 40 PSI it is still far from stealth.

The 700 cu in cannon on the other hand is close to being able to do stealth launches in a library at the right pressure.

I'm thinking the launch is too quick and the turbulence is noisy. There is a change in sound from an obvious vacuum shot to a pressure shot, but the sweet spot in the middle is the problem. I was hoping it would be silent or close to it.

[Lentamentalisk]

If you are really that concerned about it, just build a huge silencer/ expansion chamber around the end of the barrel, and drill a few holes into the end of the barrel. That will help neutralize the pressure difference, which ever way it is.

[Technician1002]

I think I have my answer. Due to the duration of the discharge provided by the small tank, it makes a pressure wave that is loud.

To test I used the other piece of the 10 foot section of pipe. These barrels were made for the t shirt competition and 6-7 feet was the sweet length for the acceleration curves. The 3 foot barrel is what was cut from a 10 foot length to make the 7 foot barrel.

In testing the suggestions, I tried a longer barrel and the results are;

50 PSI, failure to launch, but loud pop.
60 PSI same as above but louder.
70 PSI, loud pop and low energy launch.. 50 feet.
100 PSI, loud pop, lost the ball.

Let's face it, sudden high pressure releases will be noisy. The launcher is noisy due to the pressure wave created by the discharge.


Photo of the outrageous CB ratio. It does quite well at 100 PSI.

[jackssmirkingrevenge]

How tight a fit is the tennis ball in the barrel?

[Technician1002]

How tight a fit is the tennis ball in the barrel?

It is just loose enough to be shaken down without a stick.
Just tight enough that it won't drop on it's own.
Launches don't burn the fuzz.

The pipe is Crestline brand PVC SDR 26 160 PSI PR Water at 73 degree F.

[Lentamentalisk]

I am just wondering how much noise the ball makes just by rushing down the barrel really quickly. How much of a sound wave is created by forcing all of that air out... Would that be a significant amount?

In that case, a far better option for you would be to switch to a high pressure, low volume chamber, and a much shorter barrel, so that you don't move as much air.

I am quite invested in figuring this out, because I too am working on a small tennis ball launcher, for "antennas" or others such stuff, but in an urban environment, where not drawing attention is super important.
My design is basically a 8" section of 3/4" pipe for the chamber, at about 300psi, a 3/4" QEV, and 8" of 2.5" pipe (yes, it is just 8 inches long). GGDT is giving me speeds of ~180fps from that tiny setup, and ~0psi in the barrel at the release.


One thing I just thought about With such a small restriction, as it looks like you have, seeing as you are using a modded propane tank for a chamber (and like I will have with my creation) you will get a major backup in the chamber. Air will be flowing out of the chamber at the speed of sound through the restriction, because once it hits the barrel, there is a much less restricted flow. That means that the chamber will only empty at a set speed, and so the ball could leave the barrel with the chamber still having compressed air in it. Then all of that air has to escape, so might make a boom or a woomf or something, after the ball is gone. If you are pulling a pretty big vacuum, you might get two waves, one of the air flowing into the vacuum, and then a second, of the pressure catching up, and pushing it all back out.

What do you all think?

[Technician1002]

In that case, a far better option for you would be to switch to a high pressure, low volume chamber, and a much shorter barrel, so that you don't move as much air.

What do you all think?

In testing, the low pressure large volume tank is much quieter to the point of almost silent. The sudden pressure wave from the small chamber is noisy.

One thing I just thought about With such a small restriction, as it looks like you have, seeing as you are using a modded propane tank for a chamber (and like I will have with my creation) you will get a major backup in the chamber.

That was considered and I am still in the process of measuring the "Discharge time" of the tank to compare to the "Launch time" of the ball. As far as I can tell so far, all appearances is the discharge time is still shorter than the launch time. I'll know more later after some serious modeling. A 1 inch valve is good size for a 54 cubic inch tank for a very short discharge time.

Plug the value into GGDT. How long does it take a 54 cu in tank to discharge through a 1 inch piston valve and what is the launch time of a tennis ball in a 2.5 inch barrel 3 feet long?

That means that the chamber will only empty at a set speed, and so the ball could leave the barrel with the chamber still having compressed air in it.

There is a little evidence to support this. The cushion for the piston provides some recoil so after a good part of the chamber has discharged, the foam expands back out launching the piston closed. I sometimes finish a 100 PSI shot with the valve re closed and up to 20 PSI in the tank. I'm thinking of the possibility of semi auto This pressure may just be from the air hose still connected to the tank.

My design is basically a 8" section of 3/4" pipe for the chamber, at about 300psi, a 3/4" QEV, and 8" of 2.5" pipe (yes, it is just 8 inches long). GGDT is giving me speeds of ~180fps from that tiny setup, and ~0psi in the barrel at the release.

From what I've experienced, this is sure to wake the neighborhood!

I am just wondering how much noise the ball makes just by rushing down the barrel really quickly. How much of a sound wave is created by forcing all of that air out... Would that be a significant amount?

I think you have the source of the noise. The clue is the failure to launch shots are also noisy.

[psycix]

If you want to make a silent launcher, why not make a silencer?
Simply a pipe around the barrel, holes in the barrel, giving the air room to expand.

[ALIHISGREAT]

If you want to make a silent launcher, why not make a silencer?
Simply a pipe around the barrel, holes in the barrel, giving the air room to expand.

it would be phat and therefore heavy for a tennis ball sized bore.

[Lentamentalisk]

to emphasize ALIHISGREAT's point, when you look at a silencer for a gun, airsoft, or paintball, you will notice that the diameter of the silencer is several times that of the barrel. On the order of 4-6 times the diameter. That means to get a similar effect, the silencer for the TB cannon would have to be at minimum 1' diameter! Good luck even finding pipe that size, not to mention carrying it with out the help of 3 others...