Amateur radio is an eclectic hobby, to say the least. RF propagation, electrical engineering, antenna theory – those are the basics for the Ham skillset. But pneumatics? Even that could come in handy for hanging up antennas, which is what this compressed-air cannon is designed to do.
[KA8VIT]’s build will be familiar to any air cannon aficionado. Built from 2″ Schedule 40 PVC, the reservoir is connected to the short barrel by a quarter-turn ball valve. Charging is accomplished through a Schrader valve with a cheap little tire inflator, and the projectile is a tennis ball weighted with a handful of pennies stuffed through a slit. Lofting an antenna with this rig is as simple as attaching a fishing line to the ball and using that to pull successively larger lines until you can pull the antenna itself. [KA8VIT] could only muster about 55 PSI and a 70′ throw for the first attempt shown below, but a later attempt with a bigger compressor got him over 100 feet. We’d guess that a bigger ball valve might get even more bang for the buck by dumping as much air as quickly as possible into the chamber. Air Flow Pot
Looking to launch a tennis ball for non-Ham reasons? We’ve got you covered whether you want to power it with butane or carbon dioxide.
Oh joy, another compressed air + PVC build. Stop using PVC for this application and stop posting builds that do this.
I’m interested in why PVC is such a bad idea, as this is not at all obvious to me, and I may have use for one of these.
EDIT: never mind, I just read the rest of the thread
Hey, I wonder if you could make a propane filled air conditioner out of schedule 40.
a real nerd would have used a sprinkler valve for an instant release of pressure…
either way its just a potato gun, not really hackaday worthy
Public Service Announcement – Pressure test things like this while they are mostly mostly full of water. That way if anything ruptures you have far less energy involved as the water can not be compressed.
If it has enough energy to loft a ball, it can also loft shards of plastic into your soft fleshy bits….
Which is precisely why you should never use PVC for any compressed gas. Ever.
2″ Schedule 40 PVC is rated for 166 psi of operating pressure and 890 psi of bursting pressure. At the pressure you can obtain with a garden variety air compressor, you don’t likely have to worry about anything bursting. Just be careful what you aim at, though.
*Of water. None of those ratings are for air.
Pressure is pressure. The pipe doesn’t care what fluid is inside.
Granted or it does let go, something incompressible is much safer but don’t it with composed gas doesn’t make it more likely to blow
It does care whether the fluid is compressible or not, because that changes how the pipe deforms under load or impact, and that changes the stress distribution that would lead to failure.
No, what you’re thinking is, “If I apply excessive load to a piston in a fluid filled tube it will break more easily than a gas filled tube, because gas is compressible and fluid isn’t”… Where your intuition is failing is that for a given force on the piston the gas will be displaced more, or that you can’t displace the piston an equal distance against the fluid without breaking, but that will require far greater pressure than to displace against the gas. For example, if the end of the piston protrudes an inch over a solid surface, such that it only may be displaced an inch, and you fill it with gas and whack the end with a huge hammer, the gas will compress and the hammer will expend most of it’s energy bouncing off the surrounding surface. Whereas filled with fluid, the force of the hammer may be fully converted to pressure inside the cylinder in only an inch of motion and burst it. However, since the unsupported length of the cylinder wall is shorter, because of higher piston displacement in the gas example, one can quibble that a higher pressure than gas than fluid will be held. Removing that variable however, and applying equal pressure from an external source, to an equal length, otherwise sealed cylinder, you find pressure is pressure. Yes, if you’re filling it with compressed air, the empty cylinder will take longer to fill to a specified pressure, and yes if you fill a tube with water and apply the compressed air for equal time, from a source capable of achieving overpressure, it will achieve higher pressure in the cylinder than air alone and may burst, but that’s because more pressure. Time is not pressure, displacement is not pressure, pressure is pressure.
Pounds per square inch interchangeable. It doesn’t matter if it’s 100 psi of pudding, or cat hair. It simply means per square inch of volume, is a certain number of pounds of mass. Water just doesn’t compress. You can’t push more water into the same volume of space like you can air.
Water actually does compress quite a bit, just not enough to matter at 150 PSI.
PVC also has a sharp brittle transition below room temperature, so if you happen to be launching antennas on a cold morning, prepare for the worst.
Impact strength of PVC with respect to temperature: http://www.madisongroup.com/photos/impactpvc.jpg
It’s also to be noted that PVC tubing doesn’t like high temperatures either, and it starts to lose stiffness quite early at around 50 C or so. These temperatures are easily achieved by simply compressing air into the tube, since by the ideal gas law the increase in pressure must be met with increase in temperature.
70 psi of pressure pumped rapidly enough will raise the temperature to nearly 100 C and the regular plumbing type of PVC – and especially if it’s got plasticisers added to improve the cold performance – will go soft. Then, after sitting for a while and cooling down, the air is released and the temperature drops dramatically by the same mechanism. Eventually the pipe section fails and shatters.
http://www.madisongroup.com/case-studies-pvc-pipe.html
Should have gone screw pipe with a sprinkler valve. Pvc doesn’t show up on X-rays. Threading the gauge into the pvc is creating a failure point, at least tap on the engagement of the fitting where the pvc is twice as thick.
PVC shows up on x-rays. Definitely shows up on CT scans. Source: I’ve used both to localize fragments and take them out of people. On the Google Image Search below you can see two PVC pipes inserted into a cadaver. This is done by an organ harvesting team to still give the body a sense of structure, despite missing several long bones. Source: Been there, done that.
GIS: https://www.google.com/imgres?imgurl=https%3A%2F%2Fs-media-cache-ak0.pinimg.com%2F736x%2Faa%2F2b%2F7e%2Faa2b7ece6946f2bfc848c56320076e88.jpg&imgrefurl=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F448600812858694463%2F&docid=mDljmr_-ePU9qM&tbnid=7_b9Ix9MCRwFhM%3A&w=736&h=534&safe=active&bih=919&biw=1920&ved=0ahUKEwi62u7MyuLPAhXMLSYKHUFLCTwQMwgjKAUwBQ&iact=mrc&uact=8
Using PVC for explosive cannons is pretty stupid. I’ve only seen a small handful in the ER. I have yet to see an air-powered cannon explosion, and I’ve got one in the shed (purely for educational use… sure).
If you use a diaphragm sprinkler valve you can get much faster opening and higher performance.
A air dump valve would be good, though expensive for an industrisl one new. A large sprinkler valve could probably be modified alot cheaper. Sprinkler valves are really dump valves with the pilot circuit controlled by a selenoid valve.
for the price of an air gun, a couple feet of hose , a bit of epoxy, & a couple hose barbs any sprinkler value can be pneumatically activated. DIY piston or diaphragm valves ate fairly easy to construct as well. you only need 1/4 of the barrel diameter worth of clearance to get near max efficiency.
Or, you could just tie some fishing line to an arrow, but this requires having a bow. I have used this exact technique. Less “DIY,” but potentially easier to carry (no need for a compressor) and with less noise.
Just the associated danger of the return path and Mostly Bags of Water within it.
Or a slingshot/catapult, then That would be more compatible with the ‘weighted tennis ball’ projectile.
You do know that arrows have threaded holes in the front where you attach the sharp, pointy bit, right? You don’t have to put an arrow head in there. You can even buy rubber flat “blunt” arrow heads.
No problem, you can make the bow from PVC as well
Or, you could just tie some fishing line to an arrow, but this requires having a bow.
In another video he mentions that he used to use a crossbow but his neighbors gave him the hair eyeball. Hams are all about making nice-nice with the neighbors, and this seems a lot less threatening to the blue-hairs driving around the HOA Enforcement Golf Cart.
Came her to say I did both of these things as a kid, at the same time. I used a heavy braided fishing line fired from my 65# hunting bow (the only bow I had) to pull a clothesline cord, and the clothesline to pull a rope to the top of a tree so we could pull it down and away from the house as we felled it.
I added more line than I would ever need for fishing in order to reach the top of the tree and back down again, but not nearly enough for the entire flight of the arrow. After about 80 feet up, when the arrow reached the end of the spool of braided line, the massive steel-tipped fiberglass fishing arrow had so much energy it snapped the 100# test like thread (the fairly sharp right angle corner of the hole through the arrowhead probably helped cut it.) The arrow continued on its arc as if nothing happened, and the fish-stabbing end plunged nearly a foot deep into the dirt of a distant neighbor’s front yard. Fortunately, the distant neighbor’s children were unharmed, but I thought I might not be so lucky.
I use a wrist rocket type slingshot with a cheap fishing reel to shoot a 5-10 ox teardrop sinker over the branch. I attach the sinker with a swivel and a snap clip. Painting the sinker with dayglo paint helps spot it in the tree.
When the sinker hits the ground, unclip and attach some dayglo mason’s twine. Reel the fishing line back in, and unclip the mason twine, connect it to paracord, and use the paracord to support the antenna.
After a while, you get pretty good at doing this. :-)
A real nerd would have built their own quick release valve that eliminates the standard valve creates.
I wonder if a bottle rocket would do this job, and be a little safer?
You’d need one heck of a bottle rocket to pull that line, definitely not safer.
https://www.youtube.com/watch?v=q508d3k9518
Good resource for long-range FPV flying antennas, and possibly for getting your multicopters off trees. I assume the goal here is to hang something on a tree.
How to start an argument on HaD, put air in a PVC pipe. But anyway although it’s not a hack what the hell do they call those guns they use to shoot lines between ships at sea? An army (navy) surplus one of those would be perfect for this job.
When I was in the Navy, a gunner’s mate would use an M-14 to shoot the messenger line over.
https://en.wikipedia.org/wiki/Lyle_gun
i was a navy gunner. we used m-14’s or m-1’s with firing blanks, a large rubber bullet, and a firing adapter attached to the end of the rifle. btw we used to have contests on who could hit what when we shot.
I need to get lines over 100′ high trees. A spud gun would be good but if PVC is too fragile and dangerous, what material should I use instead? Or is there some way to coat or contain the PVC in case it explodes?
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