The DFTV19 Pneumatic Antenna Launcher

(Darn Fast Turbo Valve)

This air-powered Tennis Ball launcher is designed to launch tennis balls over trees towing fishing line. The fishing line is used to pull up nylon twine and then wire antennas, or heavier line and antennas.

These launchers are used by Amateur Radio groups doing Emergency Service work and preparation drills.

This is my first foray into the homemade valve area. As far as I know, the self venting and turbo features of this valve are new. I developed these ideas to meet my low flow triggering requirements. The development discussion can be found on the www.spudtech.com forums. Some credit belongs to the members there who participated in the discussion and offered their thoughts and advice.

Requirements

Specifications

Drawing

Theory of Operation

Refer to the diagram above. The orientation of the diagram matches the photo of the launcher above. The dark blue area in the center is the high pressure flow from the pressure chamber. The lighter blue area below is the pilot area. It is initially the same chamber pressure due to the small equalization hole in the center disc. The center disc is called the 'opening disc'. It generates the force that opens the valve. The top orange area is the exit port. The disc that seals this off is the exit port disc. Note that the exit port disc is smaller than the opening disc. The bottom disc is the vent and turbo disc. The venting portion of this lowest disc is the small "O" ring sealed portion. Note that the vent disc is smaller than the exit port disc. The large diameter part of this lowest disc is the turbo disc portion. The piston consists of all three discs connected together by the threaded rod in the center. The trigger valve allows pressure to escape from the pilot area via the small red area toward the left. Note that the flow limit cross sectional area in the system is the exit port disc.

The charging cycle starts by pushing the valve closed (upward) and pressurizing the pressure chamber. The pilot area (light blue) equalizes to the same pressure through the small hole drilled in the opening disc. The net force on the piston is upward since the exit port is larger than the vent port, and the opening disc generates no force since there is no pressure drop across it.

The launching cycle begins when the trigger valve opens and begins to exhaust the pilot area. The trigger valve has much higher flow capacity than the tiny equalization hole. The pressure in the pilot area drops generating increasing force downward due to the pressure differential across the opening disc. At some point this force exceeds the upward force plus the frictional forces, and the piston moves downward. The vent O ring seal opens first, allowing the pressure in the pilot area to drop further, increasing the force downward on the opening disc. The escaping pressure from the pilot area additionally pushes on the turbo disc at the bottom and generates additional force downward on the piston. This is the turbo effect. It increases the downward force on the piston more rapidly than would waiting for the pilot area to vent through the exit port. It is effectively using energy of the vent exhaust to augment the piston acceleration. The exit disc is pulled out of the exit port and the upward force drops off. The loss of this upward force also dramatically increases the net downward acceleration of the piston. The piston is accelerated downward with high forces and travels into the bumper at high velocity, leaving the exit port clear for tremendous flow into the barrel during launch.

Performance

This valve is quite fast. It makes quite a pop even at 11 psi, the lowest pressure it reliably actuates. Over about 30 psi I wear earmuffs in the enclosed space of the garage due to the volume of the dry-fire pop. (The air in the valve exit port reaches mach 1 - the speed of sound - at about 30 psi and above). Estimates of the piston travel time computed a 1" piston travel in approximately 10 milliseconds and piston acceleration of about 800 G's. The neoprene bumper seems to be extremely effective in stopping the piston safely. This valve has been tested to 80 psi. It may go much higher, that is more than adequate for my needs. I did not calculate the stresses in the discs - they are likely the limiting factor in the strength of the system. The exit port disc is the most stressed, and it is partially backed up by steel and rubber washers. It could be made stronger if needed.

The performance of this launcher is tremendous. At 80 psi the recoil of dry-firing is quite noticeable. It has more performance than the dual sprinkler valve model at just a bit over half the weight. It could be made quite a bit smaller and still have adequate performance for antenna launching. Here are a couple of measurements.

The performance was measured with 4 ounce weighted tennis ball and Saunders Zip Reel. This compares favorably with the dual-sprinkler valve launcher which reaches 120 feet at 30 psi, and weighs about 10 pounds.

Fixing a Big Leak with Vacuum

I had one big leak where the pipe from the pressure chamber goes into the tee. This was caused by the pressure gauge hole communicating the inner pressure to the cement joint fairly near the edge of the joint. This was repaired by putting a slight vacuum on the chamber and putting a bead of cyanoacrylate cement onto the junction. The vacuum pulled the cement into the cracks. The vacuum was generated by a marine oil changer. It consists of about a 2 gallon metal container and a hand operated piston type vacuum pump and tube. It works well for pulling the oil out through the dipstick on the boat or car, and now I've found a new use for it - pulling cement into cracks. The amazing result is that this launcher will hold pressure for hours...

Building this Launcher

I used a www.Grizzly.com G0516 Lathe/Mill combo machine to build this launcher. The Mill was used only for the three holes that hold the stop ring in the valve. I never thought much about having a lathe, but at one point I realized that it would be useful in several of my hobbies and I selected this model. It allows me to do so many things that I don't know why I didn't consider it before. It is a 9x20 lathe with a mini-mill on the lathe bed. The mini-mill can be moved to its own (separately purchased) mill table if one decides later that the combo is too limiting. I'm looking forward to making coil forms and winding various large coils on this lathe, among other things. At some point I will make a separate web page for the lathe-mill.

This is the first project I built with the new lathe-mill, and it came out fairly well. This was a new design and there were some design issues to deal with, but the lathe was a great help in the project. Many things that might be difficult or even dangerous with other power tools (like cutting grooves for O rings) are very easy and safe with a lathe.

The three discs were cut from 1+5/8" PVC type I rod available from McMaster. The launcher was initially built with 1.414" porting and a flat vent valve. This worked fairly well at high pressures, but would not work at low pressures. It also had a tendency to self-fire. The low pressure operation was improved by changing the exit port to 1.2" by epoxying in an adapter ring in-situ. The reliability and safety was vastly improved by changing the lower vent valve from a flat washer seal to an O ring. The valve works reliably at 11 psi if triggered within a short time of charging. If let to set with pressure in it for awhile the friction becomes too large for triggering at 11 psi. This is a function of the lubrication, I suspect. A better lube would be a good improvement. This research has not been done yet.

The small washer-bolt assembly in the photo above is the piston extraction and installation tool. After removing the screws holding the stop ring, this tool is threaded onto the bottom of the threaded rod sticking out of the nut at the bottom of the piston and used to extract it.

The pushbutton trigger valve is from McMaster and it has a Cv rating of 0.24. At low pressures there is a noticeably delay in launching but it is perhaps 1/4 or 1/3 of a second. At higher pressures it appears to be a lot less. The barrel end is a 2.5" endcap that has been bored to accept the 1.5" elbow directly. The end of the elbow has been shortened and epoxied into the endcap with about 1/8" protruding into the endcap. A good fillet inside the endcap is made of epoxy by pouring a puddle around the elbow inside the endcap. The pressure chamber to 1.5" pipe transition is similar. The endcap was bored to snugly fit the 1.5" pipe. Approximately 1/3" of pipe protrudes into the endcap, and is surrounded by a 1/3" ring cut from a length of 1.5" coupler. These items are epoxied together and into the endcap and a good puddle fillet poured around them. It is a very strong fitting, and much less weight and more compact in size than the traditional stack of bushings. All PVC used in this launcher is schedule 40 pressure rated. Joints other than the special ones mentioned above are standard PVC purple primer and clear solvent cement.

Note that the manufacturer does not recommend PVC for use with compressed gasses due to the danger of fragmentation if it breaks while pressurized.

Other Thoughts

Looking at the modelling calculations, the pressure chamber volume is much larger than needed for this barrel length. An accessory barrel extension can be employed. A smaller chamber would have almost the same performance in the standard barrel length. The smaller chamber would weigh less, so the launcher weight can be reduced considerably. The thinner SDR21 2.5" material can be employed for the barrel and this would save approximately 3/4 of a pound in weight. Thus a 3-4 pound version of this launcher could be built that would still have more than adequate performance for Antenna Launching.

Have Fun and Be Safe!

The Rest of the Story contains information about the support equipment and usage of the Antenna Line Launchers.

Please send any feedback on this document to wb6zqz at arrl.net

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