CSV19 Pneumatic Antenna Launching System
Document Notes Version 1.94 12/2/2005
CSV19 Antenna Launcher Performance Data
Measurements Measurement 9/05 using SDR21 barrel, Teflon impregnated Spectra line, Zip Reel, CO2, 4 oz ball, 5000 feet elevation, cool sunny morning - 50 psi 154 feet. Measured 40 psi 114 feet with Dacron twisted kite line (with Schedule 40 barrel) (Note that the line makes a significant difference, and changing to a new Spectra based synthetic line yielded 20% higher trajectory.) (Note that trajectory depends on many variables and will change slighly with altitude and temperature). Computer Modelled Performance* CSV-19 Compact Sprinkler Valve 19 inch Launcher system Cv 5.0 Ballwt 4.0 ounces chp oal chl bbl fps sec xbp xcp ht tofl 30 19 8 14 79 0.020 0.2 20.5 76 4.4 40 19 8 14 105 0.017 1.3 29.7 117 5.4 50 19 8 14 123 0.015 2.5 38.8 146 6.0 60 19 8 14 138 0.014 3.7 48.0 168 6.5 70 19 8 14 151 0.013 4.8 57.1 188 6.9 80 19 8 14 162 0.013 5.9 66.3 205 7.2 Cv = system flow coefficient, gpm per psi chp = chamber pressure in psi oal = overall length inches chl = chamber length bbl = barrel length fps = velocity feet per second sec = barrel time in seconds xbp = exit bore pressure xcp = exit chamber pressure ht = peak trajectory height in feet tofl = time of flight in seconds * note that these performance numbers are uncalibrated and subject to change based on measurements with your own launcher. The Cv value can be adjusted to fit your specific results. They seem to be quite close to the Dacron line measurement, however measurements with new synthetic line were 20% higher. As the line ages the performance at higher heights drops slightly due to increased drag from wear on the line.
A tennis ball with fishing line attached is pushed down the barrel using the ramrod. Tennis balls are quite tight in the barrel. The fishing reel is set to feed out line quickly.. The pressure chamber is pumped up with air using a tire pump to a nominal pressure of about 40 psi. The Launcher is aimed at a steep angle to lob the tennis ball over the desired branch or tree and triggered. The valve opens quickly and the air flows through the valve and pressurizes the barrel. The pressurized air expands into the lower pressure in the barrel and places force on the tennis ball. The force on the ball is the area times the pressure, or about 5 square inches times the pressure. For example if the barrel pressure was 10 psi the force on the ball is 50 pounds. The force on the tennis ball from the pressure accelerates the ball down the barrel. The force and acceleration are substantial. In the case of 10 psi and 50 pounds the acceleration would be 400 times that of gravity! The pressurized air continues to flow from the pressure chamber through the valve into the barrel. As the ball moves down the barrel the volume of the air in the barrel increases dramatically and the pressure drops since the air coming through the valve cannot keep up with the increasing volume in the barrel. The airlofw through the valve is limited by the diameter of the flow path in the sprinkler valve and the speed of sound - the air cannot flow faster. In this launcher the valve airflow is near the speed of sound most of the time. The tennis ball is accelerated and leaves the barrel with enough velocity to accomplish the desired trajectory, towing the fishing line over the tree. The velocity is generally lower than a fast tennis serve for the heights we need. A professional tennis player's fast serve would launch an untethered tennis ball nearly 300 feet high.
The Valve we are using is a sprinkler valve, which is a type of Pilot valve. Pilot valves use the pressure of the fluid (or gas) they are controlling to do the work of moving the valve, using a clever pressure differential system. Inside the valve is a diaphragm holding the valve seat closed. Attempting to open the valve the pressure has only the 1" circular area of the valve to work against. (The force is determined by the pressure times the area). The closing force on the valve is developed from a small spring (very minor force) plus the pressure acting on the larger area of the diaphragm (more than a 1" circle). This gives more force in the direction of closing, and seals the valve tightly to keep it from leaking. There is a small pressure equalization hole in the diaphragm that allows the pressure on both sides to equalize to the same. Since the areas are different, the force is not the same and the valve stays closed.
Triggering the valve is accomplished via the blowgun valve. The blowgun is on the pilot side of the diaphragm, and when open it allows the pressure on this side of the diaphragm to vent to atmosphere. The equalization port is attempting to build the pressure while the blowgun is releasing it, but the equalization port is very small and so the blowgun wins, and the pressure on the pilot side of the diaphragm drops. The pressure on the inlet side of the valve can then move the diaphragm and open the valve seat. This process can occur in a few milliseconds due to the large force from the pressure. The rate is primarily determined by how quickly the trigger system can exhaust the pilot area, the mass of the diaphragm, the ratio of areas, etc.
If we were to actuate the valve using the electric solenoid it would be much slower to open, and it would not open as far. The normal purpose of a sprinkler valve is to turn sprinklers on and off, going fast is not actually desired at all for water valves - water hammer is hard on things. The solenoid ports are very small and cannot be opened up as much as the blowgun. The batteries and wiring and the solenoid sticking out are not as nice a system to build and maintain, and not as lightweight and compact. I used batteries with my first launcher, and have since modified it for pneumatic operation. The performance was about doubled with this upgrade (twice the height for the same pressure). The valve oscillated and made a honking sound with electric actuation. It was clearly not opening as effectively as it does when pneumatically actuated. In fact, the pneumatic operation is what enables this CSV19 compact launcher to be so small and still be so effective - able to reach well over 150 feet of height. Electrically actuated it would not have very adequate performance for our purpose - it would have to be made larger to do the job adequately.