High-Speed Optical Diagnostics of a Supersonic Ping-Pong Cannon

We provide instructions to construct and operate a supersonic ping pong cannon along with optical diagnostic techniques for measuring ball speeds and detecting shockwaves. The optical knife edge setup is highly sensitive to small, traverse deflections of a laser beam and provides a means for detecting shockwaves associated with the cannon. To begin, power the photo receivers and laser module by connecting them to a 15 volts current limited power supply and laser power supply.

Then, attach the photo receivers to the two channels of the oscilloscope using BNC cables. For optical diagnostic setup, place the laser perpendicular to the fabricated cannon with the first beam traversing through the acrylic windows inside the pipe, and the second just outside the cannon exit. Place black electrical tape over half of the photo receiver sensor, creating a knife edge shock detection setup.

To avoid clipping, adjust the beam position on the knife edge so that the baseline voltage is approximately 50%of the maximum. Next, adjust the settings on the oscilloscope to collect 20 million data points. Use the horizontal scale knob to set the data acquisition rate to 500 megahertz.

Turn the trigger knob to trip at a voltage slightly below the set baseline acquired from the photo receiver. Before firing the PPC, wear ear and eye protection. Insert a ping pong ball into the exit of the cannon and blow lightly into the end of the cannon until it hits the vacuum fitting near the pipe entrance.

Seal the cannon's exit flange and acrylic cap using two pieces of square tape. Ensure the laser beam is centered on the knife edge, the trigger is correctly positioned, and the catching container is secure. Turn on the vacuum pump to evacuate the pipe to a reduced absolute pressure of less than two Torr.

Once a sufficient vacuum is reached, use a broadhead tip to puncture the tape at the entrance. After firing, turn off the vacuum pump and remove any remaining tape from the exit flange and the acrylic cap. For firing the supersonic PPC, close the valve connecting the driver pipe to the air compressor and fill the air compressor tank.

Blow the ball, as previously demonstrated. After sealing the supersonic PPC exit flange, insert a thin precut polyester diaphragm between two rubber gaskets and place them between the cannon's driver and driven sections. Connect the two sections using four cam clamps.

Once pressure is reduced in the pipe, release the pressure from the air compressor into the driver pipe, allowing pressure to increase until the diaphragm ruptures and the compressed air within the driver pipe fills the evacuated driven pipe. After the supersonic PPC firing, turn off the air compressor and the vacuum pump. Remove the ruptured polyester diaphragm and tape from the cannon.

The propagating shockwave, reflected throughout the cannon firing process, was represented by a change in voltage with respect to time. A positive or negative spike demonstrates the direction of the shockwave in the signal, and velocity was calculated through the square pulse width, produced by the ball cutting off the beam. A microprocessor was used to process the signal from the beam transversing the pipe interior to automatically calculate and display the ball velocity near the cannon exit.

The oscilloscope traces demonstrated the dual channel firing of the supersonic PPC. The upper trace represents the beam traversing the cannon interior, near the exit. Whereas the lower trace corresponds to the beam traversing the ping pong ball path after exiting the cannon.

A cutoff signal indicated that the ball passed and obstructed each beam. The plot of pressure differential versus Mach number revealed that 0.001 and 0.002 inch thick diaphragms rupture at sufficient pressure differential to accelerate the ping pong ball to supersonic speed. The ball speed was consistently greater than Mach 1.3 with a 0.002 inch thick diaphragm.

The presented supersonic ping pong cannon and the optical diagnostics will enhance the value of the cannon, both as a demonstration device and as an apparatus for follow-on laboratory experiments.