Fun with an Air-Mortar - Summer 2006, Finland

On summer 2005 several fellows made a pact of "lets build air-mortars to shoot tennis-balls by next summer (2006)".

To settle who made the best one, criteria was selected as "the longest shot". That was latter changed to "highest muzzle velocity" when they learned that it can be measured with some measure of accuracy. Yours truly made that measuring possible.

At the beginning of the summer 2006 rumors were heard of 3 contenders. In the end the two other contenders pulled out, when they saw the beast shown below - they also failed on one important detail: tennis balls didn't fit into their barrels...
Comments muttered were something like "this air-mortar arms-race is a horrible thing.. now we have no hope, but next year..."

Oddly all three contenders did consult yours truly on best design, but only one really implemented anything close to it. (Have a BIG pressure-reservoir, large diameter pipe from it to quick-opening valve, large diameter valve, gradually shrinking pipe from valve to barrel, then constant diameter barrel. Principal idea being that flow from reservoir thru valve etc. is _slower_ than in barrel. Air-flow related losses are related by square of flow velocity!)

The Setup

Some notes:

• This air-mortar was built by Antti Hara, OH1HIH.
• Main components are originally for pneumatic cylinders and this does not have what is known as "pressure container" meaning that it does not need to be certified as such.
• For the exit velocity test setup we rented a high-speed video camera, and built a test track with high-contrast yard-stick next to track of the the flying ball.
• There is never enough light in these high-speed photography things.
  At full sunshine we added 2x 1 kW lights at about 1 meter distance from flight track -- we should have had something like 20 kW...
• This was designed to shoot tennis-balls, and it was quickly realized that a dry ball impacts with a rather small impulse on target, thus came an idea of filling them with water...
• .. we learned also, that a water-filled ball is very fragile and tends to burst completely asunder. (But it sure issued greater impact impulse.)
• While base-ball (and Finnish Pesäpallo) is (are) same size as the tennis ball, those are way harder and were chosen not to be used as ammunition...

Velocity test shots

Ordinary video of one of those speed test shots:

• Tennisball-velocity-test-shot-movie.avi (8.1 MB) ("Three! Two! One! WHOOS!")

Nope, the ball did not reach the trafficed road visible in the movie. For some "strange" reason we shot all our tests towards the earth...

Some High-Speed Videos:

Now we get down to the business and measure the ball flight..

• Tennisball-velocity-1-atm.XVID.avi (126 kB)
• Tennisball-velocity-6-atm.XVID.avi (167 kB)

These were filmed at 395.3 frames per second and with 43 microsecond exposure time (this is why there is never enough light...)

Pressure ATM (Bar)	Dry ball speed (m/s)	Wet ball speed (m/s)
1.0	76	-
2.0	106	-
3.0	128	-
4.0	145	-
6.0	163	163
7.5	163	163

For your reference: Speed of sound at these conditions was about 330 m/s ...

Shooting at something else

• Shooting-cd-drives.XVID.avi (1.4 MB) - learning of how to aim and hit the spot...
• CD-Drives-5.XVID.avi (0.9 MB) about 1000 frames-per-second (fps)
• CD-Drives-6.XVID.avi (0.5 MB about 1000 frames-per-second (fps)
• CD-Drives-8.XVID.avi (1.1 MB) - just a show-off of hitting the CD drive end-on :-)
• beer-cans-meet-air-mortar.XVID.avi (1.7 MB) - have no fear, these were discarded aluminum beer cans refilled with water
• freeway-cola-meets-air-mortar.XVID.avi (1.2 MB) - all too lenient treatment for that "cola" drink...
• kaa-noppa-1.XVID.avi (450k) another air-mortar, but barrel was too narrow for the tennis balls, so something else is flying...
• kaa-noppa-2.XVID.avi (360k)
• kaa-noppa-3.XVID.avi (150k)

Couple of times we actually missed!:

• Missed-shot-1.XVID.avi (1.4 MB)
• Missed-shot-2.XVID.avi (0.3 MB) - yes, that was not a tennis-ball...

And now the Special Feature:

garden-gnome-1-catches-a-tennis-ball.XVID.avi (1.6 MB) (at about 1000 fps)

garden-gnome-2-catches-a-tennis-ball.XVID.avi (2.0 MB) (at about 500 fps)

Preparations for the "gnome-2" and shooting...

• We did not like looking down the armed gun barrel, so we noticed that the air-mortar did not move in between shoots, and we used discarded CD-ROM drives as test targets..
  
• Ready for Shoot! (Note 2x 1 kW flood-lights next to the target...)
  
• Shot! (The "smoke" in the picture is water mist from a water-filled tennis-ball..)
  

This web-page and high-speed videography with Citius Imaging High Speed Video Camera C10 by Matti Aarnio (OH2MQK).