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:

Velocity test shots

Ordinary video of one of those speed test shots:

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..

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

ATM (Bar)
Dry ball
speed (m/s)
Wet ball
speed (m/s)

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

Shooting at something else

Couple of times we actually missed!:

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...

Such was the fun on the weekend of 14-16. July 2006..

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