Ingenium: Ingenious Machines
"Cum catapultae proscriptae erunt tum soli proscripti catapultas habebunt."

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The Klingon chetvI'
The Medium-sized Trebuchet

It began innocently enough. I was attending a meeting of the Overnighter program staff at Carnegie Science Center. The overnighter program involves some 400 kids from school groups, scout troops and the like, spending the night at the science center. They do workshops, see planetarium, laser and Omnimax shows, see the exhibitry and, at 2:00, are hopefully asleep in sleeping bags all over the floor

Anyway, at this meeting, Cathy, the program director, was looking for something neat to do as a lobby activity. Something the kids can do during free time.

She suggested a catapult.

"What you want is a trebuchet." I said. "A catapult uses a throwing arm under torsion. It needs to be hauled back to lock and load. A trebuchet, on the other hand, uses a counterweight which could be unloaded so that kids operating it don't have their teeth knocked out by a misfire."

I got the job.

By the next week I had searched the web and discovered several sites with information on trebuchets. The Gray Company had a marvelous page and had a nice side note on a machine named "Cheesechucker." I thought it seemed a little small, considering mine would have kids clustering around, eager to be next, so I doubled the dimensions. With a plan consisting of little more than a picture with some measurements noted in the margins, I spent the next weekend secluded in my basement constructing my siege engine

[treb plans 1] [treb plans 2] [treb plans 3] [treb plans 4] [thumbnail of treb plans 4]
Complete plans for the trebuchet in five parts. [Medium Trebuchet]

It was completed late on a Sunday night. I couldn't resist and pulled it into the back driveway for a test. Two lead bars as the counterweight with a racketball as the projectile and I let fly.

It launched the racketball like an overhead serve, the ball ricocheted off the grass about 25 feet away and bounced off into the darkness. The quarter inch threaded rod I had used as the fulcrum had bent noticeably and it shimmied wildly.

By late monday night I had repaired the fulcrum with a half inch rod and made other adjustments. Again I set the device up in the driveway. Having learned my lesson of the previous night I used only one lead bar in the counterweight and found another racketball. This time the arc was more ballistic but this projectile was also lost to the darkness.

Tuesday morning and I set my machine up in the science center parking lot. Two 20-oz pop bottles filled with lead shot as the counterweight and a foam planet ball-thing as the projectile, purchased specifically to be hurled. Fwoomp! It sailed beautifully over 30 yards! I spent the next hour and a half demonstrating to all the staff coming in to work. They were all very impressed. I was impressed by the consistent accuracy. My fielder didn't need to take more than one step in any direction to catch each throw.

It exceeded all my expectations, especially considering it was intended for use inside. There were very few places inside the science center with enough overhead clearance for a decent throw. One was on the stairs leading down to the Omnimax theater. By adjusting the mass of the counterweight, one could get the projectile one floor up and over the rail. [Snooty Frenchman]Too high and it bounced off the ceiling. Too low and it would hit the windows to the Director's office. Perfect. Except that this area was off-limits during overnighters.

The next option was to set up where the ramps spiral up to each floor. I could throw up to the second floor or the third floor. I even drew up a target, a snooty Frenchman a'la Monty Python and the Holy Grail, to set upon the parapet.

As impressive as my machine was, it was also intimidating and its great range made it somewhat unsuitable for use indoors. So, it was back to the basement to make some smaller versions. Mere toys.

Read about the Small Trebuchet

MVI_0106.AVI: Video of the Medium Treb in action

30 second video clip of my Medium Treb in action. The counterweight has 22.8 lbs of lead in it and it is throwing a golf ball 150 feet. -- Revised: 13 August 2006
Copyright © 2002-2006 Kevin A. Geiselman