May 24, 2000
Sean Butler hurls water balloons
with his home-built, NOVA-inspired,
mini-Trebuchet...

trebshotA.gif (1038278 bytes)

 

Also check out www.trebuchet.com

DISPATCHES

Tuning A Trebuchet

THROWN OBJECT
Everything else being equal, if you change the weight of the thrown object my experience is that
- The heavier the object, the higher the trajectory: vertical or even backwards if the object is heavy enough (like trying to fling a pumpkin with a 90lb counterweight).
- The lighter the object, the flatter the trajectory: slammed into the ground, if light enough

COUNTERWEIGHT
Everything else being equal...
- The heavier the counterweight, the flatter the trajectory.
- The lighter the counterweight, the higher the trajectory of the thrown object.

SLING
Everything else being equal...
- The longer the sling is, the flatter the trajectory.
- The shorter it is, the higher the trajectory. This is because of later/earlier releases from the prong on the throwing arm.

THROWN OBJECT DENSITY
Lastly, thrown objects that are not "compact" like a volleyball (lots of surface area, not much mass), or very light like a beanie baby with parachute, are difficult objects to fling with any predicticality - they seem to take a different flight every time. The best objects are round or close to round that have a compact mass, like water balloons or softballs, and don't exceed a ratio of 1/50 in weight compared with the counterweight.


6 JUN 2002: Encounter at the Park

I got a phone call from Bill, a retired engineer, some weeks ago concerning an event at a local park where he and his friends were to demonstrate a 1/5 scale trebuchet he had built. Bill is into the theoretical limits of trebuchet design and efficiency - see his web site for more detail (in fact, unless you are a mechanical engineer, more detail than you would want to deal with:

http://home.att.net/~wjahsman-mta/RetroMechanics.pdf).
It was part of a medieval history lesson for some of the local middle schools and was one of several 'stations' various groups of students would rotate through to learn something about the European Dark Ages.

I had been given Bill's phone number a year ago when I entered my own trebuchet at the county fair (in the Adult Woodwork - Other category; it did not win any ribbons) by one of the staff who knew him. I called Bill up and we had a chat; unfortunately his treb's throwing arm had broken under the stress of a launch, but I left my number with him in the event that he repaired it.

Well, in the intervening year he had redesigned and rebuilt the device and now it was ready for Showtime. I put in for a day off from work and on the day of the demonstration I loaded up my own not-to-scale, not-too-historically-accurate trebuchet, the water-balloon flinging War Weasel, into the back of my truck, filled some water balloons and set off with Joanna to the park where the event was held. We found Bill without too much trouble, he and all his mates were wearing green felt Robin Hood hats. The 1/5 scale (the big) trebuchet was impressive with its 160 pound counterweight, it's throwing arm axis something like eight feet above the ground and the throwing arm proper trimmed from a 2x6 beam.

A lecture was in progress at the end of the parking lot where there were three 1/12 scale trebuchets. One of Bill's group was launching concrete-filled Ping-Pong balls with the smaller trebs; these went about 120 feet out into the adjacent field, and Bill was down range to measure distance and retrieve the missiles. Following the demonstration with the smaller trebuchets we repaired to the other end of the parking lot where the big treb stood.

The big treb was a little intimidating; not only was it larger than any trebuchet I've been next to but the missile was a two pound solid aluminum sphere - something you wouldn't want to drop on your toe, much less have zooming at you at high speeds. As part of the preparation everybody moved to the rear quarters of the machine (not directly behind, but off to the sides somewhat). Bill announced that the aluminum ball accelerated to 80 mph during the launch and that earlier launches resulted in the ball landing over four hundred feet away. If it landed on a soft patch of turf, it 'dug its own grave'. With that, he stepped on the trigger arm.
The launch was too fast for me to follow but it seemed to be very smooth; the ball soared up and away, and landed beyond the 400 foot marker cone in the field. One of Bill's comrades was out in the field and announced that the ball landed 426 feet away.

After watching another launch I made a point of not trying to follow the sphere but rather observe the action of the swinging counter-weight. It was impressive; the weight pretty much dropped vertically, whipping the throwing arm to vertical with hardly any residual motion left in the system; the counterweight dropped and stopped. This surprised me because every other swinging counterweight trebuchet I had seen (both full-sized and smaller models) tended to thrash about after launch, implying that not all of the energy in the system had been transferred to the missile. Even worse, all that jerking about isn't aesthetically pleasing; that is why the War Weasel is a fixed counter-weight machine - it might surge forward on its wheels during launch, but by launch's end it stands tall and proud like the weasel it is. It is based on Wayne Neel's design from the NOVA Trebuchet program (http://www.pbs.org/wgbh/nova/transcripts/27mstrebuchet.html).

However, Bill's design, which has a long counterweight arm and a short vertical drop, demonstrates a way to achieve better efficiency and launching aesthetics for the swinging counterweight design.

(Pipe dream here - or a Strange Interlude - an ongoing debate over efficiency in trebuchet design raged in the past - when it really mattered - and rages now . Fixed vs. Swinging Counterweight vs. Other - and there are Others. An empirical test is needed; something like requiring a 100 pound counterweight and a softball as missile but leaving everything else wide open; which design will throw the softball the farthest? I've yet to see a trebuchet simulator for the fixed counterweight with the carriage on wheels; otherwise it would be possible to settle this thing by comparing the results of the simulators and never build anything real at all. Pshaw! What fun is that? This could inspire another trebuchet construction - a Fighting Ferret treb with swinging counterweight to match the fixed counterweight War Weasel - then play one against the other.)

Then there was a bad launch. The ball took off backwards and landed perhaps a hundred feet behind - this is why Bill had his monitors keeping people from wandering directly behind the treb. The next launch also went awry, but this time the ball went almost straight up, landing perhaps fifty feet in front of the machine. The solid aluminum ball, descending from some height, hardly bounced on the asphalt. Bill borrowed somebody's pocket knife to increase the size of the hole where prong on the end of the throwing arm engaged the trigger; it appeared that the prong had been catching on something early in the launch. After Bill made these adjustments subsequent launches went without a hitch. The farthest launch of the day was around 436 feet.

Bill urged me to check with Andrea, the woman who had organized the event, about setting up the War Weasel; she had no problem with it. I assembled the beast and launched some water balloons to good effect; the sling length is tuned for throwing water balloons the size of large grapefruit, the balloons themselves are designed to be filled with water ("Water Bombs" is the commercial name - thank you Dorothy, for telling me about them) and none of the balloons popped on launch. Bill pointed out the differences in treb design to the students, but I think they were more impressed with the balloons going "Splat!".

By noon all of the student groups had rotated through, so we packed up and left.


5 NOV 2000: The Pumpkin Fling

Treb fans

Halloween came and went and a pumpkin sat on our doorstep with no immediate future. We hadn't even carved it and had no plans to make pumpkin bread/cookies/pie out of it, or roast the seeds, and yet, yet it was a gourd with a je ne sais quoi about it, un raison d'etre, a destiny.  It had a palpable aura, a faint melody exuding from the stem - "I wan-na-git-a-waaayyyyy - I wanna flyyyy away" and then its fate was obvious: it was born to be flung.

There is a hallowed event in the great state of Delaware that happens once a year, the weekend after Halloween (Nov 4,5 this year), called "The Punkin' Chunkin'" (http://www.punkinchunkin.com/index.htm).  That is the place where devices of various sorts engage in friendly competition to see which machine can fling a pumpkin the farthest - it's the world championships, I'll have you know.  The most powerful are the air cannons which have barrels on the order of 100 feet long (30m) and can fling a pumpkin/gourd over 3/5 of a mile  or 1 kilometer ( http://www.geocities.com/zbig10inch/).  There are other pumpkin chunking machine categories for catapults, trebuchets and centrifugal machines but the air cannons rule.

Maybe next year I'll try to make the Delaware event, but this is this year and I had a pumpkin on the doorstep that had a destiny and a device in the garage to make that destiny a reality.  The train of logic was inescapable - this pumpkin must be flung! Ah, to be one with the devotees in Delaware, separated by a continent but together as one in spirit!

In a matter of minutes the War Weasel was assembled in the driveway and facing the open field across the fence.  On previous launches I had adjusted the sling length to launch water balloons not so much for distance as for a towering flight.  The pumpkin was a missile of considerably greater weight and hardly fit in the sling.  Experience had shown that the heavier the object flung, the longer the sling length had to be to reach optimal trajectories, so I guesstimated the sling length and added about a foot to it.   Even so, I was distrustful of this seat-of-the-pants calculation and took cover behind the car with Joanna, whom I wished to impress.

The Quintessence of Terror made a  rather slow and majestic launch - the pumpkin did stay in its sling - but, as Scottie said, "Ye cannae deny the laws 'o physics".  My sling length estimate was woefully deficient, and the gourd made an almost vertical trajectory.  It lifted the pumpkin perhaps twenty feet into the air and then it landed two feet in front of and to the left of the Dreadnaught of Doom, and shattered the gourd.

Cortez ran into a similar problem in Mexico when he ran out of powder for his cannons.  He ordered his engineers to build a trebuchet.  On the maiden fling the missile (which I believe was massive) lifted off vertically and crushed the siege weapon that launched it.  End of game, dude!

Looking on the bright side of this escapade I realized that had the sling been shorter the pumpkin could have launched backwards either onto or into the garage.  It will take more than a 90 lb counterweight and longer sling to impress the folks at the Punkin Chunkin.

-Sean


6 OCT 2000: War Weasel Flings a Ferret

Weasels and ferrets have figured large with our team at work over the last couple of years. It began with a software component that Benoit, the company's chief architect, named sanani. When I asked him about the name he said that it was a fine tasting coffee - the most expensive coffee to be found.

However, an Alert Reader sent me an article written by Dave Berry to the effect that _the_ most expensive coffee around was Luwak coffee from Java; the beans are, shall we say, preprocessed by the digestive tract of a weasel-like creature called a Luwak. That article is attached for the curious. In any case, at $300 a pound it is certainly more expensive than sanani.

Following this discussion of luwaks and other weasel-like creatures there was a natural progression to ferret calendars, a small weasel doll (not a Beanie baby) with a tiny straw sombrero and little red deedly-balls around the brim (thanks, AnneF!), and then later a credit card issued in its name, "Sandy Weasel", with a picture of it on the card itself (thanks, MichaelH! In fact, Sandy got an increase in its credit line earlier this week - I am not making this up). Sandy has been taken on several trips around the country with pictures to prove it. KenF extended this affection to varmints in general, and coined unique products such as "Squeezle", and phrases such as "Varmint Down!" when passing critters that had made an unsuccessful bid to cross the freeway. And of course the name of the trebuchet, War Weasel, comes from this tradition.

For some time members of the team had suggested that the War Weasel had enough horsepower to fling Sandy Weasel to a satisfactory height before the critter deployed a parachute at the apex. The idea became reality today; a circular sheet of plastic from a large trash bag with fishing line for shrouds made up the reentry device. The test subject for the launch was not Sandy, but its cousin, a Beanie Baby ferret/weasel just a bit larger. We shall call this critter Randy. A couple of hand-tossed parachute deployments convinced us of the efficacy of the design.

At the appointed time I assembled the War Weasel in back of the office. As I carried Randy out back the tune of a 60's song, the Ballad of the Green Berets, ran through my head, but with different lyrics:

"Fighting ferret from the sky . . . " or "Pastel peace-sign on his chest--He is the Weasel Nation's best . . . "

The first launch did not go well; Randy was slammed onto the asphalt, causing an anguished gasp (and not a few sadistic chuckles) from the onlookers. The sling was too long for the weight making for the downward trajectory. I shortened it.

The second launch was even worse; Randy got hung up in the sling and not only body-slammed on the pavement in front of Machine of No Mercy, but then got whipped back into the works and bounced about. DanD could hardly contain himself, crying out, "The horror! The horror!".

Third time was a charm - Randy went sailing up and away, the parachute deployed at the top of the arc and the critter drifted down gracefully. Of course we had to keep on making more launches, but all the subsequent attempts were studies in vicious kinetic brutality. One particularly nasty toss ripped the precious Ty tag off Randy's ear and chipped the pastel peace-sign bead slung around his middle. JennT (not present, thankfully), the defacto Den Mother of Beanie Babies and Ty Tag Guardian will doubtless ground Randy when she gets back Monday.

We finished up launching a dozen water balloons and attempted to hit a sand-filled ash tray 140 feet away, missing it four or five times by two or three feet.

And so ended another trebuchet moment.

-- Sean


17 SEP 2000: Flaming coconut in the night

This weekend we were invited spend some time at a beach house in Dillon Beach, a small hamlet on the mainland side of Tomales Bay. The house not only has a view of Tomales Point directly across the bay and a view of Bodega Head to the north, but also straddles a ravine which provides a view of the beach about a hundred feet below.

Of course this was an opportunity to trundle out the trebuchet, complete with a half pint of Bacardi 151 rum and a coconut, and attempt an evening launch with a holistically natural incendiary device (no Iraqi crude or petroleum products) down onto the sand. After a bit of surveying the area we realized that we could not launch from the house unless we set up in the driveway and flung over the structure - too risky. Further down the bluff, though, a street ended in a cul-de-sac just above the beach so this is where we set up operations after dinner.

I brought a small pot to put the coconut in and poured the contents of the Bacardi over the hairy fruit. Shanna had a flashlight and scanned the beach to ensure there wasn't anybody below. We had a half a dozen water balloons with us, so we started with flinging them first; and Shanna tried to keep the flashlight trained on them as the survivors (half of them popped on takeoff) traced their stealthy arcs down to the base of the bluff; we could hear the impact when they hit the sand. An SUV drove down to the street to the launch site and paused for about a minute while we went on about our business, then drove off. When we ran out of balloons it was time for the main event.

The coconut needed a little last-minute rotating in the rum to freshen it up, then I put it into the sling and tried to light it with one of those lighting wands used to light grills and fireplaces. I had trouble getting the wand lit, much less the coconut so I shifted to the other side to block the wind and Roger grabbed the trigger cord. After a couple more attempts the coconut began to burn with a satisfying bluish flame which spread over the husk. Roger gave the cord the tug and the flaming coconut began an impressive ascent with the coconut hairs a-fire - for about ten feet, and then it flamed out. A few seconds later we heard it land on the sand below.

Bummer! We discussed what might be done to ensure a burn for the duration of the flight while still using an edible combustible - the ultimate goal is to retrieve the smoldering pod, crack it open and dig out the coconut meat to put on some chocolate ice-cream; kind of a kinetic baked Alaska with a medieval twist. Roger and I went down to the beach the next morning to retrieve the coconut. We found the remnants of a couple of the balloons in the sand, but no coconut. Some earlier riser must have happened across it, assumed that swallows had carried it from Africa, and pinched it.

-- Sean


Trebuchet Awareness Day: 19 JUL 2000

Back in '96 I became a member of the Hurling Society for $35 (the membership card sits in the mini-catapult on my desk), an organization hell-bent on creating a 100' trebuchet capable of flinging Buicks. Here is the URL of an article from Outside Magazine in '95:

http://www.outsidemag.com/magazine/0895/8f_bird.html

Unfortunately the enterprise ran out of money and as far as I know, no longer exists.  :(  -Sean


Sean's Trebuchet News: 09 JUL 2000

I bought a couple of 25 lb. disks from Big 5 to add to the counterweight; previously the counterweight dumb-bell carried 6 ten lb. disks; three disks to a side maximum.

Replacing two of them with the 25 pounders boosted the total from 60 to 90 lbs. The counter help asked me if I wanted them bagged and I couldn't resist - "Paper or plastic?" - only a canvas bag could have held them. I did a test when I got back home and found I could launch water balloons 163 feet; the best I had done before with 60 lbs. was roughly 110 feet.

July 4th weekend was a fine opportunity to test Version 3 with water balloons over at the Stultz-Millers; Version 2 had the shortened launch trough and symmetrical trigger. We got the kids to fill up some water balloons and launched the aqueous orbs down the street in majestic arcs, which then landed with a satisfying "Splat!"; there are few things the vanish the way a water balloon does falling from a height. Kids stood down range in the wet splotches in the street, daring the next balloon to land on them. A few socks were dampened by near misses. Karen was at both ends of the action, recording it all on her video-cam.

After a half a dozen launches we started letting the kids pull the trigger, which was a little tight. Several successful launches led me to believe the system was foolproof, but then one of the older kids set me right by not only pulling the trigger rope hard enough to initiate the launch - he then proceeded to continue pulling, skewing the carriage around so that I wound up directly behind the rising throwing arm. Karen caught the action on this one; I ducked and didn't make contact with anything. After that we told the kids to simply jerk the trigger rope rather than pull it.

When we got to the end of the 9" balloons we tried the smaller, specialty water balloons. The first attempt to launch one of them resulted in the trebuchet's counterweight tearing loose from the throwing arm. The Rube-Goldberg contraption I'd assembled to hold the counterweight had always been slightly suspect, and it let go as I hope it would if it ever did, at the bottom of the swing, going nowhere, even sparing the launch trough. That ended the treb activities for the day.

The next morning I bought a 2x4 and reworked the counterweight end of the throwing arm, leaving 6" of wood below the dumb-bell to support the weight, along with a bolt through a wooden block. This lengthened the assembly enough that I had to change the pivot point of the throwing arm so as to clear the launch trough; in other words, there was a of stuff going on; would the Device of Inter-Galactic Doom survive the operation? My first tests were a little discouraging - 143 feet with water balloons, 20 feet less than before. This is Version 4.

It was a long weekend; when I went back to work Wednesday, Ken said he had some specialty beers that he had brought back from Colorado - Trippel, a Belgian brew difficult to find in California, and that he was bringing it in Friday for TGIF at 5. The more frolic the merrier, so I passed the word that I'd bring in the treb for a 5pm launch Friday as well. On the day itself I filled up a baker's dozen plus one water balloons at lunch time and a few minutes before 5 started assembling the Beast of Ultimate Destruction.

There was a good turnout and a strong tailwind from the coast. Ken brought me a Trippel; a tasty brew and definitely not created in the British bitter style. Steve brought his video camera to record the events and throughout the activities caught us in the act. The balloons seemed to go farther than when I tested them at home, and the tape measure showed distances on the order of 170 feet. Hajo ran down range and actually caught one of the balloons; he brought it back, a pin-hole leak in it, and baptized me 'n me treb with it before Arthur took custody of it, kept a finger on the hole until we were ready for another launch, then off it went again, even farther than before.

Somebody (Terry?) suggested that we put blocks in front of the War Weasel's wheels to test the assertions made on the NOVA trebuchet program - that a trebuchet with a fixed counterweight can fling an object farther if the carriage is mounted on wheels and free to move. With the blocks preventing the treb from surging forward initially, the rear of the carriage lifted off of the ground and the distance of the throw was perhaps half of what it should have been. Brian L. watched carefully from the side and said that when the carriage was free to roll, the counterweight, rather than tracing a uniform semi-circular arc, traced something more parabolic or J-shaped - that it didn't move much horizontally. Perhaps a time-exposure in the dark, critical points lit with tiny lights, might yield something interesting, or maybe a stroboscope.

The range was such that it was possible to land a balloon just outside of one of the building's exits. No unsuspecting victims offered themselves as symbolic offerings to Instrument of Terror, so we turned our attention to our coworker's cars as they left the office. A couple of near misses (one immediately in front of the retreating car), but no direct hits.

I lengthened the sling and we tried launching a volleyball to good effect; it had the correct downwards spiking action. Lengthening the sling tends to hang onto the object and throw it flatter, even downwards if the sling is long enough; shortening tends to lob the object higher. Next week we plan to try it out on one of the near-by volleyball courts and see whether it has any use as a training aid. Other plans in the hopper are to raise the height of the pivot of the throwing arm; this will allow cocking the throwing arm to approach a 45 degree angle (it is more like 30 degrees now), increasing the potential energy of the system - Version 5. That's it - well, almost it. Below are a couple of sites you might want to check if you haven't already - one of them is the video loop that Steve video'd awhile back of the War Weasel in action - it is large, though (over 1 Mb) so it could take time to download:

 http://www.schlich.com/trebuchet/ (You are here now)

The other is an article about the eccentric landed English gentleman, Hew Kennedy, who has a trebuchet with a 40 foot throwing arm and was a consultant in the NOVA Trebuchet special in Scotland:

http://asylum.apocalypse.org/pub/u/zonker/fpp/html/trebuchet.html