Ancient Engineering
Ballistic Technologies of Antiquity

Floating Arm Trebuchet
- Hobbyist Version

The F.A.T. 3000, Guillotine-Style Catapult

Designed and made in the USA for
better quality, better performance!

This new and improved 21st century version of the trebuchet uses a modern understanding of physics and engineering principles for incredible power and range!

Also available in a
lower-cost competition version!
(click for info)

This version of our most popular kit incorporates several improvements over the old model, including our new "gap-hop" architecture - now the wheels gently hop over the gap in the tracks instead of pounding over them, eliminating the destructive "thump-bump" of the old model. This enables us to use positive tracking wheels - no more frame-strikes! The wheels are locked-in to a track system that prevents them from drifting off-course during firing. We've also added a turnbuckle based pin angle tuning mechanism, for non-slip ultra-fine tuning of the pin angle. The virtually indestructible steel counterweight boxes are included too! All major improvements for accuracy and repeatability in this one-of-a-kind machine!

Engineered using modern mechanical principles and computer aided design, this mini-powerhouse can hurl a golf ball over 200 feet. It uses guillotine action and a plunging beam to maximize potential energy and a floating arm to convert that energy into projectile motion. Precision crafted of high quality hardwood, the kit contains everything you need to build a working model.*

The trebuchet is essentially a gravity powered energy conversion machine, turning potential energy into kinetic energy and using it to throw a ball. It's a great way to see classical mechanics in action!

Similar to the forty-foot tall, 25 ton behemoth "T-Wrecks" we built on the TV show "In The Name Of Science" (2004, The Discovery Channel), except this model is only 34 inches tall, 18 inches long and 7 inches wide (12 inches wide including the steel boxes) but it hurls more than five times as far as T-Wrecks in scaled distance.

A common complaint from engineering professors is that "Students today have a great understanding of computers, but fewer and fewer of them have any real hands-on, real-world understanding of physics and mechanics." Our kits are designed and intended to spark an interest in real-world science and engineering, and maybe even a bit of history too!

The detailed instructions (14 pages!) are complete with diagrams, photos, tuning tips, equations for calculating the machine's efficiency and more! All parts are pre-cut, pre-drilled and can be assembled to a finished model in one evening (plus glue drying time).

We've put a lot of time and effort into making this kit as easy and complete as possible. It's a fun way to learn about physics and engineering!

FAT, F.A.T. and Floating Arm Trebuchet are trademarks of RLT Industries, inc.

Tools required:
Scissors, Glue, Ruler, a utility knife, small wrenches (for one 1/4" bolt and one 3/8" bolt). Sandpaper is optional.

Assembly time:
For a Master carpenter doing a sloppy job: about 2 hours.
A person with no kit building experience being extremely meticulous: two or three days.
The model in these photos was built by Ron Toms in about four hours.

Assembled Size:
- Height: 34"
- Length: 18"
- Width: 7" (frame) 12" (with counterweights attached)

Additional Counterweight Required: 10 to 15 lbs.
(Loose nuts and bolts, coins, rocks, anything that
can fit into the 4" x 2" x 6" counterweight boxes.)

- Golf balls, up to 200 feet
- Tennis balls, about 100 feet.
(Optimal range assumes a well tuned machine)

Shipping weight: 14 lbs.
Box dimensions: 36" x 12" x 4"

Quantity pricing info:
0 to 4 kits, standard price.
5 to 9 kits, 10% off
10 or more kits, 15% off

Orders of $200 or more qualify for FREE ground shipping!

WARNING! This is a functional model intended for educational demonstrations of physics. It contains a fast moving arm and projectile that can cause injury if you make contact when firing. Use only under strict, competent adult supervision.

* You'll need to supply the counterweight material. Anything from nuts and bolts, BBs, coins, sand or a good fist full of pebbles will do!

* Can throw 200 feet when properly constructed and tuned with sufficient counterweights. Your performance may vary.

* FAT, F.A.T. and Floating Arm Trebuchet are trademarks of RLT Industries.

If you are buying this model for the Science Olympiad:

There is a rule which requires the trebuchet arm to remain stationary when the counterweights are removed and the trigger is pulled. The original intent of this rule was to disqualify springs and bungee cords from the trebuchet competition.

Since the FAT does not contain any springs or bungee cords, it does not violate the intent of the rule, even though it does violate the letter of the rule. Keep this in mind if you ever become a lawmaker - good wording is difficult and extremely important in order to keep the intent, without doing harm or injustice to innocence.

If you want to use the FAT for your competition, there are options. First, I'd recommend negotiating with your instructor or judges, let them know about the problem and the intent of the rule, and see if you can get an exemption from it. This has worked for other folks in the past, but not always. It depends on how reasonable your judges are.

If that fails, here's another strategy that has worked. The rule states that the arm cannot move when the counterweights are removed and the trigger is open (unset). In this model, the arm is part of the counterweight system. Its center of gravity falls as part of the launch sequence and the arm's weight (including the wheels and wheel carriage) contributes to the total energy transferred to the projectile. You should be allowed to remove the whole arm and weight-box assembly to satisfy the "no motion" rule. In fact, since the arm is part of the counterweight, if you are not allowed to remove the arm during this test, then your judges ARE violating the letter of the rule in regards to this test!

Once the arm is removed and set aside, it will not move when the trigger is pulled, and the test is passed.

Of course, the decision whether to accept these lines of reasoning are entirely up to the judges of your competition. It would be wise to check with them first before purchasing this kit. If they will not allow it, then the SPECK or the TK3 are your next best options, although nothing performs better than our FAT!

More info about SciOly and the FAT can be found here

Good luck with the competition!

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    Price: $195.00
    Minimum age: 14
    Availability: out of stock

    Item code: 10101

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A catapult is any kind of device that shoots or launches a projectile by mechanical means. In England, a catapult is what we call a slingshot in the US. A catapult is also the part of an aircraft carrier that launches airplanes off the deck.

But for our purposes, a catapult is any of the ancient types of artillery, including Onagers, Scorpions, Trebuchets, Ballistae, Springalds, Coullards, Bricoles Perriers and more.

But most people tend to think of a catapult as the one-armed torsion machine used by the Romans. This is also known as the Onager or Mangonel.


The word Mangonel derives from the ancient Greek word "Manganon", literally meaning "engine of war". The Romans called it a Manganum. In pre-medieval French the word Manganum was changed to Manganeau, and the English changed that to Mangonel in the 1300s.

The history gets a little sketchy in the middle ages, but some historians believe that "mangonel" was shortened to the word "gonnel" about the same time that cannons were being developed, and later still, "gonnel" was shortened to "gun." And still today, in the military a "gun" is strictly a piece of big artillery.


Onager is originally the name for the wild Asian donkey. This donkey bucks like a bronco if anyone gets too close to it, and it is known to kick stones at people and predators too. So when the Romans needed a name for their one-armed torsion catapult, they called it the Onager!

The Onager (catapult) has a single arm that is powered by a large skein of twisted ropes. The ropes were usually made from hair or sinew for their elastic properties.


The word "Trebuchet" is originally French, and meant something like "to fall over or rotate about the middle" as in a see-saw rotating on its axle. It also seems to have meant a big, heavy beam. Today a Trebuchet is any kind of catapult that is powered by a massive counterweight on one end of an arm, and a sling on the other end. This includes Perriers, or "traction" trebuchets which are powered by a mass of people pulling one end of the arm with ropes.


This is a two-armed torsion device invented by the Greeks. It works similar to a crossbow, but instead of a flexible bow, it uses two stiff arms powered by twisted rope skeins like an Onager. The ballista predates the Onager by several centuries and was used to hurl stones (lithobolos style ballista) and also bolts or darts.

Obviously, this is where we get the word "ballistic".

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