Catapult, any of various engines of war used in ancient and medieval times to discharge javelins, darts, rocks, and other missiles. The larger kinds were mounted on a strong wooden platform; the trigger or projector of a gigantic crossbow was drawn back by ropes and held by a catch. Another type of catapult employed the principle of torsion to hurl heavy stones or objects over walls and across moats, cords being twisted by winches to pull back the propelling mechanism. Smaller hand-carried catapults were also used. (Encarta Encyclopedia, 1998)


Project specifications

Build a portable catapult that can launch a 10 lb. projectile 100 feet. The time duration for the project is 16 weeks.


Major Design Considerations

The catapult frame dimensions must meet the following functional requirements:

  1. Fit into elevator
  2. Fit through doors in manufacturing lab
  3. Fit through building hallways
  4. Portable weight


Material selection

Frame material

The two material choices for the frame of the catapult were wood and steel. Wood was chosen mainly because of its availability. The students also felt that a wood structure would be easier to cut and assemble. A complete list of the materials used for this project can been seen by accessing the link Bill of Materials


Source of Propulsion

A working model simulation, (see link Working Model Simulation) was created to determine the amount of force needed to launch a 10 lb. projectile 100 feet. The original plan was to purchase and test extension springs to create the desired force. Due to the cost of an extension spring, the students decided to use rubber bungi cords to produce the desired source of propulsion.


Choice of a Projectile

The students decided to launch one gallon containers full of water. The estimated weight of the projectile was 8-lb.


Initial Testing

Initial test results were as follows:



The students were obviously displeased with the initial results. The following changes were made

    1. Shorter bungi cords were purchased
    2. A trigger mechanism was devised
    3. A larger quantity of bungi cords was used
    4. A fixed stop point for the arm was set.


Final testing

During the final test the students were able to launch a 5-lb projectile a distance of 45 feet. At this point the winch had reached a maximum torque capacity and the failure was noticed in some of the wood screws. The students determined that safety constraints prohibited them from trying to achieve a greater launch distance.


Final modifications

The students replaced the lag screws in the winch with carriage bolts. A notch had to be cut into the winch arm to facilitate the carriage bolts. Extra bracing was put in place around the winch for future safety considerations


Catapult History

The catapult was an ancient siege machine that could hurl heavy objects or shoot arrows with great force and for considerable distances. Some catapults could throw stones weighing as much as 350 pounds for distances greater than 300 feet. The Greek Dionysius the Elder of Syracuse, who was looking to develop a new type of weapon, invented the catapult in 399 BC.

Catapults store energy by twisting large bundles of rope made from very strong plant fibers, animal sinew, or hair. Energy is stored by winding the catapult with a lever and gear arrangement, projectile is loaded, and the trigger releases the arm. The arm is designed in such a way that leverage converts the tremendous stored energy of the coil into kinetic energy with great speed. The catapult could fling rocks, burning pitch, or even enemy heads over the walls of a castle or fortified city. The torsion-powered catapult remained effective in medieval Europe until the advent of gunpowder, when cannons replaced it.