Trebuchet: An Exploration of Ballistics

Presentation by:Panayotis Manganaris

Project in collaboration with:Jack Mangan

Chandler Owens

Understanding: what is a trebuchet?

● Create a special sort of catapult that functions on the leverage of a counterweight acting on a projectile rather than the deformation of springs or lengths of material

L-3: sling

L-2 + L-5: arm

H: ground to axle

M1: Weight

M3: Projectile

Understanding: Criteria to meet

● Design and build a trebuchet to lob a squash ball to a recommended distance of at least 30 feet

– Be competitive to enter Trebuchet Competition

● Counterweight consists of three 305-326 grams Campbell's Tomato Soup cans.

– Create apparatus to secure weight to arm.

● Lever arm + sling hook must not exceed 0.8 meters in length

– Must be delicately balanced to pass the “pencil test”

● Axle around which the arm rotates must not be above 0.4 meters from the ground

Explore: Possibilities and designs

● Possible trebuchet types

– Floating arm

– Recoiling – Dismissed due to the observation that it probably just wastes valuable energy at this scale

– Swinging arm

● Design Objectives

– Maximize the transfer of energy from the falling weight to the projectile

● Minimize friction between the arm and the axle● Balance the lever precisely through tapering

– Maximize accuracy and distance to serve as the ranged artillery in the Trebuchet Competition

Explore: What design standards work best?

● The ideal ratio of short to long arm is 1 to 4

– Likely 1-3 to1-2 in this scale

● The sling should be as long as the long arm

● Using a lubricated ball-bearing to minimize friction in the pivot

● Inertia of components is as low as possible

Swinging Arm

Floating Arm

Define: Choosing the swinging arm

● Build floating arm?

– Floating arm more efficient, more powerful

– Not compact enough to easily pass inspection (ambiguous as to where the actual “axle” is, so maybe too tall), no guarantee of high accuracy, difficult to reload and fire

● Or, build standard swinging arm?

– Less efficient, powerful (deficits likely not noticeable at this scale)

– Absolute certainty as to the location of the axle, easy to meet requirements, can be modified in prototyping to increase accuracy more easily

– Option to Introduce a guiding rail along the bottom of the platform

● Possibly increasing the accuracy of each throw

Define: Approximating the best swinging arm

● Short arm: 20cm● Long arm: 60cm● Sling length: 60cm● Weight: 0.978 kilograms● Projectile: 0.043 Kilogram● Center of gravity:

Within 10 cm of pivot on

long arm's side

Ideate: Simulations

● Very difficult to calculate precise measurements necessary to make the best swinging arm trebuchet

● virtualtrebuchet.com helped to pinpoint best measurements from approximations

● Updated Measurements after multiple virtual trials:– Short arm: 24cm

– Long arm: 56cm

– Sling length: 56cm

– Weight: 1.05 kilograms (selected and measured cans)

– Projectile: 0.043 Kilogram

– Center of gravity: 3 cm from pivot on long arm's side

Ideate: Solving structural challenges

● The two foreseeable challenges will be the mechanics of the sling and the method of attaching the weight cans to the short arm

● secure cans using nails to form stable shelf and strap in with duct tape

● Use boline knots in fishing wire to secure sling to arm and loop around release hook

Basic Pouch Idea

ReleaseHook

Sling and pouchassembly

Throwing Arm

Ideate: Blueprint● Defining Material

Requirements● 16ft of 2x4

● 3ft of 1x4

● 1m of 1/2x2 hardwood

● Light, thin cloth

● 5 large washers for ¼in. threaded metal rod

● 3 small washers

● 8 ¼in. Nuts

● 3inch screws

● Gorilla Glue

● R4AZZ mini ball bearing

Prototype: Simulating blueprint

● 9.947m = 32.6ft simulated maximum● Note: Inertia of arm and of weight must be minimal

– Keep the structure of arm as light as possible

Prototype: Building and testing

● Trebuchet was built precisely to specifications

● Tapering of arm was done by feeling

– No time to use Inventor modeling

● First Launch:http://youtu.be/NYJJEI4Yfxk

Guiding rail

Prototype: Specific construction notes

Due to the slipping of nuts against the arm surface, the method of locking nuts against each-other was used to prevent the issue from affecting accuracy.

Showing R4AZZ shielded ball bearing.

● The optimum release angle is 45 degrees– Controlled by adjusting the bend in the hook

● SlowMo Firing Archttp://youtu.be/36lDmrNJELQ

(Also under videos tab)

● Sling secured with

boline knots as

described

Refine: Release angle

Solution: Securing Weights

● Used the previously described nails and duct tape method

Solution: Distance and accuracy

● Construction successful, surpassed expectations

● Testing to compare real distances to simulated expectations, 32.6ft simulated maximum

● 33.3, 34, 30.6, 32.2 feet experimental distances

● Testing for accuracy involved aiming for a pie pan 22 feet ahead and measuring by how many feet it was missed

● margins were by 14, 16, and 28 inches with one misfire occurring during testing

– Note: the challenge was greatest with adjusting to fire only 22 feet instead of the maximum.