Gao GuangyanDesmond Chan

Ng Kia BoonDaniel Yip

Raffles Institution

NUS Physics Open HouseProjectile Competition

Objectives

• To create a device – propelling a projectile

• With a target range of 80 metres

• Furthest possible distance for the maximum points

• Device must be:

• Accurate

• Consistent

• Method

• Consider options and conduct experiments

• Analyse and evaluate them

Design Consideration

Factors to Consider

1. Distance (Enough power for a range of 80m)

2. Projectile Size (Must consider projectile dimensions)

3. Cost (Feasible to build financially)

4. Portability (Not too bulky and easy to transport)

5. Consistency (Consistently hit 80m accurately)

6. Elegance (No unnecessary sophistication)

Design Consideration

Medieval Siege Weaponry k

1. Trebuchet • Able to propel projectile far if made correctly• Not consistent due to nature of propulsion (sling)

2. Onager • Similar to Trebuchet / but uses torsion bundle• Powerful yet small in size• But not consistent

Small-scale onager model constructed to

test for feasibility

Design Consideration

Medieval Siege Weaponry k

3. Ballista • Cross between a crossbow and an onager• However, large model needed for 80m target

4. Crossbow• Powerful crossbow required

• Projectile subjected to external influences (wind)

Small-scale crossbow model constructed to

test for feasibility

Design Consideration

Electro-Magnetic Acceleration

1. Repulsion • Several Options: Thomson’s Coil Gun, Railgun etc..• Most common: Railgun• Requires:

• Strong Rail system• Extremely High Pulsed Current• Pulse Capacitors

• Not suited for small scaling• Very expensive • Loud and Noisy operation

Design Consideration

Electro-Magnetic Acceleration

1. Attraction• Coil Gun: Linear Asynchronous Motor• Model Constructed Previously

• Powered by 729J at 450V• Only 4.35% peak efficiency• Relatively Expensive

• Projectile size too large • Consistent and accurate

• But Large model required for 80m target – too expensive to build

Design Consideration

Pressure Powered Propulsion

1. Water Rocket• Safe and relatively powerful device

• Difficult to hit our targeted 80m consistently

• Large body – affected greatly by wind

• Operators might get wet

• Design commonly used

• Difficult to construct

reliable watertight

launcher

Design Consideration

Pressure Powered Propulsion

Final Design: Air Cannon• Safe and relatively powerful device

• Similar to handgun or rifle, but pressures are lower

• Powered by compressed air (bicycle pump)

• Simple and elegant

• Original Design and completely self constructed

• Easy Operation and easy handling

• Comparatively Cheaper than other options

• Powerful, consistent, accurate.

Accelerator Design

Pressure Powered Propulsion

Final Design: Air Cannon

Accelerator Design

Construction Materials

Final Design: Air Cannon• High Pressure Industry Class AW(VP) PVC pipes

(rated 283psi)

• 1” Brass Ball Valve

• Bicycle Pumps

• Bicycle Tyre Valves

• Waterproof Plywoodboards for stand

Accelerator Design

Main Design

Final Design: Air Cannon• 2 2’ long 2” dia. Air tanks

• A 5’ long 1” dia. Barrel

• Wooden base essential

• Gives accelerator stability and supports entire structure

• Projectile: solid aluminium

• Less affected by wind due to mass

Accelerator Design

Operation • Pumped using bicycle pumps

• Simple and easy to operate

• Device is muzzle loaded

Flight Path• Both Ballistic and Line of Sight

paths possible due to power of the system.

• Low ballistic trajectory chosen

• Less affected by wind and other external influences

Accelerator Design

Physics Involved

Calculation of Output energy and projectile velocity

Air Tank Dimensions: 2"dia * 24" * 2  Barrel Dimensions: 1"dia * 12" * 5 

The barrel cross section is therefore Pi * r2 = 0.7854"2

Using Pi * r2 * h, we can calculate the volume of the chambers:Air Tank Volume = 150.7964"3  >  0.002472m3

Barrel Volume = 47.12400"3  >  0.00077225m3

Total Volume = 197.9204"3  >  0.00324425m3

Physics Involved

Calculation of Output energy and projectile velocity

We take the pressure to be 100psi, projectile mass of 0.1kg

The pressure changes since air fills up the barrel which changes the pressure. Since the volume of air does not change, We can use the

formula P1 * V1 = P2 * V2

100 PSI * Air Tank Volume = P2 * Total VolumeTherefore, P2 = 76.19635 PSI, and there is a pressure drop of

23.80365 PSI

Knowing these values, we can calculate the force acting on the projectile throughout the barrel.

Physics Involved

Calculation of Output energy and projectile velocity

The force acting on projectile = Pressure * Area100 PSI (start of barrel), 100 * 0.7854"2 = 78.54lb = 357N

 76.19635 PSI (end of barrel), 76.2 * 0.79"2 = 59.845lb = 272N

The average force acting on the projectile in the barrel is therefore (357+285.6)/2 = 314.5N

When there is force and mass, there is acceleration. And where there's a lot of force, there is a lot of acceleration!

Using the formula F=ma,314.5 = 0.1 * aa = 3145ms-2

Physics Involved

Calculation of Output energy and projectile velocity

And using the formula d = ½ at2,1.524 metres = ½ * 3145 * t2

Therefore, t = 0.0311313sTime taken for the projectile to travel out of the barrel

Hence, using these values,

Velocity = Acceleration * Time, Thereforev = 3145 * 0.0311313

Velocity = 97.90791ms-1  >  352.4685km/h! And because K.E. = ½mv2,Energy = ½ * 0.1 * 97.907912

Energy = ~479.29Joules.

Difficulties & Limitations

• Lack of funding

• Could not construct more sophisticated device

• Lack of time

• Could not carry out more experiments

• Lack of testing grounds

• Difficult to test out projectile device (large area, at least 80m long required)

Thank you.