Impact Comparison Test

Results Discussion Conclusion Calculation	Aim To compare the impact of water rockets to currently accepted SCA missile weapons.
	Method To carry out our test we constructed a rigid wooden pendulum with a 30 cm (1 ft) square target at it's end. So that we could measure the displacement of the pendulum after each impact an aluminium pointer was suspended from the pendulum's cross-arm by a spring made from a loop of mild steel. Each of four types of missile was thrown or fired at the target until 10 direct hits were scored (with the exception of the water rocket for which only 5 hits were scored). The deflection of the pendulum cased by each hit was measured between the top of the target and the equivalent position on the pointer.
	Pendulum from left of front.	Pendulum from side, note the aluminium pointer as it appears after an impact.

	The four types of missiles used in the trial are pictured below
	Arrow	Throwing Axe	Javelin	Water Rocket
	Mass:43 g (1.5 oz)	Mass:372 g (13 oz)	Mass:429 g (15 oz)	Mass:143 g (5 oz)
	Length:71 cm (28 in)	Length:46 cm (18 in)	Length: 106 cm (42 in)	Length:29 cm (11.5 in)
	Notes The arrow used was a Lochac standard war arrow. It was constructed with a 28 in cedar shaft taped with glass fibre tape, a Lochac blunt and plastic nock and fletching. The axe was constructed from a combination of carpet, foam and duct tape. The javelin was constructed of PVC tubing with a foam / half tennis ball head, foam fins and duct tape. The water rocket was constructed from a 600 ml (17 fl oz) plastic coke bottle, rubber fins, a 28 g (1 oz) wooden disk for extra weight, a half tennis ball head and duct tape. The arrow was fired from a modern laminate recurve bow with a draw weight of 30 lb at a 28 in draw. The 600 ml capacity water rockets were half filled with water and the remaining volume pressurised to 100 PSI before firing. All test took place at a range of 5 meters (15 feet). Only shots that were judged to hit the target squarely were counted.

Aim / Method Discussion Conclusion Calculation	Results The results of the trials, in the order they were recorded, are listed below.
	Missile Deflection (cm) Average (cm) Standard Deviation Arrow 15, 19, 17, 14, 15, 16, 16, 17, 18, 19 17 1.8 Throwing Axe 39, 43, 52, 43, 67, 73, 51, 51, 54, 57 53 10.6 Javelin 47, 47, 66, 39, 62, 76, 71, 67, 46, 66 59 12.7 Water Rocket 53, 53, 56, 58, 57 55 2.3
	To enable the comparison of these results with potential future tests performed by other parties the average work done on the pendulum by each missile has been calculated. For information on how these results were arrived at see the calculation section. Arrow Throwing Axe Javelin Water Rocket Energy Transferred (joules) 0.43 5.3 6.6 5.7

	Short MPEG clips showing an example trial for each missile can be downloaded by clicking on the stills below.
	Arrow	Throwing Axe	Javelin	Water Rocket

Aim / Method Results Conclusion Calculation	Discussion One of the biggest problems encountered in collecting these results stemmed from the fact that none of the participants was an experienced axe thrower or javelineer. It took around 10 throws with the axe or javelin to score a clean hit making roughly 200 attempts in all required to gather the data for the thrown weapons. The missile throwers lack of skill is evident in the comparatively large standard deviation for the axe and javelin results. It would be expected that had more skillful light infantry been involved in the trial the standard deviation would be less and the average deflection measured would be greater. The peak deflection caused by the axe and javelin, 73 cm and 76 cm respectively, are around 1.25 times the peak deflection caused by the water rocket. Only five results were collected for the water rocket, again due to the number of attempts needed to score a hit as well as the time required to reload.

	Conclusion The water rockets tested in this trial have an average impact that is equivalent to the more traditionally accepted SCA throwing axes and javelins. If the peak impact of the weapons is taken as a measure then the water rockets hit with 80% of the impact of axes and javelins.

Aim / Method Results Discussion	Calculation When the pendulum reaches its highest point after an impact it's potential energy is equivalent to the kinetic energy transferred to it by the missile. The potential energy of the pendulum is given by u = mgh where u = potential energy m = the mass of the pendulum g = the force of gravity h = the height of the pendulums center of mass above its resting point (equivalent to the length EF in the diagram opposite)
	From the length of the pendulum at which the displacement AB is measured and the length of the displacement BC the magnitude of the angle a can be calculated using the formula for the length of a chord where AB is the radius and BC is the chord length. Since ABC has the same proportions as ADE the length DF is equal to BC * AD / AB. Because ADE is an isosceles triangle b = (180 - a) / 2. b and d comprise a right angle so d = 90 - b. Knowing the length of DF and the magnitude d of allows the calculation of EF by use of the sin rule. From EF and the mass of the pendulum the amount of energy transferred to the pendulum can be calculated by use of the formula u = mgh.
	Pendulum Properties
	Length of pendulum at which displacement is measured AB 52 cm (20.5 in) Length to the pendulum's centre of mass AD 38 cm (15 in) Mass of pendulum 2.75 kg (6 lb)
	Results of calculation steps
	Arrow Throwing Axe Javelin Water Rocket Average Measured displacement BC(cm) 15.0 53.0 59.0 55.0 Displacement of centre of mass DF(cm) 11.0 38.7 43.1 40.2 Vertical displacement of centre of mass EF(cm) 1.6 19.7 24.5 21.2 a(degrees) 16.6 61.3 69.1 63.9 b(degrees) 81.7 59.4 55.4 58.1 d(degrees) 8.3 30.6 34.6 31.9           Potential energy (joules) 0.43 5.3 6.6 5.7

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