Practical Coilgun Design 

How to Measure Speed 
Measuring Coilgun SpeedHow do you measure the speed of a projectile? Is it difficult or expensive or tricky? There are several possible (and quite practical) ways to measure the starting speed. This web page describes a way to find the speed that requires no special equipment. Ways To Measure SpeedHere is a brief overview of various ways to measure speed. Although I used a horizontal ballistic speed trap, some of these would likely work better.
Ballistic PendulumThe "how fast does it go" question can be answered by using a ballistic pendulum. It consists of a capture box of known mass suspended from some light wires. You fire the projectile into the box and measure the height that it rises. You can compute the speed if you also know the mass of the projectile and the mass of the pendulum. You can also read a complete description of building a ballistic pendulum from the book "Backyard Ballistics" by William Gurstelle, Chicago Review Press, ©2001, ISBN 1556523750. The general idea is that the coilgun (or cannon or slingshot or whatever) will be shot pointblank into the pendulum's box. The projectile's momentum is transferred to the pendulum and its velocity can be determined from the height to which the whole thing rises. Suspend your ballistic pendulum from a ledge or ladder. It is important that the box swings freely after capturing the projectile. As it swings, a marker pen will trace out the path of the box on the paper attached nearby, marking the pendulum's highest and lowest points. The difference in height between the apex (top) of the swing and rest position (bottom) allows us to determine the total momentum, and therefore the projectile's speed. Speed = (1 + ^{M}/_{m}) * SQRT(2gh) Where M is the mass of the box, and m is the projectile's mass. Also, g is gravity and h is the total rise in height. You can use metric (kilograms and meters) or english (feet and pounds), but be consistent! 



Last Update 20080609
©19982021 Barry Hansen 