Bungee Ballista Part 1 PDF Print E-mail
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Saturday, 24 October 2009 20:52

Making a Bungee Ballista Part 1
 Bungee Ballista
  A ballista is basically a gigantic crossbow.  The principle behind it is that you crank the projectile backwards down the track against a very strong spring.   Then you release it, and all the mechanical energy you built up in the spring is transferred to the projectile in the form of kinetic energy.   This ballista can be carried by one person, but can throw a heavy projectile for hundreds of feet.
Operating the Ballista
  The winch in the back of the ballista pulls the shuttle back along the track.   As the shuttle moves back, it pulls on the ropes.   The ropes go around the wheels and are tied to multiple strands of bungee cord.   As the rope gets pulled around the wheels, the bungee cord stretches out.   The energy you put into the system by cranking the winch is getting stored in the stretched bungee.   When the shuttle is far enough back, the trigger is raised, preventing the shuttle from moving forward.   The projectile can then be loaded in front of the shuttle.    With the shuttle now being held by the trigger pin, the winch strap can now be loosened and disconnected from the shuttle.   Then the trigger can be lowered, releasing the shuttle.    Most of the energy stored in the bungee cord will be transferred into accelerating the shuttle and any projectile in front of it. 
!!!!  A Word of Warning  !!!!
   A ballista is a weapon.   It is a dangerous thing which can easily hurt or kill somebody.   There are great amounts of energy stored in it when the shuttle is pulled pack, and in the event of a failure, people can be harmed even when not in front of it.   The ballista can shoot for hundreds of feet depending on the weight of the projectile, how long the track is, and how springy the bungee is.   This article describes how one prototype ballista was created, but does not warrant that this design is safe, and it is not recommended that anybody else build one, especially if that anyone is not experienced at structural design and the physics principles involved.
Overview of the Design
  The ballista shown above was created in about thirty or forty hours.   It is not a perfect design, but it worked very well and has launched a projectile over 100 times with only minor problems, such as excessive rope wear. 
  Here is a drawing of the basic design.

This is the top view.   The round things are bicycle wheels.   The red cylinder is the projectile.   The shuttle pushes the projectile up the track.   Bungee cord is shown in thick black lines attached at the back of the ballista, and extend towards the outside edge of the wheels.  A rope attaches to bungee cord, and goes around the wheel to the shuttle.  The reason for the wheels is that the bungee starts out with an unstretched length of about 7 feet.   In the final design, the distance that the shuttle travel was about 7 feet total, thus stretching the bungee by a factor of 2.   Without the wheels, the bungee would have to be attached at a point 7 feet in front of the ballista which would make it rather long.  Note however, that the wheels do cause quite a bit of energy being wasted.
 This is a view from the back.   The grey things are the bicycle wheels.  They are in a little cage frame.   This frame must be strong because there will be a force twice as large as the tension trying to rip the wheels out and send them flying towards the operator.    The dark brown pieces are the main beam and the box structure underneath it which is there to stiffen the beam.   On top of the beam are the two track pieces.   These provide a slot for the shuttle and the projectile to travel in.   Note that the vertical center of the wheels aligns perfectly with the top edge of the track guide.   This is important to keep the pulling ropes from dragging excessively on the track (too low) or trying to pull the shuttle off the guide track (too high).
Last Updated on Thursday, 29 October 2009 02:15

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