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The Anvil

Sorry, there's no picture of the Anvil. This entry arrived after the photographer left :-(

Race
Order
Entry
#
Driver Name Car Name Mass
(grams)
Length
(inches)
Width
(inches)
Height
(inches)
Distance Traveled
(feet)
Running Time
(seconds)
Awards
                     
30 37 Andrew Berkun The Anvil 82000 14 10 25 0.06 1.50 Late Entry - Unofficial Heaviest


Balloon car designer Andy Berkun writes:

My entry, "The Anvil", weighed in at around 150 lbs, and had a cruising range of about 2 inches... although it only did 3/4 inch at the contest:

I realized that given a large enough piston, the air pressure in the balloon could generate a huge amount of force for a short distance. I estimated about 1/4 PSI for most of the inflation of the balloon (peaking at the start of the inflation cycle). My piston was about a foot square, giving about 35 lbs of force. I originally wanted to move a Honda Civic, which I estimated would take about 50-80 lbs of force. However the difficulty of getting the car to the test site made me settle on a balloon powered skateboard and made the design easier.

The biggest design challenge on a piston is making it seal well enough to build up enough pressure to do the job. What I used was a cardboard box and a reinforced cardboard piston which fit loosely in the box. To keep the air from leaking out, I inflated a plastic garbage bag inside the box below the piston. The garbage bag transferred force to the piston but would not leak out around the edges of the piston. There was no leakage out of the garbage bag at all.

A string was tied to the piston and run through the box to the wheel of the skateboard. I wrapped it around the wheel of the skateboard so pulling on it would make the wheel turn. The garbage bag was routed around the string so there was no air leakage around the string. The total force generated was several times what was needed to move the skateboard. The short distance I traveled was due to not taking all the slack out of the string during my run, and only partly inflating the balloon.

Now a little on an entry which did not make it to the contest:

I spent a great deal of time on a design which never worked and therefore never made it to the contest. Had it worked it would have gone several times the length of the course. I wanted to use a vane motor to transfer air in the balloon to torque for the wheels. The motor itself was only a few grams and very efficient at these pressures. However implementing it proved to be extremely difficult. A vane motor is the compressed air motor used in air powered tools. In a vane motor, flat vanes slide in and out of a cylindrical housing to touch the sides of a cylindrical chamber. The center housing is offset slightly from the chamber so the area trapped by the vanes changes as the vanes rotate. I tried to do away with the complexity of moving vanes by using a bending vane made of plastic. This would not work for the high pressures a tool sees, but should have worked with balloon pressures. The problem I had was as the vane bends, it drags on the outer wall creating friction. The pressure difference adds to that friction, which was greater than I could counteract with any lubricant. My 2 motors would not turn by themselves at any pressure. In retrospect it had to be tried, but a moving vane design is too complex to build by hand and the bending vane design is doomed to too much friction.


MB 1/9/99