|Entry #||Driver Name||Car Name||Mass
|12||30||Mike Blakely||Bottle Rocket||394||72||6||4.3||47.92||6.16||3rd Farthest Distance & Most Creative & Largest|
Balloon car designer Mike Blakely writes:
This design attempted to harness as much energy as possible from the deflating balloons by having their escaping air feed into a chamber which would grow in volume by having a surface which moved against a load. The work done on that moving surface by the deflating balloons was to be coupled to the driving axle of the car with as little loss as practical.
The moving surface took the form of a cone-shaped piston in a tube, and the force of the piston was applied against a thin line which was wound around the driving axle. The tube just mentioned also served as the body of the car, with a single wheel running on a ball bearing in front and two wheels in the rear attached to an axle which turned in two of the same ball bearings.
Starting with the balloons, they were to be exhausted efficiently by avoiding unnecessary throttling and turbulence in their outflow. To accomplish this, large-diameter tubes (3/4 inch) were installed all the way through the stems of the balloons to hold them open.
The piston incorporated a flexible skirt of overlapping pieces of Kapton sheet. Paper works as well but is much less durable and seemed to have more friction against the tube.
The body/cylinder of the car was made from 2-liter Pepsi bottles, very carefully cut, fit together and taped around the circumference. The balloon exhaust tubes came from the remains of a fish aquarium filter. The piston is made from .007" epoxy-glass sheet, rolled into a cone and lap bonded with quick-setting epoxy. A piston stabilizer ring is made from .015" epoxy-glass and prevents the piston from cocking in the tube. The apex of the cone piston is slightly truncated and capped with a small disk of .031" epoxy-glass; it is this disk which supports the tensile load of the .010"-diameter fishing line which runs back and around the rear axle.
All wheels, just under 6" diameter, are made from .031" epoxy-glass. The rear suspension yoke is also epoxy-glass (we love it) .045" thick. Rear axle bearings are a light press fit into tapered holes in wooden pillow blocks which are bonded with epoxy to the yoke. The front wheel bearing is pressed into the wheel itself. The rear axle is made from 1/4" aluminum rod and the wheels attach with #10-32 aluminum screws.
One major choice was the tubing/car body diameter. Larger diameter had the benefit of requiring less piston travel to exhaust the balloons, but would result in larger piston force and would need larger wheels to keep the tube clear of the ground. Smaller tubing had the advantage of producing less piston force, allowing lighter construction of some components, but to fully utilize the air from two balloons the overall length of the car became unreasonable with diameters under 4 inches. In the end, this issue was settled by the practical availability of light tubing; 2-liter Pepsi bottles are about 4.3" diameter and cost little. But now the problem of the inevitable imperfect joint between bottles, and variations in diameter...
The choice of a cone for the piston solved that problem. The thin cone itself is flexible and can easily be squeezed into an oval shape at its base; it still works fine if the tubing is far from round. But the tubing also had variations in its molded diameter, as much as .02", so a simple cone could never be expected to seal properly along the length of the car. That was solved by making the piston slightly undersize and then adding a flexible skirt to follow the tubing irregularities. The skirt provides a positive seal in that, the more the pressure behind it, the more the skirt presses against the wall of the tube. The friction of the Kapton skirt against the plastic tubing turned out to be low so my plan to apply a thin film of silicone lubricant to the bore was abandoned. The piston force was already threatening to break the six-pound fishing line.
Wheel diameter was mostly driven by the need to raise the car body off the ground by some safe distance, and I settled for a bit less than an inch of clearance. Larger diameter wheels were not used because six-inch wheels seemed more than large enough to travel smoothly over the course, and the car's powered travel (while piston is applying torque to the axle) was already calculated to be a large figure with respect to the course - 110 to 120 feet depending on how well the fishing line was wound on the axle. With a fixed piston travel of 50" and 1/4" axle with small groove to contain the line wrapped in 5 or more layers, larger wheels to increase the travel under power seemed unnecessary. If the car were able to clear the narrow part of the course and achieve its 110 feet of powered travel, the free-running wheel bearings were expected to allow it to easily reach the end of the course.
Test, test, test because the steering (ability to run straight) needed improvement. That is easy to say, but the car was difficult to build to the necessary standards and there was very little time left, and always a chance of damage. Steering alignment was limited to several coasting tests, seemed OK, but was compromised by a handling mistake at the last moment. More tests could have revealed that weakness.
Some interesting figures...
Pressure measurements were made of inflated balloons. A balloon inflating for the first time needed about 30 Torr for inflation. While deflating, pressure would stay around 20 Torr. A very tired balloon deflating produced a minimum of 10 Torr. Remember 760 Torr is one atmosphere, 14.7 psi, so my design pressure was between 0.19 psi and 0.39 psi.
The piston area is that of a 4.3" diameter circle, 14.5 square inches, so the force on the piston from the above pressure is from 2.75 to 5.5 pounds. Not bad if friction doesn't eat much up (it doesn't).
Piston travel is 50 inches at full pressure and is physically stopped there. The deflating balloons do work on the moving piston, and work = force x distance. Let's assume a force of 3 pounds over 50 inches of piston travel. That is 150 in.lb of work. What can that work do? If we ignore losses such as rolling and air resistance, we can do a simple calculation of the peak velocity of the car. Of course air resistance is significant with those balloons dragged along, but let's ignore it because the calculations are much simpler and the answer is more surprising.
Assume the work, 150 lb.in, is all converted to kinetic energy T, where T=1/2 MV*2, M being the mass of the car and V is the velocity of the car. (A full accounting of the kinetic energy of the car should include rotation of the wheels and axle, where T=1/2 Iw*2, I being moment of inertia and w being angular velocity in radians per second. Again, let's ignore this term.) Let's work with pounds, inches and seconds. The car weighs 0.8 pounds, but that is not M. We need the M from W=MG where W is weight in pounds and G is acceleration of gravity in inches/second*2. We find that M = 0.8/386, or .00207 "mass units". Now V = [2x150/.00207]*1/2 = 380 inches/second = 31.7 feet/second. Wow! All those spectators who were in the way should be glad the car went into the wall instead of down the middle of the track!