Bottle Rocket

Bottle Rocket

Balloon car designer Mike Blakely writes:

• Design approach taken – explain how it worked.
  

  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.

• Unique or clever features embodied
  

  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.

• Materials of construction (mention unique parts you used or fabricated)
  

  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.

• Reasons behind any significant design choices you had to make
  

  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.

• Lessons learned (what you’d do differently next time)
  

  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.

• Anything else you’d like to add
  

  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
  = [2×150/.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!

MB 1/9/99