Bottle Rocket

Feb 23, 2023

Bottle Rocket

Bottle Rocket Balloon Car

Entry # Driver Name Car Name Mass
Distance Traveled
Running Time
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:

  • 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
    cut, fit together and taped around the circumference. The balloon exhaust
    came from the remains of a fish aquarium filter. The piston is made from
    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
    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
    are a light press fit into tapered holes in wooden pillow blocks which are
    with epoxy to the yoke. The front wheel bearing is pressed into the wheel
    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
    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
    Smaller tubing had the advantage of producing less piston force, allowing
    construction of some components, but to fully utilize the air from two
    the overall length of the car became unreasonable with diameters under 4
    In the end, this issue was settled by the practical availability of light
    2-liter Pepsi bottles are about 4.3″ diameter and cost little. But now the
    of the inevitable imperfect joint between bottles, and variations in

    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
    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
    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.
    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
    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
    was limited to several coasting tests, seemed OK, but was compromised by a
    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 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

    Assume the work, 150, 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