Dr. Gadget's Science Machine
The power of a balloon can move your miniature car

By Joe Laszlo

Valerie Inafuku displays her jet dragster. You can make your own by using cardboard and bottlecaps as described in the instructions at left. Deborah Booker • The Honolulu Advertiser

Aloha! Let's try building a balloon-powered dragster!

You'll need: a long balloon, a piece of cardboard like the kind found on the back of a legal pad that is 4 by 11 inches long, two soda straws (one to serve as your wheel "bearings," another for your balloon "engine"), a bamboo skewer for your axles, a nail the same diameter as the bamboo skewer, scissors, glue, a pen, a ruler and two pairs of different-size plastic bottle caps for the wheels.

The two front ones will need to be matching smaller bottle caps, and the two rear wheels should be matching larger bottle caps.

1. Draw the shape shown here, cut it out and draw the two lines on it. Also, cut into the body on lines marked "B." Score the cardboard along the "A" lines by going over them repeatedly with a ballpoint pen. Fold on all "A" lines. The sides and rear of the dragster should be folded up. You can tape or staple the tabs in the rear to hold them in place. Fold the rear fin down slightly, but let the sides angle up. (Figure 1).

2. Using a paper punch, make holes for the wheels: Make the holes for the front wheels 1 inch back from the nose and near the fold. Make the holes for the rear wheels 1 1/2 inches from the dragster's rear, near the fold. The holes should allow the soda straw bearings to go through.

3. Cut a 2 1/4 inches piece of straw to make the front bearing, then a 3 1/2 inches piece to make the rear bearing. Put each piece of straw through the holes with equal amounts of the ends protruding on each side.

4. Cut a 2 3/4 inches piece of the skewer to make the front axle, a 4 inch piece to make the rear axle.

5. Make a hole centered in the rear for the straw of the balloon to go through.

6. Make a hole in the center of each set of the two plastic bottle-cap wheels. To find the center, draw around one of the caps on a piece of paper. Cut out the circle. Fold the paper twice, then unfold it. The center is where the fold lines cross. Make a small hole here. Place the paper onto each of the caps. Mark the centers. Repeat this with the other set of wheels. Using the nail, carefully make a hole in each bottle cap.

7. Test fit all of the wheels onto the axles.

8. To assemble the balloon-powered dragster: Put one small wheel onto the front axle, cap top in. Put the skewer axle into the front bearing. Put the other small wheel on the other end of the axle. Repeat with the rear axle.

9. Get a long balloon, stretch it and blow it up a few times. Cut the ring off the mouth of the balloon. Insert a 4 inch piece of straw 1 inch into the mouth of the balloon. Tape the straw to the balloon. Insert the straw through the hole in the rear of the dragster with at least 2 inches sticking out the back. Tape the straw down.

Find a long, clear piece of floor. Blow up the balloon and squeeze the straw to hold the air in it. Put the dragster on the floor and release the balloon! Did the jet dragster go? Why? The air escaping from the balloon exerts a force out and back an equal and opposite force is exerted to the front. This propels the dragster.

Until next time, a hui hou!

Write: Dr. Gadget, c/o The Honolulu Advertiser, P.O. Box 3110, Honolulu, HI 96802, or fax 535-8170. You may send e-mail to ohana@honoluluadvertiser.com only if you're 13 or older.