You can build an Inclinometer to measure the angle of the rocket altitude.  If you measure the distance from the launcher, you can calculate the height:

Height = Tan(Angle) * Base

For a base distance of 100 feet, this table show the height above the viewer's eyes.

Here is how I made my inclinometer:

I started with a spare scrap of wood.  The board was 170mm tall and 210mm wide (and 10mm thick). 

I drew a line parallel to the top edge of the board, about 80mm down from the top.  I printed a "protractor" , cut it out to fit the board, then glued it to the board, carefully lining the zero degree line with the line on the board. You can download (and print) this file: Inclinometer Scale.pdf

(click on pictures for larger view)

I cut a 150mm diameter disk from a paper plate, then glued a plastic washer (for a bearing) to the center of the disk, then wrapped a string around the washer and glued the string along a radius line.

I fastened the disk to the board with a short nail, making sure the disk could rotate freely

.

I used a short screw and big washer to fasten a Binder Clip to the lower corner of the board, adjusted so it could pinch the disk to lock in the angle at the rocket's highest point (apogee).

I attached a weight (about 12 ounces) to the string, which was cut to about 2 feet.  Yes, I know I'm mixing metric and English measurements.  I used the nearest rulers for measuring.

Calibration:

Rest the bottom edge of the board on a flat surface (like the edge of a table), with the weight hanging freely.  Stop the weight from swinging.  Press on the Binder Clip to release the disk, allowing the disk to rotate, centered by the hanging weight.  Release the clip to lock the disk.  Mark an arrow on the disk, pointing to the zero degree mark on the inclinometer scale.  As you tilt the board, while pressing the Binder Clip, the arrow will indicate the angle of the tilt.

Using the Inclinometer:

Position yourself a measured distance from the rocket launcher.  100 feet is a reasonable distance.  Sight along the top of the board, aiming at the rocket, while pressing on the Binder Clip.  Release the clip when the rocket reaches its highest point (apogee), which will lock the disk at the highest angle.  The height can be calculated with the formula:

Height = Tan(angle) * Base

Be sure to calculate using degrees instead of radians.

Have fun!

Safety Notice:
These launchers are basically safe.  The biggest hazard is the empty 2-liter plastic bottle falling from the sky.  Other dangers include (but are not limited to):
Tripping while trying to catch falling rockets, sunburn while having too much fun in the sun, death while running in front of moving vehicle (while trying to catch falling rockets), freezing to death while having too much fun in the winter, and bee stings.  Parental guidance is suggested.  Always shoot the rockets straight up.  Don't be stupid.

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Last modified: April 29, 2007