How high did that rocket go?

Here's a simple and inexpensive way to get a fair estimate. There are three diagrams, so expand the extended entry to read the surprisingly easy method we use.

You actually only need a couple things to figure out the altitude of your flights. A theodolite, a tangent table, and a pencil. For reasonably accurate readings, you can make the simple theodolite shown in figure 1. It's basically a 1"x2" piece of wood, 2 foot long, with a plastic protractor screwed to the side. Add a couple finishing nails to sight along, a string with a fishing weight at the end to indicate the angle, and you're set.

figure 1

The tangent table can be found in any trigonometry textbook. Use the one shown in figure 2, or find one to your liking, they're all the same.

figure 2

Still with me? Good, believe me, this is simple. In fact, this explanation takes longer than the process.

Figure 3 shows the basic concept of determining altitude:

figure 3

The 'tracker' takes the theodolite and stands a known distance from the launch pad. In the diagram, it's where the black and blue lines meet. This distance is the baseline, and the farther the better (as long as you can see the rockets from there). Our usual launch area is a football field, so our tracker is usually 300 feet (100 yards) away from the pad. The tracker on one goal line, the launch pad on the opposite goal line.
When the rocket launches, the tracker follows the rocket with the theodolite, sighting it like a rifle, until the rocket reaches apogee (it's highest point). The angle is read (where the string marks it on the protractor), and this angle is written down.
Time for some simple math. The formula is on the diagram. Look up the tangent for the angle on the table, multiply that number by the baseline, and that is the altitude in feet. Simple!!!

An example: baseline is 300 feet. measured angle is 40 degrees. The tangent for 40 degrees is .839, so 300 * .839 = 251.7 feet.

This is only one method, there are many others. But this one is cheap, simple, and accurate enough for our purposes. You can find more information about altitude tracking in the Handbook of Model Rocketry, by G. Harry Stine.

Accuracy can be improved by using two trackers placed at 90 degree angles to each other to compensate for rockets that don't fly perfectly vertical. This is the usual method used at altitude contests. We don't bother when we're flying for fun. Truth be told, we seldom worry about altitude anyway, we just guesstimate using the good ol' Mark I eyeball.

Posted by Ted at January 3, 2004 09:45 AM | TrackBack