The Estes MaxTrax rocket...

Estes MaxTrax starter set (Click to enlarge).

I recently acquired an Estes MaxTrax starter set (now out of production). The MaxTrax was a ready-to-fly BT-56 based rocket featuring an "altitude tracking capsule" that separated from the rocket at ejection. Slowed by a small streamer, it would fall to the ground at a constant speed; impact would stop the internal timer - which was triggered by a spring loaded switch at ejection - and the capsule would display the altitude, which was simply time of fall multiplied by the speed.

A 1990's application of an old idea based on some simple physics - first proposed in mid-1970, as rocketeers searched for an easier way to get the altitudes of their birds without having to use theodolites, which were cumbersome and hard to maintain. Back in 1974, Stephen Fentress suggested that ping pong balls might be a good option, as they were standardized as far as size and weight. His data and calculations showed that a ping pong ball fell at about 28.5 feet per second, so timing the ball's fall and multiplying that number by 28.5 would produce the altitude in feet. The experiments for the RX-16 rocket in Centuri's Power System outfit also suggested using a ping pong ball to get the altitude, except that the manual rounded 28.5 to 30 feet per second (page 17). The MaxTrax was the latest rocket to use the concept, creating a capsule with an internal timer to eliminate the need to time the fall with a stop watch. It's also probably the last time this will be used in a commercial rocket - accurate, small and cheap electronic altimeters have eliminated the other methods used in rocketry for many decades. 

Note - I wrote a bit more detail about this in a blog post 8 years ago. It was intended to be the first of a two-parter, but me being me, I never wrote the second piece.

My MaxTrax (Click to enlarge).

The altitude capsule (Click to enlarge).

So what to do with the MaxTrax? It's pretty obvious - stick an altimeter in the rocket body and compare its altitude at ejection (which we shall take as the "truth") to the reading on the MaxTrax capsule. Easy, and it will produces some numbers to play with. I was stoked until I happened to glance at the MaxTrax reviews on the various forums, which revealed a very important reason why the Estes concept never really worked out.

The darn thing didn't work most of the time.

It all has to do with stopping the internal electronic timer, which is started when the capsule is ejected from the body. There is a "bounce switch" in the nose, which consists of a small spring that on impact with the ground is supposed to travel forward and complete a circuit, stopping the timer. One problem is that there is too much space between the spring and the metal contact, requiring a hard impact to stop the timer. The other is that the capsule is fairly light, so it can land on its side - the bounce switch requires a nose-on impact with the ground.

Estes acknowledged the problem in a note packaged with the rocket:

Click to enlarge.

This can't be good. Still, I'm going to give it a try at my next launch. Some have suggested removing the foam around the nose cone to help close the bounce switch, so I'll start with that. I guess I can afford to waste a few B6-4 motors.

Stay tuned...