For those not familiar with the MaxTrax, it is a new starter kit from Estes with an onboard "Electronic Altimeter". This uses a capsule which drops at a fixed descent rate. The capsule physically senses ejection (apogee), times the interval until it detects landing, then outputs calculated altitude in feet and meters. Promising...

I had a chance to fly the MaxTrax yesterday (Sat.), which was a disappointing experience. Tonight, I did some analysis of the data, which was equaling disappointing. For those not interested in reading further, the short form is that this is a toy that doesn't work (at least my sample didn't), and, even if it did work, it wouldn't be very accurate.

First, a couple of bits of foreshadowing. The Estes instructions read: "NOT INTENDED FOR PRECISE MEASUREMENT". Take them at their word.

Also, the MaxTrax includes a snippet of paper to inform you: "IMPORTANT NOTICE! "Occasionally, the MaxTrax Electronic Capsule will not display the altitude after a launch. If this happens, there was an internal electronic error during the launch or descent. The capsule is NOT DEFECTIVE! Switch the capsule "OFF" and prepare another launch following the directions. "If after the second launch, the capsule still does not display the altitude, there is still an internal electronic error that can be fixed by the factory. RETURN just the CAPSULE to Estes for resetting or new replacement. DO NOT RETURN to place of purchase."

As I mentioned in a previous post, I had some trouble getting readings from my unit in ground testing -- at times under four seconds and around eight seconds. For that reason, I also hand-timed the descent of the MaxTrax capsule (ejection to landing) in my test flights, so I could extrapolate the value it would have returned if it chose to not give an altitude reading.

I made nine flights with a scratchbuilt model (18" BT-56 tube). The weather conditions were hot but calm at Mt. ASTRE. wRASP gave approximate altitudes of 220' (for Bs) and 560' (for Cs) for a Cd of 0.6. Here's data from the flights:

Several things were noticed from the start. The B flights were not giving any data, but I wondered if that might be related to the "no data around eight seconds" glitch previously noted. That's why I went to the C motors early (I had intended three B6 flights and three C6 flights [and maybe C11 flights] for NARTREK Gold data). Perhaps I should have dropped it into the nearest mailbox at that point.

Another thing noted was that the capsule had a tendency to tumble on descent. It has a factory-installed drogue streamer that you are explicitly told not to alter. However, it seems like it isn't long enough / draggy enough to stabilize the capsule in a vertical descent. My hunch is that the shock sensor that detects landing requires a decceleration along the vertical axis -- if it lands at an angle, it may fail to trigger the capsule, resulting in no data.

Things got more interesting when I moved up to C power. The unit gave readings (usually), but they were wacky. The highest value given may have been "close" to (within 33% of) the achieved altitude, but the other three readings were *way* off (>= 75% error). The capsule was still tumbling on descent, so I can only hypothesize the the low altitude readings were the result of it experiencing sufficient decceleration from the tumbling to prematurely trigger the landing sensor.

On a bright note, my hardware worked fine. The model made nine stable and successful flights with no damage aside from normal wear and tear (singed but still quite usable shock cord and mylar streamer).

While the raw data was discouraging, I had some hope that the capsule descent times that I had recorded would salvage the effort. I could use those times and the values observed in ground testing to come up with an altitude figure that the capsule would have reported if it had fallen that length of time. Tonight, I did that calculation, and the other shoe dropped. Here's the data:

Those altitudes are too high (from 13% for the B flights to 36% - 44% for the C flights). Even worse, the backtracked Cds are way too low (from 72% for the B to 75% - 83% for the Cs). My estimates may not be perfect, but the Estes data is just plain wrong.

I plan to contact Estes about repair/replacement and any further info they can give on the unit, but I don't expect it to change my opinion of its usability. If you are looking for an altimeter you have two choices: spend the money on a real barometric unit or go the cheap route with a hand-timed drop-streamer (you'll have to calibrate such a streamer yourself in drop testing from a known height and calculate altitude from descent times, but your efforts will yield a cheap, reliable, replaceable method of altitude determination).

by Jeff Vincent - Rocket Cynic™