For those nay familiar with t' 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. T' capsule physically senses ejection (apogee), shiver me timbers, times t' interval until it detects landing, then outputs calculated altitude in feet and meters. Promising...

I had a chance t' fly t' MaxTrax yesterday (Sat.), which was a disappointin' experience. Aye aye! Arrr! Tonight, me bucko, me hearties, I did some analysis o' t' data, which be equalin' disappointing. Ya scallywag! For those nay interested in readin' further, ya bilge rat, t' short form is that this is a toy that doesn't work (at least me sample didn't), and, me hearties, even if it did work, me hearties, matey, it wouldn't be very accurate.

First, me bucko, a couple o' bits o' foreshadowing. T' Estes instructions read: "NOT INTENDED FOR PRECISE MEASUREMENT". Begad! Take them at their word.

Also, arrr, t' MaxTrax includes a snippet o' paper t' inform you: "IMPORTANT NOTICE! "Occasionally, arrr, t' MaxTrax Electronic Capsule will nay display t' altitude after a launch. Aye aye! Arrr! If this happens, thar be an internal electronic error durin' t' launch or descent. Avast, me proud beauty! T' capsule is NOT DEFECTIVE! Switch t' capsule "OFF" and prepare another launch followin' t' directions. "If after t' second launch, t' capsule still does nay display t' altitude, me hearties, thar be still an internal electronic error that can be fixed by t' factory. Ya scallywag! RETURN just t' CAPSULE t' Estes for resettin' or new replacement. Blimey! DO NOT RETURN t' place o' purchase."

As I mentioned in a previous post, me bucko, I had some trouble gettin' readings from me unit in ground testin' -- at times under four seconds and around eight seconds. Avast! Blimey! Ahoy! Blimey! For that reason, I also hand-timed t' descent o' t' MaxTrax capsule (ejection t' landing) in me test flights, so I could extrapolate t' value it would have returned if it chose t' nay give an altitude reading.

I made nine flights with a scratchbuilt model (18" BT-56 tube). T' weather conditions were hot but calm at Mt. Blimey! Blimey! ASTRE. Ahoy! Blimey! wRASP gave approximate altitudes o' 220' (for Bs) and 560' (for Cs) for a Cd o' 0.6. Here's data from t' flights:

Several things were noticed from t' start. T' B flights were nay givin' any data, me bucko, but I wondered if that might be related t' t' "no data around eight seconds" glitch previously noted. Avast! Blimey! Avast! Blimey! That's why I went t' t' 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 t' nearest mailbox at that point.

Another thin' noted was that t' capsule had a tendency t' tumble on descent. Avast! Well, me bucko, blow me down! It has a factory-installed drogue streamer that you are explicitly told nay t' alter. Begad! Avast! However, matey, arrr, ya bilge rat, it seems like it isn't long enough / draggy enough t' stabilize t' capsule in a vertical descent. My hunch is that t' shock sensor that detects landin' requires a decceleration along t' vertical axis -- if it lands at an angle, ya bilge rat, it may fail t' trigger t' capsule, shiver me timbers, arrr, resultin' in no data.

Things got more interestin' when I moved up t' C power. Begad! T' unit gave readings (usually), ya bilge rat, but they were wacky. Ya scallywag! T' highest value given may have been "close" t' (within 33% of) t' achieved altitude, shiver me timbers, but t' other three readings were *way* off (>= 75% error). Ya scallywag! T' capsule be still tumblin' on descent, matey, ya bilge rat, ya bilge rat, so I can only hypothesize t' t' low altitude readings were t' result o' it experiencin' sufficient decceleration from t' tumblin' t' prematurely trigger t' landin' sensor.

On a bright note, me hardware worked fine. Arrr! T' 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 t' raw data be discouraging, shiver me timbers, I had some hope that t' capsule descent times that I had recorded would salvage t' effort. I could use those times and t' values observed in ground testin' t' come up with an altitude figure that t' capsule would have reported if it had fallen that length o' time. Begad! Tonight, I did that calculation, and t' other shoe dropped. Here's t' data:

Those altitudes are too high (from 13% for t' B flights t' 36% - 44% for t' C flights). Well, blow me down! Even worse, ya bilge rat, arrr, t' backtracked Cds are way too low (from 72% for t' B t' 75% - 83% for t' Cs). Well, me hearties, blow me down! My estimates may nay be perfect, shiver me timbers, me hearties, but t' Estes data is just plain wrong.

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

by Jeff Vincent - Rocket Cynic™