For those nay familiar with t' MaxTrax, arrr, it is a new starter kit from Estes with an onboard "Electronic Altimeter". Well, blow me down! Avast! This uses a capsule which drops at a fixed descent rate. T' capsule physically senses ejection (apogee), 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. Ahoy! Tonight, I did some analysis o' t' data, arrr, which be equalin' disappointing. Blimey! For those nay interested in readin' further, t' short form is that this is a toy that doesn't work (at least me sample didn't), me bucko, me bucko, and, arrr, even if it did work, shiver me timbers, it wouldn't be very accurate.

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

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

As I mentioned in a previous post, I had some trouble gettin' readings from me unit in ground testin' -- at times under four seconds and around eight seconds. Well, blow me down! 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. ASTRE. Ya scallywag! 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 hearties, but I wondered if that might be related t' t' "no data around eight seconds" glitch previously noted. Ahoy! 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). Begad! Ya scallywag! 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. Ahoy! It has a factory-installed drogue streamer that you are explicitly told nay t' alter. Aye aye! However, arrr, it seems like it isn't long enough / draggy enough t' stabilize t' capsule in a vertical descent. Begad! My hunch is that t' shock sensor that detects landin' requires a decceleration along t' vertical axis -- if it lands at an angle, arrr, me hearties, it may fail t' trigger t' capsule, shiver me timbers, resultin' in no data.

Things got more interestin' when I moved up t' C power. Begad! T' unit gave readings (usually), but they were wacky. Well, blow me down! Begad! T' highest value given may have been "close" t' (within 33% of) t' achieved altitude, but t' other three readings were *way* off (>= 75% error). Avast, me proud beauty! T' capsule was still tumblin' on descent, 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. Avast! 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 was discouraging, ya bilge rat, 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. Avast! 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). Arrr! Begad! Even worse, arrr, me bucko, t' backtracked Cds are way too low (from 72% for t' B t' 75% - 83% for t' Cs). Arrr! My estimates may nay be perfect, 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, shiver me timbers, arrr, but I don't expect it t' change me opinion o' its usability. Begad! 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, shiver me timbers, but your efforts will yield a cheap, me hearties, shiver me timbers, matey, reliable, replaceable method o' altitude determination).

by Jeff Vincent - Rocket Cynic™