A big part of this flight was an electronics test. Last year we struggled to get the data off the GWIZ, and found that it was interrupted by the BRB transmissions. It also had limited acceleration and altitude capability. We did not feel confident flying a bigger rocket without trialing some new electronics.
So came the search for a better altimeter. We landed on the Telemetrum from Altus Metrum. 100G Accel, 100K Baro, and a Kalman filter made it an attractive unit. Additionally it included a GPS which we needed since a friend lost my other unit. As if that wasn't enough it transmitted live data at 1Hz and was only $400 for the entire setup and would greatly simplify and lighten the altimeter bay.
Photo of the bay, it took about 30 minutes to complete this year:
Once we got the Telemetrum up and communicating with the computer we started testing it extensively on the ground. Eric's BALLS experience was fresh in my mind and we were a little concerned about flying this rocket without flight testing the unit since it was new to us. We decided to see what we could do on the ground to hopefully head off any surprises on the playa. We found the unit to be very capable and ultimately successful, but did observe a few things during ground testing:
We learned quickly that this unit did not pick up GPS lock quickly at startup, even if it was previously sitting in idle mode. It seemed to be very sensitive to metal near the patch antenna, and so we did our best to eliminate any metal near the unit (see the break in the threaded rod). Putting metal near the patch antenna seemed to cause a break in the idle communication as well. Last we also found that movement other than very careful, slow motion would cause the unit to detect liftoff when it was in launch mode. This was not overly concerning because we planned to arm remotely.
On launch day we started the altimeter and let it run in idle, confirming satellite lock. We then completed assembly and loaded the rocket into the launch rail without turning the unit off, installed the igniter without connecting the leads or arming the system. We were concerned the launch pad would block the signal, but it held lock in idle mode after we had installed it. We stepped back and armed the altimeter via radio signal and allowed it to start up. This concept of operations is really great and eliminates a very common, dangerous procedure used in high powered rocketry: arming altimeters next to a rocket, especially on a ladder.
Unfortunately, at this point we did not achieve satellite lock. We waited. Nothing. After 20 minutes we were getting 1-2 satellites inconsistently. We tried twisting the rocket in the tower to no avail. We assumed that the launch tower was inhibiting the signal but due to our perceived launch detect sensitivity did not want to move the rocket. After an hour with no GPS lock we gave up and carefully lifted the rocket until the NC was above the tower. Within minutes we had 12 satellites in lock. We rushed to launch and were rewarded with live data, and GPS coordinates. Not only does AltOS display a lot of data at once, it also verbally notes the rockets condition. The data provided by the GPS allowed us to relay the position to the flightline so we could assure them the chute was out and the rocket was not overhead another great safety feature. We got GPS until 200ft AGL at 3.25 miles and drove right to the rocket. Awesome.
Overall this flight computer is great, it fully functioned during a pretty hard flight:
-Simple (No need to tetris a GPS, Altimeter and 4 batteries into the altimeter bay)
-Great Value (GPS + altimeter)
-Capable sensors (70G max accel and the Baro tracked the GPS very closely)
-Great features (live data telemetry!)
-Works in flight (deployed charges, accurate GPS position)
The only drawback was the ability of the unit to gain GPS position on startup. I plan to shoot Keith and Bdale an email and see if we can understand what the cause was. The BRB picked up lock in the same tower last year. Overall I am impressed by the unit and excited to fly it again.
