We completed another round of flights to test my JeeNode Arduino-compatible model rocket telemetry system. The first couple of minutes of the video show the highlights. The remainder explains the details, so feel free to leave the theater at any time.
I published instructions on how to build it on Instructables.com so anyone that wants to try something similar can see how I did it. Unlike my LED bug project, this one didn't need a custom PCB or special chip programming, so it would be far easier to duplicate.
Now I've got some flight data, but is it accurate? From the read outs in the field (final flight #4 shown above), I saw a quick (off-scale) acceleration followed by an arcing altitude graph. The results are a bit higher than I would have guessed, but within reason.
One approach to validating the numbers is theoretical. Dean Wheeler has put a water-rocket simulator on-line that accounts for everything down to air temperature. To put it to use, you need to know some key parameters. My total rocket (minus water) weighed 185 grams. I used 0.7 L of water pressurized to 60 p.s.i. (there I go mixing units again). Using Dean's suggested drag coefficient, which is the primary unknown in all this, the number fall short of measured (268 feet verses 176 feet). The calculated flight time and time to apogee are also shorter. So, what is a good scientist to do? Right, massage the data. Since the drag coeeficent is the primary unknown, and all the other data seem scaled low, I adjusted it until the maximum altitude matched. The time to apogee and the total flight time are pretty close! (see the dotted lines in the graph)
There was a lucky accident here. Did you catch it? The theoretical model assumes the rocket has no parachute on its return to earth. My parachute failed to open, allowing us to directly compare the total flight time.
Of the four flights we made, only flights 1 and 4 provided data. The crash from the first flight fractured the battery holder so the micro-controller would loose power during the acceleration, leaving an eerie end to data transmission. The final flight, the parachute, making up for past transgressions, opened to early, leaving a flight in my estimate of half as high as the first.
Some final observations on the data. The calculated maximum Gs are ~60! That would kill a human, and explains why I have a hard time keeping fins attached to the rocket body. It also looks like the initial acceleration causes havock with the pressure transducer. Mounting it in a different orrientation (parrallel to g-force) might help. Also of interest is that the bottle empties of water in under 0.2 seconds, while the rocket is still just 10 feet off the ground! Some things to think about.