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After a few seconds, you'll hear a few beeps, then after about 10 seconds of silence (as it takes
baseline measurements) you'll hear the "I'm ready" beeping, approximately once per second.
Connect your igniter, test the continuity, then go back to the safety zone to await your flight.
If you're new to dual-deployments, you can see that it's a lot more involved than just stuffing some
wadding and the parachute into the tube, popping in the motor, and hooking up the igniter. Multiple
deployments require discipline in order to make them work reliably; we've seen way more than our
share of failed deployments, on everything from a small mid-power E-size rocket all the way up to
an M-sized 200 pound beast. We've seen deployments fail with the top-end flight computers and
with the low-cost units, even with redundancy. The reality is that most deployment failures are not
the fault of the electronics; it's usually something mechanical like a bad connection, a nose cone
that's too tight, a chute that gets stuck in the tube, etc. Having your blood, sweat, and tears free-fall
from 10,000' bury itself in six feet of dirt is going to ruin your whole day.
We STRONGLY recommend that you use a checklist every flight so that you don't forget anything.
We also recommend that you get a copy of the book Modern High Power Rocketry , it's full of good
information too, and subscribing to a forum like The Rocket Forum (www.rocketryforum.com) is a
really good idea, too.
In the Air...
Once your rocket is launched, barometric altitude and accelerometer samples are taken at whatever
ascent rate you've selected, typically 20 samples per second. These samples are run through a filter
to eliminate "noise" that may be caused by a number of different factors, primarily aerodynamic but
also some external factors such as wind and temperature.
Velocity readings are computed based on the difference between successive accelerometer samples
over time, as well as the difference between successive barometer altitude readings over time. In a
perfect world, this would give you a precise and 100% accurate velocity reading. The reality is
somewhat different, however. There are several things that can introduce errors into the velocity
reading: angle of attack, wind, errors in the pressure readings due to aerodynamic influences, and
time differences. If the pressure ports in your payload bay aren't sized properly, this can introduce
an error, particularly if they're too big and you have two of them opposite each other (you'll get a
crossflow through the payload bay which makes the pressure readings very noisy). Finally,
differences in the processor's timing may introduce errors, although the readings are taking at
relatively precise intervals so it's going to be very small.
The good news is that the magnitude of these errors tend to be proportionate to velocity as the rocket
ascends, so they respond well to being filtered with mathematical noise filters. Velocity-related
events are run through a digital filter to smooth out any sharp peaks or valleys that may develop due
to pressure-induced noise.
Mach Transition...
As your motor continues to burn and the velocity increases, if the velocity exceeds 800 ft/sec
aerodynamic shock wave buildup can fool the pressure sensor into thinking that the rocket is
descending when in fact it is actually ascending at a rather rapid speed. If this were not taken into
account, the flight computer might deploy the main parachute at near-mach speed, which would
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