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. This will sound like Morse code…
several beeps, followed by about one second of silence. Once you hear it, you’re not going to
mistake it for anything else, including another altimeter that you may have in your AV bay. 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, altitude 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 in altitude between successive samples and
the elapsed time between the samples. 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). 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
undoubtedly break something and ruin your day, not to mention what an object falling from the sky
at these speeds could do.
To Next Page
Loading...