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undoubtedly break something and ruin your day, not to mention what an object falling from the sky
at these speeds could do.
To prevent this from happening, the Proton uses a predictive mechanism to hold off deployments
until it's safely out of the mach "danger zone". Deployments are initially disabled when the Proton
is armed. When the noise-filtered barometric velocity drops below 100 ft/sec for at least 1 second
after the Launch Detect Altitude has been reached, deployments are enabled. Typically, this will be
a second or two before apogee, although it may be a little later in some cases (i.e. a very draggy
rocket). In any case, it guarantees that your rocket isn't going very fast when the drogue deployment
occurs, which is the ultimate goal. This is the same logic that we've used for several years with all
Eggtimer Rocketry altimeters, and in several thousand flights we've never heard of a zipper due to
an early/late deployment that wasn't due to the rocket going horizontal (or a motor-eject backup
charge firing early).
Apogee and Nose-Over
Assuming that your rocket is moving more or less straight up, it will continue to slow down during
the coast phase until it gets as high as it's going to go. If the rocket was going absolutely straight up,
the velocity at this point would be zero; it would simply start falling to the ground. In reality, this
almost never happens, because for safety you usually angle the rod/rail at a slight angle so that the
rocket takes off away from the flight line. This results in the velocity disparity that we've previously
mentioned. The rocket usually has some forward velocity at apogee; hopefully it's relatively small
so your parachute deployment happens at a low velocity and won't cause any mechanical problems
like a broken shock cord or a zippered tube. It also means that you really don't know that you've
reached apogee until after you've been there.
Accordingly, the Proton fires the drogue parachute at Nose-Over, which we define as one second
past apogee (highest recorded altitude). If the rocket is still going up, chances are that its altitude is
going to keep increasing before a one second interval elapses, so you're unlikely to get a false
apogee detection. If the rocket hasn't gained any more altitude after one second then you must be
starting on your way down, so the Proton fires the drogue at that point, for one second.
When Nose-Over is detected, the Proton begins using the Descent Samples value for altitude
sampling. This setting is very low, typically 2 samples/sec. As the rocket descends, when the
altitude drops below the Main deployment altitude the Main channel is triggered, for one second.
At this point your Main chute should pop out, to thundering applause and a hearty round of high-
fives all around.
On the Ground
The Proton detects that your rocket is on the ground when the barometrically-detected AGL altitude
varies by less than 30 feet for over 5 seconds. Once on the ground, the Proton will start beeping out
the maximum altitude continuously for anyone within earshot to hear. The beeps work like this:
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