Accelerometer Launch Detect (ALD)
Accel G's: (OFF, 3-20, default 3G)
Accel Time: (0.2 - 2.0 sec, default 0.5 sec)
These settings control the accelerometer launch detect. When the G threshold has been
reached/exceeded for at least the programmed time, a launch detect is triggered and the launch
mechanisms begin. Note that launch detect occurs when EITHER the barometric LDA altitude
OR the ALD is triggered; the first one to trigger a launch detect starts the flight process. If
you're at 50' AGL and you hit the 3G/0.5s threshold, your launch detect altitude will be 50'.
For most flights, the default of 3G for 0.5 sec will be fine. This is high enough that it's unlikely
to be accidentally triggered, and low enough that almost any motor is going to trigger it. There
are a few exceptions of note, however. First are motors like the CTI VMax motors, which
produce a very high amount of thrust for a very short period of time. For those motors, we
recommend setting the time to the minimum (0.2 sec) and setting the G's to a higher than normal
value… 10 G's should work in most cases. For relatively low thrust motors, we recommend that
you turn it OFF, and let the barometric launch detect trigger the launch. We've seen some very
underpowered rockets that never made it to the 3G threshold.
Note that events triggered from the Start of Flight are NOT triggered directly from these points.
Rather, they're back-dated to either the last sample under 10' AGL, or the last sample under 1G
acceleration. It is assumed that whichever one of these came later is an accurate start of flight.
About Sampling Rates and Memory Usage
At this point you may be wondering about how long a flight the Proton can record before
running out of memory, and what happens if that occurs. The Proton saves about 2,000 samples
for each flight. When you’re sitting on the pad or launched and have not yet reached LDA, the
sampling rotates through those memory locations, saving the last altitude reading < 10’ AGL as
the barometric “start of flight”, and the last accelerometer reading under 1G as the accelerometer
start of flight. Once you get a launch detect, the recording continues until either 1) the End of
Flight benchmark (< 30’ AGL for at least 5 seconds) is reached, or 2) the memory wraps around
to the start of flight location. In either case, recording stops there (it won’t overwrite valid ascent
data), but this does NOT affect any pending deployment events… if you run out of memory
before the main chute is deployed, it will still be deployed, but you won’t see it in the data
downloads because it can’t be recorded.
Now, you may be wondering how long a flight you can have with “only” 2,000 memory
locations. Quite a long one, actually. Let’s say that you want to fly to 30,000’ and your motor
will get you there in 30 seconds. At 20 samples per second, that’s 600 samples. That leaves
about 1,400 samples left for the “down” part of the flight. At 2 samples per second, that’s 700
seconds, or over 11 minutes. You’d have to be coming down at a rate of about 40 ft/sec under
drogue to exhaust the memory, there’s no way you’re gonna be coming down that slow from
30,000’ (unless you really like walking a lot). More likely, it’s going to be 50-70 feet/sec. Let’s
say that we come down at 60 ft/sec from 30,000’, that’s about 500 seconds, or 1,000 memory
locations at 2 samples/sec. You’d still have over 400 samples left when you land, nearly 20% of
your memory.
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