Keith et al.
Thanks for the reply and advice. I appreciate your experience and system
knowledge.
Casey and Drew pointed out that at over 100K the exact timing of apogee will
not be that big of deal. There is not enough atmosphere to zipper the rocket.
It sounds like a good idea to have both a channel for <100K and another with
longer delay for >100K. I typically use dual matches on the charges so if one
altimeter fails, I have another firing the charge. Likewise, I always have
backup charges.
The delay is another consideration that I did not take into account. It makes
sense to give the rocket time for the accelerometers to detect the change in
acceleration due to gravity. What about Drew's concern over the lack of
integration over the three axis of accel? Can we assume that as the rocket
arc's over the change in acceleration will become detectable and thus trigger
apogee detection? Is there a set threshold for change in acceleration for the
algorithm to detect apogee?
If we are fairly certain of apogee detection even in an off-vertical
trajectory, then there may not be a real need for an EFU timer. Thoughts?
I have multiple APRS radios so it is enabled on all my TeleMegas. I like to
have the audible radio beacon as a backup measure of confidence.
In 2013 at BALLS, I had the USC team set up a remote tracking station in the
hills above the playa for the very reason of end-fire degradation. The rocket
cato'd at 5,000 feet so RF degradation was the least of our problems then.
Thanks,Craig
-----Original Message-----
From: Keith Packard <[email protected]>
To: Craig Klimczak <[email protected]>; altusmetrum <[email protected]>
Sent: Mon, Jun 24, 2019 10:37 pm
Subject: Re: [altusmetrum] BALLS Project
Craig Klimczak <[email protected]> writes:
> Are there any special considerations for setting up the TeleMega for
> flights to over 100,000 feet?
Yup, 100k' is where the baro sensor ends and you end up flying on accel
alone (really, it's just coasting at that point, so even the accel is
reading 0). This means that apogee determination is much less
precise. Fortunately, apogee determination is also much less important
as there's not much air to break things at speed.
What you want is to set a fairly long apogee delay when you're over
100k' (5-10 seconds or so, up to about 150k'), and to have a backup
channel programmed to fire at apogee, but only if under 100k'. You can
wire those to the same charges by matching the +/- terminals as
described in the manual.
> Are there any considerations for rockets exceeding Mach 3?
Not for the electronics; they don't care how fast you're going.
> What are the highest altitudes achieved with the TeleMega to date?
Not sure. We've got lots of EasyMega flights up to 150k' or so.
> I've flown this TeleMega to 35,000 feet with no problems. Is there an
> altitude where the performance of the TeleMega may be limited? Has
> anyone on this forum flown a TeleMege to 100,000 feet before. If so,
> how did it perform? Did you have any issues with tracking? Losing
> GPS lock?
You'll want to do some ground testing to measure the available telemetry
range.
At the default 38400 transmission rate, the signal is good to
about -105dBm. At 2400, it's good to about -120dBm. Each doubling of
distance takes about 6dB out of the signal. So, if you have a clear
space (hilltop to hilltop) to use, you can measure the signal there and
get a good estimate of the maximum range. Measure -70dBm at 1km and
you'll have 35dB, or nearly 6 doublings of available range (64km) at
38400 baud. All of our ground stations provide RSSI numbers to use for
this.
You should also consider enabling APRS and getting some good (yaesu or
kenwood) APRS-enabled HTs. APRS runs at 1200 baud, and although it's a
pretty terrible protocol in many ways, the quality of those receivers is
such that APRS has about the same range as our stuff running at 2400
baud.
Ground testing will uncover many potential issues which you can fix
before flying.
Note that a receiver located aft of the rocket will generally be in the
worst possible reception location. Not only are usually off the end of
the antenna where the signal is weakest, you're also behind a plume of
ionized gas, which blocks radio signals. Having another ground station
off to the side can improve the telemetry reception a lot. Having lots
of ground stations is generally a good idea anyways as things can happen
fast during flight and mistakes are easy to make.
GPS will lose lock during boost, but should re-acquire during coast. At
speed and altitude, it will stop reporting correct data due to COCOM
limits, but when you slow down, it will start again.
> Apogee detection?
See above.
--
-keith
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