■ FIGURE B
■ FIGURE C
■ FIGURE D
Sidebar continued ...
acceleration is due to a change in
acceleration or due to tipping by noting if
the acceleration in another axis changes
The microcontroller inside the
Pendant G Logger can digitize measurements as fast as 100 samples per
second (the sample rate is set by the
software used to program the logger).
The logger’s memory is large enough to
hold 64K, or 65,536 measurements. At a
sampling rate of 100 Hz, the logger will
run out of memory in 10. 9 minutes.
Since a near space mission lasts closer
to three hours than to 10 minutes,
set the logger’s sampling rate to 5 Hz
or less. That’s a maximum rate of five
measurements per second.
The Pendant G Logger is inexpensive. A $59 reader programs the logger
and downloads its data, so you can’t just
purchase the Pendant by itself. However,
you can purchase multiple Pendants and
just one reader. That’s a great option if
you are running a BalloonSat workshop
where several loggers are needed.
Figure B shows some of the data
I’ve collected in near space with a
Pendant G Logger. This is a chart of the
vertical acceleration (the Y-axis) experienced by my near spacecraft. You’ll
notice that there’s more variation in the
acceleration for the first 40 minutes of
the flight. At the 40-minute point, there’s
a greater burst of shaking (variation in
acceleration). Then everything quiets
down until the balloon bursts. You can tell
when the balloon bursts in the chart. I’ve
always assumed the interior of the near
spacecraft experiences zero G initially
after burst. But from this chart, we can
see that there’s so much shaking going
on that no microgravity conditions are
experienced during the descent.
Looking closely at this chart, I
noticed that there’s a tiny decrease in the
acceleration until the balloon burst.
Therefore, I used Excel to magnify this
effect, create a trend line, and generate
an equation describing this decrease in
acceleration. The result is Figure C.
There’s about a 7% decrease in the
balloon’s acceleration during its climb to
87,000 feet. At best, one percent of this
is attributable to the reduction in gravity
at high altitude (if even that much). So, I
don’t know where the other 6% comes
from. I thought it might be from sensor
drift, but I couldn’t replicate this decrease
in acceleration by letting the Pendant sit
on the front porch where there is no
change in acceleration.
If the decrease were due to temperature effects, then I would expect the
drift to change directions as the interior