N
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A
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P
A
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to decrease as it approaches 100,000
feet. That surprises me and now I have
to wonder how accurate the pressure
and temperature equations are at
extremely high altitudes. But we’ll run
with these numbers anyway since
we’ll see shortly that the weather balloon will never get that high. By the
way, the spreadsheets I used to create
these charts are available on the Nuts
& Volts website ( www.nutsvolts.com ).
According to Kaymont (the company where I purchase my balloons),
the maximum volume of a 3,000 gram
balloon is 40,479 cubic feet. So our
astronauts’ 3,000 gram balloon
carrying a 1.5 pound payload (under
Martian gravity) can expand
40,479/13,066, or three times in volume before bursting. According to my
chart then, the balloon will reach an
altitude of 48,000 feet before bursting.
That’s the same as my lowest altitude near space flight on Earth. The
average altitude of my 54 flights (all on
Earth unfortunately) is 84,350 feet. So
it’s apparent that near space flights on
Mars are going to be a lot lower than
on Earth. This really isn’t too surprising though, since the surface pressure
on Mars is about the same that we see
at an altitude of 103,000 feet on Earth.
Let’s quickly test the accuracy of
this chart and its conclusions. So as a
test, I have created a spreadsheet for
the Earth’s standard atmosphere and
with data that was collected on one of
my flights from August 2003. I used
the temperature and pressure from
both my flight and from the Standard
Atmosphere to calculate the diameters of two balloons and developed
the chart in Figure 5.
What strikes me the most about
this chart is that the balloon volumes
follow each other very closely until
the balloon reaches 50,000 feet.
Above that altitude, the standard
atmosphere predicts the balloon will
expand faster than my actual flight
data indicates.
Why is this? Is the real balloon
having difficulty expanding above
50,000 feet? The ascent rate for this
flight is constant through the 50,000
foot transition, so the balloon can’t be
experiencing difficulty in expanding in
volume (if it did, the ascent rate would
slow down). Besides, the air pressure
changes smoothly throughout the
flight, with no abrupt transitions. But
there is one thing special about
50,000 feet: the stratosphere begins
there (the stratosphere is lower in the
winter, but this was a summer flight).
In the troposphere, the air temperature decreases with altitude. But in
the stratosphere, the presence of ozone
makes the air warmer. There’s more
solar ultraviolet radiation higher in the
stratosphere since it’s the ozone that is
removing it. Helium doesn’t stop any
ultraviolet that I know of, so I suspect
it’s transparent to ultraviolet. Also, the
balloon is white and its color may be
reflective to the sun’s increased ultravi-
olet radiation. No doubt there are
errors in my temperature and pressure
sensors. Perhaps my temperature sensor is less accurate at low air pressure.
Out of these factors, which prevails?
This calls for an experiment (I
love it when missions generate more
questions than they answer). In the
future, I’ll measure the temperature
and pressure inside the balloon and
compare it to conditions outside the
balloon. To further resolve this issue
(or perhaps to muddy it further), I’ll
also record images of the balloon as
it ascends. I can then calculate the
true volume of the balloon from the
diameter of the balloon images.
There are two minor (I hope) errors
■ FIGURE 4
Balloon Volume
(Standard Atmosphere Mars)
100000
90000
80000
Altitude (feet)
70000
60000
50000
40000
30000
20000
10000
0
1 2 3 4 5 6
Expansion (ratio)
■ FIGURE 5
Balloon Volumes
100000
90000
80000
70000
Altitude (feet)
60000
50000
40000
30000
Standard Atmosphere
TV03I Data
20000
10000
0
0 10 20 30 40 50 60
Expansion (ratio)
March 2006 93