■ The altitude chart from a real near space mission.
Notice the knee in the ascent.
■ Here’s a chart of altitudes recorded by a
flight computer during a simulated mission.
Notice the knee is there.
comparison of flights recorded by a
flight computer during a real near
space mission and one recorded
during a simulated mission.
IS THAT ALL?
I want to tighten up the code I’m
using for the GPS simulator because
in its current state, there’s too little
room left for additional features. For
example, I included the jumper in
this design as a way of simulating a
GPS getting or losing a position lock.
When the jumper is not in place, the
simulator’s code would produce
sentences with the time field blanked
out and the altitude field left
unchanging. When the jumper is in
place, it would go back to displaying
properly updated time and altitude
fields. So, before outputting each
sentence, the simulator code would
inspect pin 6 to see if the shorting
block was in place. However, before
I can make that happen, I’ll have to
work on the code some more.
At some point, I’ll have to design
an advanced version of the GPS
Simulator that produces more
THE REYNOLDS NUMBER
u = dynamic viscosity of the air
The Reynolds Number (Re) is named
after its inventor, Osborne Reynolds
who developed the concept in 1883.
Originally, it was used to help analyze
fluid and heat flow problems. By
looking at the Reynolds Number for a
balloon, we can determine if the air is
flowing around the balloon smoothly
or turbulently as it ascends. We can
also use the Reynolds Number to
determine if a scale model of the
balloon will perform like the
life size version. The Reynolds
Number is based on factors
like the balloon’s diameter, the
air’s density, and the ascent
speed of the balloon. You can
read more about Reynolds
Number at Wikipedia
Now, rather than try to calculate the
Re during a balloon flight, I went to
NASA’s website (
WWW/BGH/ viscosity.html) and
entered the balloon’s diameter, ascent
speed, and altitude into their applet.
Using this information, I created a
chart of a balloon’s Re at 18,000 foot
intervals (since every 18,000 feet the
air pressure drops by a factor of 50%,
making it easier to calculate the
balloon’s diameter). I did this the first
The equation for Reynolds
Number can be written as
follows (we’ll treat the balloon
as being spherical):
Re = pVD/u
p = density of the air
V = ascent rate of the balloon
D = balloon’s diameter
time assuming that the ascent rate
doesn’t change above the knee
altitude and then a second time with
a more realistic decrease in ascent
speed of 20% above 40,000 feet.
Take a look at the resulting chart.
According to the Engineering Toolbox
ds-number-d_237.html), the Reynolds
Number can determine if the air flow
over the balloon is laminar, turbulent,
or in between.
Drag on the balloon is lowest
when the air flow is laminar and air
flow over the balloon is
laminar only when the
Reynolds Number is below
2,300. In the included chart,
we see that the Reynolds
Number never gets that low,
although it is getting lower
during the ascent. For the
entire ascent, the air flow is
turbulent around the balloon,
although perhaps generating
However, if that were the
case, I would expect the
balloon’s ascent rate increase
throughout the ascent. Since
the ascent rate remains
constant except for the
change at the knee, there’s
got to be more than the
Reynolds Number affecting
■ Well, I’ll be darned if I can find anything that
makes sense here. I wasn’t expecting the Reynolds
Number to decrease above 40,000 feet nor for it to
get even lower in the more realistic calculation.
July 2009 85