slices, and then calculates the effect
the winds have in each slice on the
balloon’s flight. Easy. However, don’t
be in a hurry to do this for yourself
because I’ll show you shortly how to
do this online.
Descent Under a
The final phase of a near space
flight is the parachute descent. A
parachute functions by significantly
increasing the drag of a falling object.
Once the speed of descent is high
enough, the parachute’s upwards
drag equals the weight of the object
downward, letting the object
descend at a constant (and safe)
speed. Recall, however, that drag is
also proportional to air density. This
means that unlike a balloon, a
parachute’s descent speed at high
altitude is greater than its descent
speed at low altitude.
The parachute’s speed at altitude
is easy to calculate if the landing
speed of the parachute is known (by
using a model of the atmosphere’s
density as a function of altitude).
Most people don’t calculate the
landing speed of a parachute. The
landing speed is normally stated by
the manufacturer of the parachute.
That covers the four things we
need to know about a balloon flight,
ascent rate, winds aloft, burst
altitude, and parachute descent
speed. Now, we’re ready to go online
and run some predictions.
Predicting the Path and
Recovery Zone of a Near
Near space predictions can be
made at http://predict.habhub.org.
You’ll notice the inputs to the
prediction application are the latitude
and longitude of the launch site; the
launch time and date; the balloon’s
ascent rate; the balloon’s burst
altitude; and the parachute’s landing
speed. I feel the Launch Altitude is
not a significant field to fill in since
the balloon will climb much higher
than any possible launch (or landing)
One can move to the location of
the launch by zooming out the map,
dragging close to the desired launch
location, and then zooming back in
for precision. Alternatively, if the
latitude and longitude of the launch
site is already known, those values
can be entered.
Notice there’s an option called
Set With Map. If clicked, this option
lets you click on a point on the map
to use for the launch site. The
latitude and longitude fields are
automatically filled in when using this
option. I normally zoom out, drag,
and then zoom back in before opting
to Set With Map.
After pinpointing the exact
launch site, enter the remaining
fields. I chose a burst altitude of
27,000 meters based on the output
from the HabHub Burst Calculator
(and my experience). An ascent and
descent rate of five meters per
second is right around 1,000 feet per
minute and typical of my near space
Be sure to select the correct
launch day and time. The prediction
application defaults to the current
day and time. If you don’t notice this,
you may be greatly surprised when
the actual flight doesn’t match
predictions. Now all I need to do is
click the Run Prediction button.
The output of the balloon flight
prediction webpage is a Google Map
of the balloon’s travels. The red circle
is the launch site, the spiky circle is
the burst point, and the green circle
is the landing site.
As you can see from this
prediction, a launch from my nearby
airport on June 2nd and reaching an
altitude of 27,000 meters would end
up in the Boise Mountains near the
town of Prairie. Obviously, we’re
going to have to pick a different
launch site for this flight.
That’s the beauty of the balloon
flight predictor; you can move the
launch site and time around until you
get a flight that will work for you.
August 2017 15
The results of a balloon flight prediction. Here’s the webpage as I’m making a prediction for