link lines rotate independently of
each other, reducing their tendency
to tangle. The Spectra kite line is a
tough string with a smooth finish.
You’ll find it’s thin, durable, and
resistant to fraying, unlike nylon
The final benefit is the fact that
the BalloonSat Carrier is reusable.
As long as you remove and store the
carrier, you won’t lose or tangle the
Some Final Cautions
Here are some things I’ve seen
go wrong on BalloonSats or in my
own experiments (I would prefer to
learn from other people’s mistakes
rather than from my own, but alas,
that’s not always been the case).
Avoid windows over cameras.
This is an example of one of my
earliest mistakes and I’ve seen it
made elsewhere. Since it’s cold in
near space, the first response of
designers is to close off all openings
in the airframe in order to keep the
cold air from infiltrating. The result,
however, is that you end up creating a
cold surface where internal moisture
The window fogs up early in the
mission and eventually that conden-
FIGURE A. Unsquare cut.
FIGURE B. The angle on this Exacto knife
is too steep for the first cut.
FIGURE C. Holding the blade as horizontal
as possible creates the smoothest edge.
One of the difficulties in making
an airframe is cleanly cutting the
Styrofoam panels. I’ve found that a table
saw makes clean cuts in Styrofoam as
does a hot wire (which cuts through
Styrofoam like a knife through, well, you
know ...). The table saw cuts a naturally
straight line, while the scroll saw
requires care or a jig. If you’re like me,
you don’t have access to either of these
tools and can’t justify purchasing them.
Our only option then is to cut
Styrofoam with an Exacto knife.
Styrofoam is cut straight and cleanly with a metal straight edge and a
sharp Exacto knife. I lay out my cutting
line lightly in pencil and take care to
hold the Exacto knife as close to
perpendicular to the Styrofoam sheet
as possible. When I’m not careful, I
end up with an edge that looks like
what’s shown in Figure A.
Make your first cut with the
Exacto knife leaning back as far as
possible. Make several passes
through the Styrofoam and don’t try
cutting through in a single pass.
As you cut deeper, stand the blade
FIGURE D. The edge of Styrofoam cut with
a dull blade. (I used a very dull blade to
emphasize the damage done by the blade.)
A #11 Exacto blade works well for
3/4-inch thick Styrofoam. For
Styrofoam one inch and thicker, use a
2-1/2 inch long blade (this blade
requires the larger Exacto handle).
As the blade of an Exacto knife
begins to dull, you’ll notice that it cuts
one direction better than the perpendicular direction. Apparently, there’s a
“grain” in a Styrofoam sheet. I imagine
the grain is created as the Styrofoam is
extruded. A dull blade begins breaking
chunks out of the Styrofoam, instead of
making smooth cuts. Once a chunky
edge has been cut, I find that I cannot
trim the chunky edge away with a sharp
blade. Instead, I have to make an entirely new cut away from the damaged
By using a sharp blade and holding
it as close to flat as I can, I end up with
smooth cuts in Styrofoam. Since you
can go through a lot of blades, purchase
your Exacto blades in the black, plastic
box of 25 blades. The cost per blade is
low, the box protects the unused
blades, and the box is a safe place
to dispose of the dull (but still sharp)
FIGURE E. The edge of this Styrofoam
sheet is much smoother when cut
with a new, sharp blade.