BalloonSat airframes has a soft face; you can easily mar
the surface. I therefore opted to use Cellfoam 88 for the
airframe material. Cellfoam 88 is traditional Styrofoam, but
the faces of it are pressed between two hot rollers. This
gives the face a hard and smooth surface. This hardness
and smoothness of Cellfoam 88 is somewhere between a
soft Styrofoam and a solid sheet of polystyrene.
I discovered this allows you to glue a solid sheet of
Styrene to the face of Cellfoam 88 (if liquid polystyrene
glue is used). If, on the other hand, you try gluing Styrene
to traditional foam, the glue will melt the foam.
A weather balloon will carry many BalloonSats into
near space by attaching them together with one or more
cords. BalloonSats must have some type of tube through
the airframe. The tube forms a channel for the cords that
protects the Styrofoam airframe from abrasion. In my
traditional (that is, until now) airframe, students cut
channels into the walls of the airframe and glued the
tubes into them. That way, the tube does not reduce the
useable volume of the airframe.
So, Cellfoam 88 and plastic tubes are the core of the
airframe design in this article. To replicate the CubeSat
concept, I stacked layers of uniformly cut Cellfoam 88
sheets on four plastic tubes. These form decks that slide
up and down the four tubes. The holes for the tubes are
drilled near the corners of the decks where they create
the least impact to useable volume. Since the BalloonSat
is a small cube, the number of decks in very limited.
I settled for three in my first test: a top, bottom, and
middle deck. This created two bays: an upper and a lower.
I placed the camera in the lower bay and the electronics
and battery in the upper bay.
CONSTRUCTING THE AIRFRAME
I began cutting out four sheets of Cellfoam 88. I used
the 10 mm thick sheets and cut them into squares
measuring five inches on a side. The design only needed
three sheets, but I cut an additional sheet as a backup. I
then stacked the sheets and drilled four holes near their
corners. The holes were 3/16 inches in diameter — just
large enough for the plastic tubes. By drilling the sheets
while they were stacked together, the tubes passed
through the same positions on all four decks.
Bolting stuff to Cellfoam requires care because a tight
nut and bolt will crush the foam. Therefore, I glued a thin
sheet of polystyrene plastic to the underside of the
Styrofoam. I mounted the electronics to the top of the
Styrofoam with a foamed neoprene sheet beneath it.
Neoprene foam is available in colorful sheets at many
craft stores where it is sold under names like Fun Foam,
Funky Foam, and Foamies. Finally, I used a nylock (nylon
insert nut) in place of a hex nut. Using a plastic sheet,
slightly compressed neoprene foam, and nylocks seems to
be a very secure way to attach the electronics to
■ FIGURE 2. In the initial test-fitting, I bolted the flight
computer to the middle deck and mounted the middle deck
on the four corner tubes. At this point of the construction,
the middle deck was glued to the tubes. The middle deck
would divide the BalloonSat into two equally tall bays.
As you can see in Figure 2, there are two notches cut
into the middle deck. I initially cut them so I could route
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■ FIGURE 3. Next up, test-fitting the bottom deck of the
airframe. This is where the digital camera will reside. I cut a
hole in the Styrofoam for the camera lens and darkened it
with a permanent black marker.