Nuts & Volts - July 2005

Near Space

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 line.

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 strings.

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).

Windows

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 will condense.

The window fogs up early in the mission and eventually that conden-

Cutting Styrofoam

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 more upright.

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 edge.

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) blades.