Nuts & Volts - July 2005

Near Space

Figure 7. Notice the black colored cube cooled faster than the silver one.

a BalloonSat.

Insulation Changes

Next, I loaded two BalloonSats inside my Thermal Test Chamber (TTC) for testing. The first test compared the internal temperature differences between a BalloonSat wrapped in aluminum tape (the traditional material) and one wrapped in a thin black plastic packaging tape (the kind of tape used to wrap Styrofoam gliders). The second thermal test compared the internal temperatures between BalloonSats wrapped in a traditional

aluminum tape and one wrapped in aluminized space blanket.

Initially, I was a bit surprised by the chart in Figure 7 until I recalled something my father had told me. A material painted black absorbs radiation well and reflects very little of it. However, if a black colored body can absorb radiation well, it can also emit it well. I was told this is why the radar antennas on aircraft carriers are painted black and not in white or silver.

Have you wondered why potatoes are wrapped in aluminum foil before being baked in the oven? While the aluminum foil reflects a lot

Figure 8. Chart comparing the old design to the new design.

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

of the infrared radiation emitted by the oven’s heating coils, what radiation does get in, stays in, and can’t get out. The potato cooks faster as a result.

In the second test, I hoped to see that reducing the amount of metal on the outside of the BalloonSat would reduce the rate at which it cooled. In the chart in Figure 8 you can see that the results are pretty much a wash. There’s no significant difference in the rate at which the two cubes cooled.

However, the BalloonSat wrapped in space blanket is lighter in weight than the traditional BalloonSat wrapped in aluminum tape. Anything that reduces the weight of the airframe helps meet the weight criteria and permits an increase in instrument weight.

Here’s my conclusion on airframe materials and insulation. The five inch cubic foamcore BalloonSat weighs 58 grams initially and 82 grams once it’s wrapped in a single layer of aluminum tape. The same size Styrofoam BalloonSat only weighs a total of 58 grams with a space blanket exterior and stays just as warm.

Now if your BalloonSat is not using space blanket, then I recommend using Styrofoam tape in place of the aluminum tape. I’m referring to the tape hobbyists use to wrap the surfaces of their Styrofoam gliders. It comes in multiple colors and is lightweight.

Use a dark colored tape because my past near space experiments have demonstrated that dark colors on the exterior of a near spacecraft keep it warmer. See my article, Keeping Near Spacecraft Warm, in the Fall 2004 issue of Amateur Television Quarterly. (Contact the editor, Gene Harlan, at ATVQ@www. hampubs.com to get a copy.)