Poorman's Space Program

Designing and Constructing BalloonSat Airframes (Part 2)

Paul Verhage

Aside the tools listed in Part 1, you will need the following materials to assemble an airframe.

Styrofoam sheet, 1/2 inch thick

Color coded or colored poly shipping tape, 2.2 mil thick *

Wooden dowel, 3/16 inches in diameter

Polystyrene tubing, 1/4 inch in diameter **

Rubber bands

Black enamel modeling paint or black felt tip marker

Optional Material:

Foamcore

Aluminized Mylar (space blanket)

Scrim (wedding veil material)

Kapton tape (1/4 or 1/2 inch wide) ***

Black plastic model paint or black felt tip marker

* Uline sells this tape as item S-700. It's also available at many hobby stores that sell Styrofoam gliders. Modelers like to wrap the tape around their Styrofoam gliders in place of painting them. In addition to adding color, the tape makes the glider more durable.

** Plastic tubing is available at many hobby stores.

*** Also available at Uline

Figure 1. Half- inch thick Styrofoam along with a wooden dowel and hollow plastic tubing. The rolls of tape in the center are a 2.2 mil color coded tape at the top and a roll of Kapton tape below. Both rolls of tape are available from Uline. Kapton is the material used to hold thermal insulation together on real satellites. Space blanket, on the right side, is another covering material for BalloonSats that will help to give yours a real spacecraft look.

2.0.1 Dimensions of Airframe

First layout the contents of your BalloonSat, then position each item as you want it organized inside the BalloonSat. Take your time and make sure each component will work well in its place and not interfere with other components. Sensors that need to sample the outside environment must be located next to the wall of the airframe and cannot be buried inside. Items that need to be manipulated shortly before launch must to be easily accessible from the hatch of the BalloonSat and not buried deep inside the airframe either.

Once you're happy with the placement of components, measure their outside dimensions. The BalloonSat you'll now design must have an interior volume large enough for the arrangement you just measured.

Before drawing the sides of the airframe, remember that the Styrofoam has a thickness of ½ inch. Also plan for the hatch, which must cover one entire side of the airframe. It can't close the top or bottom of the airframe. Now draw and cut out the sides. A simple cubic BalloonSat will have an arrangement of sides as illustrated in Fig. 2.

Figure 2. An opened airframe displaying a recommended placement of sides.

Don't start gluing the sides together yet; test fit them to make sure they were cut correctly. The only side that should be a little larger than necessary is the hatch, as you'll trim it to the proper size after the airframe is glued together.

2.0.2 Channels in the Sides

Before gluing can begin, you must cut channels in the Styrofoam sides for the suspension tubes and the closure dowels. Remember that the suspension tubes are vertical hollow plastic tubes and the closure dowels are horizontal (usually) wooden dowels. A BalloonSat should have four suspension tubes for stability and two closure dowels for security. However, these numbers can change depending on the near space group carrying your BalloonSat and the volume of your BalloonSat. Place the vertical plastic tubes and horizontal wooden dowels so that they won't cross each other. Figure 3 shows one possible arrangement.

Figure 3. The closure dowels reside further inside this BalloonSat airframe than the suspension tubes. Because of this, the dowels don't pass through the tubes.

Draw the outline of the channels on the Styrofoam to guide your cutting. Then either cut shallow slits into the Styrofoam no deeper then the diameter of the tube and dowel and push a square channel tube through or push and twist a sharpened metal tube along the surface of the Styrofoam. Double check the depth and diameter of the channels to make sure the tubes and dowels fit properly. However, do not glue the tubes or dowels in place yet.

2.0.3 Gluing the Airframe Together

Hot glue is the glue of choice. However, since hot glue can melt Styrofoam, watch the glue gun's temperature on a regular basis. When it gets too hot and begins melting the Styrofoam, unplug the glue gun to let it cool. You can still use the glue gun while the glue cools if you monitor the glue temperature well. Check the fit of your pieces before gluing them together. Now assemble the airframe body, but not the hatch, at this time.

2.0.3.1 Making the Hatch

The hatch needs an inner layer that fits snuggly inside the opened airframe, so cut a sheet of Styrofoam to fit the opened airframe. Check the fit once again. Now glue this piece of Styrofoam on the center of the hatch. After the glue cools, close the BalloonSat airframe with the hatch and mark the edges of the airframe on the hatch. After removing the hatch you can trim it along the lines. In place of ½” thick Styrofoam, you can use thinner foamcore for the hatch's inner layer.

Figure 4. The BalloonSat hatch.

2.0.4 Covering the BalloonSat

The easiest way to add color to the exterior of a BalloonSat is with colored tape. To minimize the BalloonSat's final weight, use a thin, colored poly tape like color-coding or Styrofoam glider tape. Don't use colored duct tape as it adds needless weight. The same applies to aluminum duct tape. It looks nice, but adds unneeded weight.

The tape strips should be long enough to go around three of the sides of the airframe plus a bit longer so they end and begin inside the airframe. Apply each strip of tape with a little overlap between strips. Rub down the tape to ensure it has a good contact with the Styrofoam.

2.0.5 Suspension Tubes and Closure Dowels

Measure the length of the airframe where the plastic suspension tubes will fit. Cut the tubes to that length and sand their cut edges slightly to remove burrs. Use an Exacto knife to cut out holes in the tape where it covers the openings to the channels. A little hot glue on the plastic tube is enough to hold it after sliding it into the airframe. Don't use very much glue though, it will ooze out of the channel as you slide the tube in and make a mess that needs to be cleaned up.

The closure dowels need to be cut one inch longer than the dimension of the airframe where they will be inserted. That way the dowels will protrude ½ inch from the airframe, long enough for the rubber bands. After cutting the dowels, sand their ends slightly to round them. Use either white glue or hot glue to hold the dowels in place. After the dowel is glued you may want to add additional hot glue around the dowel where protrudes from the airframe. A small bead of glue will help protect the exposed edges of the tape around the dowel.

2.1 Variations in Design

Here are four modifications you might find useful for your BalloonSat.

2.1.1 Internal Shelves and Dividers

An experiment that needs support or separation from another experiment will benefit if there are shelves inside the BalloonSat airframe. Glue the shelves and dividers directly to the interior walls of the airframe after the airframe has been glued together (Fig. 5). This is another good use for foamcore.

Figure 5. An example of a shelf and divider inside a BalloonSat.

2.1.2 Multilayer Insulation

Multilayer insulation (MLI) is the material used to insulate real spacecraft. A jacket of MLI acts like a lightweight and unbreakable Dewar or thermos bottle. Space certified MLI is very expensive, on the order of one dollar per square inch. However, you can make an inexpensive substitute. While MLI works best in a hard vacuum found in space and won't be as effective in near space, it's still fun to make.

You'll need the following materials to make MLI:

Sewing machine

One package of space blanket

One yard of plastic wedding veil material

Kapton tape, 1/4 inch wide *

*Kapton tape is available from Uline (www.uline.com) as product S-10518

MLI consists of alternating layers of aluminized mylar and scrim. The aluminum coating on the Mylar reduces cooling by radiative cooling by reflecting thermal radiation back into the BalloonSat. Scrim minimizes cooling by thermal conduction by creating a physical separation between the layers. The near vacuum between the layers reduces the loss of heat by convection.

Make your MLI jacket by sewing alternating layers of Mylar and scrim together. The layers should begin with a Mylar layer on bottom and end with a Mylar layer on top. Cut each layer the same shape that will fold properly to cover the airframe. However, the top layer of Mylar needs to be cut a little larger so its edges can fold over the bottom layer of Mylar. Cutting MLI layers might remind you of making paper polyhedrons in middle school. After cutting the layers out, stack the alternating layers of space blanket and wedding veil material and then fold the edges of the top space blanket layer over the bottom layer. Hold the stack together and run it through a sewing machine. Only sew around the edges, there's no need to sew through the middle of the stack. When they're sewn together, you'll have a durable thermal blanket that won't come apart.

Sew a second MLI jacket for the hatch, but in this case, the MLI is shaped to wrap around the face and sides of the hatch with a little bit protruding inside the hatch.

Now you can wrap the MLI around the airframe and use Kapton tape to hold it together. When complete, the BalloonSat airframe will look more like a professionally built satellite.

2.1.3 Ports

Some experiments need to sense or see the world outside the airframe. Examples include temperature and relative humidity sensors and cameras. Ports are the openings in an airframe for these types of experiments. If they're needed, they must be cut before the airframe is covered. Begin making a port by marking its placement on the airframe with a pencil. Use a T-square if you want the openings to be square. Cut the port opening with a sharp Exacto knife and sand the edges if they're too rough. For camera ports, paint the exposed edge of the port with black plastic model paint or a black felt tip marker to reduce the glare.

2.1.4 External Sample Holders

Some experiments need complete exposure to the near space environment. For these experiments, a plastic coin tube makes an excellent holder. The tubes have screw-on caps, so they're easy to open and close. And since they're transparent, their contents can easily be observed. One way to use a plastic coin tube is to glue a Styrofoam shelf to the outside of the BalloonSat that the coin tube can sit on. By inserting additional dowels in the airframe, rubber bands can secure the tube to its shelf.

Alternatively, a hole large enough for the plastic coin tube can be drilled into the airframe. Then the tube can be inserted into the BalloonSat with al least its cap protruding outside the airframe. Be sure the hole is just large enough for the tube as it must be snug so the tube won't fall out on its own.

2.2 Completing the Airframe

Just in case your BalloonSat becomes separated from the rest of the near space shuttle during its mission, put a label on the airframe with your name and phone number. The suspension lines should never break, but accidents can happen. With a name and phone number on your BalloonSat, there's at least a chance it will be returned. The label can be made from label maker or from a PC printer and then covered with transparent tape.

Now close hatch and wrap rubber bands over closure dowels and hatch. You have a completed BalloonSat airframe ready for loading your experiments.

Figure 6. A completed BalloonSat airframe.

STYROFOAM is a Trademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow.

Previous Installments of the Poorman's Space Program Series

Paul Verhage will continue his BalloonSat series in future installments of The Citizen Scientist.

Previous articles in The Citizen Scientist by Paul Verhage on the Poorman's Space Program (including BalloonSats and Near Space) include:

Poorman's Space Program (07 September 2007).

Federal Regulations Regarding Near Space Flights (05 October 2007).

What and Where is Near Space? (02 November 2007).

What you can Expect at your BalloonSat Launch (07 December 2007).

The Thermal Test Chamber (TTC) for Near Space Instruments (01 February 2008).

Designing and Constructing BalloonSat Airframes (Part 1) (04 April 2008).