Poorman's Space Program

Designing and Constructing BalloonSat Airframes (Part 1)

Paul Verhage

Near space experiments need a container to hold and organize them. This article is about that container, the BalloonSat airframe. After a short description of BalloonSats past and present, this article discusses the materials, tools, and methods used to machine and assemble a BalloonSat airframe. In part 2 there's a suggested step by step assembly procedure. However, since the size and shape of your BalloonSat depends on the experiments you'll put inside of it, you should use this article as guidance and not as absolute rules.

1.0.1 The Recommended BalloonSat Configuration

The BalloonSat recommended here is a Styrofoam box with a hatch in front and with two rubber bands stretched over it to keep it closed during its mission. The ends of the rubber bands hook to the BalloonSat through closure dowels on the side of the airframe. During its mission, your BalloonSat and others hang below the near space shuttle on four cords called the suspension lines. The suspension lines pass through the walls of the BalloonSats, allowing each to hang in series, one after the other. Protecting the Styrofoam walls of the BalloonSat from abrasion by the suspension lines are suspension tubes, plastic tubes embedded inside the Styrofoam. Each BalloonSat is free to slide up and down the suspension lines, within limits. Split rings on the suspension lines prevent the BalloonSats from sliding too much on the suspension lines.

The near space shuttle contains the GPS receiver and radio equipment needed to track and recover the flight. Therefore, you won't add tracking electronics to your BalloonSat. Since the near space group providing the launch already provides the tracking and recovery services, your BalloonSat only has to operate its experiments and record its data for download and analysis after its recovery.

Figure 1. A chain of five BalloonSats suspended below a dual-redundant near space shuttle. The redundant modules in the shuttle ensure the mission's success even if there's an electronics failure during the 2-1/2 hour mission. Above the two modules of this particular near space shuttle is its orange recovery parachute and balloon (the near space booster).

Figure 2. A sight everyone wants to see, the near space shuttle with its payload of BalloonSats safely on the ground. The next stop for these BalloonSats is a return to their owner for data analysis.

1.1 Materials

The traditional BalloonSat is a cube constructed from foamcore and aluminum duct tape. Foamcore is 3/16-inch thick Styrofoam sheet with paper facing. It is a popular backing for printed artwork like posters. It's available in 20” by 30” sheets at many art and big box retailers. Aluminum duct tape is a thin metal foil with an adhesive coating. Most home improvement stores carry it in rolls two inches wide and 50 feet long.

Figure 3. An example of a traditional BalloonSat airframe.

Instead of foamcore and aluminum tape, the BalloonSat Principia recommends constructing BalloonSats with ½-inch thick Styrofoam and colored poly shipping tape. Styrofoam is light in weight, easy to cut and shape, and quick to glue together. It's available in four by eight feet sheets at many home improvement stores that sell it for home insulation. Be c areful; this is not the beaded white Styrofoam sheets, which are also available at home improvement stores. White Styrofoam is too crumbly and fragile to make a good BalloonSat airframe. The proper Styrofoam is light blue (not white), solid, and with a fine grain. Home improvement stores sell several thicknesses of this material, but it's the ½ inch thick material you want to use. There's no need to use the thicker stuff as it adds unnecessary weight to your BalloonSat (but it does increase its insulation). The colored poly shipping tape is the same tape hobbyists use to cover the wings and fuselage of Styrofoam gliders. The company Uline (see the later reference) sells this tape, as does many hobby stores that carry Styrofoam gliders.

Figure 4. The traditional materials of foamcore and aluminum duct tape.

1.1.1 Comparing Styrofoam to Foamcore

According to my Pelouze digital scale, a 25 square inch sheet of Styrofoam weighs 8 grams while the same area of foamcore weighs 10 grams. Therefore, a ½-inch Styrofoam BalloonSat airframe weighs 20% less than an equivalent BalloonSat airframe constructed from foamcore. Reduced weight is not the only benefit of using Styrofoam. Placing a traditional BalloonSat inside a Thermal Test Chamber along side a Styrofoam replica illustrates a second benefit of using Styrofoam; a Styrofoam BalloonSat is warmer inside than a foamcore BalloonSat.

Figure 5. A close up of Foamcore and Styrofoam. Foamcore is heavier than 1/2-inch thick Styrofoam per unit area and doesn't insulate as well. Because of its paper covering, it doesn't stand up to moisture as well as Styrofoam. Styrofoam is expanded polystyrene plastic. It's lightweight and insulates well because of the trapped air in its cell structure.

According to the experiments of Galileo, the thicker a material, the greater strength it has. Therefore, it's reasonable to assume that ½-inch thick Styrofoam is stronger (resists bending) than 3/16-inch thick foamcore. However, foamcore is a Styrofoam sheet with a bonded paper surface. That makes it a composite, and composites are generally stronger than their constituent materials.

Figure 6. The interior of a Styrofoam BalloonSat takes nearly 20 minutes longer to cool to the same internal temperature as a foamcor e BalloonSat. Its warmer internal temperature will protect its internal batteries and electronics from the extreme cold of near space for a longer period of time.

1.2 Styrofoam Tools

Most of the tools required to convert a sheet of Styrofoam into a fleet of BalloonSats are common to other hobbies. Therefore, you probably already have most of the tools you need to measure, cut, shape, and assemble Styrofoam.

1.2.1 Tool List

Metal Straight Edge

Exacto Knife

Hollow tubing *

Hollow channel *

Emory Board (cardboard nail file)

Metal Hobby Files

T-square

Pencil

Hot Glue Gun

* Made from aluminum, brass or plastic

1.3 Machining Foam

This section briefly describes the cutting, drilling, channeling, and sanding operations you need to know to make a BalloonSat airframe. In some ways, a BalloonSat is a near space version of a CubeSat. Since Styrofoam is easier to machine than aluminum, a BalloonSat is easier and faster to make than a CubeSat.

Figure 7. Airframe machining tools, except for the pencil and hot glue gun.

1.3.1 Cutting

One difficulty is cutting a clean edge in Styrofoam. Table saws cut clean edges in Styrofoam, as does a hot wire (which cuts through Styrofoam like a knife through, well, you know...). A table saw naturally cuts straight lines, while a hot wire requires a very steady hand or a jig to cut a straight line. If don't have access to either of these tools, and can't justify purchasing them, then the only option is to cut Styrofoam with an Exacto knife.

Styrofoam cuts well with a sharp Exacto knife and metal straight edge. In addition to the knife and straight edge, you'll need a T-Square to draw right angle corners and a protractor to mark out non-square edges (no one said your BalloonSat had to be a cube).

Use a metal straight edge and T-square to lay out the cut to make in your Styrofoam sheet (don't use wooden straight edges as the Exacto knife will n ick and create a crooked line). Press lightly with your pencil and don't gouge the Styrofoam when you draw a line. Load a new #11 blade in your Exacto knife handle. Place the metal straight edge along the line and hold the Exacto knife as close to perpendicular as possible. If the Exacto knife is not perpendicular, you'll cut an edge that looks like the one shown in Fig. 8.

Figure 8. Cutting Styrofoam.

Make several cuts through the Styrofoam; don't cut through in a single pass. Begin your cut with the Exacto knife held vertical but leaning back. When the Exacto blade leans back it slices through the Styrofoam and is less likely to chip it.

As the blade of the Exacto knife begins to dull, you'll notice that it cuts better in one direction than in the perpendicular direction. Apparently, a “grain” is created in Styrofoam when it's extruded. As the Exacto blade dulls, it begins to chip or break out chunks of Styrofoam instead of making smooth cuts. Be prepared to replaced the blade when the Exacto begins to make bad cuts.

Figure 9. The edge of Styrofoam cut with a dull blade (I used a very dull blade to emphasize the damage done).

Figure 10. The cut edge of this Styrofoam sheet is much smoother because it was cut with a newer, sharper blade.

Since you can go through a lot of blades making BalloonSats, purchase your Exacto blades in the black plastic box of 25 blades. The cost per blade is lower, the box protects the unused blades and it provides a safe place to dispose of the used, but still sharp, blades.

1.3.1.1 Cutting Channels

An easy way to cut channels into Styrofoam is to cut two parallel lines and then run a sharpened brass channel between the lines. Back the channel out occasionally to clear it of Styrofoam shavings. A sharpened square plastic tube can also cut channels as described in the next section.

Figure 11. Using a sharpened channel, plastic in this case, to clear out the space between two parallel cuts.

1.3.2 Drilling Holes

A drill bit doesn't do a very good job cutting holes into Styrofoam; it tends to tear it up. So use a sharpened brass tube. Sharpen the inside edge of the brass tube with a small metal hobby file then twist the brass tube as you push it through the Styrofoam. Occasionally pull the tube out of the hole to clean out the Styrofoam shavings.

Figure 12. This person is twisting the brass tube as he pushes it through the Styrofoam. He's also being careful to keep the tube at a constant height relative to the Styrofoam so the hole is straight.

1.3.3 Sanding

Cardboard fingernail files and hobbyist metal files shape and smooth edges in Styrofoam. A piece of sand paper can also shape the edges and faces of Styrofoam. There are two ways to maintain a flat surface on Styrofoam while sanding it. The first is to lay the sand paper face-up on a flat surface and run the Styrofoam back and forth on the sand paper. The second is to sand the Styrofoam with a stationary belt sander. However, be aware that sanding a glued piece of Styrofoam with a belt sander will eventually gum up the sanding belt.

1.3.4 Gluing

Hot glue bonds the pieces of an airframe together very quickly. It's also easier and less messy than either the JB Weld or silicone glue recommended in sources, like the Space Grant BalloonSat book.

The Dow Chemical Company wants you to know that STYROFOAM is a Trademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow, and that the color Blue is also a Trademark of The Dow Chemical Company. Editor.

Paul Verhage will continue his detailed instructions on BalloonSat airframes in the May installment 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).