Black solar balloon made from black plastic panels

Building a solar balloon is a fun and educational activity for anyone interested in science and engineering. It is a type of hot air balloon that uses the sun’s energy to heat the air inside the balloon, lower the air’s density, and causing it to rise. Solar balloons are typically made from black plastic, which absorbs the sun’s heat more efficiently than other colors. Black trash bags thus are a perfect material for a solar balloon.

What is a solar balloon?

A solar balloon is a type of hot air balloon that operates on the principle of thermal air currents. It is typically made from lightweight, heat-absorbing materials such as black plastic or Mylar. Compared to lighter colors, black absorbs more wavelengths (and red absorbs red wavelengths, blue absorbs blue wavelengths, etc.) and thus more energy (solar radiation).

A solar balloon is designed to rise when the air inside it is heated up by the sun and gains buoyancy, becoming less dense than the outside air.

How does a solar balloon work?

Solar balloons are flown during the day when the sun is at its strongest, and they work best in areas with clear skies and minimal wind. The balloon is usually filled with ambient air, which is heated by the sun. A solar balloon can rise to a height of several hundred feet depending on the size of the balloon (and thus, could present a hazard to low-flying airplanes or planes transitioning near airports).

The design of a solar balloon is simple and consists of a large envelope that is open at the bottom to allow air to enter. The bottom of the envelope is usually sealed with a lightweight material, such as duct tape or plastic wrap, that can be easily removed when it’s time to launch the balloon.

What are solar balloons used for?

In addition to being a fun and educational activity, solar balloons have a number of practical applications. They are commonly used for aerial photography, as a low-cost alternative to traditional hot air balloons, and to measure atmospheric conditions such as temperature and humidity at different altitudes. Additionally, solar balloons can be used for advertising and promotional purposes.

Materials needed to construct a solar balloon

To build a solar balloon, you will need the following materials:

Large sheets of black plastic (at least 6 feet wide and 20 feet long) or black plastic trash bags
Clear packing tape
Scissors
String
Black Sharpie marker
Hair dryer
Propane torch
Heat sealer

Step by step guide to building a solar balloon

Follow these step-by-step instructions to build your own solar balloon:

Use scissors to cut two large sheets of black plastic that are preferably 6 feet wide and 20 feet long. If using black trash bags, use the largest size bag you can find and cut the bag at only one of the seams so that the plastic remains double layered.
Lay the two sheets of plastic on top of each other and use clear packing tape to tape them together along the long edges. This will create a large, flat sheet of plastic that is twice as thick.
Cut a 2-foot diameter circle out of the plastic at the center of one end of the balloon. This will be the opening where hot air will enter the balloon.
Cut a long piece of string and tie it to the center of the circle you just cut out. This string will be used to hold the balloon.
Use a black Sharpie marker to draw horizontal lines across the balloon, spaced about 6 inches apart. These lines will help to create pockets of hot air inside the balloon.
Use a hair dryer to heat the plastic along the lines you just drew. The heat will cause the plastic to shrink and create vertical pockets of air. This is known as “thermoforming.”
Use a propane torch to carefully seal the edges of the balloon, making sure that there are no gaps. This will prevent hot air from escaping the balloon.
Inflate the balloon using a hair dryer until it is about 75% full. You can use a thermometer to measure the temperature inside the balloon. The hotter the air, the more lift the balloon will have.
Take the balloon outside on a sunny day and hold it by the string. The sun will heat the air inside the balloon and cause it to rise. You can control the altitude of the balloon by adjusting the amount of hot air inside.

Alternate heliotrope solar hot air balloon design

Self-made solar balloon of 4 meters height floating over a meadow

In 2020, Sandia National Labs released directions for a heliotrope solar balloon. Below are the directions they released to the public.

This describes how to build a 5-gore, 5.8 m diameter heliotrope solar hot air balloon. This is a fairly straightforward process. When making the balloons, make sure not to wear anything that can snag the material (badges, etc.). Sharp objects or corners should not be present.

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When laying out, folding, and cutting gores, it is best to wear socks instead of shoes. Tape should never be pulled off of a balloon. If it accidentally touches the balloon material, it should be left in place or cut free.

Also, when adding tape (either intentionally or not), no sticky parts should be left. Sticky parts should either be cut free or taped over. Otherwise, the sticky part will grab the balloon envelope and tear it. You are building a 20 ft sphere out of material thinner than a grocery bag — the best guidance is just to use common sense.

Space to build your solar balloon

A 40×20 ft indoor space is required to lay out and cut the gores. The floor should be smooth and clean. If the floor has things that might tear the plastic, then a tarp or other protection should be laid down first. Taping the gores together and rigging the balloon can be done in smaller spaces such as conference rooms, although having more space makes things easier. Basketball courts make the ideal construction venue.

Two people are required to fold the large sheets of plastic in the beginning of the construction process. It is possible for one person to tape the gores together, but it is easier with two. Multiple people (either individually or in pairs) can greatly speed up gore taping by working in parallel.

Materials required to build the solar balloon

12 x 400 ft roll of 0.31 mil HDPE (light duty painter’s plastic)
Clear shipping/packing tape
Scissors
Measuring tape (at least 30′)
Permanent marker
Paracord or Dacron line

How to make a heliotrope solar balloon (alternative design)

Cut five strips of plastic from the painter’s plastic roll. Each strip should be slightly longer than 30 ft.
Open the first strip until it is fully laid out. You now have a 12 x 30+ ft rectangle of plastic.
Fold it once across the long axis, then another time across the short axis (Figure 1). You now have a 6 x 15+ ft rectangle of plastic.
Repeat the previous step, then lay the rectangle of plastic on top of the first one oriented in the same way. Do this with the remaining strips of plastic. Hold them down with heavy books or gentle chip clips (binder clips can be used, but they are a little sharp).
Check to make sure each sheet is aligned. One corner should have five layers, two should have ten, and one should have twenty. This will make sense if you think about it. Also, it helps to use your arm and sweep out any air that got trapped between the layers.
Draw the gore curve. Defining Point A in Figure 1 as the origin, the width of the gore is calculated via the formula above where c is the circumference of the balloon, n is the number of gores desired, and l ranges from 0 (Point A; the center of the gore; one corner of the twice-folded sheet) to c/4. In this case c is 60, n is 5, and l ranges from 0 to 30. Recall that there may be a little excess material at the end because you cut a slightly longer sheet (better than it being too short!).
Now take a sharp pair of scissors and cut along this curve. Do this slowly and smoothly, letting the scissors glide rather than trying to hack through the layers of plastic. If you do this right, you should have five gores that look like blunted diamond shapes.
Tape the gores together. Always start from the same end of the balloon (call this the “north pole): errors accumulate as you move down, so we make the balloon’s opening at the far end (“south pole”) so we can get rid of all the plastic scraps. Leave a gap in that far end so that air can escape during the taping process. Overlap each section of tape so that there are no holes that air can escape from in flight. Also, avoid gaps of exposed sticky tape between the two gores, since those will grab other parts of the balloon after packing and cause all manner of problems. Be wary of sharp tabs of tape that can slice up the plastic as well. Usually, we tape in teams of two: one person holds the gores together and the other folds the tape over the seam. However, this keeps two people in close proximity for over an hour per balloon – not a good idea in this age of coronavirus. I have been able to tape gores by laying the two of them on a table, cutting a strip of tape, sliding the seam over top of it and rolling it over. I recommend doing this until social distancing is no longer required.
Once the balloon is fully taped, make a circular cap of plastic and tape it across the north pole. This reduces stress and makes for a more symmetric balloon.
Widen the south pole gap to a 6 ft diameter opening, this is the bottom vent of the balloon. Make a loop of paracord or Dacron line about 6 ft across. Pull the south pole balloon fabric through this loop, double it back on itself, and tape it. This loop is where we hang the payloads from. By doubling the balloon fabric across it and taping, we now make it impossible for the payload to fall off without taking at least some of the balloon with it. This slows the payload down in case the balloon experiences what Elon Musk calls an “unscheduled rapid disassembly” during flight.

Figure 1: Plastic folding and gore cutting sequence for a 60 ft circumference (5.8 m diameter) Heliotrope balloon. Point A represents the same position on the plastic sheet for each step. Step 1: Unfold each plastic strip into a 12 x 30 ft rectangle. Step 2: Fold the rectangle across its longest dimension. Step 3: Fold the rectangle across its shortest dimension. Point A is now at the lower right corner of a doubly-folded plastic sheet. Stack all the sheets on top of each other, ensuring that Point A is always in the same position for each. Then draw the gore curve on the topmost sheet and cut along the line (see Equation 1). Step 4: A gore after cutting.

Congratulations, you have built a solar balloon! With a little practice and experimentation, you can learn how to control the flight of your solar balloon and explore the science of thermodynamics. And remember, if it lifts you off the ground, let go of the string.

Image Credits

In-Article Image Credits

Black solar balloon carrying a payload via Wikimedia Commons with usage type - GNU Free. June 2, 2007
Black tube solar balloon via Catman with usage type - Public Domain. June 2, 2007
Self-made solar balloon of 4 meters height floating over a meadow via Wikimedia Commons with usage type - Creative Commons License. March 29, 2014
Black solar balloon made from black plastic panels via Catman with usage type - GNU Free. June 2, 2007