Public Lab Research note

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Solar Hot Air Balloons

by mathew |

Photo Credit: Matthew Coolidge

I've been flying solar hot air balloons since 2008 and thought I'd summarize some of my experiences here, since most of my work on them was before the PLOTS site. Here's my summary post to the grassroots mapping list back in October, 2010. Also attached is an annotated version of my solar balloon taping guide from 2009, with some good illustrations.

Here's a map I made with one back in January:

General thoughts on flying Solar hot air balloons are beautiful, fun, and really cheap (less than $5), but extremely wind sensitive and too big to fly legally above 300 feet (95 meters). They work great in deserts, tundra, and any place with lots of sun and without trees, buildings, wires, or many people.

Flying them on a line does not give one much control, and they will blow over to near horizontal in any amount of wind. a 12' (4 meter) balloon gets about 1kg spare lift. That's a lot of surface area with very little lift to keep it in place. I find 9-11AM, when the wind is usually calm but the sun pretty intense, to be the best time to fly. in order to get imagery, I look at the wind direction and pretty much let the balloon free float down wind over whatever I want to take pictures of. I then pull the balloon down, walk it up wind, and repeat.

In my attached guide, I discuss a quick deflation strategy that can be useful for free flying hot air balloons in more crowded areas. I've had mixed success using this strategy with balloons big enough to hold cameras.

Inspriration From Other People's Balloons Wikipedia has a good history of Solar balloons from the '70's. Laurence and Christophe Praturlon in France are the modern masters of DIY plastic solar balloons. Their site has great instructions. Christophe sells solar balloons for photography, albeit for a good chunk of change (he once quoted me over $800). His balloons are cylindrical. Here's a huge one for lifting a person:

Solar Jump Tignes 3

The Praturlons use really thin .3-.4 mil black HDPE. I can't find that material regularly (some cheap garbage bags are made of that) and so I use .3 mil translucent HDPE drop cloth powder coated with pigment, following the recommendations of Robert Rochte. Robert was active on the now largely silent Yahoo Solar Balloons Group. The transparent film is definitely stronger and more durable, so I really prefer it.

Tetrahedron Balloons (tetroons) flying a solar hot air balloon in Oakland, 2009

I only make tetrahedron balloons (triangular pyramid), because they're really quick to tape up. I can make a 12 foot one by heat seaming in about 30-45 minutes or with tape in about an hour and a half. I generally make tetrahedron by folding from a rectangle with a ratio of sides 1 to the square root of 3, which is the ratio to make a platonic tetrahedron. I have calculated the volume of my balloon using Steve Griffin's guidelines. He also has a great hot air balloon lift calculator.

Steve says: The volume of a tetrahedron very closely approximates the volume of a cube with a side half the length of tetrahedron's side. eg. If your tetrahedron has a side of 6 metres then the volume will be (6/2)3 = 33 = 27m3

Raven Industries recommends (article attached) making a tetrahedron out of a rectangle with a ratio of sides 1:2.31. For this shape, they say the volume of an inflated tetrahedron balloon, is 0.26*h3 where h is the length of the longer side of the rectangle the balloon is to made out of. I don't really understand why a non-symmetrical shape is more valuable, but who knows, they're pros...

Materials Acquisition

As mentioned earlier, I use thin (.3 mil) HDPE "painters plastic," a 12 foot wide, 400 foot long roll costs about $25. I coat the inside with pigment, specifically Bayferrox iron oxide black 330. Bayferrox's distributor LanXESS is a very nice company that has mailed me several 1lb free samples over the years; I don't think they sell a bag less than 50lbs. This pigment was discovered in a group of samples Molly Danielsson brought me from the concrete factory where she works, and I'm very satisfied with it. James Lent and I have tried a range of pigments and this is the only really dark one..

Call LanXESS' Inorganic Pigments department, and be honest about what you're up to!

LanXESS Burgettstown 8 Morgan Road Burgettstown PA, 15021 412-809-4730

Construction tetrahedron folding

I roll out about 20'10" of plastic from a 12' roll to get the right ratio of sides, then I fold it in half length-wise, seal the two short sides, and then seal the long side at perpendicular to the length-wise fold, like this:

tetrahedron balloon sealing

Heat seaming makes a lighter, stronger balloon, but tape is accessible and works very well:

Heat Seaming: this can be done just as well with a regular clothes iron -- no steam. I find it easiest to run the iron on its edge. A flat surface underneath is a must, and a long metal ruler works best. The hot iron melts the plastic towards the cool metal ruler, where it forms a nice bead.

Taping: I've annotated my old guide (2009) to folding and taping to correct info (attached below). If you want to tape a balloon, follow these instructions and save yourself the headache of figuring out how to line up the seams.

Pigmenting I clip open a corner of the balloon wide enough to inflate with a computer fan, and pour about half a cup of pigment into the balloon. Then, without inflating it and with the cut corner held closed, I work the pigment down and around, rubbing it all over the inside. I then pour out the excess. No matter what, you'll get a little dirty.

Inflating I make an inflation device that consists of a computer fan attached to two nine volt batteries in series. The fan I wrap with some cardboard to make a tube, and stick the tube into my balloon. It inflates in about 10 minutes, and heats up rapidly in the sun.

Sealing Just use the same system of a ring and zip-ties Aurora Tang taught us to use to close helium balloons. it works great and is totally easy. Aurora Tang's balloon closure

Files Size Uploaded
thekissballoon2_2012_annotated.pdf 3.16 MB 2012-05-30 01:16:16 +0000
RavenTetroonVolumeReport.pdf 1.2 MB 2012-05-30 02:22:00 +0000

balloon-mapping solar-balloon hot-air-balloon diy-balloon


thanks for this. I wonder whether this would work in Louisiana marshes in the summer--lots of sun. May have to make multiple tether points for the wind, though.

This is so great. Mathew - how many flights is 1lb of pigment good for? I have the 1lb jar you gave me, will probably try it this summer, but am happy to ship anyone some amount of it if they like.

If this starts to really become popular, we could buy a 50lb bag and resell, i suppose. Do you happen to know the cost of the 50lb bag?

BTW I love that their product image on the pigment website is of a pigment molecule. Talk about open source... lol

The pigment doesn't really wear off and I don't have to re-coat the balloon between flights.

Next time I make one I'll have to weigh the balloon before and after coating, but my impression is that each balloon takes much less than an ounce of pigment. Volumetrically it only takes about a tablespoon. That said, its hard to coat the balloon unless its filled with a good quantity of pigment to work around; just throwing a tablespoon in doesn't work very well.

When Molly brought me the first pigment sample, it was less than a 4 volumetric ounces,, and that served to make 3 twelve foot balloons and 5 seven foot balloons, with some left over and about half of it recklessly spilled.

I'd bet that a pound could be divided into 20 or more packets each good for a few balloons.

The phone number for BayFerrox is updated to 800-526-9377.

Hi, This is really interesting. Do you know if you can fly solar ballons over New York City, or Staten Island? If so, are there height restrictions in this area (especially Staten Island)? Many thanks

the flight restrictions are going to be the same everywhere-- stay away from the airport and under 300 feet. These balloons ARE large enough to be regulated. be careful, and keep the local FAA number handy if it gets loose.

Great set of instructions but ... the heat seaming video is sort of done from the wrong angle so you can't see what's happening. Any chance of posting one showing precisely where you put the iron? Do you put it on the metal rule and slide the rule down, or on the plastic directly? How long do you make contact?

I've tried melting plastic and just ended up making a mess. Heating the metal rule, the heat dissipated and had no impact on the plastic.

Guidance much appreciated.


Directly onto plastic will get you a melted mess. Put down silicone parchment paper (for baking cookies) and use an iron on top of that. Use the tip or edge of the iron. I have good luck using irons sideways. If you need a protective surface underneath, try butcher paper. I'd start by turning the iron all the way up, and pressing hard into the plastic. The straight-edge/form is just a guide. Chrissy gets the valve in

Ramiro on the KAP forums has posted a whole system of spreadsheets and software for making spherical gores:

also, check out the Jake III from bovineaerospace.

As a pigment, can we use charcoal as mentioned in the annotated kiss balloon pdf? As charcoal is easily available anywhere so i was hoping if i could use it? Has anyone tested with charcoal, do you recommend using charcoal?

Also what's the current progress in this solar heated balloons in public labs. I just came to know that it topic is 5 years ago.

Please share any progress, if any

Hi, @suman - this is a great question and I've had some not great experience using coarse charcoal briquettes (the balloon did not fly very strongly). But maybe vine charcoal or art charcoal would be better.

If you'd like to post this question to the whole community, try

Hi Suman, I haven't really worked on solar hot air balloons for a while. Hot air balloons, and solar hot air balloons especially, are not great for flying on strings. They don't have enough lift to resist even the slightest wind. it is a good strategy for free flight balloons.

In terms of balloon fabrication I haven't seen any new developments, but the Aerocene project is making some great strides on flight prediction software:

Pigment: A solar balloon doesn't need pigment to fly, but the pigment significantly increases solar absorption. Charcoal will work to get a pale grey color. it needs to be ground very finely. We did a series of tests back in 2010 of different pigments. The type of charcoal doesn't seem to matter much:!topic/grassrootsmapping/qdzH9mnfUo0

One thing I haven't seen tried-- static charging either the balloon or the charcoal/carbon black particles. This has been suggested a few times. I'm not sure how to do that.

good luck! I hope to see you flying.

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