# Solar Balloon

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Let's make a solar balloon that floats with the power of hot air! This is the perfect backyard activity for a sunny day. A few common trash bags taped together can become a science experiment and entertaining way to spend an afternoon.

The black color of the bags absorbs the sun's energy, heating up the air inside. The color of an object correlates with the wavelengths of light it absorbs and reflects. Black absorbs the most wavelengths, while white absorbs the fewest. Red absorbs all but the red wavelengths, and so forth for other colors. Black is the most efficient color for converting light energy into heat energy, which is then transferred to the air inside the closed balloon. The hot air is less dense than the surrounding air, causing the balloon to float up, supporting its own weight.

This same physics powers hot air that balloons you can ride in. It's the principle of buoyancy— once the air inside heats up and expands, our balloon weighs less than the air it displaces because it is less dense. This project demonstrates how solar radiation can be used to create heat, and a fun day outside.

This project is part of my free Solar Class, where you can learn more ways to harness the sun's energy through engraving and solar panels.

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## Step 1: What You'll Need

All you need are some thin black trash bags, cellophane packing tape, scissors, and a piece of string. Look for trash bags that are .3 mil to 1 mil in thickness— the thinner, the better. Likewise, thinner tape is best to reduce the overall mass of the balloon.

It's best to pick a sunny day with calm wind conditions for this project. It can be particularly frustrating to try to build the balloon when it's windy, so consider assembly indoors if it's breezy.

## Step 2: Trim Bags

Leave one trash bag with the bottom seam intact, then cut off both the seam and the flaps (if any) on two more bags. On the last bag I cut off the seam, but not the flaps, since they'll be at the opening of the balloon (however flaps are not required).

## Step 3: Tape Together

Open up and overlap the trash bags together by about an inch, then tape around each seam.

Make sure you don't have any gaps or holes.

## Step 4: Inflate Your Balloon

Run around to fill your new balloon with air.

Gather up and tie off the open end of the balloon, topping it off with air from your lungs (if you're up to it) or a hair dryer (optional).

Tie a knot in the gathered end to close up the balloon.

## Step 5: Tether & Enjoy!

Tether your balloon with some string and put it out in the sun, where it should heat up and start to float. The heat generated inside the balloon decreases the air density, which creates lift. This is the same reason objects float in water, and is described by Archimedes' principle. Since the balloon heats up slowly, the increasing upthrust from the slowly changing pressure inside can be observed in the time it takes to explain the science behind it.

It's very important not to let the balloon go. Not only is it bad for the environment, but it could be deadly if it gets into an airplane's engine.

When you're ready to deflate it, be careful because the surface will be hot. Cut a hole and start to squeeze out the air.

This project is part of my free Solar Class, where you can find another backyard project and several lessons on working with solar panels. Check it out and enroll so you can post photos of your builds!

Thanks for reading my Instructable. Check out my profile to see my other projects— here are some others you might like:

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## 19 Discussions

Hey Ms Stern!

Have you tried using an iron to "weld" the bags together? Sandwich the seam in parchment or wax paper to protect the iron to keep from melting the tube shut and turn the iron on just hot enough to melt the plastic. This would save a few extra grams of weight (maybe not worth the trouble?).

I'm glad you're content creator at Instructables! I lead an electronics and beginning robotics group at Mooresville Middle School (North Carolina) and we talked about you as an inspiring example of women in electronics.

You can use one single bag only if it's *very* big and thin, (say, some micron) Otherwise, the air inside it won't have the lift capability needed to get aloft te plastic bag, the sealing tape and the retention line. Not to speak of a possible load (balloon, candies, parachute bear, etc.

Iin the late 70s, I was just a boy, then, in Italy, a toy was sold called "UFO Solar":

http://imagorecensio.blogspot.fr/2015/01/ufo-solar... (only in Italian, sorry, hope online translators will help :-) )

It was just like that, a 6-micron thin plastic foil, opened at the ends. It had a 20-meter nylon line and a small plastic ring to attach it.

The vendor declared it could reach an altitude of 10000 m (32800 ft) and more; selling it was prohibited by Italian Laws in 1984, due to the danger it could represent for flying planes (I think it's still available on the internet from foreign vendors, however).

I still remember the (in)glorious end of my balloon: I was at my grand parents' country home; one day plastic ring broke and the balloon (which was only 2-3 meters above the ground) started gaining altitude.

I immediately grab my slingshot (yes I was a VERY BAD boy :-) ) and started shooting stones at it: the first 2-3 shots were fast, direct hits and went through the balloon's body without making great damage.

Eventually, I got an irregularly shaped stone and managed placing it in a quite angulated way. Poof. A long rip appeared and the balloon started losing altitude... very slowly, but it was going down.

It was out of reach, now, since a light wind was blowing... so I started running after it, through the fields. It touched down about 1 Kilometer away...

I never used it again, but since then I've learned some important lesson:

1) Balloons and are quite hard to shoot down (I've had luck that time, I think): the easily follow the wind and can cover long distances over a quite short time.

2) If you plan flying such a thing, have your slingshot and some sharp pebble ready in your pocket... just in case! :-)

Max - Italy

PS Remember that kites and other similar toys should not be flown over a
certain altitude (in Italy it's 50 meters, but you'd better check your
local Laws)

You are right Max and it was not only in Italy. What people forget is that these "Ufo Solar" - as well as this kind of toys, when a kid for some reason lose the wire it starts elevating more and more and more ... Sounds strange but it is. As a matter of fact after a couple of years this toy has been prohibited due the polluction generated by the lof hot bags in the high atmosphere.

We used to make UFOs out of dry cleaner bags -- even thinner and light weight.

3 threads ran from the open end to a piece of aluminum foil. On the aluminum foil we would put short piece of candle. Set those loose on calm summer nights.

They would fly until the candle burned out, gaining altitude as the load got lighter.

So use a hair dryer on "hot" to fill the balloon and it should be ready to rise! Might cool down quick though... depends on how hot the day is..

Like max1234ita, I was immediately reminded of the old comic-book advertisements for the "Solar-powered UFO" which I could tell was just this sort of thing. I don't think they made black plastic bags that were light enough back then, although I thought about trying something like this. Great to see it actually works! Thanks!

fun project , while watching it with my 7 year old kid , he noticed the ring you are wearing, and I might have made it does it have a hallmark like this inside?

Warm it up over a blacktop surface. Then move it over a grass area where the air surrounding it will be cooler. That should cause a greater temperature/density difference and cause better lift.

Fun! I wonder how big you could make something like this before the weight of the bags becomes a factor. It'd be neat to try to arrange the tube into a ring.

2 replies

According to physics the larger the balloon the more efficient it is.
Surface area/volume ratio marks how well it flies.
In case of cylindrical shape of constant diameter, the longer the better. Though a very long wiener would bend and wobble.

I've been making these since about 1960-ish. I know, I'm old! But kids are always impressed even though the lifting power is severely limited. I've also made hot-air balloons from decorators poly dust sheets which tend to be even thinner than trash bags and there is only one seam along its length. A lot of fun and guaranteed to get the kids out in the open air. A great 'ible with good illustrations. Well done, Becky.