For years I'd heard that you could make a hot air balloon from a garbage bag. When I finally tried it, I found that - as with so much of life - the "devil's in the details." Here's what I learned, in the form of the embedded instructional videos below, with the help of some of my students.
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By far the biggest problem with this project is finding bags that are thin enough. Most garbage bags sold through retail are much too thick and heavy. However, some people have sent in some solutions.
From California Science Teacher Susan Rideout: "I have a tip - I used the garbage bags you recommended, and was not able to get it to launch until I cut the straws lengthwise in half (except at the join where they overlapped to connect). This removed just enough weight that with 5 candles I launched quite well. Just passing it along for future experimenters!"
I would add to Susan's suggestion that you can use 3mm or 1/8" square balsa sticks--or even thinner if you are really careful--instead of straws. Balsa might be harder to find, but it weighs significantly less than straws.
Steve Mazzanti, a Technology Instructor in Athens, Texas has used bags from the dry cleaners, which I mentioned in the videos. But the bags have a pesky hole at the top where the hanger goes through. I've been taping the the top, but Steve welds the plastic together with a clothes iron, first putting paper under and over so it doesn't stick. That sounds like an easier and better way. He even welds bags together end to end!
Richard Brown, a math/science teacher at Watertown High School in Connecticut, informed me of a widely distributed bag that worked for him. Look for Ruffies 8 gallon Color Scents (they smell like vanilla) at home improvement stores or Amazon (at least in North America--people from other continents please let us know where you've found thin bags). If you Google it you can see what retail outlets sell them near you. At 8.9 microns (or 0.35 mil) the bags are half again as thick as the thin bags I use, but Rich got them to fly (you might have to use a fifth candle).
Finally, I knew there must be a better way to attach the candles than the way described in the video. Somebody (I can't find the e-mail) suggested hot glue, which works well for me. And when I put them all together, in a line, it's much easier to light the candles.
Part 1 : Introduction
This video covers history of hot air balloons in Asia and France, modern day ballooning, and some basic terms.
Part 2 : How It Flies
This video presents some amazing facts that allow hot air balloons to fly, woven into cool videos of people-carrying balloons preparing for take off.
Part 3 : Fuel and Safety
This video shows why we think volitile fuels are a bad idea (we use candles) and I make the case that hot air balloons, education and kids (with adult supervision) all go together.
Part 4 : What You Need to Build a Hot Air Balloon
You need aluminum foil, plastic drinking straws, birthday candles. And the only tricky part is finding a trash can liner that's thin enough, but we'll show you where to look and how very thin plastic is measured.
Part 5 : Build and Fly
This video gives the construction details for keeping the botom of the balloon open and holding the candles and we make the case for flying indoors. You do not have to weigh anything, but we do on the video just to stay conscious of keeping it light.
Part 6 : Using Smaller Bags
Using smaller bags is more of a challenge than using larger ones. We'll show you why and present some tips for making them work. Plus a (green screen) ride in a hot air balloon for the student volunteer presenters.
I put a lot of work into making these instructional videos. It's encouraging to me to know that people are putting the information to work and I improve the instructions when people point out when something is not clear ( I answer specific technical questions as best I can, too). Furthermore, hundreds of people have sent me innovations that have made all the projects on ScienceToyMaker better. So, consider sending me feedback or leave a comment at the bottom of this page about how the hot air balloon project went for you.
Here is some cool feed back.