Now that all the math and production procedures are all worked out, it is finally time to start building my first solar balloon. The first thing that We needed to work out was how we were going to cut out the pattern correctly. We brainstormed using a projector and tracing that line to make out the pattern. This idea was quickly disregarded because the scale needed and the relatively small work area we would be working in made us doubt that any projector has an area of vision that big. Even if it did, both me and andy agreed that there would likely distortions in the pattern as we got to the end of the image so a projector was out of the picture. The only thing we could think of that would let us duplicate a shape big enough was to use some kind of a template. This idea also had problems because of the sheer size we are talking about. In case you don’t remember from some of my previous posts on the solar balloon, our solar balloon is designed to be 10 feet across and so, if we do the math, we need a template about 5 feet wide and 15 feet long. Funny thing, you cant go down to a craft store and pick up a 15-foot long template for a sphere, that kind of thing is just not made anywhere we could find. because of this, we were forced to make our own. our first idea was to get an image from the internet and then printing it ours on a mega scale printer located in the printer of the library where we both go to school. Turns out trying to print a piece of paper this big would cost us over 100 dollars, which is obviously out of the question for a build that is supposed to be a very cheap project. Finally, we got the idea (from my Mom) to use a painters drop cloth. a quick trip to LOWES found us with a piece of plastic measuring about 10 feet wide and 20 feet long, just the right size for our template.
The next challenge was to put the pattern on our template. We couldn’t use a projector, for reasons mentioned earlier, so we decided to the math and draw the shape ourselves. Because we decided to make our balloon out of pieces of this template, all we needed to do was a little math and in no time we had ourselves a template. we folded the plastic in quarters just to expedite the cutting process and made sure to include a hole in the bottom so we could fill it up with air before take off.
With the template ready to go, we started an assembly line with Andy making giant sheets of plastic by fusing about 8 disassembled trash bags into one giant sheet which I then used the template to cut the sheets into balloon parts. In case you were wondering why there is a hole in the nose of our template, that was an accident where we accidentally melted our sheet of plastic to the template and was forced to cut that part away from the template and plastic sheet in order to separate the too. As an upside, the template now looks like a rocket ship!
After we completed six of these balloon parts, the next step was to fuse them into a sphere. To tackle this, me and Andy set up a system to make creating a 3D object on a 2D floor easier. we started joining each section by first joining the middle and then moving to the nose (where the hole is in the template) with Andy fusing while I was putting each piece in place so they would be ready for fusing. once we reached the nose, we then repeated the process moving about 1/2 to 2/3 of the way down the tail. We didn’t completely finish it for reasons that will soon become clear. with one side of the plastic balloon part fused, we repeated that process five more times until finally, we were at the final seam. Because this seam was the one that closed the balloon making into a sphere, we needed a different plan of attack for how to finish this. What we finally decided on was Andy was going to crawl into the balloon and I was going to drag the cloth over his back so he could position it for fusing. I was also in charge of making sure he had enough air to breath as neither of us thought to bring a fan for ventilation.
This image is a picture of Andy inside the balloon and hopefully, this shines some light on just how big this thing was. With this final seam, we started at the nose and worked our way back until we got to the tail and this is where leaving the remainder of the tail’s unfused was a good idea because it created a hole big enough for andy to work in. Once andy was out of the balloon, the only thing left to do was fuse the tails and give it a test.
In case you still haven’t gotten the scale of this balloon, your tallest friend could, if we made a hole for him, crawl inside this balloon and stand straight with feet of clearance between their head and the top of the balloon. Also, if you were wondering why were wearing hearing protection, my boss was amazing and let us use one of the backpack leaf blowers that we use for work and it is was so powerful that it filled the balloon in just minutes while still running on idle. There was hardly any sun that day so we didn’t really expect to get it flying but we still wanted to see it inflated and get some information about how hot it was inside the balloon even with almost no sun. when we inflated it, we found that our method of fusing had malted dozens or hundreds of holes in the ballon of various sizes that all needed to be taped together so it would be mostly airtight. also, we tested the temperature difference between the outside air and the inside air and came to about 5 degrees of temperature. This was worrying because our math had said that a balloon this size should be lifting almost 1 kilogram with only 2 degrees of temperature difference but in hindsight, the fact that the balloon was not flying yet was not entirely surprising because of 2 reasons.
- the balloon was probably slightly smaller than we intended it just due to manufacturing imperfections that were unavoidable
- The balloon was not full of air like the math required, it was probably only 80% full at the time we measured the temperature so the total lift would have been reduced because of that
Though this result surely surprised and worried us, we soon found that this was not going to be a problem in the slightest. If you remember, one of the purposes of this balloon was to decern an approximant temperature difference we will be getting in our balloons so we have a base for our future calculations. Because of this, we over-engineered this balloon so that it should work in almost any condition, and indeed we will see in the next Solar Balloon post that our earlier concerns were unfounded.
I hope that you have enjoyed this post. If you are attempting to build one of these Balloons yourself than plan for this build to take you multiple hours to complete, it took us approximately 10+ hours split over two days, so plan for either more people or similar build times when you do yours. Either way, I hope this post was helpful for you and until next time, Happy Tinkering!
The solar balloon is by far the largest project I have ever attempted and so an element of risk comes with it. I usually don’t like doing math when I do my tinkering projects, however, both me and Andy decided that if we were going to be making a full-scale solar balloon capable of carrying one or two people then we might want to take some safety precautions. One of the first precautions we wanted to take was to know the math so that we would at least know that our paper works on paper. If you find math to be very boring then you might want to move on to the next post but if you find this interesting or you are attempting this yourself then you should stick around.
The math behind a solar balloon or any hot air balloon is very similar to a boat. the reason is a lighter than air device like a balloon literally floats on top of the air like a boat floats on water. to solve for this, we can use the buoyancy equation which says that the buoyancy force on an object is equal to the weight of the air (or water) that you displace. in math-speak that is:
or the pressure of the displaced air (P) multiplied by the pull of gravity (G) and the volume of displaced air (V). For our solar balloon that is modified to:
(Po= outside pressure and Pi= inside pressure)
or the difference in the density between the air outside of the balloon and inside the balloon (Po-Pi), multiplied by gravity (G) and the volume (V). Though this will get you the buoyant force, we wanted the only variable to be the temperature of the air and to incorporate this we use the ideal gas law which is:
or the pressure of the gas (P) multiplied by the volume of gas in question (V) is equal to the amount you have (the number of moles)(N) multiplied by a constant (the ideal gas constant)(R) and by the temperature of the air (in Kelvin )(T). When you combine this with the buoyancy equation you get:
Once we had this equation, Andy, who knows programming, but this into a program so we could easily modify the terms and find a volume that works for our purposes. The code he wrote looks like this.
we then did some research and found the lowest temperature difference recorded in a solar balloon was about 2 degrees and so decided to use that as our test temperature. we will probably get more heat than that in our balloon and our next balloon is going to be a test platform to find out stuff like this but for now, we figured that this would be a good baseline. We started to test the program at different volumes and these are the numbers we came up with.
- A spherical balloon with a 5-foot radius (or 10 feet across) with 1 degree of temperature difference will lift 0.594 kg and with 2 degrees of temperature difference will lift 1.185kg
- A spherical balloon with a 10-foot radius (or 20 feet across) with 1 degree of temperature difference will lift 4.754 kg and with 2 degrees of temperature difference will lift 9.487kg
- A spherical balloon with a 25-foot radius (or 50 feet across) with 1 degree of temperature difference will lift 74 kg and with 2 degrees of temperature difference will lift 148kg
This data shows that a balloon with a 2-degree temperature difference and a 25-foot radius will lift a 220 lb person with ease. This is fun to imagine but for now, we will be making a 5-foot radius balloon and its cargo will be a video camera (mostly because we wanted a cargo and though that pictures might be kinda fun). this balloon is probably going to be much too big for just lifting a camera but that means that we can use it as a test balloon for a long time coming.
I recognize that math can be very boring but we decided that it was worth knowing so we don’t take weeks making a balloon that doesn’t work. I hope this post was insightful to you and happy tinkering!
I recently started a project with a friend to make whats called a solar balloon. The concept is to make a hot air balloon that uses no heat source other than the power of the sun. There have been various attempts over the past few years. Most of them have been small toys or science experiments but some people have able to get full-scale versions capable of carrying a person. If you are interested In learning more, look at this website of one that flew for 2 hours.
My friend, Andy, has been making these for a while and invited me to see what he was working on. We met up at a field one cold morning and spent several hours working to get his Solar Balloon flying. I was impressed how small the whole thing was. deflated, it was smaller than a breadbox but inflated it grew to be as tall as me and much too big to fit my arms around.
we inflated it by means of a hair dryer and kept control of it by means of a fishing pole and fishing line. We had various problems that made flying it much harder than we anticipated, stuff like the tape we used to fill the seams of the trash bags that make up the skin of the balloon coming loose, to the wind making the balloon virtually uncontrollable. After many hours of work, we finally succeeded in getting it flying!
if you want to do the math to find out how high the baloon went, both me and andy are about 6 feet tall.
It was the only flight we got that day and we had a lot of fun getting it to work but we both agreed that there was room for improvement. The next series of posts will be about our process as we strive to improve upon our last flight and to hopefully make one big enough to fly in one day (that’s the goal at least). Some of the things we decided to work on was
- A better method for joining the trash bags together. Ideally, we would like to use a heat seal like in vacuum packing but if that doesn’t work then possibly double-sided tape.
- We need to decide what shape would produce the best lift per gram of weight for our balloon.
- We wanted to see if there was a mathematical way to know how much our balloon will lift so that we know if it will both lift itself but also a cargo (like a person).
- We need to know how hot the balloon is getting on the inside so that we can get trash bags that can handle the heat
- We want to know if it is better to have the back of the balloon be reflective to help trap the heat or if it would be better to keep it all black
- Or is it better to have the side facing the sun be clear like a car window
When we start getting ready to go full scale
- We need to have a way to control of our orientation to the sun and our ascension/descent (up/down)
- We will need to know weather pattern and how things behave differently at altitude
- We will need to know what paperwork we need to do with the FAA (Federal Aviation Administration) and the NTSB (National Transportation Safety Board) so that we know that no airliner will be flying into us while we are testing our balloon.
While we are working toward a full-scale model, we decided to set ourselves intermediate goals (we are guys but we are not complete idiots) and so the immediate goal is to have a balloon that can carry a remotely-controlled camera to a decent altitude and then take pictures for the heck of it. This project will take a while and we will be taking lots of safety precautions so I will be reporting on our progress over the next while. In the meantime, thanks for joining me and happy tinkering!