Custom Homemade Plastic Bag Birthday Candle Powered Fire Balloons -- overflite --
Big Bag Balloons were inspired by Dry Cleaner Bag Balloons. By increasing the volume, balloons can get more net lift, climb faster, fly higher, shine brighter and burn longer. Constructing Homemade Plastic Bags is simplified by "Hot Pizza Cutter Heat-Sealing Technology."
All of the Overflite Balloon Designs are based on the concept of a Theoretical Universal Balloon. Here, "Virtual Balloons" are imagined to exist mathematically, as a Reasonable Proxy of Reality. From these theoretical balloons, real balloons can then be designed and built, with predictable results.
NOTICE: All Balloonists are warned that they build and fly Fire Balloons completely at their own risk, and that they are completely responsible for determining all risks, and for any consequences that result.
Remember, you are completely responsible for every balloon you fly. Review Warnings and Cautions.
Synopsis of Theoretical Candle Balloon Design
three-dimensional object, such as a balloon, if
the dimensions are increased evenly, then the
volume increases by the cube of the dimension
change, while the surface area increases by the
square of the dimension change. This means
that the balloon's surface area and weight
increase by the change in volume^2/3. For
modelling purposes, equivilent heating is
assumed if the candles are also increased by
around the change in volume^2/3. See
Model for Equivilent Heat Rise.
So, the gross lift increases, relative to the
gross weight. Hence, the burntime of the
engine can be increased:
As a theoretical example, a neutrally buoyant balloon is imagined, that has no weight, except for the candles. It gradually increases in volume and candles, keeping the same heat rise, from the size of say a soap bubble, until it is huge. At first it has almost no burntime. Gradually its burntime increases. Hence, every volume can be associated with a particular theoretical burntime, depending on the ambient temperature, elevation, the amount of heating, and the burning efficiency of the candles.
If a balloon is made from lightweight plastic, a certain amount of its theoretical burntime has to be devoted to getting the bag and frame off the ground. After that, its theoretical burntime can be applied to candles. Similarly, if a balloon is made from paper, which is heavier than the plastic, even more theoretical burntime has to be devoted to the bag and frame. As a rough approximation, a tissue paper balloon probably needs at least 30-40 cubic feet of volume to hoist a set of birthday candles.
Overview of Model Hot Air Balloon Designs
The simplest way to make a balloon bag is to fold over a sheet of plastic, so the corners are matching, then to heat seal two of the sides together. Scotch tape reinforces any weaknesses. Ventilation holes are made. The result is a pillow shaped bag that looks like a giant dry cleaner bag. Dropcloth material is 9 feet wide. This means that the simplest bag shapes have either a height or width dimension of 4 1/2 feet. See Volume Calculations for Cylinder Shaped Balloons.
The engine is made with a single-file of birthday candles, melted onto the flat side of a balsa wood stick, with dripped wax and a heated fork. This makes an "engine stick." Balsa wood sticks are then tied onto each end of the "engine stick," to make an "H-frame." The "H-frame" is then attached onto the bag with scotch tape, to assemble the finished balloon.
A "14" cubic foot plastic balloon is a good size to hoist a birthday candle engine. A "24" cubic foot plastic balloon is a good size to hoist a padded-wax birthday candle engine. It is also a reasonable starting size for Homemade engines. Stay tuned for information here, as well as for information on candle powered paper balloons.
Optionally, a "self-destruct mechanism" can be added, by attaching lengths of scotch tape to the bag and frame, to carry the fire, and break up the plastic, after the engine burns down.
Optionally the engine can be made with dripped wax alone, which increases its weight and lengthens its burntime. To conserve space, double-file engines can also be made. Advanced homemade candle or two-stage engines can also be built.
Optionally the frame can be made with drinking straws instead of balsa wood sticks. Optionally the bag can be cut into a traditional hot air balloon shape, using multiple "gores. Optionally the bag concepts can be adapted to making static-heated balloons, which fly without attaching any fire.
Aerostatics of Birthday Candle Balloons
As described, different sized balloons will heat by similar amounts if the change in candles equals the change in volume^2/3. Hence, if the heating for one balloon is known, then the heating for the other balloons is reasonably predictable.
For modelling purposes, at 69 degrees Fahrenheit, twenty birthday candles are liberally assumed to heat five cubic feet of air by 139 degrees, to 208 degrees. See Suggested Number of Candles for Different Sized Balloons. This creates a 1/4 ounce of lift per cubic foot. At 28 degrees Fahrenheit the heating is assumed to be slightly less, or 134 degrees, to 162 degrees Fahrenheit, creating .28 ounces of lift per cubic foot. See Calculations for Model Hot Air Balloon Lift.
In reality the heating is not so precise. For mathematical purposes though, exact calculations are assumed. More candles increase the lift, but the overall heating is less efficient. Fewer candles decrease the lift, but the overall heating is more efficient. Also, it is believed that equivilent heating is achieved with somewhat fewer candles than predicted by the model.
It really does not matter all that much
exactly how many candles are used. Since
it is the volume rather than the candles that
create the lift, a little extra volume, or
somewhat fewer candles than suggested is
Model Hot Air Balloon Construction Materials
Thin Plastic Sheeting
(Painters Dropcloths) -- 1/3 Mil ( 8
Micron ) High Density Plastic
(Opaque) -- is similar to grocery
bags. It is strong and resists flame
damage. It may resist overheating
too. Alternately, 1/4 mil ( 6 Micron
) clear plastic is also okay, but it is
vulnerable to flame damage. Material is sold as
9 x 12 dropcloths, or as a 9 foot roll.
Suggest ordering a roll, for around $12, from a
paint supply store. Brands include
Balsa Wood Sticks -- 1/8 x 1/4 inch (3 x 7 millimeters) -- are sold in three foot lengths, in hobby stores and art supply stores, for about about fifty cents each. For "Standard Balloons," break the sticks in two, so they are around 1 1/2 feet long, except for the "engine crosspiece," which you may want a little bit longer.
Birthday Candles -- 1/4 x 2 1/2 inches -- 24 to a pack. A pack of candles weighs around an ounce.
Pizza Cutter, wood and metal, used only for balloons. Heat it over a stove or sterno and roll it over the plastic material, layed out on a wooden board to heat seal the bags.
Wooden Board. Ideally the board should be at least ten inches wide and close to six feet long.
Large Kitchen Fork, and optionally a Pie Server, used only for balloons. Heat over a stove or sterno to melt the birthday candles into an engine.
Sheet of Paper, to place under the birthday candles. Notecard, to free the melted birthday candles from the paper. Utility Candle, for dripping wax. Nylon String, for tying frame together. Pieces should be around 1 1/2 feet long. Scotch Tape, for repairing bag and for attaching to the frame, and for use as "burnup tape."
Scissors, for cutting the bag material. Pencil or Pen, for punching holes in the bag. Magic Marker, for marking the bags. Measuring Tape and Ruler, for measuring the bag material and the balsa sticks. Postage Scale (Optional) , for weighing the components and for science experiments. Pocket Thermometer (Optional) , for science experiments and potentially, for advanced balloon design.
Basic Construction Designs for Birthday Candle Balloons
As described, dropcloth material is 9 feet wide, and can be cut to any length, to make "blanks" for heat sealing into bags. This means that the simplest shapes have either a height or width dimension of 4 1/2 feet. With a 9x12 foot dropcloth, 4 feet or 6 feet of material are good lengths to cut for "balloon blanks."
For the Standard Bags or Fat Bags, the fold is at the top of the balloon, and the sides are sealed, so the balloon is 4 1/2 feet high. The Standard Bag uses 4 feet of material, and is figured to hold around 14 cubic feet of air. See Volume Table. More material can be used too, especially with a roll of plastic. Lift will improve, and there is no need to increase the engine power. The balloon is figured to carry around 40 Birthday candles and to lift around 3 1/2 ounces in 69 degree weather.
The Standard Bag is figured to weigh around an ounce, with a fair allowance for burnup tape. The frame is figured to weigh a little less than 1/4 ounce, with an allowance for attachment tape. The basic engine is figured to weigh around 1 3/4 ounces, with a small allowance for extra wax. This makes the initial weight around 3 to 3 1/4 ounces. So the starting net lift is figured at 1/4 to a 1/2 ounce in mild weather, and at close to an ounce in cold weather. See Candle Lift Table.
The Fat Bag Balloon uses 6 feet of material, and is figured to hold around 24 cubic feet of air, including a reasonable amount of tapering. This balloon is also known as the Ross Perot, or Hogie. It is figured to carry around 56 candles and to lift around 6 ounces in 69 degree weather. Optionally Fat or Tall Balloons can be powered by 48 candles, ie. two 24/packs of candles
The Fat Bag, with burnup tape, is figured to weigh around 1 1/2 ounces. The frame is figured to weigh around a 1/4 ounce. The candles are figured to weigh around 2 1/2 ounces or more, with an allowance for extra wax. This makes the initial weight around 4 1/2 ounces or so. So the starting net lift is figured at around 1 1/2 ounces. This is a significant amount of net lift. Hence the engine can be made with dripped wax alone, to weigh 3 1/2 ounces or more, and get significantly more burntime.
For the Tall Bag, also known as the Rocket Bag, the fold is at one side of the balloon, so it is 6 feet high and 4 1/2 feet wide. The top, other side, and bottom taper are heat sealed. The Tall Bag Balloon is figured to hold around 24 cubic feet of air, to carry around 56 candles, and to lift and weigh around the same amount as the Fat Bag Balloon.
Engine, Frame, and Bag Taper Design
24/pack birthday candles are usually slightly less than 1/4 inches wide, or around 7/32 of an inch. So, 40 candles for the Standard Bag Balloon make an engine that is around 9 inches wide. 56 candles for the Fat or Tall Bag Balloon make an engine that is around 12 inches wide.
A 36 inch balsa wood stick, broken in two, is about 18 inches long. Here a 40 candle "engine stick" would get around 4 inches of clearance between the bag and the flame. This clearance can be increased by making the "engine stick" around 20-21 inches long. Hence the other stick would be 15-16 inches long, which is still usable for one of the "outer sticks."
With the 56 candle engine it makes sense to make the "engine stick" around 24 inches long, thereby providing around 6 inches of clearance between the bag and flame. But the other stick will end up too short to be very usable, unless two of them are taped together to make a longer stick.
CALCULATION:With the "H-frame" design, the width at the bottom of the balloon bag should be rougly equal to the length of the "engine stick" plus the average length of the "outer sticks." So, the bottom of the balloon bag gets tapered towards this width. To get the center of lift high up in the balloon bag, the taper can be started fairly high up in the balloon bag.
For the Standard Bag Balloon, with a 20 inch "engine stick," and an average length for the "outer sticks" of 17 inches, the bottom of the bag should be around 37 inches. Hence, if the bag width is around 48 inches, then narrowing the bottom of the balloon bag by around 5 inches on either wide will enable the frame to fit properly.
For the Fat Bag and Tall Bag Balloons, with a 24 inch engine stick and 18 inch "outer sticks" the bottom of the bag should be around 42 inches. For the Fat Bag, being 72 inches wide, this means tapering and narrowing the bottom of the balloon bag by around 15 inches on each side. This is a lot, so the taper should start quite high up in the balloon bag.
With the Tall Bag Balloon, being 54 inches wide, the taper will only need to be around 6 inches on either side. Again, to concentrate the air high in the bag the taper can start fairly high in the bag.
Five to six inches of clearance between the bag and frame does not sound like a lot. In windy conditions the plastic will tend to blow into the flame, causing it to shrink and develop holes. But the plastic does not readily catch on fire, as long as there is not scotch tape nearby. As long as the holes don't become huge, the balloon will still fly just fine.
Also, getting an exact bottom width is not very critical. If it is too wide, the extra plastic can get taped together into "pleats." If it is too narrow, the ends of the "outer sticks" can be broken off to get the frame to fit correctly.
Procedure for Constructing Balloon Bags
The important factors in bagmaking are: a) to make sure the heat seals are strong and properly reinforced with scotch tape. b) to size the bottom of the balloon bag so the frame fits properly. c) To start the taper for the bag at around its halfway point to concentrate the center of lift high up in the balloon bag. d) To punch in ventilation holes towards the top of the bag, and optionally at the bottom of the bag, to give the engine fresh air, and to create "negative draft," so hot air does not "billow" out of the bottom of the balloon bag and destabilize it in flight.
BAG MAKING PROCEDURE:
2) Fold over the plastic, and match the corners. Lay out the plastic, on top of the wooden board, so all the creases are smoothed out, so it is even, and so it is positioned where you want to make the heat seal. Try to smooth out the plastic so there are no air gap. Wherever there is an air gap there will be a flaw in the heat seal.
3) To heat seal the plastic, first heat up the pizza cutter, on the stove or sterno, turning it frequently until it gets very hot, but not red hot. Then slowly roll it over the plastic, somewhat slowly, somewhat lightly, watching the roller to make sure it is turning. Be aware that the pizza cutter will tend to "seize" unless the pressure is just right. No matter what happens, keep rolling the pizza cutter, and don't stop until the cut is finished. You can make any repairs later.
4) After finishing all cuts, carefully inspect the seal, and repair any gaps, holes or weak spots with scotch tape. You can also use scotch tape to reinforce the seal at the bottom of the bag so it doesn't pull apart. For final inspection,. pressure test the bag.
5) Then, puncture small ventilation holes into the top of the bag with a pencil. Around 1 to 1 1/2 per cubic foot seems about right. Hence, Standard Bag Balloons, at around 14 cubic feet, should get around 15 to 20 ventilation holes. Likewise, Fat and Tall Bag Balloons should get up to around 25 to 30 small puncture holes. If you find the holes getting to be too big, just make fewer of them. More holes makes the balloon more stable, but at some point valuable hot air is lost.
Ventilation holes are very important. They help stabilize balloons by providing "negative draft," and by reducing how much hot air "billows" out of the bag. Balloons are also stabilized by good tapering. Try to make the fattest part of the bag around 3/4 up, narrowing slowly towards the bottom of the bag. Ventilation holes can also be made at the bottom of the balloon bag.
Attaching the Burnup Tape to the Bag
Optionally, lengths of scotch tape can be attached to the bag to help it burn up better when the flight is over, and to break it up into smaller pieces. The idea here is to get flames to travel from the frame to the bag, along strips of scotch tape. You can think of this as a "self-destruct" mechanism. It is important to locate the "burnup tape" where it won't contact the engine candle. Otherwise there is a possible risk that the bag could catch on fire in mid-flight.
If the frame is attached at the center of the bag, then the "burnup tape" can be located around 8 inches away. This way the tape is located at the least likely place to brush the fire. If you like you can start the tape up about a foot high, then attach a scotch tape "fuse" to it later, when assembling the finished balloon.
Four pieces of tape, four feet long or so, up each side of the balloon should break up the balloon to a fairly high degree. It will also provide a pyrotechnic display, as the burning plastic falls off the balloon. This amount of tape will probably not increase the bag weight very much. More complex arrays are unnecessary. Also....balloons with burnup tape should not be flown where they are likely to crash, since the burnup tape encourages the bag to remain on fire. Without the tape the plastic is not very flammable, so crashed balloons usually go out reasonably quickly.
Construction of "Engine Stick"
As described, the Standard Bag Balloon, with around 14 cubic feet of volume, is modelled to carry around forty birthday candles. Here the balsa wood "engine stick" should be around 20 inches long. The larger Fat and Tall Bag Balloons, with around 24 cubic feet of volume, is modelled to carry around 56 candles, with a 24 inch "engine stick." Optionally, if the Fat and Tall Bag Balloons only carry 48 candles, then the engine stick can be made a little shorter. See Candle Table.
It is important for the balsa wood sticks to be strong enough to carry the weight of the engine. The 1/8 x 1/4 inch sticks should be strong enough. They are also flat enough so the engine won't easily tip over. If the engine weight is increased significantly, then an extra piece of balsa wood can be scotch taped to the engine stick to improve its strength.
Engine Making Procedure:
2) Light the utility candle and drip wax lightly, onto the candles and the balsa, just enough to pin everything together. Heat the fork or pie server over the stove and brush it lightly along the candles a bunch of times until they are all melted together. In the meantime drip wax from the utility candle across the gap between the candles and the wood to strongly secure the engine.
3) Let the engine cool. Once it is hard, free it from the paper with a notecard. Then turn the engine over, and repeat the procedure for the other side. Optionally you can spread wax along the engine stick, to protect it from falling wicks, to provide incremental extra burntime when the candles burn down, and to help it burn up better.
Optionally, if you can afford significant amounts of additional engine weight, you can make engines with melted wax alone, without using the heated fork. Hence you can make engines "in the field," with only a bag and the other components. Here if you use a "hard" candle, the additional wax will have a higher melting temperature, hence cutting down on drippage, and extending the engine burntime. Alternately, you can make completely custom made engines, with more weight, a taller candle, using larger wicks, and high temperature wax. These engines are more efficient than birthday candles.
To package the bag, engine stick, "outer sticks," and string, everything can be laid out on top of several sheets of newspaper, then get rolled up inside, and get taped shut. This way the balloon is ready to go when it is time to assemble it for launch.
1) Cut a length of string, around 1 1/2 feet long. Make a slipknot, in the middle. Then loop the slipknot, around the end of the "engine stick," and around the middle of one of the balsa wood sticks. Tighten securely. Then wrap the string around the two sticks in both directions, and knot it, until the two sticks are tightly tied together. Make more wraps and knots. Repeat the procedure for the other side. Make sure the knots are tight or the engine might tip over or the frame could come apart.
2) Open up the bag, upside down, and place the "H" frame and engine inside. Orient the frame, so it is symetrical with the bag, or so the "burnup tape" is located towards the ends of the sticks. Attach scotch tape to the ends of the sticks and to the bag, around 1/2 inch from the bottom of the bag. Try to angle the tape, and squeeze everything together securely.
3) Judge the fit of the bag. If the opening at the bottom of the bag is "too small," plan to break off the ends of the balsa sticks. If the opening at the bottom of the bag is "too big," plan to bunch up the loose plastic, as you tape, so that the loose plastic won't be able to wave too close to the flame when the bag fills with hot air.
4) Continue this process, with longer pieces of tape, taping the middle, the very ends and in between. Make sure everything is secure. Optionally place pieces of tape along the balsa wood, and to attach to the "burnup tape. Inspect everything.
Launching Model Hot Air Balloons
First, if the weather is very hot, or if you are at high elevations you should view the balloons as experimental, and increase the volume, to get some extra lift, without letting the temperature of the balloon get past 220 degrees Farenheit.
In cold weather you can figure balloons to rise fairly fast. In warm weather they will tend to rise slow. Both of these tendencies also depend a lot of actual liftoff weight. As a general rule, figure that each 10 degree change in ambient Fahrenheit will raise or lower the lift by around 4%. Also figure that each 1,000 foot rise in elevation will reduce lift by around 2-3%.
Windy conditions make it more difficult to get the balloon to fill with hot air, make it more vulnerable to flame damage, send the balloon into a lower trajectory once it is released, and may cause temporary loss of lift in gusting conditions, which could make it impossible for balloons to clear specific obstacles. In general though, if you can get the balloon to clear obstacles it will fly. Balloons will also generally fly okay in light rain, even though the raindrops add weight, and cool down the bag.
As said before, also don't launch near airports, or in any areas where there is a fire danger, especially not if they are hilly or mountainous and it is gusting. Also avoid any terrain induced "wind tunnels." They can come from unexpected sources.
Try to figure out which way the wind is blowing, and try to get a clear view of where the balloon will go. Avoid flying it into any obstacles, but in close quarters, if there isn't much wind, they should tend to clear most obstacles.
There are basically two launch methods. The first is to light the balloon in an area which is somewhat protected from the wind, then once it is buoyant to bring it out into the open and launch. The second method is to light the candles then slowly walk the balloon downwind, as long as you aren't marching it into obstacles.
The typical launch method is to have one person hold the top of the balloon, and have a second person light the candles, starting in the middle, then as the balloon fills up towards the ends. In windy weather keep the plastic away from the flame with your hands. Once most of the candles are lit the rest of the candles will light too.
Within around a minute, assuming the weather is mild and the balloon is not overweight the balloon will seem like it "wants to fly." At this point it is generally advised to hold the balloon down a little bit longer so it can build strong starting lift, to be able to get through its first blast of wind. Once you are satisfied that the balloon can be released without hitting any obstacles let it go. Keep an eye on it throughout its flight, as a basic safety precaution.
By Thomas Taylor -- firstname.lastname@example.org