How to build and launch candle balloons

Birthday candle powered hot air balloons are a lot of fun. Sure, you can buy kits on Amazon. But there’s nothing better than building the balloon yourself and watching it drift off into the night sky, finally burning out miles away and thousands of feet in the air. This post provides an equipment list and instructions for making the candle balloons in the video below:

Equipment List:

0.31 mil clear plastic paint dropcloth (get it at Home Depot or any large hardware store)

one pack of birthday candles

another spare candle

drinking straws

scotch tape

paper clip or sewing needle

clothes iron

lighter

How to Build the Balloon:

1.  Cut a 6 x 6 ft (~2 x 2 m) square of drop cloth.

2.  Fold it in half lengthwise.

Folded over piece of plastic ready to be sealed with an iron.

Folded over piece of plastic ready to be sealed with an iron.

3.  Seal the top and side by melting them together using the iron.  Leave the bottom open.  It may take a bit of adjusting to get the iron hot enough to weld the plastic together but not so much that it melts.  If you’ve not done this before, I suggest practising on a spare piece of plastic.  Once you’re done with the balloon, it will form a cylinder 6 ft high and about  2 ft in diameter.

4.  Make a frame to hold the candles by taping straws together to make a “+”sign.  Disassemble the paper clip to make a pin (or just grab a sewing pin).  Push the pin through the centre of the frame.  This pin will hold the candles onto the frame.

100_0546

5.  Make a double row of birthday candles (or just take the back off the pack of candles, leaving them lined up in a convenient package).  Drip wax from the spare candle onto the rows of birthday candles to bind them together.  This pack of candles is your heat source.  I call it an “engine” out of habit.

100_0544

6.  Attach the frame to the balloon (I usually poke holes through the plastic, then tape the frame to the bottom of the balloon on the outside).  Stick the engine on the pin.  The balloon is ready to fly!

Launching the Balloon

Call a friend and wait until the winds are very light.  In general, the best policy is to make sure that the wind is imperceptible and that it is not disturbing leaves on trees.  Go to a wide open area without nearby houses or isolated trees.  I’ve lost more balloons than I care to count in trees – and nearly hitting a house is almost as nerve wracking as explaining to the owner why his house burned down.  Thankfully, I have not had that experience!

Have one person hold the balloon up and another lay on the ground.  Light the candles, being sure to keep the plastic off the flames.  The balloon should reach neutral buoyancy in about thirty seconds.  Gently release the balloon when it is positively buoyant.  Do not try to fight the wind – if it starts blowing, walk with the wind to avoid it pushing plastic onto the flames or collapsing the balloon entirely.

Having hot wax dripped onto your jacket's just part of the experience!

Having hot wax dripped onto your jacket’s just part of the experience!

Safety and Legal Considerations

There are county level regulations in some parts of the USA that prohibit candle balloons.  The FAA regulation on unmanned free balloons permits the operation of balloons with payloads less than 4 pounds without restriction, but there is a catch-all clause stating that the balloon must not jeopardize health and safety.  So if the balloon starts a forest fire, burns down a house, or even causes a public disturbance, then you are liable.

I have had the police called on me once.  The homeowner who placed the call thought we were committing arson.  Once I explained what we were doing, the officers were perfectly fine with it, but they did point out that they’d be getting in contact if there were any house fires that night.  Our very first launch started a small fire when the balloon got stuck in the tree and the engine fell onto the ground.  I have seen another, poorly constructed balloon land in a parking lot when the frame fell apart.  Also speaking from personal experience – never, ever use the kind of candles that don’t blow out!  They keep going all the way down.

The take home message:  use common sense.  Launch after rain or snow.  Consider where the balloon may land if something goes wrong.  Remember that most people who see these have no idea what these balloons are, and they tend to assume the worst.  In any case, treat the balloons like fireworks, and you’ll have no problems.

Advertisements

Solar Balloon Buoyancy and Trajectory Predictions

So you’ve built a solar balloon. But will it even fly? And how high will it go? This post outlines a method to model the buoyancy and 1D trajectory (elevation vs. time) of a solar balloon. Quite a few formulas go into the complete the model, so a full description is left to a longer PDF report, which is available here. Additionally, the code written to compute the trajectory is available here.

Modeling solar balloon buoyancy is an exercise in heat transfer. Generally, the balloon is heated by the radiation it absorbs and cooled by the fluid flow around it and infrared emission. The sources of radiation absorbed by the balloon include direct solar radiation, reflected and diffuse solar radiation, and infrared radiation emitted by the earth and sky:

radiation_sources

A contour plot of the radiation intensity falling upon a balloon vs. time and elevation on Nov 1 is shown below.  The amount of direct solar radiation reaching the balloon increases with elevation due to the thinning atmosphere, however, the overall radiation intensity is highest near to the ground due to the IR radiation emitted by the Earth.

Rad_Cont

The other important source is heat transfer is due to fluid flow. As described in the report, there are fairly accurate engineering level correlations which can be used to determine the rate of heat transfer from the surface of the balloon to interior and to the exterior atmosphere.  In conjunction with our model of the radiation environment, we use these correlations to determine the balloon buoyancy and ascent rate.

An example of the trajectory of a solar balloon 5 m in diameter with a 3 kg payload is shown below. The balloon takes off slightly after dawn (0 hrs), reaches a peak elevation of ~15 km, and then descends after sunset (indicated by the red line).

bl_el

One interesting prediction of the 1D trajectory model is that certain configurations of solar balloons should be able to fly at night, powered by the infrared flux from the Earth alone. There is precedent for this, as French scientific balloons powered by solar/terrestrial radiation alone have achieved flights of up to 70 days, circumnavigating the globe several times.

There is still a good deal of work to be done on this model. Most importantly a validation against real solar ballon flight data is needed. Additionally, there are a number of issues to be addressed, such as modeling the effects of non-spherical balloon shape and finding a convenient way to determine the optical properties of the balloon film (such as solar absorptivity). We eventually plan on combining this model with rNomads to compute 3D balloon trajectories.