Re: Solar balloons

["John S. Denker" <jsd@MONMOUTH.COM>]

"Dwight K. Souder" wrote:
> Most of the student opted for the tubed shaped balloons because it is faster
> to make, but some took on the challenge of making the "traditional" hot-air
> balloon shape.  The advantage with the traditional shape was that we could
> use a small hair dryer to inflate it and have it fly up in the gym.

I've made a balloon or two in my time.  I had one that went for many,
many miles before we lost sight of it.  It was still climbing.

> We've had limited success with the tube balloons.  The biggest problem are
> the winds.  A nice, sunny, calm wind day in Ohio may only occur once or
> twice in the year.  :)  So now we are just waiting for such a day.

Don't wait.  Launch from a the lee side of a building or a stand of
trees, where you will be protected from the worst of the winds.

> I'll send you the plans I used (written in MicroSoft Word).

In general, it is best to avoid sending anything in MS Weird format.
Why not use HTML format?

========================

Specific suggestions:

Make it big.  A few years ago some Italian guy wrote a book
called _Discourses on Two New Sciences_.  One of said sciences
was kinematics, the laws of motion.  The other was scaling laws.

Think about the scaling in a balloon:  The lift goes like the
volume.  The weight goes like the surface area.  The bigger
the better, until you start running into serious strength
problems.  Which you won't, unless you've got an unusually
good supply of ultra-thin material.

The smallest one I ever bothered to make was 12 feet tall and
about 10 feet in diameter, made from five 9x12 plastic drop
cloths.  (Four of them joined to make a 9x9x12 square tube,
plus a 9x9 cap.  Of course the cross section didn't stay
square when it was in use;  it bulged into a tube shape.)

It pays to shop around for the thinnest material.  You
want the stuff that the emperor's new clothes are made
of.  Usually I like to shop at high-quality stores, but
for this you want to shop at the cheesiest stores, to
find something that is too thin for any normal purpose.
(It's hard to special-order what you need in anything
less than industrial quantities.)

Joints were made by forming a cuff on each piece (to get
quadruple thickness) and then heat-sealing with an old
clothes-pressing iron.  Takes some skill, but goes really
fast once you get the knack.  Really strong.

Small boo-boos can be fixed with cellophane tape.

Launch on a sunny morning.  You want strong sunlight, but
cold air.  The day after passage of a cold front is ideal.

For something that big, a hair drier is not going to do much.
For the initial fill, just turn the mouth into the wind.
In the unlikely event of no wind, just run with it.
Then to heat it up, you need something on the order of a
small campfire.  (A good kerosene heater would do it, I
suppose, but I've never owned such a thing.)

If this is really going to be a _solar_ balloon, you need
to blacken it.  Soot works beautifully, which is why a
genuine campfire is desirable.  Throw some damp straw
on the fire.  It'll make heavy black smoke which will
blacken the inside of the envelope, with minimal added
weight.

Don't do it near an airport.

Come up with a clever payload.  You'll need something
by way of ballast to make sure it doesn't just roll
over and spill.

There will be strong temptation to add a "sustainer engine"
attached to the balloon.  But
 a) There are safety problems with this, if you don't do
it right, which is why
 b) Airborne flames are really, really illegal in most
jurisdictions.  Mention this to the kids;  otherwise
they'll be doing it next weekend.