Re: Balloons without gases...

["Richard W. Tarara" <rtarara@saintmarys.edu>]

A quick calculation will show the problem.  Try to make the balloon out of
aluminum to lift a load of 500kg plus the balloon material.  Let the
balloon be spherical, and let the radius be 10 m.

Then    Vol*density(air)*g = (500 kg + Mb)g

Using density = 1.286 kg/m^3  you get   Mb = 4887 kg

Dividing by the area of the balloon you get  3.89 kg/m^2

Dividing that by the density of aluminum (2700 kg/m^3)  you get an average
thickness of 1.4 mm.

Now you have this 5000 kg (50,000 N) sphere with a thickness of 1.4 mm.  It
will collapse of its own weight.  Evacuated there is will certainly be
crushed like the pop cans I use for demonstrating air pressure.  You can
make the balloon smaller (maybe 5 m in radius) but then it becomes even
thinner.  You might try titanium, but it will be heavier and therefore
thinner.   

Good luck,

Rick

----------
> From: Doug Craigen <dcc@cyberspc.mb.ca>
> To: phys-l@atlantis.cc.uwf.edu
> Subject: Re: Balloons without gases...
> Date: Tuesday, March 04, 1997 10:14 AM

> > >        Greetings everyone.  I've been working on a few labs dealing
with
> > > the gas laws when a thought occurred to me.  Is it possible to make a
> > > "floating" balloon without the use of any "light" gases (hydrogen,
> > > helium, etc.)?  What I was wondering, would it be possible to take a
> > > rubber balloon like material and seal it all the way around so that
air
> > > cannot get in or out of the inside envelope.  The inside lacks any air
> > > (vacuum or near - vacuum).  If the inside of this balloon had some
sort
> > > of light, strong rib material that flexes outward pushing the outer
walls
> > > so that the balloon has a greater volume.  I know the pressure inside
> > > will greatly decrease and the "skin" of the balloon will be "sucked"
> > > inwards, but if it were rigid enough, could this work?  Has it already
be
> > > done?  Am I onto something that could bring me instant success and
early
> > > retirement?  :-)
> > >
> > > Dwight
> > > Ashland, OH
> >
> > Dwight--
>
> Dwight your proposal certainly sounds like it would work provided the
> right materials could be found.  That is a pretty "heavy" provision
> though.  The first question to pop to mind is whether balloon materials
> themselves hold up very well to 15 psi (oops, I've slipped out of metric
> mode).  My recollection is that the excess pressure that they hold is
> typically much less than atmospheric, but I don't have a source on hand.
> Perhaps someone else has a number for the pressure difference that
> would burst a balloon skin.  In general, I'm sure we've all seen what
> atmospheric pressure can do to fairly rigid materials.  I used to do a
> demo for elementary school kids where I boiled water in a big metal can,
> then screwed the lid back on.  It twisted and collapsed, but didn't float
> up.  Clearly you need what are on average much stronger and lighter
> materials than a metal can provides.  i.e. strong ribs and a balloon type
> side.