From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: sci.energy
Subject: Re: Popular Science Fly Wheel Article
Date: 21 Dec 92 16:13:52 GMT

In article <1h41jqINNn2j@gap.caltech.edu>, carl@SOL1.GPS.CALTECH.EDU (Carl J Lydick) writes:
> In article <1h3t1nINNddb@sportster.ksu.ksu.edu>, camelsho@sportster.ksu.ksu.edu (   James A Seymour) writes:
> >The American Flywheel Systems (AFS) battery contains a pair of 
> >counter-rotating "rim wheels," suspended by magnetic bearings in a
> >vacuum housing.  The rim of each wheel is made from a high-tensile-

Ok, magnetic suspension is good - no bearings or axles to fail.  How do they
plan to guarantee that the rotors cannot slip out of the suspension?
A strong impact might force them out, or the magnetic suspension could
waver for a variety of reasons.

> >power a car, a flywheel battery must be capable of rotating at 150,000-
> >200,000 rpm.  If the flywheel is a metal disc, it could break into 
> >shrapnel at that speed.  And if it is a spoked wheel, the rim pulls
> >away from the hub at high rpms.
>  
> So far, so good.  Though their emphasis on "shrapnel" makes it appear we're
> soon going to run into problems.

Also, 200,000 rpm sounds too slow to me.  Some crude calculations I made
suggest it's closer to 200,000 revolutions per second.  To determine for
certain, we'd need to know the diameter and mass of the rotor.  We know that
they plan 600 miles, but I assume they plan that on a specially constructed
lightweight vehicle.  If we assume that an IC engine could power their
vehicle at 300 mpg, then all the foregoing discussion about 2 gallons of
gas and 100 miles still applies.  Their 20 batteries store enough energy
to completely vaoprize 40 kg of iron, or completely melt over 200 kg.

> >that it fails, the Kevlar fibers simply turn into "cotten candy,"
> 
> Yup.  There we go:  "If it's fluffy, it's safe."  It won't be very effective
> shrapnel, at ranges of dozens of yards (just how fast do you think the
> atmosphere will stop hypersonic cotton candy?).  BUT THAT "COTTON CANDY" STILL
> HAS ALL THE KINETIC ENERGY THAT YOU'D STORED IN THE FLYWHEEL.  You still need
> to enclose it in some sort of containment that's capable of absorbing all the
> stored energy.  If the containment can't absorb the energy, then, at the risk
> of repeating myself,
> 
> 	BBBBBB    OOOOOO   OOOOOO  M       M !!!
> 	B     B  O      O O      O MM     MM !!!
>	B     B  O      O O      O M M   M M !!!
> 	BBBBBB   O      O O      O M  M M  M !!!
> 	B     B  O      O O      O M   M   M !!!
> 	B     B  O      O O      O M       M 
> 	BBBBBB    OOOOOO   OOOOOO  M       M  !
> 

And we aren't talking about "cotten candy".  There is no proof yet
that that is what we'd get.  I think we'd get molten drops of very hot
stuff that used to be Kevlar.  The shock wave from the failure should be
strong enough to melt most of the rotor before it hits anything.  The shrapnel
that we need to worry about is not the rotor itself, but bits of the
housing, the car (and its occupants?).

> OK, by using several flywheels, they've dealt with two problems:
> 	1)  Have half spinning one direction, the other half spinning the
> 	    other.  Then if you've bolted them down well enough, the car as a
> 	    whole won't have any problems due to gyroscopic effects (other than
> 	    metal fatigue where the housings of the flywheels are attached); and
> 	2)  They've reduced the energy stored in each flywheel.  I'd be happier
> 	    if they were using about 500 flywheels, but 20's a start.  They're
> 	    now only talking about something like 2 sticks of dynamite
> 	    equivalent in the event of a flywheel failure.

There is another problem with many flywheels - the probability that
one of 20 will fail is much higher than that for a lone flywheel.  Of course,
this is mitigated by the possibility that a flywheel with 1/20th the energy
capacity could be constructed more safely.  Still, I imagine a strong impact
could cause them all to slip off center simultaneously and touch the
walls of the their containers.  BOOM.

> >For now, the flywheel battery exists only as a computer simulation.

Of course.  Computer simulations don't generally kill you when they go BOOM.

As an aside, I've never been a big fan of Popular Science.  I find that it is
Popular, but not always Science.  I suspect the flywheel battery people are
either unqualified to be doing the work they are doing, or are hucksters
out trying to con "investors" out of cash, along the lines of the "free energy"
people.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov