🚀 go-pugleaf

In article <1992Dec23.180140.7552@enterprise.rdd.lmsc.lockheed.com>,  writes:
> The tensile strength of carbon carbon fibers is currently > 1 million psi,
> the producers won't tell you the real numbers as this field is very competative.
> This means that if a carbon carbon composite is 80% fiber the tensile strength
> of the composite is 800 kpsi.  The ultimate tensile strength is the tensile
> strength of the graphite plane, meaning one has made a perfect single crystal
> coiled graphite sheet fiber.  This perfect fiber would have a tensile strength
> greater than that of perfect diamond which is 14 million psi.

Ok.  Let's make a great big flywheel to give the flywheelers the benefit
of the doubt.  We'll make it 1 meter in radius - that's mighty big to fit
in a car.  We'll assume the rotor weighs in at 100 kg.  Pretty doggone heavy.
We'll assume that the rotor is a cylindrical shell infinitesimally thin.
This is the configuration for minimum stress and maximum energy.  (It means
that there are no spokes, etc., but we'll assume that the designers are
brilliant and can suspend this ring, spin it up and pull power off it).
In this case, energy is easy, it's just 0.5 * M * R^2 * da/dt^2.  The energy
in 2 gallons of gas is 92,000 kCal, and I feel this is reasonable to
get their supercar the 600 miles that they claim for range.  a is angle in
radians.  This results in 442 revolutions per second (2.8 km/s, probably
larger than the sound speed of the composite).  The radial stress is
not very important here, the hoop stress is larger.  The hoop stress in
this case is just rho * da/dt^2 * R^2.  rho is density.  I don't know
the density of carbon composite, but it is considered light, so I'll
give it 2 g/cc (I don't think it floats).  Thus, hoop stress is 2240 kpsi.
Thus, the flywheel will fly apart before it can store the
requisite energy.  Given diamond, a flawless flywheel would hold
together at this immense size, probably a bit impractical, but I
will not say that it will remain impractical.  However, reduce that
wheel in size just a little, and the diamond, too, will fly apart
before it could be spun fast enough to store the required energy.


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