From: Blair.Haworth@launchpad.unc.edu (Blair Haworth)
Subject: Re: A gripe, and Re: Flywheel batteries...
Organization: University of North Carolina Extended Bulletin Board Service
Date: Thu, 31 Dec 1992 15:15:28 GMT

In article <1992Dec31.031634.13720@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>In article <1992Dec28.165210.28613@samba.oit.unc.edu> Blair.Haworth@launchpad.unc.edu (Blair Haworth) writes:
>>
>>Why not start with a static device, like a peak-load
>>storage or a backup power source, where you don't have to worry about
>>accelerations and you can put it in a pit, just in case?
>
>Such flywheels exist. The one at MIT has been cited. [...]
>Multi-ton flywheels, turning at a few thousand RPM aren't remotely
>suitable for auto use.

I didn't say they were; I suggested that the developers of advanced
flywheels were missing a bet by not coming up with static applications
(preferably less demanding than firing the Alcator) instead of going to
automotive applications with the associated difficulties.  I repeat, I
don't expect automotive flywheel energy storage to become a reality this
time around (this is what, the third time around sice the 'sixties?); I
just don't rule it out forever-and-ever, unless technological change
renders it irrelevant.  A distinct possibility.   

>>I'm not totally convinced by the energy-is-conserved-and-that's-that
>>crowd, although I think containment will be daunting.  I suspect though,
>>that at least a partial solution exists today, in production-line
>>automotive technology, in several thousand examples.  Consider the M-1
>>tank: it carries its forty rounds of main-gun ammo in an armored
>>compartment designed to vent upward in the event of an explosion: the
>>rules say energy is conserved, they don't say which direction it has to be
>>conserved in.  The total energetic content is typically about (40 x 10kg=)
>>400kg of propellants and very roughly, assuming half the warload is
>>HEAT-MP, (20 x 10kg x 15% explosive filling by weight=) 30kg explosive
>>(numbers from _Jane's_ via memory).  Do these sound like familiar numbers? 
>>Yes, we're talking about a 60-70 ton tank, here, but keep in mind that
>>we're talking about what happens to be one of the most thinly-armored
>>parts of the tank.  It would also be foolish to ignore the, ah, distinctly
>>directional character of flywheel failure, but consider that the same tank
>>is designed to defeat just that sort of attack (shaped-charge warheads). 
>>Both these features worked pretty well in combat; their descendants might
>>(for all we know) work on the road.  I won't bet the rent money, and of
>>course, the relevant technology is heavily classified, but it is there. 
>
>The M1A1 has never survived a main ammo locker failure to my knowledge.
>Like the Hood, once the main magazine lets go there's little mere steel
>can do to resist. The crew is history.

Actually, that's not accurate.  The ammo storage system was tested to
destruction repeatedly and had to be redesigned when the tank was
upgunned.  I believe it functioned as designed at least once (N=1 here, so
let's not get carried away) during Desert Storm, although not with the
crew in the tank.  And, as it happens, the whole idea is that the crew
_isn't_ "history".  The whole idea is to preserve the crew, albeit maybe
not in pristine condition.  Morale aside, tanks are an industrial
commodity; experienced crews are harder to replace.  "Mere steel" isn't
the right way to look at the design philosophy, by the way.  It's a
weak-link arrangement to allow a controlled blowout.  The _Hood's_
magazine, being essentially a closed steel box, would likely have tended
to tamp the explosion, making things worse, in contrast.    

>Note that the main threat to MBTs is the kinetic penetrator called a
>APDS round. This round uses pure kinetic energy to destroy enemy tanks.
>Mere HEAT rounds are considered ineffective against modern armor. That
>armor, called Chobham after the military establishment where it was
>developed, is not in any way light. Most of the mass of a 65 ton tank
>is armor designed to defeat a 10 kg penetrator travelling at 5,000
>m/s. Since the flywheels we are talking about would have a rim velocity
>of 5,000 m/s or greater, and would mass more than 10 kg, it should become
>obvious that containment would need to exceed 65 tons of armor to stop
>it should it fail.

I'm afraid this is a bit facile.  The main threat to MBT's is APFSDS (APDS
is becoming an obsolescent technology) precisely because of the
development of composite laminated armor arrays ("Chobham" is a slight
misnomer) optimized against HEAT attack.  "Optimized" is the operative
word here; these armor systems can be tweaked to accommodate different
threats; the M-1's protective system has been changed two or three times. 
I'm not suggesting the direct application of Chobham-type armors; just
that their principles might be applicable.  Until someone builds some
flywheels and tests them to destruction, we don't know the nature of the
insult to be protected against: HEAT-like (small fragments, gas) or
APFSDS-like (lower velocity, bigger chunks).  My main point, in any case
is that it's not entirely valid to generalize the effect of a
non-specialized attack from a specialized one, by definition a worst-csae
scenario.  The most advanced 120mm APFSDS round I've seen numbers for
(German [Diehl?]) fires a 10kg-ish tungsten penetrator 28mm in diameter at
around 1800 m/s muzzle velocity - I think you got that 5000m/s figure from
the 5000_ft_/s often quoted for APDS rounds as a ballpark figure - so that
the whole three-and-a-quarter megajoules energy is being deposited in
about 6cm^2. The energy density in a flywheel failure isn't going to be
nearly as much, especially if the flywheel is engineered to bias its
failures toward small chunks (admittedly likely a nontrivial task) and
multiple small units are used.  It may well be containable with "smart"
armor at typical automotive weight and bulk.  It also may not.  Or the
whole thing may be irrelevant.

Apologies to the group if this is too much like sci.military for their
taste; I'm mired in a dissertation on modern armored fighting vehicle
design and it's warped my brain.

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