Re: Railgun Research: Barrrel Design or Power Supply

mmeron_at_cars3.uchicago.edu
Date: 02/28/05 

To: sci-military-moderated@moderators.isc.org
Date: Mon, 28 Feb 2005 10:23:34 GMT

In article <cvljjn$q99$1@rainier.uits.indiana.edu>, glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) writes:
>In article <l8eTd.6$25.1286@news.uchicago.edu>,
> <mmeron@cars3.uchicago.edu> wrote:
>>In article <cvj96a$vfo$1@rainier.uits.indiana.edu>,
>>glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) writes:
>>>In article <0xUSd.10$45.5968@news.uchicago.edu>,
>>> <mmeron@cars3.uchicago.edu> wrote:
>>>>In article <cvfg26$lnt$1@rainier.uits.indiana.edu>,
>>>>glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) writes:
>>>
>>>>>One of the attractions of rail guns is that the projectiles are smaller
>>>>>and they don't have the powder charge, which is hoped to simplify
>>>>>logistics and improve autoloading systems.
>>>>>
>>>>Quite the contrary.
>>>>
>>>>Take the amount of propellant in a the cartridge of a single tank
>>>>shell. Estimate the amount of energy stored in it. Note that once
>>>>put into place this energy remains stored very stably and for a very
>>>>long time, with little or no maintenance, yet can still be released
>>>>within a milisecond or so, at any moment you wish, using robust
>>>>mechanisms. Now, estimate the weight, size and complexity of a system
>>>>capable of storing comparable amount of electrical energy, and
>>>>releasing it on demand within comparable time scales. "Simplify" my
>>>>ass.
>>>
>>>I think existing systems fall short by a factor of 1.4.
>>>
>>>The powder is replaced by diesel fuel. Some system like capacitors or a
>>>flywheel have to be able to store energy for a few shots in rapid
>>>succession. But you're not thinking it has to be a flywheel or batteries
>>>carrying energy for 40 shots, are you?
>>>
>>No, not at all. But just for a single shot, you've to be able to
>>store few MJ and be able to release it, in controlled fashion, within
>>few miliseconds. Not a small requirement and the solutions do not
>>come in small, simple packages. Ah, and if you put it on tank, you'll
>>better be able to recharge the system and be ready for the next shot,
>>within seconds. So, try to estimate the size and complexity of the
>>capacitor, the power supply charging it and the switching system
>>discharging it. Or, if it is a flywheel, the size and complexity of
>>the flywheel, the power train which brings it up to speed and the
>>system capable of converting all the flywheel's mechanical energy into
>>electrical within miliseconds.
>
>A four pound dart going 2300 m/s is around 5.5 megajoules. Target muzzle
>energy 10 MJ at 3.5 km/s.
>
>Flywheels have been built that can achieve energy densities of 130 kJ/kg
>with a safety factor,
>
Yes, that is about reasonable. Whish is nearly 2 orders of magnitude
less than what a decent modern 'gunpowder" will get. And, once you
discharged the energy fro this flywheel, how long before it is ready
for another shot?

>[Robert Acebal, "Energy Storage Capabilities of Rotating Machines
>Including A Comparison of Laminated Disk and Rim Rotor Composite Designs",
>IEEE Transactions on Magnetics, 35, 317 (1999).]
>
>That's the most recent reference I have.
>
>If you figure 10 MJ per shot that's around 80 kg of flywheel per shot, not
>including bearings, structure, electrical windings, etc. That's pretty
>weighty. But when you consider this is about main battle tanks, and
>dropping more than a thousand kilograms from the standard ammunition load,
>I wouldn't be so sure that it can't happen. That brings us to the level
>of worrying about factors of two or four or so, not orders of magnitude.
>
>Not that I think deployment of such vehicles is imminent, but you seem
>pretty categorical in saying it's not going to happen. There's a
>difference between saying such a technology will and must be fielded, and
>saying that it might be, and without technology that is indistinguishable
>from magic.

I would say that it'll take something pretty close to magic to make it
worthwhile, YMMV, of course.

Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"

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