Re: Rail guns (was: US SOCOM desires...)

["Robert J. Hansen" <rjhansen@xxxxxxxx>]

> <snip>
> 
> I think the advantages of a railgun are:

Well, given I've built a couple of them (the coilguns were more
successful, the railgun was prone to lots of mechanical failures), there
are a fair number of problems with the general notion.

> - you can deliver the same energy by using a smaller mass that travels
> faster - so less space per shot for ammunition storage (all relative to
> a conventional weapon of the same type);

This one is false for the forseeable future (20-30 years). 
Electromagnetic acceleration of projectiles works, but it's horrendously
inefficient--most of the energy gets discharged as heat and utterly
wasted.  To get 30,000 joules of energy delivered to the flechette, you
need 300,000 or more joules of energy delivered to the weapon system. 
This means they overheat in terrible, awful ways.  A gunpowder rifle
needs a bullet, a propellant charge and a primer charge--pretty simple
and light.  A railgun needs a high-energy power supply, coolant, and
sophisticated power electronics--all of which are heavy.  The power
electronics may be able to be miniaturized; the dry ice and the kilowatt
generator have less likelihood of being miniaturized anytime soon.

The flechettes are smaller, yes, but the overall weapon system is
amazingly bulky.  Even room-temperature superconductors will still need
to be cooled (to keep them at room temperature), and even RTS will need
tremendous sources of power.

> - 2x the velocity, 1/4x the mass gives the same energy but half the
> momentum, which is also relevant to recoil;

This is very true.

> - some systems (e.g. tanks) can be built to absorb more recoil, and
> those can get bigger weapons than are practicable with chemical fuel;

Tanks get better bang per buck with chemical propellant.  The specific
impulse of ammonia-based propellants is immense; you're talking about
something that can propel a tungsten dart at 3500m/s already.  Adding
power electronics, coolant, and massive capacitor banks to the tank is
unlikely to give much in the way of improvements.

Naval vessels are a different story.  Given that many Navy cruisers have
their own nuclear power plants, the rail guns can just patch into the
cruiser's own power system.  Cruisers also have a wonderful, wonderful
coolant system called "the ocean"; drop a couple of hoses into the water
and you've got an effectively unlimited supply of fresh coolant.  From
an engineering perspective, a cruiser's the perfect place to do it.