Dan/Grimmund wrote:
> Tim-  Any significant difference for just using scrap iron?  Maybe
> with a little DU or tungsten cap?

Not really.
You'll need a bigger penetrator, but it will get through an Earth-type
atmosphere largely intact e.g. metallic meteorites.

Kurt Feltenberger wrote:
> A nuke could go as fast, but once past a certain
> velocity you'll have more energy released on impact
> than a similar sized/mass nuke.

The projectile has to be moving extremely fast to be
competitive with nuclear weapons, mass for mass.

TNT equivalent velocity is ~2.9km/sec.
(1kg TNT has an energy of 4.18 x 10^6 J).

Nuclear weapons start at 1000 times this:
http://en.wikipedia.org/wiki/Relative_effectiveness_factor

with an upper limit of 6 kilotons per kg barring some major
design improvements.

http://blog.nuclearsecrecy.com/2013/12/23/kilotons-per-kilogram/

So the 'nuke equivalent' velocity is 2.9 x sqrt(3 x 10^6) ~ 5022 km/sec
for a mid range (3kt/kg) nuke.

This is not in the 'orbital deadfall ordnance' category, which limits you to
11km/sec for an Earth-like planet.

Beyond this, projectiles will need independent propulsion to get them up
to speed for eventual impact at the target.

Solar escape velocity from Earth is about 43km/sec.
Galactic escape velocity starting from out solar system is only ~500km/sec.

> Fourth, they make a very big hole in the ground and will pretty much
> collapse must subsurface structures within 100+ km of the impact point.

This implies gigaton-level yields. Subsurface structures are hard to cave in
at a distance.
Think about ICBM attack vs. hardened targets and the controversy over Earth
penetrating weapons.

For 'Rods from God', the RAND report MR1209 'Space Weapons, Space Wars' has
all the relevant detail in appendices B and C.

http://www.rand.org/pubs/monograph_reports/MR1209.html

For Earth penetrators, the National Academy's report "Effects of Nuclear
Earth-Penetrator and Other Weapons". Look at chapters 2-4.

http://www.nap.edu/openbook.php?record_id=11282

Rob O'Connor