I mean, one'd need to accelerate the rods to immense speeds to use them for carpet bombing, no? Needing enormous accelerators and/or spess mahics? Or am I missing something here?
I think the idea with tungsten rods is that you release them from orbit and gravity accelerates them to a high speed. Due to their high mass and speed they don't need any explosives either. Though, the planet would have to have sufficient gravity, so it might not work on asteroids unless the ship is already moving itself. At least, this is how they claim that they would work on Earth.
I dunno if they'd be worth it when used like that. I mean, it's a High-altitude gunship, not an orbital warship, and normally would still operate inside the atmosphere. Longer range is possible, but then you loose accuracy, and the main use for this is precise artillery support, not indiscriminate bombardment, though when that is needed I think a complement of nukes should do the trick very well in a pinch. But if the main goal is glassing a site, then an orbital warship using a big rail/coilgun seems better.
From this video:
http://www.escapistmagazine.com/videos/view/reel-physics/7296-GI-Joe-Retaliation-Tungsten-Rod-DropA tungsten rod of 8500 kg going 8700 m/s from an altitude of 800 km gives 32800 mJ, or 78 ton (0.78 kiloton) TNT equivalent.
If we assume a gunship is flying at, say, 100 km (a Lockheed AC-130 gunship has a service ceiling of about 9 km, by the way, and 100 km is the Kármán line on earth, aka upper limit of the atmosphere), and that it wouldn't be using rods of 8.5 ton a piece (that lockheed gunship only weighs about 55 ton itself), these rods would have substantially less energy. I could do a quick calculation, but you get the idea.
In the armory you can buy a 1 kiltoton nuke for just
7 tokens. The bomb on Hiroshima was
12-15 kilotons. So really, when not using very big coilguns to actually accelerate your ammo to high speeds (which would cost
a lot and probably be big and heavy, an issue for ships in atmosphere), just using nukes is much more efficient (a nuke is a lot of power in a relatively small package).