OK. Let's be done screwing ourselves over. Wind will NEVER do as much damage as flames, not for the same amount of power. Most you MIGHT hope for is a perfect roll with the flaming projectiles, which would allow you to not spend a revision and then spend it on the gale stones. Maybe. But probably not. So, here's my plan:
Design Phase: Make the simple, mechanical, utterly non-magical pottery, well, bombs, I guess.
Revision: Retroactively add the magic. This splits the difficulty up. We'll make progress into incendiaries (Not that much should be needed. As I pointed out, everything suggested is common-knowledge oh-hey-this-lights-on-fire sort of stuff), THEN add an enchantment. Also, if we get screwed, again, on the roll, we can just fix the flammable part. We'll be using Lucky Strike, of course. So we'll hit with large numbers of these things.
Also, it will
absolutely require no revision, once they're working, to grab one of these bad-boys and chuck it over the walls of a fort. So we do NOT need to redesign ballistae. Because logic says if you can blow it up one way, you can blow it up another. Heck, perhaps as an order we could have soldiers construct wooden troughs on the walls of the fort, so they can merely push these off the walls, without needing protection from the flaming casing.
TO SUMMARIZE:
Is everyone voting for the firestorm shells OK with splitting the design between revision and design phases? And possibly sacrificing the expense part as well, in order to get these things out in greater numbers really fast?
Precognitive Rings: (1) Devastator. {Although Nuke 13's idea sounds similar}
Blast of Wind: (0)
Zapstones: (1) Egan_BW.
Firestorm Shells (3): Taricus, Happerry, Madman198237