The folding fin unguided rocket, that could be used for custom rocket pods, underwing rockets on WWll planes, MLRS rockets, RATO boosters, or even flares. Rockets would be based on S-5 aerial rocket, with fins spawning when they are launched. They would also have a very short or adjustable delay between firings.
Completed in update 1.7:
Really nice idea! LuukPlanes
Zachary Barnes commented
Makes sense since we already have guided rockets