TNT Fireworks

I'm trying to make TNT fireworks, but I'm not having much luck.
I made a vertical redstone cannon, (didn't follow any tutorial so the redstone's really messy) it has 2 sections, bottom has propellant and top has fireworks. The plan is that the fireworks fly, and then 1 or 2 TNT timed differently to the rest explode earlier, pushing all the other TNT sideways. The problem I'm having is that for some reason, the differently timed TNT flies higher, so it pushes the rest of the TNT downwards.

Help?

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