Metroid: Father Hunt

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Thanks. There's a red door in the first vertical shaft to the right of the start point. Shoot it with five missiles to open it.

I've always wondered how one is supposed to know that it takes five missiles. That's rather unintuitive and the expected reaction to the first missile not doing anything would be to stop trying, huh.
I know it's a flaw from the original games but maybe it could be helped by making it move obsious that a missile is doing something, like having the door open just a tiny bit and close again until you've hit it with enough missiles where it'd open fully.