

You can see this in the movie for yourself in one of Dan O'Bannon's tactical displays:

The SFX team apparently took this problem and envisioned them more "realistically" as something akin to traditional bombs. (You may interpret it differently, though.) That might make sense when you're writing something, but it's obviously harder to ignore the laws of inertia when someone asks you to animate it. I think by "ninety degrees" that Lucas was probably imagining that the pilot had to be directly above the exhaust port in order for it to drop go all the way down into the reactor.

In the fourth draft (the one just prior to filming) of the script, Lucas gives us a hint about how he envisioned the Proton Torpedoes working: DODONNA Or rather, they probably weren't supposed to. Short answer: They don't really "turn 90 degrees".
