system
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human
[Star Trek 3] Why didn't the Enterprise's self-destruct blow up the whole ship?

It seems that the self-destruct explodes the bridge, and then a few seconds later, the saucer section. The rest of the ship tumbles into the atmosphere, which certainly destroys it, but I wouldn't think the computer was planning on that to happen.

A dedicated destruction for the bridge would make a certain amount of sense, but why not just detonate the warp core like the Enterprise-D does?

EDIT: After thinking for a bit, the ship is extremely damaged at the beginning of the movie. It's possible the self-destruct just didn't work properly.
gpt
The primary purpose of the self-destruct system is to prevent sensitive information from falling into the wrong hands. While the complete destruction of the ship is usually the desired outcome, it is not fully necessary (or even possible due to the limitations of the conventional charges in place). The conventional charges on board were nowhere near enough to destroy the entire ship, but they are located to trigger secondary explosions of the ship's warp, impulse, power grid and fuel system.

The 1701-nil had taken significant damage in the battle against the Klingon, and as a result her warp core had automatically shutdown and purged to prevent a catastrophic breach. Without an active core, the self-destruct system was much limited. 

The charges in the saucer section detonated the fusion reactors that were still online as intended, resulting in a secondary explosion sufficient to destroy that section. However, it seemed several of the charges failed to go off, particularly those meant to destroy critical engineering section components and breach the antimatter containment system. 

It's worth noting that the ship had been rushed back into service after two serious engagements, only to stumbled into a third, so even though the self-destruct system is independent of the main system, it's likely to have seen some damage as well. It's also possible that the self-destruct system had its command and control network poached for spare parts, since the system would've been viewed as expendable in trying to get as much of the ship back online as possible, especially after her 'theft' deprived her of Starfleet technical assistance.