At the end of the film, the Enterprise is falling toward Earth, but they can't do anything about it because the matter/antimatter injectors aren't aligned. Then when Kirk gets the warp engine working again, they proceed to use liquid propellant thrusters (we even see the flames!) to regain attitude control. Liquid propellant must be hugely inefficient given the size of the Enterprise (each thruster must have the power of a dozen Saturn V's), but still. Even if they weren't using liquid propellant, why not just fire the impulse engine to prevent entering the atmosphere in the first place?
How do you know they're liquid propellant? They could be impulse rockets or something. Also, the ship had NO power outside of emergency lights, not even artificial gravity.
That's not really a plot hole, so much as a technical oversight regarding whoever built the propulsion system. A similar problem occurred during the TOS episode, The Naked Time. When Kevin Riley shuts down the engines, the Enterprise's orbit begins to decay, mainly due to the breakup of Psi 2000. Without engineering online, there is no way for the ship to maneuver, as apparently all systems are tied into the warp engines.
Because of the flames. If not chemical propellant, the logical alternative would be plasma (ion drive like the one NASA is currently testing), but even that wouldn't require the warp engine.
I'd be willing to agree they were flames we saw. Or some kind of plasma stream. The vents glowed for a few seconds after they shut down, so they were plenty hot. However, referring to the Next Generation's Technical Manual, you'll note that the Enterprise D maintains a low-level Cochrane field to reduce the ship's apparent mass for impulse engines and maneuvering. I suggest that perhaps this ship has a similar system to enable it to land and take off from an M-class world. Without the warp core functioning, there wasn't enough power to generate such a field, and the thrusters would have been useless. Does that work?
This... I swear some people have never actually seen the show. In TOS, the warp engine was sometimes treated as the power source for almost everything on the ship. Also, in addition to the phasers, Andrew Probert and Mr Scott's Guide to the Enterprise had the TMP-era ship's impulse engine hooked up to the warp engine too.
Exactly. Also, your post reminds me of Guy Fleegman's line from Galaxy Quest: "Did you guys ever watch the show?!"
I presume without antigravity supplied by the warp engines as stated already above everything else is useless.
After the thrusters slow the Enterprise's descent and it drops below the clouds there's a loud start-up sound, presumably the impulse engines activating. It then lifts above the clouds, the thrusters disengage and the ship stays put, ie. under impulse power.
This is not a "plot hole" by any stretching of the term. It's a technical nitpick based on your interpretation of a visual effect.
The thrusters would be uselss if they were just solid or liquid propellent anyway, as without any other main drive working, ie the impulse drive, those jets would run out, and the ship would plummet again seconds later. Either way they needed the core back online.