We're taking the tech manual as evidence here and that describes....
TNG TM p141 wrote:The deflector field itself is emitted and shaped by a series of conformal transmission grids on the spacecraft exterior, resulting in a field that closely follows the form of the vehicle itself.
It also says that each nacelle includes two shield generators projectors.
But on-screen evidence is almost always a bubble, so I would assume it is normally a bubble. So a hull-hugging field is certainly possible for a GCS. I have no idea how quickly they can change over, and obviously they usually run in bubble mode in the series. I have no idea what's required to change over, though they do expand the shields a few times in the series that I recall, to protect the romulan scout in The Defector, at least. And yeah, reconfiguring it to protect the nacelles from the saucer seems like it would not be something they'd do normally.
Eternal_Freedom wrote:On the more substantive argument you made about the computer core being locked out of the autodestruct activated, those are good points but my primary objective is to disable the ship's engines (as stated in the OP) which my plan has a reasonable chance of doing. The OP also states that the ship must be as intact as possible. If the computer cores are locked out, oh well, we can if needs be cut them out of the ship and take them back to base to be decrypted at our leisure. If the autodestruct is activated, well that depends upon the method of autodestruct this GCS refit is using - if it is using the "detonate warp core/antimatter storage" method, then once we storm engineering we should be able to emergency vent the antimatter and/or eject the warp core.
If it is using the E-Nil "random explosive charges in the saucer section" method seen in TSFS, then I've no idea.
It can do both, according to the TM.
TNG TM p141 wrote:In the worst-case examples, all propulsion and defensive systems are irreversibly disabled or destroyed, and there is no possibility of assistance by other Starfleet or allied vessels. A high probability that the ship can be boarded or tractored then exists, and the activation of the auto-destruct sequence within the computer is to be the final event. To a lesser degree, certain scenarios judge a disabled starship to be a catastrophic danger to a greater number of living beings; e.g., the population of a planet, requiring the abandonment of the ship and its subsequent destruction to prevent the disaster.
In the preferred configuration, the starship undergoes rapid vaporization from thermal and mechanical shock caused
by a deliberate release of warp engine reactants. Remote computer system decryption algorithms generate one final set of cascade failure commands, and all engine safety interlocks are compromised. Matter from the primary deuterium tankage and the total supply of antimatter from the storage pods on Deck42 are expelled simultaneously, producing an energy release on the order of 1015 megajoules, roughly equivalent to 1,000 photon torpedoes.
If the command links to the engine systems are severed, the secondary destruct system is automatically selected. Ordnance packages are located at key locations around the vehicle, including the antimatter storage pods. These are detonated in concert with intentional overloads of all fusion reaction chambers. The release yield of the secondary system is calculated to be 109 megajoules, roughly equivalent to 500 photon torpedoes. The secondary destruct system becomes the primary system for the Saucer Module in Separated Flight Mode.
As an aside, the 1021
megajoules) number is interesting, as that implies the photon torpedo is a highly respectable 1018
J (239 megatons) which is somewhat higher than calculations that have been derived from this same book based on the antimatter load of the standard photon torpedo; perhaps there is a 239 megaton 'heavy' torpedo or some such.
Amusingly that also means that the entire engine system of a GCS is... 239 gigatons in a self destruct scenario, a little bit more than the Acclamator guns in the ICS. I guess we know what that'd look like then
(Not quite, as a beam weapon would go through the other side rather than exploding, but still).
In any case, it can overload the warp core, or use Ent-nil style destruct charges. There's a map in the book that shows dozens of large and small destruct charges throughout the saucer, three in the engineering section, and three in each nacelle.