Brilliant! In my ignorance, I imagined either a single super-science I-beam or a quad of box beams, but I like the look of this. The only thing I'd caution you about is the four grills visible on the inside of each original pylon. These suggest to me that there's some kind of open space beneath them.
Well, The inner "beam" isn't going to be the exterior skin... there will be a bit of hullwork on top of this, if only to provide a means for handling deflector shielding and so forth. It's not anything comparable to the "hull thickness" I've got elsewhere (which consists of mechanical elements, exterior skin, interior skin, probably some self-sealing compound, etc, etc).
I haven't forgotten about the grillwork. But, Vektor's version aside, I haven't seen any indication on-screen that the grillwork need be inset INSIDE of the hull skin. Rather, it looks to me as if these grills are applied to the skin.
My take on those grills is that they're radiator panels... probably the primary energy rejection system for the entire ship. There are also dedicated radiators for the warp nacelles and for the impulse reactors... the impulse one being the "fabric" we see on either side of the impulse deck, and the m/am reactor cooling system being on the nacelles themselves, since I reject the idea of the m/am reactor being in the secondary hull, and rather prefer the idea that the power is being generated on the 1701 in the nacelles themselves. In fact, I intend to implement a variation on the fan-pub "nacelle interior blueprint" from the late 1970s... which itself borrowed a bit from an episode of the animated show.
As radiator panels, they can be very thin. They're usually highly-conductive materials (copper, aluminum, etc) with fluid-flow capillary networks to transfer heat in, and a black surface (since black radiates better than any other color... just like it absorbs better). The thickness is really governed by "how thick do they need to be in order not to be too fragile" than anything else. So my plan is to have these be right there on the surface.
Torsional? Do you imagine the engines tending to corkscrew around their axies or did I just read something into your description you didn't actually put there? I'd argue the warp engines don't add any significant force to the vessel. Rather, they "just" warp space and everything within that warp moves. The only forces I see operating on the nacelles would be inertia when the ship accelerates using impulse engines or turns/banks. And even that may be greatly diminished depending on your view of intertial dampers.
EDIT: Yeah, I definitely think I misread you ... I don't know why the image of each engine "wanting" to twist around its long axis popped into my head on the first read.
Well, imagine you're holding a wrench. You apply a translational force to the handle, but where it become torsion over the length of the wrench. Force x radius = moment. A longer wrench handle lets you generate more torque with less force.
Now, imagine that the Enterprise turns. The nacelles aren't symmetrical, front to back, and the attachment point isn't at the centroid, either. So, the inertia of the nacelle will generate a torque. Same thing for a climb or a roll. (This is part of the advantage of the TMP nacelle pylons, and part of what is so much worse about the Abrams-prise.)
If the ship is operating in an inertia-less mode (as I tend to assume warp drive normally is) then this isn't an issue. But when moving under impulse power, the slightest manuever in any direction will apply a significant torque load on the structure, greater than the associated translational force.
Does that make sense to you? It's basic "mechanical engineering 101," statics and dynamics. But I know most folks in here haven't had the benefit of that sort of an education, so it's not surprising that it might not be well-understood by lots of people here.
Just imagine that you have a home-made "Enterprise" model on the hood of your car, and start driving around wildly, and imagine the forces the model will see as you squeal around corners.
Maybe it's best that Google failed to enlighten me on this issue, because I'd really rather not be caught faking any understanding and BS'ing my way through a reply. The depth of my exposure to engineering was writing programs for instrumentation engineers and reading P&I diagrams. So I'll ask: does "a little FEA problem" (finite element analysis?) mean you're going to calculate assumed loads and try to find an arrangement that gives you good numbers?
My bad... another "mechanical engineering" concept.
FEA (as you correctly surmised, "finite element analysis") is a computational method of determining the effect of loads on objects and systems. You break the system up into a tremendous number of very small, very simple elements, each of which is governed by a small number of simple equations. The equations, however, form a very large matrix, which can only plausibly be solved by computer.
FEA allows you to determine the stress, strain, deflection, etc, etc, to an object or assembly when under a given mechanical load. It can also be used to determine thermal flux throughout a system, or magnetic field densities... all variety of things which are effectively impossible any other way.
Basically... think of it as a "what if" tool... "if I apply a lateral force of 600N at the aft end of the nacelle, how much of an impact does it have on this pylon structure? And how much does it have on this OTHER pylon structure?" That's sort of what I'll be doing here... applying simple loads and determining which structural approach is the most effective... the strongest.
Would it help matters if you assumed the rigid components of the dorsal went through the primary hull with the latter attached to the sides? Maybe that structure above the impulse engines is also involved.
Oh, it is, it is. But think "torque" again. The saucer is very wide. Any force applied at the far port or starboard edges translates into a tremendous torque at the attachment point. Apply an upwards 10N force at the starboard edge, say. That results in a translational upwards force at the attachment point... of exactly 10N. But (and since I'm at work and can't measure things right now) let's say that the point of the force being applied is 200m to port,and 250m forward, of the dorsal centerline at the point of attachment. This means that there will be a "roll torque" (that is, around the forward-to-aft axis) of 10N x 200m, or 2,000 N-m, and a "pitch torque" (that is, around the left-to-right axis) of 10N x 250m, or 2,500 N-m. Now, suppose that the leading edge and trailing edge elements of the dorsal-to-primary attachment are 50m apart. Each element will, as a result, see 2,500 N-m/50m, or 50N of force. In one case, it will add, in the other it will subtract... so the vertical members of that attachment system will see +60N and -40N.
That's not bad... but now, consider the roll torque.
Let's say that the dorsal is 5m wide. And the torque applied to those elements is 2,000 N-m. That means that port and starboard elements will carrycarry (2000 N-m / 5m = ) 400 N, from that little 10 N applied force. As above, it will add or subtract depending on the side it's applied on, so you'll see +410 N and -390 N loads.
That joint is inevitably the weak point of the entire ship... much moreso than the nacelle attachment points. Do a hard port or starboard turn, and the saucer would be probably rip and rend away from the dorsal. So this particular detail is the one where I really can't see a realistic mechanical solution, and have to accept "filmmaking magic."