A side bar trying to explain hull geometry, time barriers, and nacelle numbers.
Geometric Complexity: Warp Nacelle Combinations
Once a starship design team has chosen the specific model of nacelle and general hull proportions based on mission parameters, the next step in design involves choosing the number and orientation of nacelles along with precise hull geometry. Both subspace and normal space physics have conflicting impacts on performance that designers must balance.
Basic structural efficiency favors spherical hull forms, maximizing internal volume while minimizing surface area. However, a true spheroid introduces substantial subspace field distortion, resulting in a phenomenon known as "subspace drag". The drag occurs as the geometric effect of the hull shape on subspace fields force overlapping warp fields to compress. The compression then distorts the field geometry at the trailing edges of the field, disrupting the coherence of the warp field and reducing its overall effect on the hull within.
The subspace drag phenomenon is more pronounced at higher warp factors and field intensities. At a high enough power level, the distortions in the warp field are no longer concentrated on field's edges and instead begin to impact segments of the propulsive bubble. Some parts of the space frame become exposed to normal space physics effects as these "gaps" in the field increase in number. Eventually hull segments begin to dissociate as relativistic effects begin impacting hull materials caught in the compression effect.
The dissociation effect occured at different field intensities depending on hull configuration, but was almost universal at fields past 400 cochrane intensity in 22nd and early 23rd century engine configurations. Originally labelled the Time Dilation Barrier due to the fact that relativistic effects were responsible for asynchronus component dissociation by the 2180s it began to be referred to simply as the "time barrier". It would not be until the 2230s that universal application of duotronic processor technology allowed field control software to compensate for subspace compression effects, effectively breaking the time barrier.
Although time dilation dissociation is no longer a factor with modern nacelle control software and coil designs, subspace drag remains a consideration in ship design. High drag coefficients begin to eat into the effective field strength of a warp field, leading to reduced propulsive performance. Starfleet's favored compromise between lower subspace drag coefficient and structural efficiency remains the saucer shaped primary hull. Greater understanding of subspace field dynamics since the 2340s has led to substantial changes from the basic truncated cylinder popularized in the 23rd century to more ellipsoid hulls.
Warp nacelle orientation and number have several distinct effects on starship performance. A typical Starfleet vessel dedicates roughly 1530% of its total mass on warp coil. Even modern subspace field coil based impulse engines and RCS assemblies face challenges from the weight distribution of large nacelle structures. Nacelle placement makes an enormous difference in sublight performance. Designs with nacelles closer to the centerline make more maneuverable sublight combatants.
The opposite is true for warp maneuverability. The asymmetry between subspace fields projected by warp nacelles is the primary driver of warp maneverability. The further apart the nacelles, the more flexibility a ship has in adjusting its field intensity to maneuver at warp speeds. Most Starfleet vessels attempt to balance the positioning of warp nacelles to combine acceptable sublight performance with effective warp maneuverability.
The number of warp nacelles impacts the overall mass of a starship and the efficiency of its drive components. A single nacelle greatly decreases the weight distribution problems at sublight velocities and generally helps produce the "cleanest" warp field in terms of hull geometry's influence. The result is a lower subspace drag coefficient and smoother acceleration between peak transitions. Single nacelle configurations however suffer from a crippling lack of maneuverability at warp, and a difficulty with producing more than a small number of field layers.
Horizontal paired nacelles offer the greatest tradeoff between warp maneuverability, general propulsive efficiency, and peak performance. Ships with horizontal nacelle pairs tend to have substantially better yaw performance, a trait often making threedimensional tactical maneuvers at warp more difficult. This partly explains why Starfleet fleet deployments tend to feature formations with ships oriented toward horizontal rather than vertical station keeping.
Trinacelle designs are generally an attempt to compensate for hull forms with greater volume to surface area ratios. The third nacelle helps provide a reinforcing field layer to reduce the impacts of subspace drag. Unlike in single, dual or quad nacelle designs, the third nacelle in these configurations are not used as a main propulsion unit. This often leaves them underutilized and overequipped for such limited use. (It has also spawned the term "third nacelle" to mean an awkward tag along to a group who serves little purpose)
Finally, quad nacelle configurations offer a design flexibility at the cost of increased complexity. Quadprojected warp field geometry requires substantially more tuning to negate the impacts of subspace drag. Even the standard compression effects from drag are much larger, and a ship equipped with 4 nacelles is required to substantially reduce the amount of hull space located near or behind its warp coils. Both the Constellation and Cheyenne class starships demonstrate this principle by omitting or substantially downsizing their secondary hull. However, quad nacelle configurations offer superior warp maneuverability, particularly in unorthodox pitch and roll maneuvers, making the ships well suited to eluding pursuit or exploring regions of space "offaxis" on the galactic plane.
Coil longevity and cruise endurance are both increased in quadnacelle designs. Lower field intensities can be used from each nacelle to sustain a given speed, allowing the use of fusion power sources to sustain integer velocities or reduce the strain of high energy plasma use on coil elements. In addition warp field manipulation software can operate the nacelles in pairs, idling the remaining pair to reduce coil strain. The cost for this increased superluminal flexibility is significantly greater ship mass. The heavier ship requires much more powerful impulse drive units to achieve acceptable sublight maneuverability.
