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Old October 12 2013, 01:53 AM   #60
Nob Akimoto
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Re: Starfleet Procurement Policy Draft

New side-bar. Warp nacelle design and field configuration.

Nacelle Design and Field Design
Starship design is always a compromise between competing design goals. In warp propulsion the characteristics in the balance range from cruise endurance, energy efficiency, field density, and coil life. Modern warp nacelle designs generally have a balanced combination of the requisite properties, but ship design also plays a significant part in determining the capabilities of a warp nacelle.

For example, the Nebula and Galaxy class starships share a common warp nacelle in the GN-41, but have substantial differences in their hull, mass, volume configurations and as a consequence significantly different field geometry. Variable field geometry has been a goal of starship designers for the last twenty years as subspace physics are better understood, but a practical in-flight configurable geometry with greater than 10-20% variability in field configuration remains in the realm of speculative fiction and holonovels. The most flexible field dynamics currently exist on ships with variable pylon geometry like the Intrepid class, but even their highly efficient warp systems are incapable of reconfiguring field geometry by more than 25% without substantial replacement of nacelle elements like coils.

Perhaps the greatest trade-off required of modern warp nacelle designers is between integer and decminal warp factor power consumption. Raising the energy efficiency of a nacelle at integer velocities increases its power consumption near peak transition thresholds, while engines optimized for lower power transitions require greater power to sustain an integer speed. The GN-41 series features greater integer efficiency while the LF-42 in the Sovereign is focused upon peak transition efficiency. The ability for a coil to generate a high cochrane field is not as important in modern engine design as its power consumption at a given field strength.

The GN-41H Mod.3 (Galaxy-class 2380 spec) and LF-42B (Vesta-class and Sovereign 2382 spec) each have similar top end performance, theoretically capable of sustaining a field of up to 10,000 cochranes without substantial coil stress. However, the GN-41H's power consumption past Warp 9 increases at a substantially higher rate meaning a ship with that engine can only sustain a maximum speed of Warp 9.9 (3,053 cochranes) for 12 hours compared to a LF-42B equipped ship's Warp 9.99 (7,912 cochranes) for 12 hours with a standard tanker fuel load of 6,000 m and identical output Class XII warp core. At integer velocities, the gap in endurance is reversed, with the GN-41H capable of sustaining Warp 9 (1,512 cochranes) for 3 months compared to the LF-42B's 1 month with the same fuel capacity and warp core configuration.

As a consequence nacelle choice and warp geometry calibration are highly dependent on the needs of a given starship. Long range starships of explorer configuration are equipped with high integer efficient engines, while ships with a greater requirement for rapid deployment have peak transition efficiency boosted engines. A typical Sovereign class starship equipped with a LF-42 series nacelle pair requires nearly three times the fuel stowage compared to a Galaxy to attain similar mission endurance of three years at cruise velocity (warp 8 as of 2381). However, in a crisis situation, the Sovereign is capable of sustaining Warp 9.99 for 36 hours compared to the Galaxy sustaining Warp 9.9 for 12 hours.

In general greater Z-axis compression increases integer warp factor efficiency while peak transition thresholds are easier to cross with a smaller x-y axis profile, helping visual cues identify a starship's emphasis. Some ships like the Intrepid class attempt to combine properties, using high density warp coils and variable pylon geometry to switch from z-axis compressed fields to a narrower warp field to sustain speeds in the Warp 9+ range. In practice this leads to a substantial decrease in coil life, requiring regular refurbishment every 400,000 cochrane-hours due to inconsistent field stress on the coil structures and the greater density of the coil resulting in greater material stress. By contrast the typical coil life of a GN-41 series warp nacelle exceeds 1.2 million cochrane-hours between refurbishment and nearly double that between replacement.

Finally mention must be made of coleoptric warp "nacelle" configurations. Typically used by native Vulcan craft, a small number of Starfleet ship designs utilize this design of nacelle configuration. Coleoptric drives have excellent acceleration and peak transition crossing capabilities at lower warp velocities and have extensive coil lifespans in the order of 2-3 million cochrane-hours. The drives are inefficient at higher warp factors and do not sustain decimal warp velocities well. As a consequence they are most useful for ships that require long endurance at moderate warp factors, low-end acceleration with large mass burdens, or long deployments without coil refurbishment. They are usually found in workhorse civilian designs with a rated output of 350 - 650 cochranes. Currently Starfleet does not operate any mainline starships with coleoptric warp drive configurations.
Credit: Timo Saloniemi on the "GN" series nacelle name.
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