star trek - Why did Starfleet choose saucer-shaped starship design?

Purely in-universe speculation.

Warp fields are regularly represented as a squashed torus. This represents the volume of space the warp field can encompass. The filled can be stretched and spindled to wrap closer to the ship for various reasons (power usage, speed, defection, etc.) but at it's lowest complexity it is torus.

So a encompass the most amount of mass in the warp field you will shape your ships to fill the field as much as possible. That gives you a few options,

1. Making purely spherical, or elliptical ships.


2. A half circle crew section vacuum gaped from nacelles.


3. Saucer crew section vacuum gaped from nacelles.

The first is materials intensive, and does not allow for vacuum gaping the nacelles (highly energetic and hazardous to health) requiring more shielding. And the saucer shape is more structurally stable than the half circle.

So the early ships be designed to fit in a warp field as low energy and stable as possible, be as structurally stable as possible to get the the most mass and volume moved with the minimal amount of energy expenditure. From this you can estimate the class of a vessel by it's geometry.

• Slower cargo vessels have shorter travel distances between refuels would not need the efficiency of Explorer/Military vessels.


• Shorter range vessels are more wedge designed, have a less round footprint (Intrepid, Olympic)


• Combat vessels are more blocky as their armor/reinforcement can double as shielding from the nacelles. (Defiant, Nebula)


• Long range vessels are larger, but fit evenly within the torus with minimal wasted space. (Galaxy, Ambassador)

As with all design it is a balance between performance, power usage, and stability.

And we see other races following similar guidelines:

• TNG Romulan Warbirds fit the torus footprint almost perfectly and while maintaining the vacuum gap and fitting more mass into the same volume as Federation vessels. They get one of the best balances of power, range, and stability of any warship. It is one of the reasons why they are some of the most capable warships.


• Klingon vessels tend to be guns with a power-source and nacelles stuck on. They don't fill the torus footprint very well, probably trading vacuum gap for shielding, but are very flat (bird of prey arms swing horizontal for warp) allowing them to gain torus efficiencies allowing further range. Older style cruisers were less flat but they did not need the range of Federation explorer vessels as they were closer to base and do not need the efficiency. Also they have a history on not worrying too much about shielding their crew from the effects of the "Engine Pit", so vacuum gaping is preferable to added shielding.


• Ferengi vessels also fit the torus footprint running for range, but their nacelles are part of the main body with only the forward section vacuum gaped. So either more shielding or they use that section for cargo which would fit their idiom.


• Cardassian ships tend to break the torus more, but they are patrol/defense ships. They tend to be sorter range and more reliant on supply lines. On a side not there seems to be 2 almost traditions of shipbuilding, clockwise from earth they tend to be wedges with engineering and nacelles combined, while counter-clockwise they tend to be vacuum gaped.


• The Dominion - similar philosophy and limitations of Klingon but more structurally sound.

A big divergence from this is the Vulcans, but their ships seem unique as the use nacelles perpendicular to the line of motion as opposed to parallel. I don't know what benefits that would bring other then the Vulcans being able to smug that their warp fields are 90 degrees of everyone else's.

-Side note
It was specified in-universe (Specifically ST: Voyager) that the Borg do not use the same warp methods as other species. I believe they used trans-warp conduits to travel. Therefore, because they use different methods than usual species, they may have no need to create designs that are as "efficient" as other ships.