# Nacelles: if two are better than one, then why are three not better than two?

In Star Trek, the vast majority of Starfleet vessels (and prior warp-capable Earth vessels) are equipped with two warp nacelles.

Those very familiar with the Star Trek universe need no convincing about this, but for those whose Trek awareness is a little more tangential, some examples include:

The Phoenix  (first warp-capable Earth vessel)

Enterprise NX-01

USS Enterprise NCC-1701

USS Reliant NCC-1864 (with one nacelle blown off)

USS Grissom NCC-638 (about to meet its end)

USS Enterprise NCC-1701-A

USS Excelsior NCC-2000

USS Enterprise NCC-1701-D

USS Voyager NCC-74656

USS Defiant NX-74205

Even shuttlecrafts and runabouts have two nacelles:

On occasion, we have seen one nacelle...

...and three nacelles...

...and even four nacelles...

...but these instances are most definitely in the minority. This leads to the question:

Why are two nacelles optimal?

Equivalently:

If two are better than one, then why are three not better than two?

For answering, I'd like to stay in-universe wherever possible.

• Since you want in-universe, will leave as a comment: if two are sufficient, adding a third does not necessarily improve the system enough to make up for the cost and complexity of building and maintaining 50% more systems
– TZHX
Jun 7, 2015 at 14:38
• I have a vision of you designing a starship with 4000 nacelles. Jun 7, 2015 at 14:56
• @Praxis - "If 4000 nacelles is a good design, wouldn't 4001 be better?" Jun 7, 2015 at 15:04
• FTR note that it is perfectly common in real-world aircraft design, that, two large engines are better overall than four large engines (just look at actual large passenger airliners today). Similarly it's by no means certain that (say) 8 cylinders in a petrol engine is necessarily better than say 6. You're trying to optimise a set of equations: the optimum figure will be some value ("2" for modern very large aircraft engines, for example). Jun 7, 2015 at 16:15
• I feel like maybe we don’t need nine pictures of two-nacelle ships to get the point. Jun 7, 2015 at 23:01

This is discussed in the Star Trek TNG Technical Manual (considered a canon source of info about the Star Trek universe).

In short, different engine configurations have been tried in the past but for ships over a certain size, a twin-warp-nacelle design seems to be the easiest way to achieve a well balanced warp field as well as offering the best opportunity to come out of warp in a controlled way in the event of a catastrophic failure of the flight systems:

Second, a pair of nacelles is employed to create two balanced, interacting fields for vehicle maneuvers. In 2269, experimental work with single nacelles and more than two nacelles yielded quick confirmation that two was the optimum number for power generation and vehicle control. Spacecraft maneuvers are performed by introducing controlled timing differences in each set of warp coils, thereby modifying the total warp field geometry and resultant ship heading. Yaw motions (XZ plane) are most easily controlled in this manner.

During Saucer Module separation and independent operation of the Battle Section, interactive warp field controller software adjusts the field geometry to fit the altered spacecraft shape. In the event of accidental loss of one or both nacelles, the starship would linearly dissociate, due to the fact that different parts of the structure would be traveling at different warp factors.

Obviously we do see ships with multi-nacelle designs so this isn't a 'hard and fast' rule, but it does offer an in-universe glimpse at the rationale behind why we see this configuration so very often in both human and alien ships.

Out of universe, this was largely a design consideration based on Roddenberry's own prejudices. In a 2005 interview for Trekplace Andrew Probert (Concept Artist for Star Trek TOS and ST: The Motion Picture) gave them a solid overview of the design process as well as his own additions to the treknology canon:

Probert: Gene specified to me, in fact, that starship warp engines operate in pairs... only in pairs because they're codependent. If you had one warp engine, you'd probably go in a circle, I don't know... (laughs) So in the same breath he negated the three-engined dreadnoughts along with the single-engined destroyers, on the edict simply that, to achieve warp drive, you had to have codependent warp engine pairs. As far as the line-of-sight requirement, that was my edict, that, in order to be codependent, the warp engines had to "see" each other, totally. I'm taking about the power combs, not necessarily the Bussard collectors but the bulk of those combs have an energy path between them. And then for other starships, just like in World War II, where all the nations had fighter aircraft that all looked different -- you know, a cultural distinction between, say, a German aircraft and an American aircraft or a Japanese aircraft -- they all operated in the same way having the same basic components of wings, body, and engine, so I applied that thinking to the alien ships I designed as well, so the Ferengi ships, and Romulan Warbirds, have twin warp engines that have to see each other in order to operate. Even my shuttlecraft having a very shallow clearance, still see each other. That's why designs like the Romulan scout ship, where the engines cannot see each other, aren't consistent. There are also some cool Starfleet designs like the Nebula Class ships, but their warp engines cannot see each other. Even those runabouts ignore that ruling which messes up the continuity. Science fiction in particular NEEDS to be consistent. If you negate that,...it all falls apart.

• Interesting. This also seems to imply that those 2-nacelle warp engines are heavily biased towards yaw maneuvering - perhaps the three- and four-nacelled designs were meant to address this and evaluate how useful would the extra control be in practice (Stargazer in particular with it's long mission would seem like a good fit)? This would explain why they were not eventually fully pursued - the galaxy being mostly flat and combat being mostly "sea-like", pitch and roll aren't all that useful, especially in warp. We know the starfleet is pretty silly about their testing conditions, after all... Jun 8, 2015 at 8:22
• You could do a timed maneuver in any direction by rolling so it is a yaw maneuver. Jun 8, 2015 at 15:54
• @Richard - Great answer. In universe: So rather than increasing the number of nacelles to boost warp power, better to just increase the size of each nacelle and stay with two nacelles to avoid instabilities. Out of universe: Perhaps starships with lots of nacelles (10, 50, 100!) would have taken too much time and material for the modelers to build, would have been cumbersome, and may not have looked as aesthetically pleasing as only two nacelles. Jun 8, 2015 at 16:15
• This really is an amazing answer. Kudos once again, Richard. Jun 8, 2015 at 21:20
• Does that mean you could stop a ship going to warp by sticking a sheet of plywood between the nacelles? Jun 12, 2020 at 8:52

The first ship we see with an odd number of nacelles is the Niagara-class wreck at Wolf-259 http://www.trekplace.com/article15.html (although, the most famous is the Enterprise in the future timeline from the TNG episode "All Good Things...").

This is because of an old production design directive that ships had to have two nacelles - with an in-universe reason involving warp fields and so on. http://www.ex-astris-scientia.org/articles/design.htm (although, apparently, some ships with two nacelles could warp with only one nacelle being operational).

However, in TNG, we do see several vessels with four nacelles - the Constellation class. These were deep-space vessels, and the idea was that one pair of nacelles were a redundancy due to the lack of repair facilities in their operating regions. http://memory-beta.wikia.com/wiki/Constellation_class Some other classes also had four,

So, how do we explain the Kelvin and the alternate-future Enterprise?

Apparently, the Kelvin's single nacelle held two parallel warp field generators (annoyingly - I've seen this referenced somewhere on the internet, but can't find the reference to link to).

So - to answer the question:

The most common configuration of starships are dual-nacelled (even alien species follow this convention [Klingon and Romulan and Dominion being the most obvious]) - but there are other configurations. In the end, it comes down to engineering - two nacelle configurations must fit some optimal design providing efficient and reliable warp travel without too much engineering.

• Regarding that production directive: the prevailing wisdom is that Roddenberry implemented that directive after seeing some 3- and 4- nacelle designs in published material, because he didn't like the people who drew them :\ Jun 7, 2015 at 14:40
• interestingly: there have been quite a few "violations" of Roddenberry's rules:trekplace.com/article15.html Jun 7, 2015 at 14:46

The dominance of twin nacelle designs makes sense from an engineering perspective when you consider aircraft design.

IRL, trijets (jets with 3 engines) became really popular during the 70s and 80s because of their superior safety margin and FAA flight range rules that gave trijets more leeway than twin engine designs. Because of this, by 1980, trijets became the standard configuration for airliners, and they were considered the cutting edge designs of the time.

So why are they so rare compared to twin engine airliners today? (There are only 2 trijet designs in production today, and they're both business jets.)

Well, having three engines provides more redundancy and can enable greater performance than twinjets, but this comes at the cost of design complexity and cost/fuel efficiency.

These days, few airliners are designed with three engines because:

• Newer ETOPS (Extended-range Twin-engine Operational Performance Standards) rules lobbied by Airbus and Boeing enable twinjets to fly the same distance/duration as trijets.
• This change came partly because of improved twin-engine safety since the FAA introduced the 60-minute rule for twin-engine aircraft. When the rule had been conceived, most aircraft were propellor- and piston-engine-based. When jet engines came along, they provided much greater thrust and reliability, which have both steadily improved over time.
• Twinjets can be mounted symmetrically on each wing. Trijets require a third engine to be mounted along the central axis, which increases the complexity/cost of the intake duct design and engine mountings.
• A jet can really only mount the third engine on the tail, which typically requires a T-tail design. The T-tail has some advantages, such as better pitch control and glide ratios. However, they're also much more expensive to build and maintain (due to greater design complexity, stronger/heavier materials needed because of the higher forces generated by the tailplane and greater difficulty repairing/replacing the third engine because of its high placement) and are prone to deep-stalls.
• But the greatest disadvantage is the fuel efficiency and thus operating cost of trijets. Generally, it's much more efficient to run a single large jet engine than multiple smaller jet engines with the same combined thrust (in general, larger turbines are more efficient than smaller ones). So if you can get an adequate level of reliability and performance out of two engines, it's much preferable to adding a third one. And since turbine engines have gotten ever more reliable and powerful, trijet designs have understandably fallen out of favor.

So based on real-life experience, it makes sense that twin nacelle designs are more popular than other configurations. It likely provides just enough redundancy and warp capacity while balancing against production/operating/maintenance efficiency.

• "The dominance of twin nacelle designs makes sense from an engineering perspective when you consider aircraft design." Does it? Aeronautics is not astronautics. The former has more far more in common with fluid dynamics than the latter. The only reason some spacecraft share engineering similarities with airplanes is that they are reentry atmospheric vehicles. Ships built entirely in and for space will be totally alien to us air dwellers. Nov 2, 2018 at 0:04
• @bishop If you read the rest of the answer, it should be pretty clear I wasn't talking about aerodynamics. Nov 2, 2018 at 6:41
• You describe features of AIRcraft, and propose those would apply to SPACEcraft. Then you conclude "So based on real-life experience, it makes sense that twin nacelle designs are more popular than other configurations." Do we have real-life experience with warp fields? Nov 2, 2018 at 12:36
• @bishop I described tradeoffs between redundancy/reliability and efficiency/accessibility/cost in systems design for propulsion systems. Aerodynamics were not part of the argument. Your whole argument seems to boil down to "space" and "air" are not the same and therefore no experiences or knowledge from one industry can transfer to the other. I can tell you, for a fact, that you are severely misinformed. My real-life experience is working with aerospace engineers who've done both systems/propulsion design for both aircraft and spacecraft. Nov 3, 2018 at 22:09
• I'd also recommend looking up major AEROSPACE companies and see what range of products they build. Nov 3, 2018 at 22:11

I recall reading an in-universe story, in which it was explained that: In the early days of warp travel, some ships were built with three nacelles because the mathematics of warp propulsion suggested that such configurations would have better performance - BUT in practice these ships never performed as well as theory suggested, indeed they were nearly on par with two nacelle designs, so designs with three nacelles were quietly abandoned. Presumably later the theory improved to the point of explaining why these older designs failed and how to do three-or-more nacelles right.

• Welcome to SFF.SE! This looks like a good answer, would you be able to back it up with a direct reference to the source you speak of? Nov 8, 2016 at 15:58
• I would love to plus one this, if it had backing evidence from canon. Nov 2, 2018 at 0:05

Having more nacelles does not necessarily increase the forward reach of the warp field. So no real advantage. But a small quad cell design could create an exceptionally maneuverable warp drive fighter craft. Consider the field shapes obtainable with timing and frequency shifts. Consider the effectiveness of harriers in using their vectored thrust for improving their maneuverability.

• Could you go into more detail about the "field shapes" and why more nacelles don't necessarily increase the forward reach of the warp field. Aug 21, 2017 at 7:21