star trek - Why aren't Federation ships built with two warp cores?

20th century humans learned the value of redundant engines on aircraft. If one fails, you can keep flying.

But in the Star Trek universe, Federation ships like the Enterprise have only one warp core. (AFAIK — I can't say I've examined every Federation ship before asking this question.)

Why is this so? If the warp core is so important, why wouldn't ships be built with two, so they have a backup? That way, if one fails, you wouldn't need to, oh I don't know, send key crew members into an irradiated area to fix it.

I understand that a second warp core would, in practice, deprive Star Trek of many storylines. What I'm asking is whether this restriction has ever been explained in-universe.

Answer

Brandon and gnovice have made some good points, and I just want to add that:

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  If a single-engine plane loses that engine for just a minute or two, it's going down. This isn't the same situation with vessels like submarines and cars, which generally only have a single power supply. The Los Angeles class attack subs only have a single nuclear reactor, though it powers multiple steam turbines which in turn create electricity and drive the propeller. The same is true of the Ohio class ballistic sub. If the reactor were to lose power or need shutting down, they'd probably just surface using battery power (or their emergency diesel backup). Losing the reactor in itself isn't a catastrophic disaster.

  
  

  Similarly, starships don't actually "fly" most of the time. They're in deep space or at least high orbit. If the ship suddenly went dead, it would just be adrift, not fall out of the sky.

  
  
  


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  Indeed, if a starship's warp drive were to go offline, it would likely be fine for several days, which should be enough time to make repairs, evacuate, or ask for assistance. In fact, the saucer section seems to be able to travel for quite some time using its own impulse engines and warp sustainers completely separated from the warp drive. So there must be enough leftover drive plasma circulating through the saucer section or enough leftover electric energy stored in batteries for the Saucer section to continue operating under normal power for at least a few days.

  
  


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  On the other hand, if a ship like Voyager were flying within a planetary atmosphere, and it suddenly lost impulse power, it would likely crash even faster than a plane. But most starships seem to have multiple impulse engines.

  
  

Also, warp drives are much more reliable than jet turbines or internal combustion engines. They do occasionally need to be shut down for maintenance or get taken out of commission by spacial anomalies or enemy attacks, but these are incredibly rare events that are similar to nuclear reactor meltdowns.

Ideally, yes, you would have redundancy for all critical systems (an extra main computer, extra main deflector, extra dilithium matrix, an extra captain, etc.), but you have to weigh the costs of operation and maintenance (and the additional dangers of operating a second warp drive) against the actual benefits you'd gain from it. And it's not clear the benefits of a second warp drive are worth doubling the size of the engineering section and engineering crews on each ship.