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I know nothing about the military, so I apologize if this is obvious. Why is this practical?

CRUSHER: I recommend we evacuate and seal off all non-operational areas, and group the families and crew on the odd-numbered decks.

Booby Trap

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The ship is experiencing a rapid energy drain and is operating on reserve power. Since we know that a very sizeable chunk of the ship's energy output is channelled into keeping the crew alive (59.2%?), taking out half of the ship's decks would result in a dramatic lowering of their energy needs and usage for heating, gravity control, lighting, life-support, etc and hence increase the length of time that the shields can remain up.

GEORDI: With the engines idling, the energy loss has been limited. But our reserves will be depleted in less than three hours. We won't be able to hold our shields in place.

As to why they have them move to the odd-numbered decks (rather than having them all cluster in the centre of the ship, for example) one assumes this is because a) Now only half the ship's crew have to move at all b) The half that have to move only have to move up or down one deck, minimising turbolift use.

(h/t to ApproachingDarknessFish)

Valorum
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    @EikePierstorff By using odd-numbered decks, half the crew doesn't need to move, and the other half only needs to move one level. In order to use, for example, the upper half of the ship, half the crew doesn't move, and the other half has to move on average about 10 levels. – ApproachingDarknessFish Nov 13 '18 at 08:26
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    the life-support requirements should more depend on the number of lifes, not on living space. everything exept gravity at least. – ths Nov 13 '18 at 12:14
  • @ths heat regulation as well – user20310 Nov 13 '18 at 12:16
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    @ths - Actually it's not entirely clear why a) Why life-support takes such a colossal amount of power and b) Why when life support shuts down every starts choking instantly instead of just breathing the available hour for several hours. Chalk it down to the need to inject urgency into a scene. – Valorum Nov 13 '18 at 12:33
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    @user20310 dissipating body heat depends obviously on #bodies. (in space, you don't need heating) – ths Nov 13 '18 at 12:42
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    @ths yeah, but you do need more cooling – Christopher King Nov 13 '18 at 13:55
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    59.2% is for a much smaller ship. I'd imagine the absolute percentage of power used for the tiny cargo ship referenced in that episode would be different from that of a Galaxy class ship (bigger integrity field, etc). – forest Nov 13 '18 at 14:17
  • @forest - Indeed, and hence the ? to indicate that it's not an absolute figure – Valorum Nov 13 '18 at 14:20
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    The 59.2% statistic is a good find, but seems ridiculously high when you consider the physics. We're comparing the energy needed to bend space time to the energy needed to perform menial life support. You'd figure they must be doing something supremely wasteful, and sure enough we do know the replicators on a ship rely on mass-energy conversion, and they're probably using replicators for a lot more than what we see on screen. – 16807 Nov 13 '18 at 16:24
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    @Valorum That number may also include gravity plating and heating – Izkata Nov 13 '18 at 19:20
  • Given the usage of replicators and holodecks, especially in TNG and later, I think it's safe to say that they are pretty casual about energy use. They're either very wasteful or they have a staggering amount of energy generally available. – Upper_Case Nov 13 '18 at 19:30
  • Could it be that you don't want to unbalance a ship by having everyone concentrate on one side of it? Like having everyone on a naval ship on the right side - that would be bad. Same idea, in 3D? By using odd floors, people are kept all through the ship, but vacate half the floors. (I'm not saying it makes a lot of sense in microgravity, but Star Trek spaceships do operate often like naval ships or airplanes.) – Galastel supports GoFundMonica Nov 13 '18 at 20:23
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    Also, having had to run to a bomb-shelter more than once in my life, I can tell you that you want the place you evacuate to in a dangerous situation to be as near as possible. If you need to move to a distant location, there's panic, and children getting lost, and bottlenecks that slow everything down causing more panic and more lost children... Going one floor up or down is faster and easier, thus safer. – Galastel supports GoFundMonica Nov 13 '18 at 20:31
  • Because ST writers weren't known for their technical accumen. – RonJohn Nov 13 '18 at 21:32
  • @Valorum https://scifi.stackexchange.com/q/4461/16157 – congusbongus Nov 14 '18 at 03:14
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    @ths "Life support" means something very different in space than it means in a hospital: it's not so much about keeping individuals alive as it is about maintaining a space in which people can live. You need atmosphere of the right composition and pressure, climate control with a livable temperature, gravity at a comfortable and useful level, and so on. These things don't depend so much on the number of people on the deck, so by isolating the crew onto half the decks, you can turn off the power to the other half while supporting the same number of people. – The Spooniest Nov 14 '18 at 16:40
  • @TheSpooniest atmosphere and temp changes are in fact very closely correlated to the number of people. i don't expect them to have a lot of combustion engines running. the only unknown is the energy requirement of grav plating. – ths Nov 14 '18 at 17:55
  • @ths The total volume of atmosphere is probably more dependent at that scale of a ship on the number of decks than the number of people, so you could probably get some savings there too, even if it's just in the amount of air the life support systems have to circulate. – Austin Hemmelgarn Nov 14 '18 at 18:45
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Using odd numbered decks is simple, convenient, and easy to communicate.

The key point here is to reduce the number of occupied decks. By specifying odd-numbered decks, people only have to move one deck up or down. Only the people on even-numbered decks have to move at all and it is easy for people to verify for themselves that they have moved to a safe deck. This simplicity should make the evacuation process easier and reduce the risk of anybody being on an unsafe deck by mistake. If you add the additional instruction that people on even-numbered decks move one deck up, then everybody is moving in the same direction.

It is a bit like the problem of separating a group of people into two equal sized sub-groups. If you just say get into pairs, they will take forever. If you line them up and give them each a number, it is much easier to divide them up as you wish.

DonBoitnott
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Chris Johns
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One good reason would be to minimise risk if the situation escalates further.

Escape pods are evenly distributed across the ship.

Getting people onto even-numbered decks means that if they need to evacuate, then people can get to the escape pods quickly and efficiently, and with minimal overcrowding.

On the other hand, if everyone is moved to the upper decks and you need to evacuate, the escape pods on the lower decks would go unused and the ones on the upper deck would be overcrowded.

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  • Putting everyone on the upper half of the ship doesn't overcrowd the decks any more than putting everyone on odd numbered decks. In the the first case, the lower deck escape pods go unused; in the latter, the even deck escape pods go unused. Either way, the escape pod capacity needs to be double the ship population. – Nuclear Hoagie Nov 13 '18 at 15:23
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    @NuclearWang but if you assume escape pods on every deck with some surplus capacity but not huge amounts, people are 0-1 decks from a pod in one situation, while in the other some of them may be half a ship away – Chris H Nov 13 '18 at 15:41
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    @NuclearWang - if everyone is on even decks, they can get to the pod on the adjacent decks reasonably quickly; even in the worst case, if the turbolifts are inactive, the next deck is only a short jeffries tube away so if the pods on the deck you're on are full, then you still have options. If everyone if on upper decks, the pods on the lowest decks can't be accessed in a timely manner at all by anyone; you don't have another in an emergency option if the pod is full. – Spudley Nov 13 '18 at 15:43
  • My understanding is that they basically shut down the decks not in use. Once you've sealed off the unused decks, you can't access the escape pods from there anyway. Not much of a "seal" otherwise. – Nuclear Hoagie Nov 13 '18 at 15:57
  • Escape pods can only be distributed near the surface of the ship, which means rather un-evenly... – einpoklum Nov 13 '18 at 16:23
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    @einpoklum - The ship also has emergency transporters situated all over the ship including in the centre of the saucer section – Valorum Nov 13 '18 at 16:29
  • @NuclearWang shut down could well mean no lights, uncontrolled temperature, no oxygen supply/CO2 removal, but would be unlikely to mean venting to space. It would still be possible to pass through these areas for some time after they were powered down. Sealed access point can be unsealed to allow passage if safe – Chris H Nov 14 '18 at 14:01