Tyranids are Bull

Yeah, Warhammer 40K isn't really serious, but of all the factions in the game the Tyranids are the least serious of them all.

Based on the number of passes on the laws of nature and intelligent behavior that they are granted, the Tyranids aren't science fiction, they aren't skiffy, but are firmly in the fantasy camp.

First of all, telepathy.  Sure, telepathy is part of the WH 40K universe, but since telepathy is bunkum by any measurable standard (not involving gullible people), their being telepathic moves the 'Nids one step away from science fiction.

Step two comes in a form that Games Workshop has probably never given any thought:  The energy requirements of looting a planet of its entire biosphere.  To take a certain mass and lift it out of earth's gravitational pull requires enough energy to boil an equivalent mass of water, many, many times over.  Since, we are told, the Tyranids are a completely biological race, making no use of other forms of technology, all of their energy comes via chemical means.  This mean enough energy to accelerate the biomass (and the forces sent down in the attack) to the escape velocity of the planet and then of the system from that planet's orbit.  For Earth, this is 42 km per second, or 882 MJ per kilogram of mass.  A kilogram of octane releases 45MJ when it is burned, so for the Tyranids to take a given mass of a planet's biomass with them to their next victim requires an energy equivalent of burning twenty times that amount of fuel.  Sure, they could capture the energy of the planet's sun, but that would require the overwhelming majority of Tryanid biomass to be devoted to photosynthetic organs.

Taking them further away from reality is the unavoidable fact of Newton's laws of motion.  In order to propel themselves through space, the 'Nids must push against something that itself must be left behind, which means leaving mass behind.  They must eject some form of matter from the fleet in order to make the fleet move.  The amount of ejected mass is variable (the more energy put into a given amount of mass, the more momentum gained for the rest of the fleet), but it cannot be completely ignored.

In a space-based conflict the Tryanids are at their weakest.  Regardless of their hive mind, vast numbers, or other advantages that serve them well on the ground, their complete reliance on organic technology means that their weaponry will use chemcial or biomechanical propulsion, meaning that the very fastest Tyranid projectile will be no faster than the very slowest projectile used by any opponent able to conduct military operations in space.

The Tyranids are by no means undetectible in space, notwithstanding their non-use of electrical or mechanical power or metallic components.  Everything in the Tyranid fleet is well above the temperature of the background of space, and if they are anywhere a planetary system they will more than warm enough to give a nice IR picture to anyone capable of navigating space.  To an opponent with any form of energy weapons, the Tyranids will be sitting ducks in space.  One nuclear warhead--against which a purely biological space fleet will have utterly no defense--will be more than enough to destroy any hive ship within several kilometers.

Finally, compared to the vast amounts of material available from nebulae and gas giants, raiding earth-like planets for the material of their biospheres is like Tiger Woods giving up golf in order to run a convenience store in Detroit; the risk is far higher and the gains far lower.  You would think that the eons-old Hive Mind would be smarter than that.

So the Tyranids aren't nearly the unstoppable force that the WH40K portrays them to be, and they certainly don't belong in a science fiction setting.

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