As far as your specific example of IR sensors, presumably the sensor elements themselves would be as well isolated from warmer parts of the ship and thus tend to be not much warmer than open space to start with. I don't think there would need to be tens of thousands of meters of coolant loops, but neither of us can know with any certainty.
They would need to be actively cooled otherwise they'd be unusable any time they were exposed to significant irradiance. Being anywhere near a star and not kept on the dark side of the ship for long enough to cool down would effectively blind them. Even their own power consumption could do that if they were isolated from the rest of the ship, because they couldn't benefit from the ship's radiators...without expansive and delicate radiators of their own, that could be shielded from incoming solar irradiance, they'd overheat themselves. It's hard to passively cool things in a vacuum.
So I'm just going by typical mass distributions in vessels up to this point in history.
Which is silly in and of itself.
Real vessels up until this point in history are mostly chemical rockets with enormous fuel fractions and extremely poor specific impulse. Modern spaceflight is dominated by limitations to delta-v imposed by such constraints.
Ships in Elite, are predicated on this power/impulse limitation being virtually absent. We can power ships for protracted periods of time and run engines with enormous thrust that require trivial reaction mass. That sort of paradigm shift in power and propulsion would make vessels that bore significant resemblance in mass proportions to our current spacecraft unthinkable.
There have been similar shifts in other propulsion technologies in the real world that can be used as an example.
Imagine going back to the late 19th century or early 20th century and telling shipbuilders or naval strategists, who are used to the most efficient warships of the era burning 10-20% of their mass in coal every couple of weeks that, in less than 70 years, there will be ships that can, with under 0.1% of their mass devoted to fuel, steam at full power for a decade straight. It would sound ludicrous, and it would allow for equally inconceivable sorts of vessels. It's also exactly what happened when nuclear power was applied to naval propulsion.
So the notion that the sensors would weigh as much or more than the propulsion system on a vessel that can travel from one end of the galaxy to the other just seems silly at a glance to me. Especially sensors that fall off at less then 20km. Likewise that they'd weight twice as much as heavily armored bulkheads built to withstand combat. Of course you're welcome to disagree and I respect your technical observations.
I don't disagree with this, never did.
I'm just saying that this was never the point, because neither I nor Ramius were comparing sensors to anything else. We were comparing sensors on one ship to sensors on a vastly larger ship.
Kinda feel the same way about shield gens offering more or less protection based on the ships theyre outfitting in.
I never had any problem with this. Shield emitters are part of the vessel itself, not the shield generator. Diferences in configuration and efficiency of those emitters, combined with different shapes and volumes that need to be enclosed in a shield bubble, make different shield strengths between different ships with the same generator about the least implausible aspect of shielding.
Afterall, no one expects the same amplifier to result in the same sound or signal or whatever when attached to different speakers, radio antennae, etc.