Exposed engines and fuel lines

Yo I like Heinlein dawg, but that's one fucking book compared to the whole of 40K.
Not to mention 40K has things that are in Heinlein as well.
Is it even possible considering the now Chaos Space Marines used to be in the Legion, as well as more than ten thousands muhreen died in the Heresy?

If those are the only sci-fi works you can come up with I seriously doubt that you have more than a passing interest in science fiction to begin with.
You fail to realize that science fiction has a definition that comes from the term itself: Science that is based on fiction.
Tell me a single aspect of 40k that is based on science. Give me a single piece of research into physics, engineering, biology or sociology (the main four sciences most science fiction works are based on) that a major aspect of 40k was written around.
I assume you will fail to provide a single one.
Here are a few "fantastic" elements that can be found in 40k on the other hand:
A parallel universe that reacts to the thoughts and emotions of all living and warp-sensitive beings. It also has an effects on said beings, and can alter their state of mind. It is populated by manifestations of the most extreme emotions and is used for long range transportation and communication.
A piece of equipment on any warp capable vessel that uses SECRET AND ANCIENT TOTALLY SCIENCE AND NOT MAGIC FOR REALS! that allows the vessel to pass into the warp and survive the journey.
A person who is TOTALLY NOT A MAGE, who can see and feel the warp, and can navigate a vessel using a third eye that mutated to their forehead.
People who can use TOTALLY NOT USE MAGIC and instead rely on the powers of the warp to bend reality. They suffer great consequences for their immense powers though as the warp can end up bending their minds too, or attract dangerous demons.
Super Soldiers wearing knight armour, but in SPACE and with super duper totally science and NOT MAGIC armour that can withstand most blows from mere mortal foes, but fails against melee attacks from other, evil knights, or deadly monst–er–XENOS

Don't get me wrong. I agree with you. WH40k is a fun setting for YOUR DUDES, but it's not a scifi setting.

Reminder that 40k is just Warhammer Fantasy IN SPACE.

Durr-hurr, those are just science we do not discover yet.

There is nothing far fetch about 40K tech actually, only the stuff related to MAGICK/Psy that is outside of our comprehension.

Nobody is going to deny that.

...

Great crusade numbers
20 legions, with at least 10 chapters each.

'present day numbers'
No legions

The difference between the two only gets sillier if you assume that every players custom marines represents an additional chapter that's not mentioned in official records, and that's before we take the Primaris nu-marines into account.

Most of it is based on conventional tech, except with NEW ADAMENTIUM SUPER HARDENED ALLOY or nuclear fusion energy.

These aren't far fetch, they are just undiscovered.

Source on the several thousand chapters?

You are thinking of navigators. Astropaths, as their name suggests, are psykers capable of telepathic communication at interstellar distances.