Since Space Hulks are basically ship wrecks, fused together in the warp and having some dimensional instability, I can only assume that they lack any functional Gellar Fields.

So how can xenos like Orks or human renegades even "hitch a ride" without being torn apart by certain entities of the warp, etc?

Some ships that have been amalgamated to Space Hulk might still have working Geller Field generator. So they might have bubbles of Geller Field protecting specific areas of Hulk where Orks/Stealer/Humans hitch a ride.

Also, Orcs use their wards to protect themselves. Whether this is a Gellar Field like IoM utilizes, talismans, chalk, or a rat's skull, is left vague and at the GM and Players interpretation. So, theoretically, Orcs could create wards in certain areas to protect themselves. For Genestealers that constantly seem to thrive on Hulks, well, no one really knows. Can't really ask them, you know. For Human Renegades, probably what Routa-maa mentioned is the best guess on how they survive. And simply because your vessel losses it's Gellar Field in the warp, does not mean all 100% of your crew are instantly dead (however, it could happen). It could be 99% get eaten/tormented/captured/shredded/devoured/turned-inside-out/head-exploded/killed-themselves/etc, so that 1% is still hiding on the ship (and probably gone bat-crap crazy).

Yeah, I think we can flub that both the Orks and Nids are protected by the gestalt power that makes each cool. Put enough Orks together, and they start deciding how the warp works, while the Shadow in the Warp might shield some Tyranids, and their lack of souls, more or less, might also stave off the Ruinous Powers' minions; in either case, the daemons are likely to say "not worth it; not gonna get anything." While I think that the Tyranid Codex was sort of crud (I'm no expert, but I don't remember any happy reviewers, personally), there is a wonderful description of the Fall of Shadowbrink (Codex Tyranids 6E, p.28) While the Nids presented might not fare as well as this description illustrates, it does a wonderful job of showing how the warp daemons and the Hive Mind duel, and while neither side has it easy, the Hive Mind does, eventually, persevere, as the Daemon, even the four Greater Daemons running the show, run out of power feeding them.

If nothing else, Daemons don't do much with Orks or Genestealers, anymore, so they probably aren't attracted much to those signals on a Space Hulk. Don't know.

For Genestealers that constantly seem to thrive on Hulks, well, no one really knows. Can't really ask them, you know.

Actually you can. Genestealers are intelligent and many are capable of speech, just not the purebreed ones (like those found in codex tyranids). Although getting answers out of them might be kind of problematic.

Also, Orcs use their wards to protect themselves. Whether this is a Gellar Field like IoM utilizes, talismans, chalk, or a rat's skull, is left vague and at the GM and Players interpretation. So, theoretically, Orcs could create wards in certain areas to protect themselves.

To a degree but my undertsanding is that their primary protection from the warp aboard their ships are the large snarling faces and spikes and whatnot they build out of scrap and attach to the things. The ork logic being that such intimidating features will frighten away da pansy warpy thingwutzits and because there are thousands on orks present on such a vessel with their WAAAGH energy constantly being soaked into it and they believe the method works....then it works.

Or it works comparably to the crap-ass geller fields that get cranked out in Imperial factories anyway but from an ork perspective having a deamon occasionally get aboard is probably seen as a source of entertainment.

To my knowledge, as has been said, the orks' gestalt psychic field likely functions as a rudimentary form of gellar field. Essentially, because orks are orks...and orks dont fret 'bout nuffin, warp entities won't pose the same danger to orks that they do to 'oomies. Plus, forgive me if I'm wrong, but weren't chaos orks sort of retconned out of the fluff?

Note, this is not to say that these warp entities wouldn't rip orks up in a mundane, teeth and claws sort of way.

Finally, with respect to gene stealer... aren't they essentially soulless. Thus, immune to warp fuckery?

Finally, with respect to gene stealer... aren't they essentially soulless. Thus, immune to warp fuckery?

My assumption about tyranids as a whole is that each have fleet is itself such a powerful intelligence/being/soul that it rivals or dwarfs those already present in the warp. I mean that's why they cast the warp shadow right? Because they're locally excluding the Emperor's signal in the immediate area and those of the usual warp entities by all the synapse creatures broadcasting and rebroadcasting the hive mind's thoughts/orders.

Now as for how fifty genestealers stranded on a hulk avoid getting posessed or ****** with I couldn't tell you, maybe it just is a combination of being so individually soulless and mindless but that doesn't really follow on it's own since the fluff says demons can sometimes posess machinery or even inanimate objects like for instance a scaffold so that they can wait for a bunch of ratings to get on it and then make it collapse.

I believe that Genestealers don't exactly travel through the Warp, but rather aim at the star and do some sort of crazy gravity manipulations to slingshot themselves through space at a target. I've never actually thought about how Genestealers survive on a Space Hulk that's hurtling through the Warp since they usually don't have Gellar fields, but they do also enter a kind of hibernation state when there's nothing to feed on, and they don't really have very much that they want , at least not that Chaos could grant them so that might also afford them some protection?

Now a ship that's actively under Genestealer invasion that's thrust into the Warp... that has potential...

I believe that Genestealers don't exactly travel through the Warp, but rather aim at the star and do some sort of crazy gravity manipulations to slingshot themselves through space at a target. I've never actually thought about how Genestealers survive on a Space Hulk that's hurtling through the Warp since they usually don't have Gellar fields, but they do also enter a kind of hibernation state when there's nothing to feed on, and they don't really have very much that they want , at least not that Chaos could grant them so that might also afford them some protection?

That could be it. that when in the warp or during unitersting times that the genestealers create nests or perhaps cocoons and then enter a state of stasis so perfect and absolute that it's indistinguishable from death even from a deamon. Personally I'll just go with that it's a fantasic handwave and it allows for the explorers to enter a room and a big glob of drippy goo to fall on their head from an empty cocoon,

Actually most hulks DO have gellar fields that still function, otherwise what leaves the warp occasionally wouldn't be recognizable as hulked ships smushed together with asteroids but instead be a giant warp infused ball of puttee that the denizens of the warp had shaped into whatever they desired.

The gellar fields aren't hull-hugging on a ship, instead projecting a kilometer or two around the ship. If a hulk is densely packed enough then the working gellar fields will protect a sizable portion of the hulk, usually the 'core' where interesting fights and discoveries happen anyway, however the extremities will be exposed to the warp and thats why you get rock, adamantium, plasteel and other materials all fused together and looking weird.

Interesting and that makes sense.

What kind of power sources are most commonly used on Imperial ships that would last so many years?

Edit: Then again, time in real space vs the warp is another matter.

Well imperial starships generally run on plasma reactors. I dont know much about the science behind it but if a fuel rod can spend 6 years in a nuclear reactor i'd imagine the plasma source in an imperial ship can run for decades. Obviously it would normally be refuelled and so if it runs dry it could be damaging in the long run... but yes warp time passes quicker than real time so while that hulk may appear only every decade from real space perspective it may only have used up a years worth of power while in the warp.