Why don't the space marines just open fire on the space hulk with their own ship and destroy it from the outside? If they need to enter then why don't they just use viral bombs combined with their boarding torpedoes or other weaponry first to clear out the interior?
I don't think you quite appreciate the sheer size of Space Hulks. They are truly massive which makes conventional means of destroying them impossible. Virus bombs would also be worthless as soon as they hit a sealed door.
Actually given enough ships and enough weapons, a Space Hulk can be destroyed. However, the reason that they often send in elite teams is because Space Hulks are potential treasure-troves of Lost Technology. Old ships that they cannot build anymore, data vaults with information of Standard Template Constructors, etc. Despite the risk, there is usually great value to be had by trying, and the Space Marines are the most qualified to do it. However, if a Space Hulk presents a clear and present danger (like it is heading toward an important and populated system) then the Imperium will try to make arrangements to have it destroyed. Even then, they will usually try to extract what they can before it passes the critical threshold where the destruction must happen.
Keep in mind that having enough ships and enough weapons is going to be impossible 99% of the time, since the Imperium is locked in war on all sides and simply can't spare the amount of ships required to reduce a space hulk to nothing unless it posed a risk to something vital; some Hive Worlds at risk is tragic but acceptable, losing a forge that makes Titans much less so.
And there's always one detail that needs to be accounted for: 40k's minimum Idiot Ball quotient. Each faction must always be throwing some caution to the wind in order to make the conflict happen, similar to the Anthropic Principle. In this case, the Blood Angels have literally no reason to set foot on the Sin of Damnation the second time except to avenge their own pride, rather than actually protect anyone or anything.
Why don't the Imperium send servitors created for combat to clear out the Space Hulk? Servitors are created by removing a part of a person's brain and keeping bits necessary for the job. In this case leave the combat relevant stuff and remove the other stuff including reproduction. The Genestealers can't use them to create cults because they can't reproduce and although this would be expensive Space Marines are far more expensive and hold irreplaceable items such as armour which gets chewed through pretty fast by Genestealers. Once the hulk is cleared, or the lions share of the work and damage has been absorbed by the servitors, send in someone else to collect and analyse any artifacts around.
Because while you can make a servitor that can be a nightmare for a regular human to face, they wouldn't pose a threat to a Terminator, who as you noted get chewed through pretty badly already. You also have it in reverse; it's cheap to make combat servitors, but skilled enough humans still reduce them to scrap. Genestealers would rip through them without even slowing down.
Servitors have this bad tendency to become mind-locked when trying to figure things out on their own. Their tasks either have to be very routine, or else have a techpriest around to instruct them. In short, they're just not smart enough.
The Ciaphas Cain novel The Emperor's Finest involves a force from the Reclaimers (a Meaningful Name due to their focus on recovering Lost Technology) chapter doing a Space Hulk boarding mission (which Cain gets roped into going along with). They actually do employ a large number of cheap servitors or very simple bots which they deploy into the Space Hulk before any human boarding takes place. The purpose of these servitors is not to fight, but rather to map the interior and give the Space Marines and idea of what the internal layout of the place is, where they should focus their intensive scans from the ship, and what objectives they might achieve by boarding. Sure some of the servitors could get scrapped in the process, but they reason that is a small price to pay for locating any sources of hostility they can then decide to either make a concerted effort to destroy or avoid. They did not count on genestealers being clever enough to avoid them instead of attacking on sight though, until more suitable hosts present themselves.
According to the Space Hulk backstory, the last time the Blood Angels tried to cleanse a space hulk, 950 of the chapter's 1000 Space Marines died. Other chapters have been wiped out from less significant losses than that (specifically the Astral Knights, who were declared destroyed after losing 700 of their number destroying the Necron World Engine), and the Blood Angels have the added complication of flawed gene-seed on top of that. So how does the Blood Angels chapter still exist?
It does make sense if you remove the last zero from all those numbers.
Jossed once and for all by the 2013 video game. "Fifty survivors out of one thousand" is canon. Oh, and as for the Astral Knights: they were "understrength" when they attacked the World Engine, with only 772 battle brothers, and every single one of them died on the World Engine.
The Blood Angels are one of the fist founding chapters, they continue existing because they have enough political clout, being founded by the emperor himself and what not, and prestige to not be disbanded by the high lords of Terra like the Astral Knights, at least that's how i think it works.
Do space marines send scouts into the space hulk as well? The standard codex chapter has 1000 marines with a 100 being scouts, this would mean at least 50 of the lost marines would have been scouts.
Most chapters would probably be considered beyond recovery after being reduced to 50 brothers (the Celestial Lions come to mind as well), but most chapters aren't from the first founding. While they don't quite have the support network of the Ultramarines, most of the Blood Angels' second founding descendant chapters would probably consider it an honor to give them aid in the form of promising recruits, battle-brothers on loan, war-gear, apothecaries to help manage the gene-banks etc while they recover. Not to mention the clout that any of the first-founding chapters have with the other organisations of the Imperium.
Remember as well that the Blood Angels skirt around the Codex Astrates by having some of them in the Death Company (I think) and not counting towards the 1000 man limit. Combine that with what the above troper said and while losing 950 Blood Angels would be a major blow, they could survive and pull themselves back together.
It's true that the Blood Angels are "mostly" Codex standard, but the only thing that the Codex states is that you have one thousand ACTIVE Marines. They still probably have dozens if not hundreds of neophytes waiting for the implantation of gene seed and thus promotion to Scout; this has been described in-canon as the case for multiple chapters. The major loss isn't the number of bodies but the near-irreplaceable pieces of technology they may be carrying, as well as the centuries of experience a single Battle-Brother (especially if he's earned Terminator rights) can possess.
Considering Genestealer's ability to just shred through all but the best armour (and even then, still having good odds) and there being very little space in the corridors of a Space Hulk, why do they send in troops with the largest, heaviest and most difficult to maneuver armour they have available?
As Genestealers shred through all but the best armour it's a good idea to send those that have the best armour. In addition all kind of power armours increse the strenght of the wearers which means that the armours are not only for protection but also allows the marines to use bigger things easier.
Also, Honour Before Reason. Space Marines aren't known for taking the low-key, intelligent course of action.
And nothing else can really clear out a genestealer infestation besides Space Marines; whatever advantages one gains from being faster from not wearing power armour is outweighed by the disadvantages of not having power armour when a Genestealer attacks. Sure a regiment of Imperial Guard could move faster and easier than a group of Space Marines, but against Genestealers on a Space Hulk they'd either get slaughtered quickly or implanted and sent off to create a genestealer cult.
I was not talking about Power Armour in general, but specifically Terminator Armour. It's like sending tanks through back alleys - there's no space for them to maneuver, they can't bring all their weapons to bear, and they suffer from limited fields of view. If they were more lightly armoured it wouldn't take an entire action point just to turn ninety degrees, they'd be able to fire past one another, and even - wait for it - turn their heads. And apart from the (admittedly devastating) assault cannon, there use no weapons that aren't portable when clad in standard power armour.
Gameplay and Story Segregation. According to the fluff Terminator armor was originally designed for use in boarding actions and is significantly tougher than power armor. Additionally while marines in power armor can carry storm bolters now back when the first Space Hulk was originally made the fluff stated that storm bolters were only usable by terminators which would mean that the terminators did have a pretty significant firepower advantage over power armored marines. Finally going by the fluff (and the rules for the full game) Terminators should be a LOT tougher in melee against genestealers than they are in Space Hulk. The melee rules for Space hulk are designed to favor the 'stealers to make for a more interesting game.
I suspect that the teleport homers built into every suit of Terminator armor also play a role here. While such technology does exist in more modular form, the homers built into the Terminator suits are considered among the most reliable, and this is of a huge value when then need to deep strike within a hulk, especially one with heavy interference of teleportarium auspex scans. This also allows very rapid extraction once the Terminators reach their objective, they can be whisked away the moment they send the "Mission complete" signal. Heck, even if the marine himself is killed during the mission, they can still use the teleport homer to bring his armor and body back, so that the armor might be repaired and continue to serve the chapter and that his geneseed might be extracted to raise new marines to one day replace him.
Speaking of teleport homers; why are there ever rescue missions? One makes sense, you're going in to find a battle-brother who is carrying something important, but his homer is busted. The mission is a success the *second* you give it to him. And every time a Terminator goes down, he's instantly teleported out (granted that's the justification for being able to walk through the square where he fell), so can't they be triggered manually or something? Honor Before Reason is one thing, but even the Space Marines realize "fight until your last breath" isn't a brilliant tactic when the next mission is "Send in ten more marines to recover your corpse, of which six will probably die, and then we'll have to send in more to get THEM"...
Even in the good old days of the pre-Hersey Imperium where stuff worked right Teleporters weren't exactly widespread, and it's all been downhill since there. Odds are you aren't manually activating their homer, you're physically taking a new one to them, with the mission's end being you slapping it on them so they can be teleported out. That the teleporters are working so well to begin with is pretty incredible, so it failing half-a-dozen times is pretty lucky all things considered.
Why do they even use Space Marines? S Ms are geared towards fighting Orks, Daemons, Chaos S Ms, etc, they have issues with numbers and non wide-open fields, which are space hulks in a nutshell. Terminator armor is rare and if lost cannot be replaced, plus it is designed to move forward and not defend. Imperial guards are much better suited for Space hulks, Death Corps of Kreig are even trained to fight in the same kind of enviroment from birth. If a genestealer can go through any armor then why use the rare can never be replaced armor that gives no protection, when guard flak vests will give jsut as much protection and in the time it takes for a genestealer to rip through the vest 10 craft worlds have made another 1000 each. While the storm bolters and assault cannons the marines carry can put an ungodly amount of fire down a corridor at once, they have no accuracy, guard lasguns on the other hand are accurate and can drop a genestealer in 1-3 shots depending, and if the stealer gets in close, guards carry bayonets, knives, and if they have an nco or officer with them they can have some [[BFS]] on their side, while S Ms generaly only have power fists which are terrible for this situation. Genestealers rely on stealth and numbers to win, so two guys in power armor can't defend themselves enough, while guards come in large numbers so they can be everywhere, eliminating the genestealers advantage. And even if a space hulk jumps out with a battalion of IG on board, the Imperium has reserves.
That would require the higher-ups at Games Workshop to apply logic to anything.
Imperial Guard regiments, Navy boarding teams and PDF garrisons do sometimes board space hulks. It's just that nobody makes games about them. More's the pity.
"Storm bolters are innacurate"? What?! At their respective effective ranges, storm bolters and lasguns are practically identical as far as user handling goes (interface with helmet's HUD, very low recoil, box magazines). One just has explosive rounds.
In game a good 4 times out of 6 the bolters will miss unless the player has luck dripping off their head, that is bad accuracy.
A lasgun would be even worse (and by my calculations the Storm Bolter hits 55.56% of the time).
A friend who's been playing 40K since Rogue Trader explained it like this: Assuming the Imperium has an actual reason to board a Space Hulk (which the Blood Angels don't, they're stuck in an Honor Before Reason scenario) you need to scout the Space Hulk first to figure out which rooms still have atmosphere and which don't, as well as which sections are and are not filed with deadly radiation. For Guardsmen this is tricky, but Space Marine armor conveniently has radiation shielding and self-contained atmosphere. Long story short, if you want a better example of how Space Hulk boardings are usually done, check out Death of Integrity.
Death of Integrity also justifies a lot of the potential headscratcherness involved in a Space Hulk operation. For example, the marines were prepared to simply destroy the hulk outright until the Adeptus Mechanicus objected and asked them to investigate it first. The Terminator armor was used because some particular areas of the hulk were so irradiated that without the additional radiation shielding such suits possess even a normal Powered Armor-clad Space Marine would not last long. Thus the non-Terminator marines were used to secure some of the less irradiated areas of the hulk, and even scout marines (wearing voids suits) were employed to explore the outer surface of the hulk for entry points and to assist techmarines.