Not really a rule question, but hey!

What are the roles of each class in RT? And I dont just mean the basic design of each class, I mean what sort of things can they delve into (not including any alternative ranks ofc, just the basic model)

I noticed DH is really nice for explaining what class does what, as well as being pretty explanatory on their own, but RT seems more about the fluff explanations rather than the crunch explanations.
Maybe its just me but meh

Dark Heresy is set up as more of a mystery, survival game where each character class brings specific skills and talents that the players combine to each fill a role needed to figure out what the heretics are up to and barely save the day (or more often, escape with their lives as a Hive City burns). They're outmatched, outgunned and outnumbered by their foes so they have to rely on those things they are good at to survive.

Rogue Trader characters usually carry enough gear that would cost as much as a Hive City, and have wave after wave of their own men that they can throw at problems, or at least distract their problems while the PCs accomplish their actual goal. They have individual things that they do better than everyone else, which is hinted at by their special powers though. The other important difference between RT and DH is that RT characters get more experience than DH according to the book, and as a result although their Skill/Talent advances are more expesnive, their characteristic advances are NOT. Thus characters will frequently and quickly max out their cheap advance track, and even their mid-cost advance track whereas with DH it was a struggle to save up experience to buy that Toughness advance you really needed.

Also the PCs being paragons of humanity is hinted at with the adventure structure, where instead of being sent to deal with a problem (DH and DW) the players are encouraged to come up with their own adventures for fun and profit, and there's no "wrong" way to deal with a situation, as long as they emerge with something they can sell later.

As far as the basic career paths go, there are certain things that each class does really really well. But as a general rule, Rogue Trader is a very open game in terms of being able to do whatever the hell you want. For a quick rundown here is a brief list.

Rogue Trader - All around great leader, sort of a jack of all trades. Can buff allies, also gets to be the ROGUE TRADER!

Voidmaster - Three different types, (Pilot, Guns, Small Craft) - Take your pick, whichever one you are amazing at you will be AMAZING at!

Navigator - Can be played as Navigator or Navis Scion at Rank 1. Traditional Nav is more focused on powers and being a navigator. Navis Scion is more focused on social aspects, but is also still a fine navigator. Kills people by looking at them...

Astropath - Probably the most powerful class, kills things dead and gets to be creepy as hell while doing it.

Arch - Militant - Excels at the fine art of making problems disappear in a spectacular fashion. (Explosions, Gore Fountains, Bisections, etc...)

Seneschal - Primary social class, excels at spymaster role as well as adding to the profitability of any endeavors being undertaken.

Explorator - Tech Priest, usually a blend of Dr Frankenstein, Nikolai Tesla, Doc Oc, and Robocop...also helps to be insane!

Missionary - Can be played MANY different ways, but particularly fun if you enjoy giving rousing speeches at the table! Also helps to be a pyromaniac

If you happen to own Into the Storm, it has a list of possible ship roles for the various archetypes in RT. Other than that, RogalDorn and Erathia summed it up nicely. To expand on some though

Rogue Trader: Can be a pirate, legal warrant holder, heir to a warrant holder, THE warrant holder, and/or somewhere in between.

Arch-Militant: Bodyguard, man-at-arms, bounty-hunter, head-hunter, Ex-Storm Trooper, etc. etc.

But you have the jist of it Quoth. It's really more about the fluff.

Here's my take.

Navigator - Can be built as combat mage, but utility mage works better IMO. Can also be sniper supreme.

Seneschal - Social fu, doubles as the Sage equivalent.

Rogue Trader - Facilitator. Leader, negotiator. Can get anyone to do what they do even better. Gets to sit in the Captain's chair and wear cool hats.

Arch-Militant - Sergeant and general, gunner. Honestly, the Voidmaster does everything this guy does almost as good and much more besides. Looks silly in cool hats.

Astropath - Spy, advisor, counselor to the Rogue Trader. Reads the minds of friends and foes alike and keeps the Lord Captain informed. Becomes even better than the Voidmaster as the sensor operator, given time. Needs sunglasses.

Explorator - Another facilitator (unless THAT guy plays it). Can make sure everyone on the ship is well-equipped (unless it's THAT guy, in which case he's got a shoulder launched ICBM and every finger-tip mounts a Power Swiss army knife) and every landing party is ready for every situation (or leads every landing party and solves every problem by killing everything on the planet single-handedly). Is responsible for most broken games.

Missionary - Comic relief. Space him as soon as you can. He will rat you out to the Ministorum for smuggling, dealing with xenos, or anything else rogue traders do to turn an honest profit. Invite him on a space walk and cut his safety cord. Or trade him to some natives.

Navigator - Critical character. If THAT guy plays the navigator, he will be a combat monster that kills with a glance at short range, and kills with a long-las at long range. If Timmy the Trollkin plays the navigator then he will save the entire ship at least once every endeavor, save you months of time, and find things for you like **** Tracy on steroids.

Seneschal - Does many things the Rogue Trader does. Some things the Rogue Trader does at an earlier rank, some things the Seneschal does at an earlier rank. The Seneschal is a better investigator, though.

Voidmaster - Like Rogal said, except the Voidmaster can actually do four (add sensor operator) and do all four simultaneously. They'll just be fantastic at one of them, and merely really really good at the others. Could run the entire ship alone if they had a cloning machine.

Edit: Seriously?!? **** Tracy got censured? It's a NAME! It's a British name, you silly Brits! I didn't make it up. You guys did!

Laughed so hard I woke the wife. Screw To the Warp with you, Errant Knight. *Fingerwag-shaming*

Edit: wait, it censors **** but not screw? By rights they should both be axed! Feh. Edited for propriety, I guess? I don't even know.