In theory, If one were to acquire the Honor the Chapter splat, could they potentially create something like a Novamarines Rogue Trader or a Flesh Tearers Arch Militant by using a Space Marines chapter in place of a homeworld and applying Marine bonuses as appropriate, or to play Space Marines in RT using DW rules, would one have to obtain the DW core rulebook and use DW character generation rules as detailed in the DW core rulebook?

My RPG group's diving into a new RT campaign and we tend to use a bit more 1st Edition 40k style lore in some spots, making an SM Rogue Trader or crew member more appropriate, and we thought we might give it a go. If the splat idea as detailed above won't work, I'm thinking a character with the Hulking mutation, some implants for unnatural strength and the like, Powered Armor and a Locke Pattern Bolter as starting gear, and stunted fellowship akin to a Death Worlder might not be a all bad road to go down to simulate Space Marine players.

Mmn, this is all opinionated, but I don't think Space Marines have any place as a Rogue Trader/Rogue Trader's crew. Space Marines are created with a purpose that is decidedly not traversing the stars for fame and fortune. I would advise against picking up the Deathwatch Books just to play Rogue Trader. Don't let me stop you from playing the way you want to though; I'm just sharing my thoughts on the matter.

<Rogue Trader - Core> and <Into the Storm> have Origin Path (character creation) options that let you pick up mutations and other goodies by RAW. That is probably the best way to have a Space Marine-like character without actually being a Space Marine.

All you need to do to make someone a space marine is add the unnatural characteristics and give them bolter and power armour. You can pretty much use any existing career path in Rogue Trader beyond that; just replace the homeworld step with a "space marine chapter" option. A space marine doesn't actually get all too much beyond that, and FFG's marines aren't too different from humans, statwise, just better soak and melee damage. They have a higher characteristic modifier, too (+30), but since you're homebrewing anyway, it's really best to come up with your own vision of marines more according to your interpretation of fluff. You'll have to heavily houserule Deathwatch's material and, truth be told, it doesn't translate too well to a rogue trader environment, given every progression path assumes you're in a regular marine unit. RT, on the other hand, lists general careers that can be easily used by someone with marine racial characteristics.

Techmarine? Explorator

Chaplain? Missionary (or whatever it's called)
Any other marine? Arch militant, rogue trader etc. The only career that might be a genuine stretch is seneschal.

Wow space marines on a roguetrader vessel- that is very oldskool 40k!

But in the lords of mars novel you have a rogue trader/mechanicus/ Black templars alliance, so i guess some overlap is still possible.

Well in my Expanse Harak Fel was somehow able to call on a couple of squads of space marines during the later stages of the hut for the Dread Pearl. They not only helped him survive the attentions of Lord Admiral Bastille's little fleet but were Key to Fel's survival on the Pearl after his landers were destroyed by eldar.

Since then a single space marine and his scout marine “squire” have joined Fel's crew.

The players haven't actually looked into it yet but it's the Marines Errant. Fel once performed a daring rescue of a group of marines and some very valuable equipment stranded during the Kadyron Voyage. The space marine on his ship is serving as his military adviser slash champion. It also gives the battle brother, a prospect for ship command, a broader perspective and a glimpse at the methods and tactics of those he might face when in command of a ship.

I actually have a Space Marine as my Arch-Militant in the Rogue Trader game I run. He uses the Deathwatch rules, house ruled that he cannot use Squad abilities as there are no other Marines to squad with. (Note: He only started with unnatural toughness at RT Rank 1, and didn't become a 'full marine' until the appropriate xp level, in part because Deathwatch hadn't come out yet, in part because of complicated origin story that took multiple endeavors to resolve.) I haven't had any statistical problems having him in the party, but that may also be partly because I have a very-non-game-breaking group, so the fact that he tended to dominate in personal combat wasn't anything people had an issue with. Indeed, several 'random encounter/speed bump' combats were solved by "let the Marine handle it" while the Navigator and Rogue Trader stopped in the previous room for tea. ( Literally , in the case of the players once - they made a dinner run while the marine was fighting.)

Having a Space Marine in the party is something you should really discuss/clear with the entire table, because it does greatly affect the game dynamic. They're super-warriors even more than your standard Arch Militant , and are going to stand out and change the dynamic of social situations & encounters. As a GM you have to keep them in mind as well, both in terms of peoples reactions and in terms of combat potential. They're large and dangerous, they're going to suffer the majority of the fire.

Thanks for all the feedback! By the sound of things, house-ruling is the way to go.

This time around the GM will be rotating, but we should able to handle having a Space Marine running around, as our party's had its share of Lawful Good Paladin types amongst a den of thieves and scoundrels. For us the key is to set the players against each other not negatively, but in a fashion that compliments the talents of each PC and helps them keep one another from taking their character overboard.

As a fantasy example, maybe the CN rogue picks a lock to search the royal quarters for clues to a murder while the Pally is busy keeping the guards at bay with his, ahem, unique negotiation skills. Or it would be entirely conceivable for the Pally to act as a moral compass when the Rogue wants to rob a merchant of all his valuables, potentially bringing in too much attention from the powerful mercantile guilds. Standard kleptomania and overuse of checks to seduce aside, the group's mature enough to handle 'special' characters.

For the record, the full 'crunch' rules for playing a Space Marine in WH40KRP exist only in the DW core rulebook (well, the DH supplement Daemon Hunter has rules for Grey Knights, but they are too specific to represent standard Astartes); none of the DW supplements contains enough info to make a Space Marine character without the rulebook.

In my last RT campaign one of the players was SM.

backstory was the deathwatch was tired of his shenanigans

and attached him to "xeno monitor duty on the frontier"

which being translated is "kill aliens in the koronus expanse"

which being translated is "kill aliens in the koronus expanse"

How do DW Marines 'scale' against Rak'Gol? It always seems to me that they would be well-matched, but I've never gotten around to testing that...