…if one were to fire a Nova Cannon round at the surface of a planet? (assuming that it is Earth-sized) Would it pepper the planet with superheated plasma?

On another note, what combinations would be more ideal on a Battlecruiser? Dorsal Lance + Macro Broadsides +Lance Broadsides or Dorsal Lance + Macro Broadside + Landing Bays?

The efficiency of your battle cruiser's loadout depends on the purpose you are building it for. I would reccomend against the some of everything approach personally though. I find it inefficient in almost all cases as attack craft are far less effective in small numbers. My favorite battlecruiser load out is dual bombard cannons in the prow and dorsal slots, and hecutor plasma broadsides everywere else, but thats just my opinion. I have never mathmatically crunched the numbers for efficiency on it.

Cryhavok said:

Well, attack craft are pretty good at what they do. Fighters counter torpedoes or bombers (and if I remember the rules right, fighters are actually super effective at fighting when outnumbered, since they destroy a number of enemy squadrons equal to the degrees of success on your command check, meaning that if 3 fighter wings escort 2 bomber wings, and they encounter 1 fighter wing, and your Rogue Trader hits five degrees of success on his command test, you've successfully wiped out their attack force unless they rolled nine degrees of success)

Bombers are, admittedly, less useful in small numbers. Assault Ships, however, work just fine individually, and they're a great tool if you don't want to actually destroy the enemy ship (particularly if the whole party climbs in and your GM runs the boarding action as a normal adventure)

Yeah, in the end we went with an Armageddon-class with a Mars Pattern Nova Cannon, dorsal Star-Flare lance and Port/Starboard Sunhammer Lance Batteries. Still a port and starboard slot open, so we're thinking of eventually acquiring Sunsear batteries just to deal with shield.

The concept is that you would be able to fire the nova cannon, possibly with the Star-Flare as a follow up (or opener) or be able to knock down the void shields with Sunsear before Racking up fire with the Star Flare and Sunhammers. The sunhammers and sunsears have 9 range and the Star Flare at 7, so engagements at range would be a possible strategy. Takes up an enormous amount of power, but the Plasma Drive can support it.

I'm not sure if you should just treat it as a wide-area orbital bombardment, though, as a nova cannon shell is far more powerful than a single lance strike or macrobattery hit. I would imagine the damage to be quite catastrophic, as say, comparing a macrobattery to a nova cannon would be akin to comparing a rifle to a rocket launcher.

On another note, what is the point of ground battles then, as a ship in low orbit can blast armies to kingdom come with the push of a button?

Your novacannon/lance boat sounds interesting. The sunsear broadsides are a good choice for knocking down shields. I'll be curious to hear how it fares in practice, if you are willing to keep us posted when you get it up an running.

The point of ground battles, despite the ability to obliterate everything is simple: profit. Planets generally have some way to make profit, and a ground war can take those things and place them into you profit factor. From facilities, to labor forces, to the rare artifact the enemy possesses. Cleansing the planet of life is a collosal waste of most of it's value, which is why exterminatus in generally left to inquisitors and the like. (Not debating which has the power, just say one might have alot more motivation.) Infact, if you have a particular lack of piety, you could likely even manage to get profit factor from chaos scum and xenos filth infested planets.

There are several things to think about when determining if obliterating the surface of the planet is worthwhile. How much will a war cost me versus how much will I gain. How much will destroying everything lose me, and will I gain anything from it? Will expending a novacannon round cost me more or less than sinply leaving and finding somewhere else to play? Life is relatively cheap and soldiers can take and hold objectives that a novacannon can only explode.

It's worth remembering that Fighters tasked to the Escort role (and, in theory, fighters tasked to Intercept that time their attack run correctly, although that does require house rules/a co-operative GM) can be used for SEAD (SEVD? SEOD?…enemy turret suppression anyway), making the assault boats/bombers that follow up more effective. And torpedoes work wonderfully when combined with small craft operations, as you can only use turrets against attack craft (fighter/bomber/assault boats) OR torpedoes in a single turn (or that was the case in BfG, anyway. I'd need to re-check BfK to see if it applies in RT).
A moot point if you're going for a Nova cannon rather than torpedo tubes, true, but it might still come up if you go for torpedo bombers.

And unless you're going for the Hold Landing bay (which an Armageddon-class can't) or the Escort Bays (and why would you, with a battlecruiser to play with?), you've got no real reason not to launch 4 squadrons of attack craft per turn, and you can carry 12 squadrons. You can even stack launches over several turns to combine for a complete strike wing of 12 squadrons (admittedly, that does require you doing so at rather short ranges, as your first-launched squadrons will be running low on consumables by then).

Ground battles are still fought, despite the sheer destructive power of the starships in 40k, for the same reason we don't just wipe out armies with airstrikes today- sometimes the objective is to wipe out an army, but mostly you're fighting because you want something they have, and blowing up them/the area they hold from orbit tends to limit your ability to seize that afterwards; it's very hard to loot a glowing crater…
Plus, of course, anything really important is going to be hidden in underground bunkers, which will either be sufficiently hardened (the defender hopes) to resist orbital bombardment, or simply hidden from view. Unless you know where your target is, you can't rain nuclear fire on his head.
Therefore, you need ground forces to find your enemy, draw him away from your objective and hopefully get out of range of the incoming death from the skies (so you can use those ground troops again); or to find your enemy and take the fight to him in an area you can't afford to have your starship fire upon.

In regards to what would happen if you fired a nova cannon at a planet…. at the very least you'd have an instant extinction event. Any object traveling at even a fraction of the speed of light would hit the planet with enough force to crack the mantle and kick up sufficient debris to create a nuclear winter. On top of that, the warhead explodes with enough force to destroy almost anything within several thousand kilometers. Combined together that's enough power to annihilate any planet. whether it would pepper the planet with plasma would be pretty a much a moot point by then.

I wouldn't necessarily want to argue this assessment, as I feel a Nova Cannon fired at a planet SHOULD cause catastrophic damage, but I'm not so sure. Every once in a while, the Imperium reminds us that this is the grimdark future, and that they are really no better than anyone else on the "good people" scale. If things go south enough, the Lords of Terra, or the Inquisition, can pass the order of Exterminatus on an entire planet, consigning a billion souls to the other side. When they do this, they dip into a scret little closet they have, and pull out a terrifying weapon. This might be a cyclonic torpedo, able to ignite the planet's atmosphere, virus bombs, keyed to rapidly exterminate all life down to a cellular level, vortex warheads, which tear holes into the Warp, and suck things in, or even a magna-melta, which bores to the core, and makes the planet fracture. These are all rare, ill-understood, and possibly impossible to replicate methods of wrecking an entire planet. A fleet of vessels with special munitions might also be able to do this.

So, why have i just rattled off all of this stuff, which people already know? Because these are what it takes to blow up a world. Comparatively, Nova Cannons are commonplace. If all it took to bake the planet was a hit from a NC, from a single ship in space, the event wouldn't be nearly so big, and these arcane doomsday weapons would just be a relegated to a few additional conversation pieces in Prof. Farnsworth's laboratory.

A Nova Cannon would do collossal damage to a land-based target, and probably take out a city-sized span, underground stuff and all, no problem, but even it doesn't have the AoE oomph needed to obliterate the planet, the same as the Hulk doesn't.

Afterward, the issue might be "what would happen if you did do this?" Exterminatus is rare, and not something even the draconian Imperium often employs. If your RT were to NC a planet, or actually crack it, and mess it up, the Imperium would come and find you, and do the same to your ship.

venkelos said:

Not necessarily. The Koronus expanse is wilderness space and the Rogue Trader is THE LAW. If he decides to go Exterminatus or "just" Nova Cannon, then THE IMPERIUM has decided that this was necessary. Of course, your peer may moan that the planet was salvagable, that it wasn't really necessary and go "WTF dude?", but after all that was the RTs call. Of course, if you go around and scorch every habitable planet in the Expanse down to the crust, the other RTs might deem you insane or worse and try to stop you. But one planet (that the Imperium has never heard of before) usually shouldn't be much of an issue.

Emeror save you if you try that on an established imperial world though.

eBarbarossa said:

I agree with you, but I was trying to make the case for a planet, any planet. If the Exterminatus Weapons weren't considerably more powerful than a Nova Cannon, then the Imperium wouldn't need EWs; they'd just have big ships with nose-mounted guns. While the Imperium might not care if you, the RT, obliterate the world they've never heard of, and even welcome it if Eldar, Orks, or worse were down there, the scale of "blow up the entire planet" doesn't seem like the thing a ship is carrying; physically obliterating a planet is a daunting challenge. If the Moon struck Earth, I think we'd be toast, but maybe not every person, and much of the Earth would be intact, if fragged. That would be the case for any planet of approximate mass, I would guess. So, a single Nova Cannon blast wouldn't be "the rock that made the dinosaurs go away", thus not that big. Anyone down there at Gr0 is dead, hands down, and I assume even underground bunkers will breach (so the Governor's Proteus-class bunker was a nice idea, but not enough), leaving nothing of the area you demolished, but you wouldn't wreck Rhode island with it, much less a continent, or the planet.

And with that, my prattling is done.

Deciding whether a nova cannon kills a planet or not is difficult. Game-balance-wise, it shouldn't. Game-mechanic-wise, it would. Realistically, it probably would, but I'm no physicist. 

Balance: It'd a bit ridiculous to give the player the ability to willy-nilly destroy planets. 

Mechanic: NCs have a 30,000km explosion diameter. Earth would be toast.