hi all. i am getting ready to start the "endgame aventure" with my pcs ( they want to start a radical campain and move it into black crusaide when it comes out) anyway it is going to take place on a carbon planet with a admech forge/ hive city on it dedicated to starship production. the benifits of the planet are such that ships can be built within the atmospher low enough that breathing is not a problem, this has the twofold benifit of a more secure shipyard and much faster build times.( think decades not centuries to create a new battleship) unfortenitly the forces of chaos hace discovered the shipyard and wish to destroy it, they will do so by hijacking the newly completed crusier" ashfull wake" and firing its nova cannon directly on the hive.

my question is just how much damage will this do to the city, planet ect? so far i have found the method of firing it( super dense mass speed to a fraction of light speed) so i know it would be devestating, but just how much? further more would a void shielded fortress underground possibly survive?

any thoughts welcome

The planet is going to be wrecked, (as in lose an entire side turned into molten rock and its atmosphere on fire) but that's the whole point of a hive is that they're an enclosed, self-sustained environment which is designed to do two things- Survive anywhere and last long enough for help to arrive sometime in the next 12-24 months from friendly forces. They do have shields and a fairly large amount of space-ship grade guns, torpedoes and other nasty weapons hanging off them to make people who annoy the inhabitants, die. So it may not be as completely fragged as people may at first think, its certainly going to be a bit dented and unhappy though to say the least!

As MKX says, a Nova cannon would certainly destroy a major chunk of any unshielded hive city. Most major Imperial hive cities would be heavily shielded, however.

If you were running a RT game, you could stat up the city using RT rules, giving it a profile like a stationary, planet bound starship. As you're running DH, you could as an alternative simply set out a fixed number of turns for the Nova Cannon's firepower to whittle down the shields and start inflicting damage on the hive proper.

In terms of raw firepower, I'd always viewed Nova Cannon as being area effect weapons on a scale of thousands of kilometres square - so we'd be talking the equivalent of gigatonnes of TNT. A single shot would devastate a big chunk of the Eastern seaboard of the USA, for example, or the whole of southern England. They're not necessarily planet busters, though; even dozens of shots, while scouring the surface of any world of anything not buried deep in a mine, would still leave it more or less intact. However, the equivalent of a nuclear winter would probably set in, thinking it through.

It also depends on the type of hive city. Imperial hives are often (in my view excessively often) portrayed in the Necromundan style, with wastelands surrounding spires miles high. Once the shields are down, one imagines that a city with quite a small "footprint" might be very vulnerable to orbital strike. It may be that a different type of hive city - maybe a gigantic version of LA, or Mexico City for example, a city spread over thousands of square kilometres, with a very diffuse infrastructure, might withstand a Nova Cannon strike - or indeed any type of orbital bombardment - rather better than a Necromundan style hive spire.

If someone uses a nova cannon round on a planet/city consider this. in space the radius of the attack explosion is in excess of ten thousand kilometers. Atmospheres of planets hit by this weapon would be set on fire. Any biosphere within ten thousand kilometers would be catastrophically affected. The shock wave inside a planet's atmosphere would likely go around the planet several times killing anything standing, wounding anything hiding and wrecking anything of less than stout structure. Surviving the hit, fires and shockwave would leave you in a planetwide apocalypse, good luck surviving without food or clean water.

Each nova round would do 1d5 hits (to a voidship) on a near miss ( with 1 hit negating a voidship's void shield array, and any hits through the voidshields doing 2d5+4 or +5 damage to a voidship's hull - so to answer your question a multi kilometer long voidship can be destroyed in one hit if you are lucky at rolling dive = 15 HI plus criticals ). A direct hit does 1d5+2 hits, each hit negating a void shield. Even if a city is massive and even if it has layered void shields (lets say 3 minimum on the area directly struck) nova rounds, on average, will penetrate them on a direct hit every time. They also completely ignore armour so its armored shell is worthless. Then, if it penetrates the shield, you get to consider what just happened: an explosive device accelerated to a fraction the speed of light just exploded against the city, set the air on fire, wrecked the infrastructure and biosphere of the entire planet via shockwaves/earthquakes and ..... yes you're screwed.

The resulting direct hit would shatter any city described in Warhammer 40k fluff if it penetrated the shields. Even if it didn't woudl the city still be in the same place? Or would the explosion move it half a kilometer in a random direction? That sudden rapid movement of the city would likely kill every living thing inside.

The only saving grace, if you can call it that, is that the cannon is an inherently inaccurate weapon. A direct hit is very unlikely. But the planet would be wrecked in new and interesting ways, even if it isn't already.

Nova Cannon are a terror weapon, and somewhat effective at killing voidcraft too.

I would generally assume “close to light-speed” to mean at least 80-90% of C. The size of a Nova cannon shell is never given precisely, but the diameter of the shell is given in other sources (Fifty meters in “Warriors of Ultramar”), though a 30 meter diameter nova cannon is mentioned. Mass can be derived by assuming the length is at least equal to the diameter, or (more probably) a multiple of the diameter (2-3x longer than the diameter, for example. A fifty meter diameter shell would be a hundred and fifty meters long).

Example: Going by a 50×150 meter shell made of Iron (assume 30% solid, its supposed to be packed with explosive of unknown type and density) fired at .9c yields a shell mass of around 770,000 tons and and a kinetic energy rating of 90,000,000,000 petajoules (Holy ****!).

Giving the blast of a Nova cannon (The most powerful ship mounted Imperial weapon) a staggering yield of 22 petatons. For those of you struggling to comprehend these figures

A petaton is a unit of mass that is equal to 1,000 teratons. It can also used as a unit of energy equivalent to 1×1015 (one million billion) tons of TNT. This latter use is usually restricted to astronomical events such as meteor impacts or large science fiction weapons.

The energy released by the explosion of one petaton of TNT, 4.18×1024 joules, is equivalent to the energy of an earthquake of magnitude 12 on the Richter Scale, or to the energy of a 60 km rocky meteorite impacting the earth at 25 km/s.[citation needed]

A02034040 said:

Its actually 'a fraction of light speed'...that is not even close.

Schwarzie said:

It is a relatively 'new' weapon (in 40k terms and time spans at least; 'rediscovered' in terms of FFG fluff as there is a archeotech version and that indicates the nova canno nhas been around for a while at least in FFG material). To my knowledge no one has used one on a planet in the fluff. I haven't read every novel but I don't recall a nova cannon being used in that manner. Don't give on e to your players because they will eventually say, "Hey. Why don't we just shoot the good guys/bad guys planet with it?"

Regarding void shields, simply comparing them to (relatively small) shipborn shields would be a mistake I think, even if a battleship can mount 2-3 void shields. A hive would be able to mount much larger shields and have the power to...well 'power' them. The siege of Vraks (Imperial Armour books) mentions ultrapowerful void shields impervious to orbital bombardments and WH40k stories mention space marines having to take down the void shields with an assault so I think we must assume that a properly defended Hive world is indeed able to withstand bombardment by a combination of defence emplacements capable of destroying void ships and heavy void shield defences....

Even if the voids are 'bigger' the total destruction of the planet's biosphere - such as it is - and the movement the city would suffer after a hit - even a near miss, would assure its destruction. If your town were to suddenly fly up into the air forty meters the landing would do all the damage. One hit and move on to the next hive world.

thanks all. i knew it would be destructive, now i just have to find a way to descibe his scene to get the magnitude of this across lol. oh and A02034040 i think anything that hits a whole number percentige of light speed ( ie 2%) is considered a fraction so the blast might be much smaller, but thanks for an actual number i can put to the "potential" blast.

It is described in the rules as hitting anything within 1VU of impact (thats 10,000 kilometers in space) as a indirect hit and causing 1d5 hits. Considering nukes would do much less in vacuum (energy dispersal over a perfectly spherical area being the cause) on an indirect hit...10k kilometers is a gigantic explosion in space in terms of energy per square kilometer so fudge on the >gigadamage. I'd be interested to see your description of the results to your players

ok if anyone has read the black library book "Cadian Blood" by Aaron Dembski-Bowden, they would know that a nova cannon shell is a implosive device, essentially creating a minature, short lived black hole, and the shell is essentially the size of the ships cross section. so basically the void shield generators in the city would overload and cook off the nuclear reactors within the heart of the hive, and basically byebye hive city

oh and im also forgetting the various munitions, power cores, and fuels for the ships being built cooking off aswell

wulfenite said:

I'd rather take Battlefleet Koronus's word seeing as it's the actual game mechanics for Nova Cannons in the 40k RPGs. I'm not entirely clear why an implosive Nova Shell would overload shield generators, and power plants. They certainly don't do anything like that in Rogue trader.

wulfenite said:

As much as I love Aaron Dembski-Bowden's work, he does tend to stumble a bit when it comes to the details of starship combat - he's previously mentioned torpedoes being stopped by shields, for example.

That said, the concept of some Nova Cannons firing shells that produce singularities is entirely reasonable - as Battlefleet Koronus describes, there are a few different patterns of Nova Cannon, which employ a variety of different types of shells (some are explosive, some are plasma-based, while a rare few produce momentary warp rifts (being essentially colossal Vortex charges).