Page 4.
andrewgpaul wrote:
Ford Prefect wrote:
As an aside, are there any numbers given in regards to the number of Orks Thraka has commited to the Armageddon campaigns?
From Codex: Armageddon:
"... in excess of 2,000 ships and at least twelve space hulks ..." Is pretty much it.
Well, there's still a thing to notice. I have been reading many typical BFG battle scenarii, and what stroke me is just how small fleets are, up to the point they're just a few ships above what is good enough to call them flotillas.
Page 5.
white_rabbit wrote:
The 3rd "battle" was an attack across the entire armageddon sub-sector, Codex Armageddon describes an initial main wave of attacks on over two dozen systems. Based on the warband estimates given in the codex, we have a rather paltry 850k-4.5 million orks in the first wave, approximately 100 Gargant weight warmachines, about 300 Battlefortresses, 20-25% of the warband strength is artillery and armoured vehicles, with about 90k-450k of that main warband number being Artillery or Speedkult warbands, which are almost 100% crews of armoured vehicles and artillery.
Obviously this is only forces attacking armageddon,not the other dozens (at least) of worlds in the sub-sector.
12-16 Spacehulks, 250-400 Kroozers, (as well as the battleships that are indentified later on) 2100 escorts, about a hundred Roks, 3000+ squadrons of fighter bommas (this could be 15k to ??? )
At this point Armageddon is a beacon for all orkish activity in the immediate area, and is drawing orks in probably from across a significant portion of the imperium.
Orks based on Armageddon are also likely to have 10% more muscle mass than the average ork, akin to orks "fighting for tribal leadership" rather than your basic grunt, at the moment, there are estimated to be 30 orks for every human on Armageddon, and without drastic action, its estimated to rise to 50 Orks per human over the next 25 years, and regions "cleared" of active orkish warbands are suffering Orkish population booms, with the orky life cycle of squigs, Snots/Grots, Orks being tripled in speed due to the harsh conditions.
An Ordo Xenos Inquisitor advocating Exterminatus mentions that there were "millions" of orks on Armageddon prior to the second orkish invasion.
For anyone interested in more details about those elements, I'll encourage you to read the thread on your own, especially if you're looking for more numbers.
It gives an idea of the scope of such battles. The troop numbers are high, but the fleet sizes, in general, are not, and the industrial might of the Imperium is extremely limited when it comes to the heavy fleets. Despite Forge Worlds. It's logical that a lot amount of ships are bound to local defense, less powerful and only a fraction of the entirety of the Imperium's fleet, counting industrial and merchant fleets.
The Imperium may be able to deliver a good many ships at once, but it takes a hell of a time to build them.
But one point needs to be made, though. Sectors of the Imperium are established from a top-down view of the galactic plane. Sectors are cubic zones, 200 LY wide (along the edge), on the average. Some smaller, some perhaps larger. They're divisions of the Segmenta, large swathes of the galaxy subdivied for administrative purpose, notably.
The typical mileage of a sector's battlefleet is 50~75 warships. A sector represents several sub-sectors, themselves covering around eight to ten planetary systems, sometimes more, sometimes only one.
Even if you pick two subsectors per sector, and give each subsector 8 systems, that's 16 systems per sector.
The Astronomican's 50K LY range pretty much defines how far the Imperium can extend before being unmanageable due to the miseries of the Warp. Then, of course, we should keep in mind that perhaps a fraction of that number could be FTL capable. There could be many warships maintained for local defense.
The circular area of the Astronomican is 3.1416 e10 LY². A sector has a top-down square area of 40,000 LY². There would be 785,400 sectors in the Imperium, more or less.
At the very least, 39,270,000 warships in total.
When considering the entire galactic volume (~3.3 e61 m³) and a sector volume of 6.17268 e50 m³, we get a maximum of 53,461,387,941.4 sectors.
Yet, how can it be that so many battles, even some crucial ones, notably during the massive Chaos campaigns, involve so few ships?
It's relatively difficult to pretend that most of the warships would be FTL incapable since even escort ships can move through the Warp.
Now, the concept of warship may have to be revisited, once used in the context of space combat, as a space capable ship built, armed and properly armoured for combat would fit the definition. This could be encompassing any small craft such as a bomber, which in WH40K are well armoured, heavily armed and often engaging much bigger warships, rather relevantly.
Let's also notice that the whole of the human fleet capacity could cover both local patrol fleets, and the long range capable and generally much heavier warships of the Imperium.
Today, coast guard fleet include ships just over 60 meters long, and easily fall under the warship definition.
The "superfast, large stealth missile"
Skjold-class patrol boat is a 47.5 meters long
fast attack craft, which considering its armament and hull, leaves no doubt about its battle capacity.
As similar examples, with different configurations, comes the
Protector-class,
Armidale-class and
Fremantle-class patrol boats, respectively 32.6, 56.8 and 42 meters long.
The
Osa-class missile boat was 37.5 meters long. Or the old
HMS Lightning 26.7 meters long torpedo boat.
With WH40K bombers being plus 40 meters long, that works rather well, and could cover a whole range of even smaller crafts, and with some large Imperium carriers capable of launching 40 bombers, we can come to the understanding that a large carrier or smaller ones, plus planetary defense forces comprised of their own bomber squadrons, and then a few warships, battleships, cruisers or escorts, or smaller cratfs, easily explains the 50-75 average warship count, and fits with the scope of the typical BFG space battle.
However, there is a major issue with the rationalization proposed above, in that the Imperial Navy doesn't include crafts under frigates and destroyers as naval ships. Bombers are typically classified as aircrafts. The capacity of use faster than light drives doesn't seem to be the factor of difference, since non-FTL system bound larger crafts are considered warships.
The obvious element of difference there is merely a question of carriage and low altitude atmospheric fly.
Then, again, if some small warships were capable of hovering close to the ground, we could ditch that element as a factor in naval nomenclature, and bring everything down to a question of transport, in that if it can be transported, it's an aircraft, somehow.
Even the term boat doesn't necessarily imply the belonging to the classical imperial nomenclature under "warships": assault boats are launched from Emperor-class battleships for boarding operations.
Then remains the gunships, which I don't know where to put.
Which would mean that the other rationalization would be that a great bulk of the Imperium's naval power is about ships, perhaps smaller, which have traded FTL capacity for horsepower, weapons, shields and armour, making them just as good as bulkier FTL-transit capable ships.
It fails to explain, however, why so many systems seem to be literally underarmed, waiting for the Navy to come to the rescue. It would imply that those local crafts aren't up to the task of fending off anything greater than a single or few destroyers.
Connor MacLeod wrote:
BFG Magazine #7
Thugh they were outnumbered by over three to one the Imperial ships blasted their way through the hive fleet and scattered the bio ships into small groups. The Tyranid piecemeal counter-attacks were beaten off by the awesome firepower of the heavily armoured and shielded Imperial ships. Their lances of fusion fire transfixed the organic hulls of the bio-ships and clouds of Imperial Navy fighters darted in to tear apart the crippled vessels. Within an hour the remnants of the hive fleet were dead and drifting, charred hulks spinning slowly through the void.
The battle had taken its toll of the combined fleet. Calgar had lost half his remaining ships and several Imperail ships had to be destroyed by the weapons of their compatriots because they had been boarded and overrun by the Tyranids.
Note the "fusion fire" lances (like mentioned in the BFG rulebook) as opposed to laser lances. I would presume that fusion lances are shorter ranged but more powerful than laser lances (much as a melta/multimelta is more powerful than a lascannon, arguably, but shorter range.)
Also note the battle with the Tyranids took the Imperials at least an hour, even though they were outnumbered 3:1)
On the topic of what a lance is, Connor's suggestion makes sense. From what I read on SBC, I remember a few members arguing that lances had also been presented as lasers. I found myself wanting to know the difference, and through my reading of several sources, it is rather clear that there are two types of lances, with the emphasis being put on the long range and heavy laser lances though.
13th legion clearly defines lances as "high-energgy lasers". The high energy is reminescent of the "high powered" bit quoted much earlier on, from the BFG rulebook.
That said, "lances of fusion fire" could also be the "fusion beamers", and therefore not as high powered as the laser lances.
Connor MacLeod wrote:
Page 14
Bio plasma si treated like a lance shot...
..
Because it is a relatively slow-moving attack, like that of a bomber squadron, bio-plasma ignores shields.
The "slow moving/ignore shield" bit is curious for a number of reasons. One being, that to be an effective direct fire weapon it has to hit very rapidly (matter of seconds) voer thousands or tens of thousands of kilometers. This in turn implies a minimum velocity of thousands/tens of thousands of kilometers per second. Yet it ignores shields? there ware weapons that are just as fast/faster but do NOT ignore shields (weaposn batteries.)
Likewise it implies similar velocity for bombers and ordnance.
Void shields being partial to slow moving objects is nothing new. It's been noticed on Titans, and in BFG, torpedoes are said to ignore shields, although I don't recall speed precisely defined as the decisive element.
A Gargant's Ball munition can continue its course by rolling on the ground, and it also utterly ignores a Titan's Void shields and a Gargant's power fields.
Perhaps not ultra-consistent, it stills seems to be a recurring effect.
Connor MacLeod wrote:
Page 6
The Endeavour class, lacking the speed for anti-pirate operations, ,is preferred as a covnoy flagship or a heavy escrot to larger capital ships of the line. Its heavy gun batteries can almost match the firepower of larger cruisers and it is capable of holding its own in the midst of a pitched battle.
Endeavours are light cruisers but pack nearly as much firepower of a heavier cruiser.
From time to time, I tend to be a bit lost with imperial sub-class nomenclature, notably when there are revisions. Following a fact from a couple pages earlier on, where we were told that power was mainly divided between engines and weapons, here we see that basically a lighter vessel is actually acting more as a defense platform rather than what its size might suggest: a light interceptor sort of bulky frigate with low firepower.
The bottom of the 7th page and the top 8th page of the GUT thread have info about the evolution on nomenclatures, but I would certainly advise to get some of the free PDF files from Games Workshop and then try to puzzle the things up with references from the SDN threads.
Connor MacLeod wrote:
Page 7
Operating in this manner [One Endurance light cruiser plus 2-3 Endevaour light cruisers], the Ad Liberis was responsible for the destruction of nine Ork escorts as the War for Armageddon began above the high-g world of Pelucidar during a savage three-hour engagement before being ordered to withdraw by Admiral Parol.
"three hour" battle between nine Ork escort ships and a handful of light cruisers.
The escorts are generally weaker ships, Ork ones possibly even more, and the Imperium had four light cruisers.
Either the ships didn't have enough firepower to take down defenses that fast, or they followed a pattern of quick fire exchanges, then regrouping, moving around and even perhaps trying to find a hole into a flight formation, but that is not really "savage".
Boarding parties, however, would considerably lengthen the duration of combat.
When you see how, in general, the Imperium has enough firepower per ship to take down a ship of similar tonnage in a very short time, or how some dozens of bombers can cripple a massive warship in one fell swoop, it is very hard to consider that ships exchanged fire for three hours on and on without trying to find mitigating factors.
Connor MacLeod wrote:
PAge 26
One of the planets in the sub-sector completes its capital ship construction project. The Planetary Governor presents your fleet with a brand new cruiser.
This is off of a game chart, so it may count as game mechanics, but it implies that many planets (Feral, Feudal, or other more advanced) can create at least cruiser-grade vessels with their available facilities.
Without the full context (the prior quotation was from the seventh page of the BFG magazine #12), it's hard to take this at face value. The implication would be valid if there were more details proving that this ability can actually be replicated on many worlds.
The capacity of very low tech worlds to be able to build new cruisers appears remarkable.
Connor MacLeod wrote:
Page 27
An Inquisitor calls for an Exterminatus immediately. The next time you win initiative you will fight an Exterminatus scenario as the attacker. The Inquisition sends one of its ships along ot help. Treat the ship as a Dauntless class light cruiser with an Exterminatus weapon in the prow.
Light cruisers can do exterminatus, at least as per the exterminatus scenario. This is gameplay, so it may be considered conservative. We already could surmise from BFG that light cruisers could do so (they fall under the minimal definition of a "capital ship") of course, but this is a nice confirmation. Then again, the above also notes that this is an Inquisitorial ship, and those are notoriously more powerful than others.
Besides, we know in the fluff, for example, that Cobra class destroyers can conduct exterminatus with the right loadout too.
The vastest damage achieved by Exterminatus, in a short amount of time, is through the use of cyclonic warheads (previously virus bombs). They're totally technobabble weapons.
It is most important to notice that contrary to Connor & Co's claims, an Exterminatus may also be achieved as a localized operation, and could start with low power weapons, the final blow brought later on with the help of technomagical weapons:
"Xenos" features two Exterminati which start with low firepower. When I mean low, it's
very low.
It's a key element in the understanding of an Exterminatus procedure.
I even recall some people at SBC arguing that there's been limited Exterminatus ops.
Not to say that since there have been planets which only had their surface scorched by the Exterminati, at any time a particular target would require a greater application of firepower, like a mining complex or fortress, said target would be dealt with adequatedly; this is not a reason to pretend that a fleet or flotilla would have to slag the surface of a world to a depth of several meters down with their conventional weapons.
Connor MacLeod wrote:
Lost Soal wrote:
The last pieces comes from the musings of Larkin about his Long Las.
Only In Death, Pg29
Kill-shot at four thousand metres. He'd managed that once or twice.
Their is enough power range and accuracy in a Long Las for a skilled marksman to make a kill at 4km. While I don't know much about fire arms, if any modern rifle can match this I would be very surprised.
To get multi-km ranges you generally need very large-calibre rifles, like a .50 BMG sniper rifle. 7.62mm sniper rifles typcially won't go out more than 800-1000 meters or so IIRC. Even the .50 calibre rifles typically don't hit out to much more than a mile (though it has been known for skilled sniprs to hit out further.. in 2002 a Canadian sniper made a 2400 meter kill shot with a .50 BMG sniper rifle.) To reliably reach out to 2+ km though, you generally need a 14.5mm or larger round, and even then that's unusual (2.5 km would be tops.)
What's more, those are HEAVY sniper rifles - the ones designed particualriyl for heavy duty armour-piercing duties and whatnot. I doubt the long-las is even remotely comparable to something like that.
This doubt would be based on what?
WH40K weapons aren't exactly of the smallest kind.
Lexicanum wrote:
The lasgun mounts a bayonet lug, allowing the weapon to be fitted with bayonets or combat knives. The sniper variant of the lasgun, known as the long-las, is the preferred weapon of Imperial Guard sharpshooters. The barrel of the long-las is extended to bolster the weapon's accuracy. The barrel requires replacement every 20-50 shots depending on the power setting and cool down time. For this reason, the long-las is outfitted with a "slide-lock" barrel, which is easily locked and unlocked from the weapon's housing. Mad Larkin from the Gaunt's Ghosts novels by Dan Abnett carried one such modified Lasgun.
Plus this is a long range sniper weapon that largely ignores gravity over such short distances, and this Larkin guy only scored a kill once, perhaps twice (he's not sure) at 4 km.
Compare this to the
M82A1 heavy sniper rifle:
Specifications:
- Weight: 31 lbs
- Length: 48 or 57 inches
- Barrel Length: 20 or 29 inches
- Cartridge: 12.7 x 99mm, NATO
- Caliber: .50 BMG
- Action: Recoil operation, rotating bolt
- Sights: 10x telescopic
- Rate of Fire: Semi-automatic
- Muzzle Velocity: 853 m/sec
- Maximum Effective Range: 1,800 m
- Maximum Range: 6,800 meters
- Feed System: 10-round box magazine
The record by one of the Canadians was over 2400 meters with a
McMillian Tac-50, with low drag ammunition. The maximum effective range of this weapon (assuming the builder didn't lie about the stats) is 75% of the record held by the Canadians.
As it's easily noted on rifle/shooting forums, official claims are often exagerated.
So basically a long-las in a skilled sniper's hands is between 2-4x longe ranged than a modern rifle.
An extremely rare kill, for a weapon that would logically be more massive than a traditional lasgun. The lasgun has the advantage that what you see is what you can hit, simply because no matter if gravity or difraction affect the photons' path, since light managed to bounce off your target back into the ocular, in theory the lasgun's bolt will travel back along the same route.
Then, of course, comes the question of how well the lasgun is calibrated.
We don't know the size of the target.
As a side note, you can generally expect a regular rifle to have roughly 1/2 (7.62mm or .308) to 1/3 (most .50 BMG) the range of a sniper rifle for precision shots, and probably comaprable range against groups (Point target vs area taget IIRC. For example, an M-16 has a point target range around 450-500 meters, and an area target of around 800-900 meters.)
This would mean you could probably expect a regular lasgun to hit a "point target" at between one and a half to two kilometers, and "Area targets" of around 3-4 km, roughly.
Or perhaps not. The evidence for this reasoning is rather sketchy, at best. If we pick the percentage from above (75%), then by extrapolation the maximum effective range of the sniper lasgun would be three kilometers, and thus, half of that would be 1.5 km, and 1 km for a third.
That is without knowing how a long range lasgun has its range improved. The loss ratio between a sniper lasgun and a traditional lasgun could be very different from the one that applies to current projectile rifles.
It would actually
need to be different, to fit with books, wherein ranges rarely reach beyond half of the highest value presented just above, and kill-shot ranges are even shorter.
Connor MacLeod wrote:
Battlefleet Gothic magazine #15
Page 18
The Seditio Opprimere was reduced to a near hulk at the Battle of Prandium. It was rebuilt as a gunship with powerful long-range plasma lances to give the Ultramarines stand-off fire support when fighting Tyrannids using the new fleet tactics. This severely compromised the ability of the barge to deploy trroops.
Mention of "plasma lances" - the third kind of lance (the other two being laser and fusion lances) I have seen for Imperium warships. It's also implied to be both mass/volume intensive and very long ranged/powerful (how much so isn't sure.)
Nothing really excludes the idea that plasma lances couldn't be just another wording of the fusion lance/beamer. Plasma, after all, can be obtained by fusion, and infantry weapons shooting plasma are related to fusion from time to time. Perhaps only their increased range would make them different.
Perhaps they are not capable of prolonged uses, with considerable cooling down timespans.
Connor MacLeod wrote:
BFG magazine #16
Page 4-5
In seconds, Urthwart became a victim of the aptly named Planet Killer - the world and all it had ever contained committed instantly to history by this greatest of Abbadon's machineries.
Planet is destroyed in "seconds" by the Planet killer. This probably qualifies as an "outlier" since usually it takes around half an hour to an hour to achieve that. Unless it was a very small planet, or unusual in some way. Given the warp/daemonic properties inherent in the Planet Killer, its also quite possible that some "Exceptionally fast" kills do occur, just like in a few cases we have exceptionally slow ones.
A second possibility is that something on the planet itself blew up. We know from the St Josemane's hope example that planetary powerplants can be rigged to blow up and mass-scatter a planet in an Exterminatus, so its possible that the Planet Killer can take advantage of this somehow.
Alternately, since IIRC this was durinb the 13th Black crusade, the Planet Killer may have simply been upgraded, and this just represents a vast improvement in its firepower.
Or the weapon's power was used differently, to affect the whole surface instead of that 30 minutes long drill thing.
It's not like weird spreading/technobabble effects of weapons at such scales would prevent the capacity of a planet killer to spread over the surface of a world. After all, if the video game Firewarrior cutscene is to be taken at face value (aside from the extremely messed up scales), that's pretty much how the cyclonic torpedo spam ends, with a big fireball and stuff that runs over the planet's surface.
I may quote myself in a possible APK thread.
Connor MacLeod wrote:
PAge 20
There could never be enough warships to fortify all the millions of worlds in the galaxy that make up the Undying Emperor's domain, and some planetary populations may go decades or generations without once having the privelege of an Imperial Cruiser silently gliding into orbit overhead.
"millions of worlds" in the Imperium.
There could "never be enough warships to fortify all the millions of worlds" If we assume "fortify" means half a dozen vessels (sevearl cruisers and a number of escrots, ther could be no more than 12 million ships total, 1/3 of which cruisers. Note that there IS a differencec between "fortify" and "patrol" - a single cruiser or a flotilla of escorts could not realistically be considered sufficient to fned off any sizable naval force save pirates.
As far as the last part: "decades" or generations implies a visit of a cruiser once every 20-100 years. If we assume each planet is viisited in that timeframe once, this would mean between 20,000- 100,000 cruisers, minimum. At 10 million planets its 100,000-500,000 cruisers.
10 million planets? Did the figure go up by one order of magnitude between the first capital of this paragraph, and the final period?
There are sources that peg the Imperium's world at "a million".
And do we know that
all worlds are inhabited?
14; a celestial body (a planet).
A system under imperial control would typically count several planets, even several stars in some cases (up to 6 or 7).
If you consider our system, there's between 9 and 10 planets (allow me to keep my distance from ever changing definitions), with only one naturally inhabitable. Ours.
That's simply 10%, roughly.
There's the following interesting part from the BFG rulebook which didn't make it in Connor's thread:
BFG rulebook, p.45 wrote:
PLANETS
Less than 1% of systems have planets orbiting a solitary star in the manner of ancient Terra. Even so, there are millions of star systems containing billions of worlds scattered across the galaxy. Most planets are either desolate, empty and airless, or surrounded by an atmosphere too noxious to support life. In the Gothic sector there are over two hundred inhabited worlds and tens of thousands of other planets. Planets often becomes the focus of space battles as opposing fleets attempt to establish forward bases or extend their control throughout a contested system.
So, basically, we learn that a system is considered acquired when a central world is under control. Worlds in the life band of a star system are where "the bulk of a system's defences are built" (p. 43).
Some colonies and hive cities can be found closer to the local star in certain systems.
This sufficiently narrows the scope of worlds that really need to be defended or even visited.
Gothic sector has +200 inhabited worlds, and at least 20,000 more worlds. So on a total of 20,200 worlds, only 0.99% are inhabited.
Perhaps even more telling, is that on the other side of the spectrum, if the Gothic Sector counted a total of 100,000 worlds, then there would be only 0.2% of its worlds that would be inhabited.
Now, let's return to Connor's ship count based on the scarce visit of ships to planetary populations, and let's get some numbers here.
With 1 million worlds, say you need 20 years for all planets to see a cruiser once (and that's picking the fewest decades possible), it means that within 10 years, only half your worlds will have seen a cruiser, and only a quarter within 5 years.
It puts the maximum number of cruisers at 50,000, so that at the end of the 20th year, and for the 21th year, your ships can return to the first batch of planets they visited for another round.
That's in the absolutely best conditions imaginable.
Less worlds with noticeable planetary populations, and with time cycles greater than 20 years, will significantly cut the numbers, by a rather considerable deal.
For example, 100 years instead of 20 brings the number down to 10,000 cruisers.
10% of the Imperial worlds being significantly inhabited brings the number down to 1,000 cruisers.
Or only a hundred cruisers when only 1% (well actually 0.99%) of the Imperium's worlds are inhabited. 1% is obtained from the ratio of inhabited worlds in the Gothic sector, as seen above.
Now,
IF the Imperium did control 2 million worlds, that would make a couple hundreds or thousands of cruisers tops.
On a mildly related note, a sub-sector such as Bhein Morr contains many systems:
Balaam, Bhein Morr, Duran, Elysium, Fularis, Kharlos, Krool, Luxor, Mastado, Oechalia, and Stranivar. (11)
The Lysades subsector contains the following systems: Arimaspia, Bladen, Boetia, Coimbra, Corlini, Saviour, Schindlegeist, Sicyon, Skagerrak, Tarantis, and Vindalex. (11)
Source.
You can find Connor's fleet numbers
here.
This is clearly another big topic, so at some point, I may take a deeper look at this subject.
Connor MacLeod wrote:
Page 20
Despite these portents of doom, Belis Corona was not without its defences. It is home to vast orbital dockyards, decomissioned ship reserve fields and repair facilities that dwarf even those of Armageddon's St. jowen's Dock in importance ot the security of the Imperium.
Seems to be a sector base if not the naval sector base. Has its own reserve fields as well as its own major shipyards (bigger than aRmageddon's which means they're significant.)
This also implies that reserve fleets and major yards are present at least one per every sector (thousands or tens of thousands of such yards in the Imperium, in addition to the Segmentum bases, whch are perhaps even bigger and the Forge world yards.) Given the probable size of such yards, we might anticipate hundreds of thousands, if not millions of additional vessels in shipyard reserves, easily.
There's no proof that the conditions that apply to Belis Corona establish a standard.
Connor MacLeod wrote:
page 28
Deep within the bowels of the ship, archaic weapons of unimaginable power hummed to life, and the Master of Gunnery personally supervised the loading of the awesome Nova cannon as gangs of men numbering in the dozens opened the thirty-meter wide breech to accept the massive, multi-ton shell being trundled into place by more lash-motivated gangs pulling on an enormous overhead gantry under the careful eye of gunnery chiefs and Mechanicus adepts.
This Nova cannon, mounted on a Victory-class battleship, is thirty metres in diameter. This is smaller than the fifty metre diameter Nova cannon mounted on a battleship in Warriors of Ultramar (then again that one traveled at barely 2% of c, whereas in BFG and Rennie's novel Shadow Point specify that nova cannons travel at near-c. Go figure.)
In any case, assuming a 30 meter diameter, 60 meter long cylinder the density of water traveling at .8c, the shell will mass about 42,500 tons (4.25e7 kg). At .8c the KE of the round is 2.55e24 joules. Momentum for the shell is 1.7e16 kg*m/s. This is very conservative, given that the projectile could arguably be longer (as noted, if the shell is of similar proportions to a torpedo, then it will be much more massive) and my absurdly low density (it should be at least several times as massive on density alone.)
The velocity could be much higher as well. At .9c the KE of a 42,000 ton shell goes up to 5e24 joules. (momentum 2.633e16 kg*m/s) At .99c the KE goes up to 2.33e25 joules (momentum of ~9e16 kg*m/s)
What I find also confusing is that, unlike a great many other weapons (bombardment cannon, lances, torpedoes, etc.) a nova cannon fires only a single shell. If Nova cannon come in different sizes (in a battleship you should be able to easily mount several nova cannon) why they can't do more than one smaller type. It would seem to make more sense for the weapon (especially if, as noted before, Nova cannon are of similar scale to Bombardment cannon or the larger kind of torpedo.) Heck, maybe they do, or it depends on ammo or design....
Note that given the recoils (momentum) above, we can estimate the accleerative capabilities of a warship. Assuming iron, 95% empty space, and a 7 km box (height/width 1/5 the length), the estimated mass of a battleship would be ~5e12 kg (5 billion tons) with a recoil between ~2e16 and 9e16 kg*m/s would be between 4,000 and 18,000 m/s^2. which again as I noted is very conservative for various reasons (the density of the shell alone could justify the accelerations being considerably higher.)
All these Nova cannon calcs are good, as long as you focus on a limited segment of the whole information about this special weapon. They don't take into consideration the problem that the weapon cannot slow down on its own, making a poor blast effect, and certainly not one as suggested by the BFG rulebook.
Also direct hits against Roks are hardly consistent with the calcs above, yet no mention was made about Roks being shielded, reinforced or benefiting from any Waaagh effect whatsoever (which is an excuse used rather often to defend high numbers).