Hi,

I've been doing some thinking.
What would be the speed and mass of a Macro Cannon projectile and would be its effect on a planet.

Doing some research I found out a average projectile, as stated in the BFG manual, would weigh approx. 1 kilotonne (1.000.000kg).
Looking at rogue trader if a projectile would have to travel a distance 12 VU (120.000km), a Macro Cannon's maximum range it would have to travel at least 240.000 km/h to reach the target in half an hour (Space Combat turn).

The fastest traveling meteorites are rumored to have hit earth at 240.000 km/h (nice coincidence) so the speed is not unreasonable.

So for our science friends out there....what would a Macro Cannon volley do to a planet?

Santiago...

Depends on how much mass it loses during entry, and if it explodes at altitude.

I highly doubt it will lose much mass during reentry since that will take less that half a second, and any mass it loses will produce gigantic amount of heat.
Though doing some further research I do think the projectiles will have a lower mass since RT states a Frigate for example being 6 megatonne.

But even if those projectiles would be 1/20th or even 1/40th of the weight say 25.000kg it would still equate to something the size of an aircraft carrier hitting the ground at an insane speed...

It would do nothing at all to a planet. Assuming the projectile is a sphere made of iron or something of similar density it'd be about 6m diameter, that comes to 904 tonnes. At 67 km/s (240.000 km/h) such a projectile would break up in the atmosphere, if you are unlucky some of it might hit you, but that's about it.

Sure, a kilotonne SOUNDS awesome. But it's really not that much when it has to go through the atmosphere. Macrocannons won't work on ground targets.

www.lpl.arizona.edu/impacteffects/cgi-bin/crater.cgi

Graspar said:

If you're using the same munitions as in space combat, probably true. I personally houserule that effective planetary bombardment uses specialised munitions that are better at penetrating atmosphere at the expense of being less effective against space-based targets. I've stated that any ship with a Munitorium will have carried as standard.

However, barring Exterminatus munitions, which Rogue Traders won't have access to, as Graspar said, Macrocannon shells along with other RT-rulebook weapons won't do much on a planetary scale. The players can devastate cities and wreck havoc on a human scale, but none of the weapons we have stats for so far will wipe out the Biosphere like a Dinosaur-killer asteroid. Which is as it should be- the Imperium of Man didn't survive this long by letting weapons of that magnitude enter common usage where any Captain with a grudge can depopulate a whole world.

However, barring Exterminatus munitions, which Rogue Traders won't have access to, as Graspar said, Macrocannon shells along with other RT-rulebook weapons won't do much on a planetary scale. The players can devastate cities and wreck havoc on a human scale, but none of the weapons we have stats for so far will wipe out the Biosphere like a Dinosaur-killer asteroid. Which is as it should be- the Imperium of Man didn't survive this long by letting weapons of that magnitude enter common usage where any Captain with a grudge can depopulate a whole world.

I wouldn't be too sure about that. A Rogue Trader isn't exaclty "any Captain", but one of the most powerful individuals at least as long as we're staying below sector-scale. Converting, conquering or killing worlds is exactly what they do and considering that the US and the SU were probably capable of making earth enter nuclear winter during the cold war era, I think it's pretty safe to assume that a RT setting his mind to it has at least the same ability.

Graspar said:

Try changing your macro-cannon shells to be made out of Tungsten (19.25 tonnes/m^3 and a 4.6m diameter shot):

http://www.lpl.arizona.edu/impacteffects/cgi-bin/crater.cgi

You get a few hundred kilotonnes of boom at the planet's surface.

Edit:

Oh, and aircraft carriers tend to displace tens of kilotonnes (supercarriers displace over a hundred) so a kilotonne for a single macrocannon shell sounds pretty reasonable.

Edith The Hutt said:

D'oh.

But you've got something wrong there. A kilotonne tungsten sphere would have a radius, not diameter of 2.3. A 2.3 diameter tungsten sphere would weigh ~114 000 kg, so it's about a tenth of the weight given by the rulebook.

www.lpl.arizona.edu/impacteffects/cgi-bin/crater.cgi

But yeah, it's going to hurt a lot more with tungsten.

Graspar said:

Oooops. I spotted that error a few minutes after posting and tried to correct before anyone else saw it. Just what I deserve for doing mental arithmetic after midnight. Thanks for the correction

Hehe, I'm just glad I got to get something right after being so horribly wrong.

Now, how would laser (sunsear) and plasma (ryza) work for ground targets?

Who says the Shell 'weighs' a kiloton? The Mars Macro-batteries subsystem states, “Pg.202....these are reliable hard hitting weapons firing kiloton ordnance...” To me, the kiloton refers to impact strength or denotation of explosive or non-explosive ordnance. To quote the Gothic Blue book, “pg.20: Each battery consists of rank upon rank of weapons: plasma projectors, laser cannons, missile launchers, rail guns, fusion beamers and graviton pulsars.” There is also a story on the next page about a gun crew loading a gun that fires shells of only 7 tons. It would be safe to say that some types of weapons that make up the myriad of guns that within a battery, there is something that can make it through reentry and cause havoc akin to a nuclear detonation.

Still, nothing says the group can't park their ship above the polar caps of a planet and melt it with nuke-like firepower to cause a global flood, blow up large swaths of rainforest to lower the oxygen level so it can't sustain human life, or pummel an entire continent into a plasma blasted wasteland. The issue is that it takes time, lots of ammunition, and doesn't guarantee complete eradication of life. Exterminatus weapon's work on a whole different level of ugly. From setting off all the oxygen on a planet and turing it into a small sun for a few hours, to the life eater virus that liquidizes all biological matter and passes through atmospheric filters and re-breathers as if they didn't exist... then turns everything into methane before then setting the planet on fire. The other option is bombardment cannon's of the space marines, but those just powerful Melta-ordnance to cut deep even into mountains to get at strategic targets, and torpedoes that could burn their way through the atmosphere if properly heat shielded, then detonate with the energy of a mini-sun over a city.

In my books, I would allow the players to use Weapons batteries on planets, but only 'accurately' (Using a sledge hammer instead of a scalpel during brain surgery accurate', but at least they will hit closer then from 60,000km ou in space) from orbit, where they are vulnerable to ground based space defence weapons... And if the planet doesn't have any, then it brought this upon itself.

I was more under the impression that munitions with a kiloton rating would be their explosive effect aka a kilotonne of TNT = 4.1 terajoules... 1 gram of TNT is around 1.1kcal, so its a pretty impressive bang, especially considering macrocannons puke out quite a few of them per volley.

Rather than solid shot, this would make them effective against ground based targets as like most warheads they'd have a defensive outer casing, if it was me, I'd use depleted uranium, which is commonly used on an ICMB's warheads re-entry casings now, to lower the impact of the atmosphere you could also use a shorter charge in the cannon itself.

As to the 'bang' itself, that doesn't necessarily mean that the explosive is of a nuclear variety, the effect sans the radiation would still be much the same and its kind of up to conjecture with the ye olde written lore of 40K just how they go bang. They could well be some super-chemical reaction or even some exotic, critical ionization velocity wrapped up in a shell casing.

As much as it might be seemingly practical and romantic to throw cannonballs at other ships and planets, the Imperium of man does maintain a pretty solid technology foundation when it comes to killing. If it is a kinetic killer, the kiloton would refer to its impact capability, rather than its size and that would require factoring its mass and velocity the cannon can chuck it.

While I aggre so far with what people are saying I do have one question. Why use cannons and batteries when you could use mass drivers. Those shells serve a very important role for defense where a mass driver just takes space debrie super accelerates it and then boom.

In my books, I would allow the players to use Weapons batteries on planets, but only 'accurately' (Using a sledge hammer instead of a scalpel during brain surgery accurate', but at least they will hit closer then from 60,000km ou in space) from orbit, where they are vulnerable to ground based space defence weapons... And if the planet doesn't have any, then it brought this upon itself.

Well, there's evidence saying that a starship's weapon batteries are reasonably accurate to about 3km when aimed from about 6 VUs, given a Mars Pattern Macrocannon.

Sure, the kilotonne projectile might refer to the explosive yield rather than the actual weight of the projectile. But that's not as much fun seeing as how the tungsten round releases over 500 kilotonnes of TNT worth of energy at impact.

But once again that's probably a more reasonable interpretation, in which case I go back to "macrobatteries won't work on ground targets."

Lorewise, any Imperial vessel with enough firepower to engage in ship-to-ship combat can turn a fortified mountain complex into a crater, I don't know if the science would support this assertion, but that's what an Imperial Warship is supposedly capable of in the 40k setting. From what my gaming group(s) has/have learned from various 40k-canon novels, Rogue Trader and many matches of Battlefleet Gothic, this is my understanding of what Imperial Starship-grade weapons systems are capable of. These are all my own estimates and educated guesses, based more off what I know of the 40k setting and personal experience than any insightful mathematical analysis. But either way it's well established in the lore that however Weapon Batteries and Lances do their thing, they work extremely well when used for planetary bombardment.

Weapons Batteries
The most common types of Weapons Batteries are the large solid projectile cannons that have already been extensively discussed, usally referred to as Macro Cannons. Throne only knows what standard Mars-Pattern macro shells are made of, but as amusing (and thematically appropriate!) as the idea of upscaled lead cannon balls are, it's probably one of the many made-up materials commonly used by the Imeprium.

Macro Cannon bombardment is messy and will cause catastrophic collateral damage for potentially dozens of kilometers in every direction (the "collateral zone" if you will), but it will annihalate the target and anything near it, hardened underground targets would be highly resistant to Macro Cannon shelling, however, due to munitions concerns. Having full-sized Mars Pattern Macrobatteries would speed the process, but I would imagine accuracy would suffer if you didn't have a particularly good master of ordanance.

Laser Batteries are considerably less destructive per shot, but longer ranged, more penetrating, more accurate and not constrained by limited mutions as they run off a Starship's power grid. A warship armed with batteries of Sunsear Lasers would be able to effectively bombard a medium sized target such as a town from very high orbit with accuracy, and could equal or exceed the mass destruction that can be wrought by Macro Cannon given enough time. If the Explorers were trying to bombard a planetary target during a space battle, it would probably take 25% longer or even 50% longer to accomplish the desired result with mass laser bombardment than with macro cannons. But with the luxury of time they could devastate whole continents without needing to re-arm, and would be capable of more precice bombardments than any other type of weapons battery, making it one of the better choices for bombarding invaders on Imperial soil.

Plasma Batteries will simply annihalate whatever you point them at. The rules for them as presented in Rogue Trader suggest that Ryza-Pattern Plasma Batteries would quickly vaporize and/or glass large areas of terrain in short order, I'm not sure what ballistic properties plasma weapons would have in an atmosphere, but I imagine they would be fairly accurate compared to macro cannons, but still far too indescriminate for precision bombardment. If the Explorers were trying to bombard a planetary target during a space battle, it would probably take as little as half as much time to destroy the target area with plasma fire as opposed to macro-shelling, but the Lord Captain couldn't expect any result other than total destruction of the target and anything else in the collateral zone. These guns are also ideal for mass bombardment of an entire planet due to them running off the ship's plasma drives, though their power draw would possibly make annihalating a continent slow work.

Other Weapons
Lances are considered perfect for presicion destruction of a target from orbit due to their precise nature and enormous yield, but they are next to useless for mass bombardment of a target area. There is at least one recorded example of a Lance Strike being used to destroy a single wing of a larger complex without harming the Imperial Guard forces less than a kilometer away.

Astartes Bombardment Cannons "consist of a turret mounted linear accelerator that fires re-entry hardened magma bombs of much higher calibre than regular torpedoes at a much higher velocity. Its primary function is planetary bombardment, which is used in support of Space Marine landing forces. This suits the battle tactics of the Adeptus Astartes quite efficiently. Thus a single vessel can bombard an area of planet, then immediately deploy drop-pods containing battle-brothers to mop up the confused opposition. They can also be deployed in ship-to-ship combat to devastating effect. The high power of the warheads will literally render armour plating irrelevant, and can tear an enemy vessel to shreds with a single salvo."

Really says it all there, they combine the best of Lances and Weapon Batteries to create a weapon that wrecks whatever you point it at, and only what you point it at, but we can't get them.

"The ammunition fired by the Nova Cannon is a specially prepared imploding charge, timed to detonate after it achieves a set distance from the vessel that fired it. The distance is calculated by the ship's weapons officer. Though the high speed of the projectile prevents detonation at short ranges, as it will have already obtained such a high distance in a very short space of time."

Nova Cannon are undoubtedly weapons of mass destruction, even when compared to the above weapons. The ship muct present the bow to it's target to use the Nova Cannon however, as it is always a Prow armarment which precludes the use of other weapons on the planet while employing the Nova Cannon. Also, the warheads are designed to go off after a predetermined amount of time once fired to increace the chances of scoring a hit, though when firing at a *planet* I expect they could be set to detonate on impact easily enough. Accuracy would also be greatly improved by the fact the the planet isn't a moving target, or at least, a slowly and predictably moving target. I'm not even going to speculate on how catastrophic a Nova Cannon impact on a planet's surface would be. But I expect the destruction could be described using words like "total" or "biblical."

Simply firing solid shot on a shallow re-rentry path at a target from orbit would net you some massive damage. Normal macrocannon ordinance is explosive. Sheath it in an ablative shell and fire it such that it coasts through the atmosphere to the target and you'd annihilate anything you want - fire as many shells as needed. If it has defenses thats what your troops are for, insertion to destroy said defenses.

Lances fire through atmosphere just fine....theres a IG book out there where a Navy vessel uses its lance(s) to destroy a large demon on a planet after it is encountered by an IG unit.

In the end, in Lure of the Void there is a use of weapons batteries to bombard a planet. One of the Rogue Trader competitors parks his macro-battery only cruiser above the target planet, and starts making it hail with ordinance, eventually with his lance armed escort joining in after. So while were free to give and take from what are players are able to do, the game itself supports (Narrative) the use of weapons batteries to target planets and instillation on it. Due to the epic scale and space opera style of Rogue trader, the effects of such bombardments should be considered equaly so in devistation.

Macro-cannons are no planet busters, but imo, a pretty standard frigate could cause some pretty terrifying damage to a planet's surface.

Firstly, I think the 'kilotonne ordinence' thing is more descriptive than an exact measurement of the projectiles, which I suspect would vary greatly in size.

Also: Accuracy, if you want to hit anything at 60,000 kilometers, you also need to be able to hit it at 1,000 kilometers. A Macrocannon in orbit can target and hit a single tank, it has to be able to in order to hit other spaceships (though it may blow up more than just a single tank).

Also, there is no reason why a macrocannon could not fire differing ordinance, Tzar bomba was only 8m long by 2 wide, you could conceivably squeeze a few of those out, bingo 1/2 gigaton bombardment. an extreme example though. in general, with specialist orbital bombardment shells, even if just heat shielding, you could do appalling damage.

though there is no canon for it, I suspect that 40k ships would conceivably have lighter orbital attack weapons ICBMs, Kinetic kill vehicles, masers and the like. tactical bombardments to the main ship weapons strategic.

You could also conceivably make good use of emp attacks, given the haphazard levels of 40k technology, a high altitude detonation(or bombardment) could have pretty crippling effects.

My group has had a general rule that firing from high orbit, only lances are able to conduct planetary bombardment. You need to fly into low orbit to use macrobatteries and this makes you extremely vulnerable to counter fire, places the ship under great stress and makes otherwise routine manoeuvres potentially deadly. Its possible, but you have to be somewhat skilled and brave to do it. Doing it over a hive city will get you shot down by the cities defenses very quickly.

Lances on the other hand are ace at destroying stuff from high orbit. Reasonably precise, as in, you can hit the district you want in that city. Extremely deadly (as in anything not a hardened fortification is instantly toast.

Imperial fortifications are often void shielded and hardened against attack from orbit, one of the primary reasons you need to send in the (space) marines sometimes. There are plenty of examples of defensive structures holding out for months or years in the lore when there are plenty of orbital warships available for lance striking. The reason they don't is void shields and ground defenses.

Lasers aren't actually that useful from orbit: thermal blooming effects and the inverse square law mean that "precision strike" quickly turns into an area-effect weapon. I read an estimation somewhere (and I will cite it properly as soon as I remember/find it again) that a laser with a 60cm aperture, fired from geosynchronous orbit (and so firing through the entire atmosphere), will hit an area of roughly a hectare (roughly 2 American football fields) at sea level. The beam energy (and hence destructive effect) of the laser strike will likewise be spread over that area. The advantage of shooting at a planet is that it can't dodge, so you can afford to keep that laser firing for longer (heat limitations notwithstanding).
In space, that same laser is going to have a beam diameter roughly equal to its' aperture for the majority of its' effective targeting range, as it's only the diffraction effects of the actual aperture to deal with (plus any drifting wisps of dust/vapour, but you can pretty much ignore those).

Plasma weapons, interestingly, get more accurate in an atmosphere, for reasons which I don't have the math to understand (something to do with damping electrostatic blooming and virial theorem), but would almost certainly start losing range and power much faster as the plasma bolts dissipate in atmosphere. My personal take is that plasma weapons in the 40k style are one of the "soft" sci-fi parts of the setting, and that they really should be relegated to ultra-deadly high-tech flamers and (relatively) cheap drive systems, but hey, it's all good.

Macrocannons are an interesting one- targeting shouldn't really be a problem (you've got optics and auspex capable of spotting an object less than 5km long, over interplanetary distances, travelling at a ridiculous relative velocity; hitting the stationary target below you? Your margin of inaccuracy is going to be less than a centimetre, barring stupidly strong winds).
Given the necessary muzzle velocity for space combat (66.666...km/s, going by the OP), and assuming the projectiles are plain ol' BFRs (Big Frakkin' Rocks, ie: pure kinetic weapons, with no propellant stage or warhead), each kilogram of mass in the shot is going to impact with roughly the same energy as a 494 kilogram hunk of TNT*.
That being the case, even a 2kg hunk of junk is going to impact with over a kilotonne of force (taking into account the gravitational acceleration of the planet). If the macrocannon shell is 40kg (as suggested above, that makes an impact of 26.95Ktonnes (assuming a planetary gravity equal to Terra).

Now, for extra BLAM factor, consider that a Lunar class cruiser has (per BfG, via the short story Cross the Stars ) 40 guns per deck in its' macrocannon battery (equivalent to a single Mars-pattern Broadside, in RT terms), with an unknown number of decks, but probably at least 2 or 3 (spaced and braced for recoil). That means over a megatonne of kinetic force, which can be targeted to well within a metre, before needing to reload even one gun (assuming only one gundeck in the battery).

Of course, anything in a suitable orbit to shoot stuff on the planet below (and have a hope of hitting its' target; despite my comments earlier, you really have to get fairly close to pick it out with all that atmospheric crud in the way) is going to be in a predictable orbit. All it takes is a single rock (or small craft) sent up on the other side of the planet on a reciprocal orbit, and your day can be completely ruined (or, for a more boring option, a powerful gun on the surface, firing from over the horizon and letting simple ballistics hit so the work).

*Robinson's 1st Law of Space Combat: An object impacting at 3km/s delivers kinetic energy equal to its' mass in TNT; 1 Rick=1kg of TNT's worth of kinetic energy per kg of projectile, found with Ricks=(0.5*V²)/4.5e6), where V is the velocity of the projectile in m/s

so Ricks=(0.5*66666.667²)/4.5e6
Ricks=(0.5*4.444e9)/4.5e6
Ricks=2.222e9/4.5e6
Ricks= 493.82717

Given that a 2 kg rock imparts a megatonne of kinetic energy at the minimum speeds, we can safely say that one of the following three is true:

1) A mars pattern macrocannon fires a projectile that weighs no more than 2 kg or

2) "Kilotonne ordinance" in the description of mars pattern macrocannon means "weighs 1000 tonnes" not "does damage like 1 kilotonne TNT" or

3) The word kilotonne in the description is an ad hoc "sounds dangerous" description rather than an attempt to give the weapon actual parameters that can be used to calculate stuff.

Alternative one is ridiculous, alternative two is massively overpowered and alternative three is boring but probable.

So it is safe to assume that the space weapons presented Rogue Trader/BFG/40k are more than capable of whiping out small cities (New York) with a single volley and larger cities (Hive Cities) with one or two hours of sustained bombarding.

Off course I could imagen planets having Defense Satalites or ground based missie batteries or macro batteries.

Too....much....math.......cant........go on....

Off course I could imagen planets having Defense Satalites or ground based missie batteries or macro batteries.

Considering that planets avoid quite a lot of the hassles of space ships (having to lug all that stuff around, bleeding off heat,...), that seems very likely.