Thursday 8 October 2015

BFG: A Sense of Scale

Realism and consistency have never been considered to be strengths of the Warhammer 40,000 setting and Battlefleet Gothic is no exception. No wonder then that the games designers and story writers play quite fast and loose with silly concepts such as size and distance.

One thing that I've always enjoyed about the game however is the sense of scale. There are space-faring cathedrals the size of cities that traverse the warp and are defended by high-tech shielding while they attack each other with... conventional cannons mostly. Yet these cannons are actually surprisingly accurate because they are able to hit targets, relatively frequently, from vast distances. But just how vast are we talking?

I'm going to try my best to tackle this question. Make no mistake that my guess is no better than anybody else's, but I like having a logically consistent setting to play in so this is mostly for my own benefit.

A Blackstone Fortress. The 40k equivalent of the Death Star.

Weapon Ranges and Scale

The first question I want to try and and answer is what scale the battlefield uses.

According to the rulebook chapter First Principles (p5) "Battlefleet Gothic takes some liberties with scale" to make the ships look good, so we clearly can't use that as a starting point. Fortunately it then goes on to give us our first bit of hard data by saying "the actual base of a ship model represents very close range around the ship, no more than a few thousand kilometres." Normal sized ship bases have a radius of about 1.25cm. We could argue semantics until the cruisers come home, but I'm going to have to assume that a 'few' is a value between 3 and 9 inclusive.

To support this, on p78 there is a description of Torpedoes being shot down by turret fire at a range of 2,000km so 1.25cm is clearly more than 2,000km.

1.25cm is a few (3-9) thousand kilometres. 

The next piece of evidence we have is this note by Andy Chambers which states a scale of 1cm to 1,000km for planetary features, although he doesn't explicitly mention the battlefield itself. He also notes that "the scale is basically there as a rule of thumb." and that we shouldn't take this as read for weapon ranges, speeds and so on. Nor does it agree with the statement about base sizes. Clearly this isn't entirely reliable then and we should look elsewhere, such as the stories within the setting.

There aren't many instances I could find where engagement distances were specifically mentioned for space combat, let alone any kind of maximum. One example is in reference to two Blackstone Fortresses which "took up station some seventy-five thousand leagues from Fularis II and just out of range of the world's weapons platforms, except for the torpedo launchers." If we make the reasonable assumption that the weapon platforms are in a relatively near orbit of the planet this gives us an upper boundary for the effective range of conventional weapons.

So lets work on the assumption that these defences would have had an effective range of 70,000 leagues. That's about 388,920km. Given that the maximum range in this game, with a few exceptions for Warp Cannons and the Planet Killer main weapon, is 60cm we have a rough tabletop scale of 1cm = 6,500km.

This just about fits with the First Principles, meaning that very close range around a ship (represented by the base) is within about 8,000km.

Another source of distance is the Nova Cannon. "Nova Cannon shells are not armed for a fraction of a second after firing, allowing them to travel many tens of thousands of kilometres through the void before they become truly deadly." The minimum range of a Nova Cannon is 30cm. This works out to around 195,000km in our scale which definitely accounts for "many tens of thousands of kilometres".

Other weapon ranges mentioned are on BFG p119 where a Frigate is describing shooting at Iconoclasts from about 5km and closer and stating that they couldn't miss at that range. The Frigate rammed the iconoclast soon after implying that this was unusually close range indeed.

One nice side effect of all this, is that it makes the Imperial 'dumb-fire' torpedoes seem much better than you would assume from reading the rules. Effectively, after being pointed in the right direction and getting up to speed, they can hit a fast moving, <1km long target within a 16,000km diameter volume of space with very reasonable accuracy. In fact once they are that close they are only inhibited by the target's armour and point defences! For reference the longest ranged ICBM is the R-36M at about 16,000km and it has an accuracy of 220-700m CEP. Pretty comparable really.

  • 1cm = 6,500m

Weapon Ranges:
  • Turrets: up to 8,000km
  • 15cm Batteries: up to 97,500km
  • 30cm Batteries: up to 195,000km
  • 60cm Batteries: up to 390,000km
  • Nova Cannons: 195,000km to 975,000km
  • Torpedoes: 1,170,000km and more - at least the length of a 6' table.

Comparative Distances:
  • Light second: 299,792km
  • Earth to Moon: 384,472km
  • Cruiser turning circle: 170,000km assuming no special orders.

If these values are even close to accurate then it strongly implies that excellent computational facilities assist with aiming and/or conventional munitions are guided to some extent. Fortunately this disposable technology already exists in Seeker bolts and Executioner shells so it wouldn't be much of a stretch.

Celestial Features

Unfortunately the planet sizes given in the rulebook come into conflict with the figures above. We're actually given "typical planetary templates" in the book and they don't match these calculations even closely. Fortunately I think we can ignore them safely as they don't even make sense relative to each other!

Below is the chart of planet sizes and which planets from our solar system they might represent. I've added the diameter of each planet in brackets.
  • Small Planet - up to 15cm: Mercury (4,879km), Pluto (2,372 km) or Mars (6,779 km)
  • Medium Planet - 16-25cm: Venus (12,104 km) or Earth (12,742 km)
  • Large Planet - 25-50cm: Saturn (116,464 km) or Jupiter (139,822 km)

Even assuming Earth was being represented by a 16cm planet, Jupiter would have to be 175cm across in comparison (and Pluto about 3cm). The largest discovered planet, the catchily named ROXs 42Bb, has a diameter about two and a half times that of Jupiter so would be over 4 metres across in this scale. Clearly this doesn't work and something has to give. Maybe one of these three can be correct, but certainly not all of them.

Gravity wells (10, 15 or 30cm respectively) aren't a great measure either as they suffer from the same scaling problems and depend a great deal on what the individual planet is made out of.

Interestingly the book goes on to talk about moons as well. It states that "most planets have many small moons around them and most of these are no larger than generously sized asteroids." and that these examples are not dealt with further in the rules. Larger moons are set at a size of 5cm and a distance of 2D6x10cm from their host planet.

Again the size is far off, but curiously the distance seems about right.

The distance range on my proposed scale is 130,000km to 780,000km. You'll note that the distance between the Earth and the Moon fits neatly in-between these values. For a few other examples of large moons in our solar system, Ganymede and Io orbit Jupiter at around 1,070,400km and 421,700 km respectively, Titan orbits Saturn at 1,221,870km and Triton orbits Neptune at about 330,000km. As such this seems to fit the proposed scale pretty well although it could probably be d20x10cm instead (and generate multiple moons) if the table were big enough to accommodate them.

Therefore it's my belief that the planet templates, much like the ships themselves are considerably oversized for what they represent. A more realistic set of values would be:
  • Mercury (1cm)
  • Venus (2cm)
  • Earth (2cm)
  • Mars (1cm)
  • Jupiter (21.5cm)
  • Saturn (18cm)
  • Uranus (7.5cm)
  • Neptune (7.5cm)
  • Pluto (<1cm)
  • ROXs 42Bb (54cm)

Which would very roughly give us the following values:
  • Small Planet/Moon - 1-5cm
  • Medium Planet - 6-15cm
  • Large Planet - 16-50cm

Ideally there should be a Very Large Planet category as well in order to split out that last group a bit too.

A Small Planet is the only one that would be considered to be habitable and would be the most likely candidate for a planetary assault scenario. Anything larger would likely be a ice/gas giant or a rocky world with too great a gravity (and the associated effects thereof) to be commonly habitable. Having said that, invading facilities orbiting at the edges of gas giants would be a pretty cool scenario!


Conveniently, you can just about represent a habitable world using a ping-pong ball and its moons with marbles. Unfortunately, this doesn't look very cinematic.

If you assume that Blackstone Fortresses and unspecified Orbital Defences can shoot farther than the rules would indicate (maybe 120cm or more) then you could probably double the scale (1cm = ~3,000km) and still get some fairly consistent numbers. These values definitely seem to represent the upper and lower limits for scale though, beyond which things start seem awry.

These measurements are of course subject to scrutiny as the sources are all somewhat anecdotal and I have picked and chosen from among those that present the most cohesive result. No doubt there are counter examples, especially since the Rogue Trader RPG books may have more information, but it's good enough for me.

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