In our last game session we were subjected to a situation that got me thinking.

We picked up a message from a rival dynasty and decided to check out the location it was being broadcasted to. Knowing full well that it could be a trap we decided to spring it. We made a warp jump, wound up at our destination and was hailed by a ship.

The ship that hailed us was cloaked by Dark Eldar technology and once we got a lock on them they turned out to be outside our weapons range. This makes me wonder:

- How do you detect an incoming ship in the vast blackness of the warp, unless you are purposely using active augery? And if you detect a vessel within 20 VUs how far is the reach of subsystem vox transmissions? Is it safe to assume that the ships sensors and systems cannot really work with anything outside of 20 VU, thereby autofailing attempts at Locking on targets or Jamming communications? Or do entirely different rules apply to these circumstances?

Thanks in advance

The obvious way to detect an incoming ship is to look for it's drive flare. A ship actively under thrust is going to be visible at interplanetary ranges. Given the size of the ships and the speeds they are listed as being able to accelerate at, they would be comparable in output to a nuclear bomb detonating, but sustained rather than just for a moment.

Eldar, of course, have no such tell-tale and combined with Holo-Fields this means that sneaking up unseen is the default for them, rather than the exception that is is with other races.

Even with modern technology we could send radio transmissions to any planet in the solar system, so anywhere in the same star system will therefore be within Vox range. The thing to consider here is how long it takes for the signal to reach the destination. It takes minutes for speed-of-light radio signals to reach from one planet to another, for ships at the extreme and opposite edges of a system it could even take up to an hour. On the space combat scale, 20VU would mean about half a second of time delay.

I therefore rule that combat ranges (20VU or less) are the limit for having an actual conversation, such as two Captains hailing each other and talking directly. At longer ranges, the ship is recording a message and then sending it, and waiting however long it takes for a reply. Unless you want to get a calculator out, then assume each time a message is sent and a reply received anything up to an hour has passed.

I run it as a cross between modern space detection tech and submarine warfare.

When propulsion drives are activated, an active sensor sweep is performed, weapons are fired, or vox signals are emitted in open space you can be detected by pretty much anything in the system that has a line of sight. You light up like a christmas tree on most standard passive sensors against the void.

However, these signals are also easy to mask with background interference. Nebula, stars, astral phenomena, and asteroid fields can make certain kinds of signatures difficult to detect.

Active scans can give you a basic, radar style view of a solar system, but again, light you up for the whole world to see. I typically assume that most ships not in a civilized system default to "run silently", and mostly just coast unless they absolutely have to thrust. Unless my players are willing to perform active scans it is VERY difficult to detect anything not actively transmitting some form of energy.

I also assume ships engines have a 'spotlight' effect and don't emit thrust energy omnidirectionally, but instead use shielding to minimize energy outflow. If a ship is on the edge of a system and begins to accelerate in-system I don't think anyone is going to see the main thrusters fire, though if they were to accellerate away use make significant use of maneouvering thrusters they might be detectable.

Comparisons to modern scientific understandings of spaceflight physics don't tend to represent the 'reality' of the WH40K setting. Within the setting, space combat is much more like the Age of Sail, and it's quite possible to be sneaky in space.

riplikash said:

While ship's engines do not emit omnidirectionally (that'd be remarkably stupid for a drive system), they do emit an exhaust plume of sun-hot incandescent plasma that can be several kilometres in length. And as these things tend to do, it will spread out (think, cometary tail, or a ship's wake) before/as it fades and cools.

Which means that unless the ship has a base course pointing straight at you (and parallax effects are negligible), you're going to be able to see the drive wake as a streaked line, even at distances of several AU. And even if it is pointing straight at you, you'll probably be ably to see a glowing halo with a tiny occluded spot in the middle (thanks to that spreading plasma glow), although without a comparative star chart for your current position, it'd most likely be able to make its way a decent distance in-system before you realised it was a ship and not a new star.

And, of course, even when running silent (main drive shut down, etc.), a starship is still going to be lit up like a christmas tree, albeit not necessarily in the visual spectrum: thermodynamics ensures that your ship will be radiating a depressing (if impressive) amount of waste heat (although probably still not enough for the proper comfort of the crew), and any electrical motors aboard the ship (plus internal vox, live cogitators and possibly quite a few mechanical motors, thanks to natural dynamo effects) are going to turn the ship's hull into a giant RF antenna (although this is probably not going to be noticeable unless there is someone specifically watching that particular area of space for it, with specialised or especially sensitive passive instruments).
The waste heat is going to be a bigger source of detection, and yes, the Imperium does use stuff that can pick that up: the short story Cross the Stars describes the role of the opticon (essentially, spotters for the ship's guns) as, and I paraphrase: sticking a telescope out the side of the ship, looking for something hot and moving relative to the starfield.
Admittedly, this requires someone pointing their telescope in the right direction, which, given the reliance on manual labour, reduces the odds of detection somewhat, but it apparently does produce results: in the aforementioned short story, a recently pressed (and so barely half-trained) gunner was able to locate a Hades-class heavy cruiser at around 140,000 km without much trouble (although, obviously, the fact that it was in his assigned area of search was luck).

Alasseo said:

That's pretty much how I'd imagine that, too. And if you own "Fire Warrior" for the PS2, you can see that in the game's intro cutscene - I very much like the idea that even the Imperium's starships pollute the void, just as Imperial economy pullutes its planets.

But that already gets slightly off topic. If you take a look at the Battlefleet Gothic rules, ships get a morale bonus if any of the opposing ships is on special order, meaning that certain systems of a void ship are supplied with a higher amount of energy, which, in turn, makes it easier to locate for enemy augers, be it because of heat (as mentioned before), radioactivity, plasma emissions &c. Of course all that is simplified in BFG for reasons of playability, however one gets an idea of how such things might work.

As a framework, just assume that augers work on a planet system's scale, being affected by planets, asteroids, radiation, sunstorms, warp activity aso. Whatever you do as a GM, it's fine. It's of no use to make it too complicated.

Even a ship running silent can still move at half speed. For some of the faster ships, that's still equal to a cruiser at full speed. It doesn't make sense using real-world physics, but as long as you keep it consistent within the game, it'll do.