I was thinking about interesting space phenomenons that might be a base of a curious plot in the session or a "terrain feature" in voidship battle. Haven't you?

I came up with idea of a planet with Kessler syndrome. That's when the low orbit is covered with masses of space debris floating with speed of a bullet in all directions, slicing and dicing everything - pretty much the same as in the movie "Gravity" but on a much larger scale. Let's say players have to land there, but they can't approach the planet with their main ship - the damage would be catastrophic. A special lander has to be dispatched...

Or it can serve as an "bottomless pit" in voidship battles.

Aren't void shields supposed to avoid such problems?

In BFG, capital ships (anything above frigate-class) couldn't enter low orbit anyways - they'd get sucked into the planet's gravity and crash on the ground.

I wasn't aware of Rogue Trader describing it differently, so I guess I simply assumed it would be the same. >_>

But yes, I would assume void shields to help against debris. Of course, that doesn't mean you as the GM could not come up with some technobabble way to render void shields useless, such as a thick cloud of energised gas (which also affects sensors and communications, for additional fun).

Aren't void shields supposed to avoid such problems?

Looking at their description I'd say they would have some trouble to do this. Voidship would be bombarded by zillion streams of small objects moving at high speed - shields work properly against single high power shots, they kinda suck against salvos. Besides those shields have to do something with the impact energy - it cannot vanish. If bombarded for long enough by hundreds of tons of debris per second they may overheat and break down.

In BFG, capital ships (anything above frigate-class) couldn't enter low orbit anyways - they'd get sucked into the planet's gravity and crash on the ground.

I wasn't aware of Rogue Trader describing it differently, so I guess I simply assumed it would be the same. >_>

Well that's a shame, because physics would allow a voidship to enter and keep low orbit. There's no reason not to if it'd stay out of the atmosphere and maintain 1st. cosmic speed. It's mass doesn't really matter here.

Well, the description says:

Void shields create barriers of energy around a starship to protect it from stellar debris and incoming fire.

It doesn't specify how big and fast debris is, but we can suppose that most of it will simply be obliterated by the shield.

For bigger objects you could use the rules for asteroid fields. You could give the players the opportunity to make some tests in order to avoid the most dangerous orbits (it's not likely that ALL orbits are going to be covered) and pass through the safer ones.

Or they could simply wait for the orbital decay. "Aaaaand, next planet on the list!"

Well that's a shame, because physics would allow a voidship to enter and keep low orbit. There's no reason not to if it'd stay out of the atmosphere and maintain 1st. cosmic speed. It's mass doesn't really matter here.

Well, part of low orbit is the atmosphere - I think BFG just threw both terms together for the sake of simplicity, because the next step after low orbit was "landed". I believe ships approaching a planet after warp translation spent weeks or even months slowing down; capital ships in that version of 40k were no super-manoeuvrable Star Trek ships but humongous unwieldy devices that took a load of time to change heading or adjust velocity. So I thought it made sense that it'd have trouble maintaining low orbit, as the necessary speed to counter drag might have the vessel "overshoot" its target and leave the planet behind, simply because it can't turn fast enough to maintain a rotation.

But as I said, I have no idea if RT actually kept this and for all I know this game might have even grand cruisers be perfectly capable of landing on a planet's surface. But even if RT had actually adopted this - it's your game, that means you make the rules. No shame necessary!

No, I don't think void shields protect from low-velocity debris. That would negate the special ability of a Repulsor Shield. I guess the key question now is to determine at what velocity a Replusor Shield fails to stop debris. After all they don't work against certain types of debris (e.g. torpedoes).

Hmm, this starts to sound very Dune'ish to me now ...

Hmm, this starts to sound very Dune'ish to me now ...

What, you thought GW had actually made them up themselves?!

Void shields only protect against larger impacts of debris and directed weapons fire. It's why small craft and torpedoes bypass shields. Also if you imagine a carrier having to turn off its shields in order to launch small craft - nobody would use them if it were the case!

You are possibly thinking of Repulsor Fields, which aid with stellar phenomena that impact on your ships maneuverability.

GW and FFG both borrow heavily from Frank Herbert. Navigators always come to my mind first.

They borrow from everything.Apart of real physics xD

So I thought it made sense that it'd have trouble maintaining low orbit, as the necessary speed to counter drag might have the vessel "overshoot" its target and leave the planet behind, simply because it can't turn fast enough to maintain a rotation.

No turning around is really needed here. If a space vessel wants to prevent escaping of the planet's orbit it has to slow down by burning in the direction opposite to their movement.

Sure, I just didn't think that the cogitators and the thrusters of massive cruiser-sized ships and larger are capable of making the necessary "fine-tuning" to achieve stable orbit. Basically, Imperial ships have really, really crude engines that don't do for fine manoeuvring, especially once the vessel is close to the middle of a planet's gravity well.

I'm having images in my head of ships docking at space stations with the aid of giant cables, similar to sea ships nowadays.

Same as GW's writers I'm no astrophysicist, but that's how I rationalised it. ^^

No turning around is really needed here. If a space vessel wants to prevent escaping of the planet's orbit it has to slow down by burning in the direction opposite to their movement.

Sure, I just didn't think that the cogitators and the thrusters of massive cruiser-sized ships and larger are capable of making the necessary "fine-tuning" to achieve stable orbit. Basically, Imperial ships have really, really crude engines that don't do for fine manoeuvring, especially once the vessel is close to the middle of a planet's gravity well.

I'm having images in my head of ships docking at space stations with the aid of giant cables, similar to sea ships nowadays.

Same as GW's writers I'm no astrophysicist, but that's how I rationalised it. ^^

Well it would be a no brainer for a voidship designer to put smaller engines in it, which could be turned on at the lower orbit to maintain stable, circular orbit and turned off entirely when it's ready.

In fact after you leave planet's atmosphere, all manouvers become piece a cake and don't take much energy and wit.

All ships have maneuvering thrusters, even imperial warships. The discussion should be diverted to point out that while ships can easily maintain an orbit (well how else would they perform exterminatus?) the problem lies in the fact that as pretty much all starships are constructed in space they aren't designed to be under the pressure of an atmosphere and gravity of a planet as strong as it is on the surface not to mention a lack of landing gears! I mean they certainly don't have vertical take off and landing capabilities, don't have anti-grav hovering capabilities either, so imagine how long a runway they'd need to take off even if they somehow could!?

This is all backed up by the planet bound for millenia background package - ships that 'land' on a planet never do so voluntary and rarely in one piece...

The bigger the ship, the more distant the orbit it needs to maintain. Bear in mind that our moon orbits the Earth- it's just a really, really high orbit.

If a cruiser, for example, were to get in a low orbit- similar to the orbit of the International Space Station, I'd imagine- stresses on the keel might start to get noticeable. For an example of how pressures can affect the form of a ship, see the movie Down Periscope- as the sub of the film dives, a taut string strung across a compartment starts to sag as water pressure compresses the hull of the ship. Gravitic pressure, rather than being a distributed compression, would try to bend the ship and snap it like a twelve-year old trying to make a s'mores stick. That's... not good for any length of time.

If a cruiser were to get in too close to, say, a gas giant (or anything else with enough of a gravity well for you to really "slingshot"), those stresses may well result in dramatic fractures.

In fact after you leave planet's atmosphere, all manouvers become piece a cake and don't take much energy and wit.

In BFG, they still take ages to turn, specifically because those small manoeuvring thrusters are so powerless compared to the momentum they have to deal with. Andy Chambers once wrote an article about how 1 cm on the board equals 1.000 km, and a game turn represents between 15 minutes and an hour.

And as mentioned before, low orbit partially includes the atmosphere.

I didn't really consider stress on the hull as those ships are built to withstand the impact of powerful weaponry - but following the posts of Kasatka and Annaamarth, I've come to realise that it would not be so much about the amount of force exerted on the vessel, but the omnipresence and duration of it. When the entire ship is put under this sort of pull, this may indeed lead to microfractures that eventually threaten the ship's structural integrity.

There's a great futurama line about pressure on starships here http://www.youtube.com/watch?v=O4RLOo6bchU

Yep. He's got it right.

I mean in theory you could construct a voidship within an atmosphere that had a drive powerful enough to take off and break orbit, but it would be such a monumentally massive undertaking and i don't think the Imperium really has the technology for it...

Take for example a cobra destroyer:
It has a mass 5.7 megatonnes and a max acceleration of 7.6 gravities.
Now assuming that when we refer to gravities in 40k we are referring to a Terran standard gravity, so 9.81m/s squared. 7.6 x 9.81 = 74.556 m/s. That squared is 5558.597136 m/s maximum sustainable acceleration. That is barely half the acceleration needed to break earth's orbit and when you consider that the Cobra is one of the fastest ships the Imperium can field... well little chance of one ever landing and taking off again on its own. Now i'm no good at physics but if somebody would like to solve for how much thrust would be needed to propel 5700000kg at 5.558597136 km/s i would be interested to know how many planets worth of fuel it'd take

I think you confused velocity with acceleration. I'm an aerospace engineer, so I could work this out, but I'm really lazy and, besides, sci-fi engines are out of my league. We can assume that the Imperium doesn't build spaceships designed for atmosphere because they're not worth it but I personally assume that they could do it if only they applied.

I mean: you can open gates through HELL, so who cares about gravity? Screw you, Newton, your laws are HERESY because there's only the EMPRAH's Law.

I mean in theory you could construct a voidship within an atmosphere that had a drive powerful enough to take off and break orbit, but it would be such a monumentally massive undertaking and i don't think the Imperium really has the technology for it...

Take for example a cobra destroyer:

It has a mass 5.7 megatonnes and a max acceleration of 7.6 gravities.

[physics stuff]

Yeah, I can kinda tell physics isn't your bag.

If you can sustain any amount of acceleration more that the strength of local gravity, then your acceleration should be sufficient to go where you want. When we talk about a standard G being 9.8 meters per second squared, the number 9.8 doesn't get squared- the squared part is the "per second." It's another way of saying that the acceleration is "9.8 meters per second of velocity gained per each second of acceleration," so after 2 seconds of acceleration you'd be falling at a rate of 19.6 meters per second (ignoring wind resistance etc. Only perfect systems in this classroom).

FYI, Jupiter is stated as having as having roughly 2.5 Gs of gravity- so that Cobra could get escape velocity from Jupiter very easily.

The problem here isn't acceleration, the problem is a matter of scale. It's the same reason we don't have a space elevator on earth, or tanks the size of office blocks- engineering on that scale is a cast iron *****.

Thanks guys, that's the kind of knowledge i simply don't have when trying to explain why stuff can or cant exist within a game setting. For the most part i hate bringing real world physics into gaming (trying to use IRL zero-g knowledge to try and move through an anti-gravity trap in a D&D game is a big no-no for example) but sometimes you need to just to quash some silly suggestions by others.

Real world example, mate: The now-retired US Space Shuttle pulls about 4 G's on takeoff, and that can make orbit. In theory, all you need is acceleration > planetary gravity. In practice, you need enough extra margin to deal with atmospheric drag and to get yourself upstairs in a reasonable time.

In 40K, you're adding things like contragrav and inertial dampers to the mix, so feel free to toss Newton's laws out the airlock.

Given that, a Cobra should be able to make orbit. It has the thrust to reach orbit even without high-tech shenanigans. Might irradiate / glass the heck out of a planetary surface, though.

IIRC, the old BFG rules mentioned that escorts and transports could make planetfall, so there is fluff precedence...

Cheers,

-V.

Yeah fluff wise the precedent for transports landing exists, though that was largely because within the context of a wargame transports were transporting military stuffs like troops, armour and supplies to warzones and so during scenarios involving planetary invasion something needed to be capable of landing. I'm not sure why you'd ever want to land an escort though... they aren't big enough to transport anything large and critical enough to require a landing that couldn't be drop pod'ded, shuttled, teleported etc to the surface. Perhaps 'landing' on an asteroid and using it for covering while in silent running, but that would involved so much gravity and an atmosphere as we've been discussing.