Once ardanium was discovered, and its properties were understood, it turned out that friction was everything and there was no such thing, really, as the speed of light.

There is an upper limit to how fast things can travel under circumstances even vaguely resembling normal, and light has a tendency to travel at (or near) that upper limit, but this arbitrary velocity at which acceleration becomes impossible and funny things happen to time actually comes from another factor entirely. Dark matter.

Dark matter is everywhere, literally, it actually occupies all of space, including the tiny proportion also taken up by other things such as nebulae, solar systems, dust clouds and ambient vacuflora drifts. Dark matter is the substance which prevents anything from exceeding 299,792,458 metres per second. Dark matter is the term physicists have historically used to describe anything which causes universal effects that they cannot otherwise explain.

It's as good a term as any, though, for the crust on spacetime that locked the Human race and the other two local spacefaring biologicals into tiny spheres around their home systems for the first few centuries of their respective exploratory eras. Each in turn then found a small deposit of ardanium, and discovered that mineral's interesting attitude to gravity under different radiation exposures.

Existing somewhere below the early 21st Century periodic table (and on a slightly different layer), ardanium still has a relational link to the alkaline metals. However, the electron shells are directly interfaced with a side-on lattice (theoretically impossible, and nobody is particularly happy with any model used to explain it) of the Habgood boson, also known as the pseudo-graviton or - due to an unfortunately timed joke by a top particle physicist in hearing of members of the press - popularly and erroneously, the Quirky-Quark.

This unusual interaction of particles allows this extremely irregular element to act in a manner almost indistinguishable from stable matter, despite the fact that it should have a half life measure in fractions of one percent of a nanosecond. It is literally held together by 'fake' gravity from the Habgood boson and it is this effect which also gives it its unique capacity to manipulate gravitational effects when exposed to streams of composite Higgs quasiwavelets and depressed semistrange fermions, shunting the electrons around in their alignment and causing them to 'fall'.

For Humans the luxury of small scale, plane applied artificial gravity came first, so that when people finally slid down their first gradient to a distant world, they did so with a comfortable one Gee on the deck and with the option of the same on the ceiling or any other available surface if they really wanted.

Pick a direction to be 'forwards', pick another to be 'right' and you've by default defined 'up'. Expose the ardanium core of your gradient drive to the correct mix of exotic quanta and unlikely-sounding wave forms, and the arbitrary two dimensional plane you defined with your two decisions becomes stretched into a three dimensional concave-sided wedge, although not in any way that makes sense to the perceptions of any entity yet encountered. On the surface of that wedge, the dark matter becomes stretched thin and 'cracked', so that as your ship “pulls the piste” and slides down the slope back towards Normalised space, it can exceed the speed limit normally imposed. Get it right - and most people do most of the time - and (when friction reasserts itself and the gradient under your ardanium hull layer flattens out) you will have traversed many times the distance light normally travels in a year in a matter of days or weeks, and be close enough to your target to complete your journey on chemical or ionic engines in a similarly reasonable period. Get it wrong and you'll be in the middle of nowhere and have to start again, assuming you can.

Early experimentation proved that you can't pull too close to a planet, star or other massive body, most of the time the gradient fails to form. Sometimes it does form and the ship vanishes, never to emerge. The accepted theory is that the addition of the gravity well distorts the wedge and breaks the slide at which point a variety of terrible exciting and terminal physics terms happen to the unfortunate vessel. While there is an equation to predict minimum safe distance based on the mass of travelling ship and mass of destination all but the most daring and desperate military actions like to aim for a good couple of days distance just to be on the safe side.

While ships can traverse a gradient, it has proven impossible to transmit a signal that can maintain coherence during the slide. As a result, all interstellar communication needs to be delivered via ships, leading to lucrative mail services and and a thriving courier culture.

The ardanium layer must be maintained across the whole contact surface of the ship. Any non-ardanium parts will lose physical coherence due to the distorted spacetime of the gradient, destroying the ship without a trace. Commercial and personal ships can be constructed with the bottom of the ship coated in a thin layer of ardanium, also used to provide gravity on-board (this configuration has the advantage of making a lot more sense to the crew than having a surface above them or off to the sides on contact with the gradient). This really only has to be thick enough to survive micrometeorite and intra-atmospheric particle damage. Warships, on the other hand, must maintain a layer thick enough to sustain weapon damage, which means that a combat vessel intended to leave its home system is astronomically expensive to build and maintain while very easy to cripple.

It also places limits on ship design - because of cost you are only coating one edge in ardanium, but you need to make sure that will be the only edge ever in contact with the gradient, so it is often the long side and ships tend to be long and low rather than short and high. If the bottom of your tall ship clips a clump of dark matter on the gradient, theorised as possible as it only cracks the dark matter not removes it, there is a risk that ship would “topple” and one of the non ardanium surfaces would touch the gradient and be destroyed. At least that is what is theorised because any ship or test bed that did happen to vanished into whatever collection of obscure quantum waveforms you become when the gradient goes wrong. No one is entirely happy with the word “topple” either to describe a process of rotational geometry that is happening at a point where normal physics and geometry literally don't apply, but once again nobody's managed to come up with a better term that can be understood by people without 3 PhD's in theoretical physics.

Because of the way the need to coat the “underside” of a ship in expensive Ardanium works, economy of surface area encourages the building of interstellar ships that are as large as possible: surface area increases in proportion with the square of the ship's dimensions, but its capacity increases in proportion to the cube. So, if you wish to transport X capacity of stuff from A to B in one journey, less Ardanium is required if you build a single ship with capacity X, than if you build two ships each with capacity X/2. So in purely ardanium consumption terms, it's more economical to build big.

However, other issues discourage over-large ships, and as well as the conventional engineering issues of building super-massive structures large ships suffer other practical problems: inertia makes a massive ship more difficult to control during the slide and takes longer to decelerate, reducing accuracy when aiming at the target destination, then when you are in normal space inertia and maneuverability become issues. So there is a trade off between Ardanium economy (bigger is better) and the ship's primary function (often better served by being smaller).

Thus the vast majority of personal ships vary between truck and mansion size, although couriers and others happy to risk travelling without a gradient drive of their own can go down vessels the size of a car or old earth fighter plane.

In terms of large transports, passenger ships, freighters, two to three kilometres is the upper limit for the richest builders and owners, around the one kilometer is the current standard for “large”. Those will be bulk-freighters that will always remain in high orbit and have passengers and cargo ferried to them. Cost for your postal system is often more proportional to volume than mass - the bigger your object to be shipped, the bigger the courier it needs, so the fewer ships that can carrier it and the more ardanium they needed to build.

Military designs prefer their gradient capable ships to be smaller because they need a much thicker layer of ardanium and the bigger the ship, the bigger the target (and the easier to strand).

Similar considerations exist with the ship's shape, the most economical shape is the one with the best volume/surface area ratio, which is a sphere. However to maintain stability in the gradient the contact surface needs to be flat, so civilian freighter ships typically adopt a half to three quarter sphere shape with the ardanium surface being the flat edge. Passenger ships are usually more lozenge shaped for aesthetic reasons.

These enjoy being massively cheaper due to the lack of Ardanium, but suffer from the lack of Gravity that comes as a side effect from the Ardanium plating. Ardanium being much to precious to waste on something that isn't pulling a gradient.

Whether to add gravity varies by function and by species, in general for long term civilian space stations and orbital facilities an element of gravity is considered nice, especially around the habitation sections, but with modern medicine and supplements to manage bone wastage its hardly essential. Zero G for your docking area and cargo bays just makes life easier for everyone provided they remember to wear field suits: impact force from moving objects is based on mass, not weight.

Military stations will often forgo gravity entirely - while rotation is fine for civilians, for a battle station a damaged rotating module becomes an enormous sheering force. Some careful use of ardanium floor plates allows certain key areas to have gravity - doctors complain that it is hard enough to work already when the patient doesn't drift off the table. Stations that do have it will include the ability to slowly “break” the rotation should action stations be sounded, safe in the knowledge that thanks to the impossibility of gradienting next to planets, they will have time to come to a stop before trouble reaches them.

In system ships both military and civilian tend to be gravity-free. For the civilians the moving parts make the whole thing more expensive, for the military it's a huge target. That's not to say that there aren't some nice luxury liners that will cruise you round Sol at a comfortable 0.75G on the outside as you rotate and take in the sights, but it's very much a luxury item.

The Many are famous for ignoring gravity on their ships altogether, often even on ardanium equipped ships and just having webbing on all surfaces they can grip on to.