The so-called “gravity drive” isn’t an actual propulsion system. It’s a combination of an internal gravity environment and the external gravity screening.
Early spacecraft equipped with internal gravity showed that the internal gravity field has the effect of making the interior of the ship resistant to the forces of acceleration while the ship is in motion, allowing for higher levels of acceleration that would normally have an unhealthy effect on the crew. In addition, the gravity rotors angle the internal gravity to help compensate for acceleration. Gravity field realignment easily compensates for typical maneuvering. However, sudden changes in direction and speed can temporarily exceed the rotors’ speed at realigning the internal gravity. In some cases, the onboard computer’s attempts at matching gravity alignment with existing natural gravity, during hard maneuvering, can actually magnify the adverse effect of sudden changes in acceleration3. This is why almost all lockers and cabinets on a ship have latches, and most shelves have some sort of guardrail or lip to keep items from sliding off.
The external gravity screen system is composed of two primary elements: gravity dampeners at various places throughout the ship; and the primary gravity rotor assembly, consisting of the gravity wave amplifier and the primary gravity rotor. The primary gravity rotor is actually two counter-rotating rotors. This creates the effect of a “gravity gyroscope” that helps balance the forces of inertia and acceleration. The assembly creates a bubble of stabilized gravity around the ship. Its orientation to the design of the ship doesn’t matter and the rotor assembly isn’t always a visible part of the ship, but every ship in the Verse has one. The gravity rotor assembly is different from the smaller gravity rotors that provide the ship’s internal gravity.
In some ship designs, such as the Firefly design (all models) the primary gravity rotor assembly is visible as a rotating ring around the waist of the ship. The dampeners and rotor assembly work in concert to negate the force of gravity acting on the ship, and help dampen the effects of acceleration on the structure of the ship. One of the ways that gravity screening changes ship design has to do with weight.
On a firefly, for example, the outrigger engines are connected to the ship by a complex hinge-axle mount that allows a variety of motion. The hinge that allows the engines to fold down to parked and maintenance positions, is not strong enough to support the weight of the ship. Without gravity screening to reduce the weight of the ship, the engines would snap off of their mounts when lifting the ship. In fact, the hinge is rated for just the weight of the engine under typical stresses. The axle that allows the engine to rotate for Vertical Take-Off and Landing (VTOL) is strong enough to support the ship during take-off and landing, but only just. The axle would fail when the engine produced enough thrust to fly at speed. Without gravity screening, the engines would be directly connected to the axle, and it would be much thicker. With gravity screening, a firefly is quite nimble for a cargo transport. Without, it would be an ungainly hulk unable even able to lift itself while empty of any cargo. Additionally, the landing legs of a firefly cannot support the weight of the ship without gravity screening. The only time the gravity screening is entirely powered down is while the ship is supported by a maintenance gantry.