Gee, what movie did Bax steal this idea from?
Gravity Drives are essentially portable jumpgates, allowing ships equipped with them to cross between real space and hyperspace whenever and wherever they want.
More advanced gravity drives can also generate powerful gravity shields and allow ships to travel at incredibly high speeds. Gravity drives are massive devices, so any warship smaller than a heavy cruiser is simply too small to carry one.
Types
Gravity drives have been around for decades, and the technology has evolved and advanced over time. There are currently four types of gravity drives still in service:
- Type I: Can create jump points, but not gravity shields, and cannot be used for propulsion.
- Type II: Can create jump points and gravity shields, but can't be used for propulsion.
- Type III: Can create jump points, gravity shields, and propulsion.
- Type IV: Can create jump points, gravity shields, and propulsion. Firing ports for weapons can be opened in the gravity shield, allowing the ship to fire while still shielded. (Although this is the standard Jurvain grav drive, the Poseidon-class Star Control Ship is currently the only human warship equipped with this type of gravity drive.)
Jump Point Projection
Gravity drives can generate a tight gravity well at a pinpoint location in space strong enough to rip open a "jump point," or hyperspace portal. The jump point can be projected up to 100 kilometers away from the ship (this may sound like a great distance, but in space combat this is actually very close range) to allow other, smaller vessels to cross through the portal first. Generating a jump point takes between thirty seconds to a minute to form. While forming a jump point, a ship can NOT use its gravity drive for propulsion or shielding. In a modified form, a jump point projector is the basis of the capital weapon, the Gravitic Ram.
Gravity Shielding
Gravity drives can generate a field of gravity-distorted space around their ship. This gravity shield can also be expanded to protect other, smaller vessels clustered around the ship, although this comes at the price of proportionately expanded power consumption and reduction in the shield's duration.
Any projectile (torpedoes, missiles, shells, plasma, etc.) attempting to penetrate the shield will be stopped in its tracks and crushed by the intense gravity. Beam weapons (chemlasers, grav lasers, and particle cannons) can penetrate a gravity shield, but their effects are highly degraded. Gravitic weapons (graviton cannons, graviton torpedoes, gravitic rams, and grav lances) can disrupt gravity shields and ultimately overload them, forcing the defending ship to either drop their gravity shield or be destroyed when their gravity drive explodes.
The main disadvantage of gravity shields is that although your enemy can't fire through your shield, neither can you. To return fire, you'd have to drop your gravity shield. Furthermore, a defending ship must maintain at least 90% hull integrity to avoid being literally ripped apart by the powerful gravitic forces of its own shield. Therefore, any damage from weapons that do manage to penetrate the shield can quickly become catastrophic.
Gravity Propulsion
A gravity drive can generate a focused artificial gravity well in any direction, and the ship to and literally "falls" toward the well at speeds relative to the gravity well's power level. Gravity propulsion works in both real space and hyperspace, and can reach acceleration rates of up to a hundred gees (or Earth-standard gravities), much faster than those of an ion drive. Like gravity shields, gravity propulsion well can be expanded (with a corresponding power drain) to pull other, smaller vessels along with the main ship. While using gravity propulsion, a gravity drive can't simultaneously be used to generate gravity shielding or a jump point.
Limitations
It is important to point out that gravity drives can be used for propulsion OR shields OR creating a jump point, but NIOT all three (or even two) functions at the same time. In other words, a ship cannot maneuver AND maintain a gravity shield at the same time. It CAN, however, coast on a ballistic trajectory while maintaining a gravity shield (although it would have to drop the shield in order to accelerate, decelerate, or change course) or creating a jump point. Likewise, a warship trying to withdraw from combat would have to drop its gravity shield before it could form a jump point to escape to hyperspace.
Cost & Availability
Because gravity drives are huge, energy-intensive technomagickal devices, they are prohibitively expensive. In the case of military-grade Type II, III, and IV gravity drives, they must be integrated into the design of the ship and installed during construction. Consequently, only governments, megacorps, and upper noble houses can afford to buy or build gravity drive ships (and especially gravity drive warships). Even then, they would probably only own a handful. Lower houses might be able to afford one.
The dismantling of the commercial jumpgate network within the Holy Terran Empire sparked a huge demand for gravity drives to keep the economic and transportation infrastructure operating. The K'Nes corporate clan Miao Mercantile (among many others) met this demand by flooding the market with simple Type I drives only capable of jump point generation (really more of a jump point projector than an actual drive system). Small, self-contained, and (comparatively) cheaper, these can be installed in big cargo pods or in the modular cargo bays of larger commercial freighters. Retrofitting one of these gravity drives into a warship usually requires sacrificing one of its capital weapons systems or fighter bays.
Larger commercial freighters with built-in gravity drives are slowing appearing on the market as the Imperial economy and shipyards recover from the Caal Invasion and Ascension War.
Legality
Under the old Earth Federation, gravity drives were limited to military vessels, and it was illegal (and carried severe penalties) for civilian ships to be equipped with one. Although gravity drives are legal in the Holy Terran Empire, it has not yet been established in-story if the Emperor controls or regulates their construction, distribution, or use.
Behind the Scenes
Gravity drives were introduced by Nathan Bax, inspired by the gravity drive system in the sci-fi horror movie Event Horizon. Martin Hohner greatly expanded on them during season 4.3 (as his character was a naval officer), and he in turn was influenced by Weber's David Weber's Honor Harrington novel series.