The phased array gravity drive, more commonly known as the impeller drive, was the preeminent sublight propulsion mechanism for spacefaring vessels of the Post Diaspora era.
While impeller drives could be made small enough to drive a man-portable missile, such a drive could not change its acceleration or be turned off until it burned out. The smallest craft that could fit a non-single-use drive were shuttles and recon drones.
The impeller drive created a pair of stressed gravity bands above and below a ship, known as the impeller wedge. In order to generate the wedge, a pair of impeller rings, located at the fore and aft ends of the ship, were required. The impeller rings contained 16 beta nodes. Around 1900 PD, the best acceleration for the impeller drive starship was well under 600 gravities.
The drive "tamed" gravity waves to be used in normal space, but was highly dangerous in hyperspace, until the invention of the Warshawski sail, which helped detect other gravity waves and objects. (HH1)
An impeller ring, also sometimes called an impeller band, was a ring structure made up of numerous beta nodes. The two rings at the bow and stern of a vessel came together to form the impeller wedge.
Military-grade starships actually generated two layers of bands. This had the added effect of preventing analysis of the inner band, which reduced the enemy's ability to track a ship through the distortion of the bands. (HH1)
Alpha nodes were the component of the drive's band that propelled a vessel to near light speeds. Their size varied based on the size and strength of the impeller wedge to be generated. Alpha nodes were the primary nodes of the impeller drive.(HH1)
The beta node was a secondary component of the drive. (HH1)
The Royal Manticoran Navy developed the Beta2 node, a more powerful version of the standard beta node. The Beta2 node was used to great effect on the RMN's new generation of LAC's, starting with the Shrike class.
The wedge was a pair of extremely powerful gravity bands located above and below the generating ship. Both bands were inclined such that the forward end, or "throat", was far deeper than the aft end, the "kilt". The wedge was capable of accelerating a vessel to near-light speed. A wedge was much larger in length and width than the ship that generated it.
Due to the immense twisting and warping of space that occurred in the band, no known weapons could penetrate an impeller wedge in either direction. The dorsal and ventral aspects were invulnerable to attack as long as the wedge was up. The flanks, forward, and aft aspects of the vessel were exposed.
The flanks of a wedge could be protected by covering them with sidewalls, although these were not as strong as the wedge and could be penetrated by some weapons. Either the fore or aft aspects (but not both at once) could be sealed in the same way, but this would result in the ship being unable to accelerate. Accordingly, bow and stern walls were not used until late in the First Havenite-Manticoran War, and then only when the tactical situation warranted it. The development of the two-stage bow wall allowed partial protection without affecting acceleration.
The nature of the wedge meant tacticians would most prefer to attack "down the throat" of an opponent, with an "up the kilt" attack being slightly less desirable due to its narrower aspect. (HH1)
Any physical object touching a wedge would be destroyed. Planetary combat missiles used their wedges to destroy the target. In 1911 PD, Horace Harkness destroyed the PNS Tepes by rigging a pinnace to bring up its wedge while still in its boat bay. (HH7). During Operation Oyster Bay, Manticoran tugs attempted to protect Manticore and Sphinx from falling debris by blocking the largest pieces with their wedges. (HH12)
A collision between two wedges would cause catastrophic damage to the nodes of both ships or craft, unless one wedge was much stronger than the other. Counter-missiles destroyed incoming missiles in this manner. On a ship of the wall, width of the wedge "was measured in hundreds of kilometers," requiring plenty of distance between ships in formation. (HH8)