From the beginning of hyper travel, it was known that there were multiple hyper bands and that the "higher" the band, the closer the congruity between points in normal-space and thus the higher the apparent FTL speed, but their use was impractical for two major reasons. First, translation from band to band bleeds off velocity much as the initial translation. The bleed-off for each higher band is approximately 92% of the bleed-off for the next lowest one (that is, the alpha band translation reduces velocity by 92%; the beta band bleed-off is 84.64%; the velocity loss for the gamma band is 77.87%, etc.), but it still had to be made up again after each translation, and this posed an insurmountable mass requirement for any reaction drive.

The second problem was that the interfaces between any two hyper bands are regions of highly unstable and powerful energy flows, creating the "dimensional shear" which had destroyed so many early hyperships, and dimensional shear becomes more violent as band levels increase. Moreover, even the relatively "safe" lower bands which could be reliably reached were characterized by powerful energy surges and flows—currents, almost—of highly-charged particles and warped gravity waves. Adequate shielding could hold the radiation effects in check, but a grav shear within any band could rip the strongest ship to pieces.

Hyper-space grav waves take the form of wide, deep volumes of space, as much as fifty light-years across and averaging half their width in depth, of focused gravitational stress "moving" through hyper-space. Actually, the wave itself might be thought of as stationary, but energy and charged particles trapped in its influence are driven along it at light– or near-light-speed. In that sense, the grav wave serves as a carrier for other energies and remains motionless but for a (relatively) slow side-slipping or drifting. In large part, it is this grav wave drift which makes them so dangerous; survey ships with modern sensors can plot them quite accurately, but they may not be in the same place when the next ship happens along. The major waves in the more heavily traveled portions of the galaxy have been charted with reasonable accuracy, for sufficient observational data has been amassed to predict their usual drift patterns. In addition, most waves are considered "locked," meaning that their rate of shift is low and that they maintain effectively fixed relationships with other "locked" waves. But there are also waves which are not locked—whose patterns (if, in fact, they have patterns at all) are not only not understood but can change with blinding speed. One of the most famous of these is the Selkir Shear between the Andermani Empire and the Silesian Confederacy, but there are many others, and those in less well-traveled (and thus less well-surveyed) areas, especially, can be extremely treacherous.

The heart of any grav wave is far more powerful than its fringes, or, put another way, a "grav wave" consists of many layers of "grav eddies." For the most part, all aspects of the wave have the same basic orientation, but it is possible for a wave to include counter-layers of reverse "flow" at unpredictable vertical levels. Despite the size of a grav wave, most of hyper-space is free of them; the real monsters that are more than ten or fifteen light-years wide are rare, and even in hyper the distances between them are vast, though the average interval between grav waves becomes progressively shorter as one translates higher into the hyper bands. The great danger of grav waves to early-generation hyperships lay in the phenomenon known as "grav shear." This is experienced as a vessel moves into the area of influence of a grav wave and, even more strongly, in areas in which two or more grav waves impact upon one another. At those points, the gravitational force exerted on one portion of the vessel's structure might be hundreds or even thousands of times as great as that exerted on the remainder of its fabric, with catastrophic consequences for any ship ever built.

In theory, a ship could so align itself as to "slide" into the grav wave at an extremely gradual angle, avoiding the sudden, cataclysmic shear which would otherwise tear it apart. In practice, the only way to avoid the destructive shearing effect was to avoid grav waves altogether, yet that was well nigh impossible. Grav waves might be widely spaced, but it was impossible to detect them at all until a ship was directly on top of one, and with no way to see one coming, there was no way to plot a course to avoid it. It was possible to recognize when one actually entered the periphery of a grav wave, and if one were on exactly the right vector, prompt emergency evasion gave one a chance (though not a good one) of surviving the encounter, but the grav wave remained the most feared and fearsome peril of hyper travel.

Then, in 1246 pd, the first phased array gravity drive, or impeller, was designed on Beowulf, the colonized world of the Sigma Draconis System. This was a reactionless sublight drive which artificially replicated the grav waves which had been observed in hyper-space for centuries. The impeller drive used a series of nodal generators to create a pair of stressed bands in normal-space, one "above" and one "below" the mounting ship. Inclined towards one another, these produced a sort of wedge-shaped quasi-hyper-space in those regions, having no direct effect upon the generating vessel but creating what might be called a "tame grav wave" which was capable of attaining near-light speeds very quickly. Because of the angle at which the bands were generated relative to one another, the vessel rode a small pocket of normal-space (open ahead of the vessel and closing in astern) trapped between the grav waves, much as a surfboard rides the crest or curl of a wave, which was driven along between the stress bands. Since the stress bands were waves and not particles, the "impeller wedge" was able, theoretically, at least, to attain an instantaneous light-speed velocity. Unfortunately, the normal-space "pocket" had to deal with the conservation of inertia, which meant that the effective acceleration of a manned ship was limited to that which produced a g force the crew could survive. Nonetheless, these higher rates of acceleration could be maintained indefinitely, and no reaction mass was required; so long as the generators had power, the drive's endurance was effectively unlimited.

In terms of interstellar flight, however, the impeller drive was afflicted by one enormous drawback which was not at first appreciated. In essence, it enormously increased the danger grav shear had always presented to reactor drive vessels, for the interference between the immense strength of a grav wave and the artificially produced gravitic stress of an impeller wedge will vaporize a starship almost instantly.

In the military sphere, it was soon discovered that although the bow (or "throat") and stern aspects of an impeller wedge must remain open, additional "sidewall" grav waves could be generated to close its open sides and serve as shields against hostile fire, as not even an energy beam (generated using then-current technology) could penetrate a wave front in which effective local gravity went from zero to several hundred thousand gravities. The problem of generating an energy beam powerful enough to "burn through" even at pointblank ranges was not to be solved for centuries, but within fifty years grav penetrators had been designed for missile weapons, which could also make full use of the incredible acceleration potential of the impeller drive. Since that time, there has been a constant race between defensive designers working new wrinkles in manipulation of the gravity wave to defeat new penetrators and offensive designers adapting their penetrators to defeat the new counters.

The interstellar drawbacks of impeller drive became quickly and disastrously clear to Beowulf's shipbuilders, and for several decades it seemed likely that the new drive would be limited solely to interplanetary traffic. In 1273 pd, however, the scientist Adrienne Warshawski of Old Terra recognized a previously unsuspected FTL implication of the new technology. Prior to her Fleetwing tests in that year, all efforts to employ it in hyper-space had ended in unmitigated disaster, but Dr. Warshawski found a way around the problem. She had already invented a new device capable of scanning hyper-space for grav waves and wave shifts within five light-seconds of a starship (to this day, all grav scanners are known as "warshawskis" by starship crews), which made it possible to use impeller drive between hyper-space grav waves, since they could now be seen and avoided.