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.
That, alone, would have been sufficient to earn Warshawski undying renown, but beneficial as it was, its significance paled beside her next leap forward, for in working out her detector, Dr. Warshawski had penetrated far more deeply into the nature of the grav wave phenomenon than any of her predecessors, and she suddenly realized that it would be possible to build an impeller drive which could be reconfigured at will to project its grav waves at right angles to the generating vessel. There was no converging effect to move a pocket of normal-space, but these perpendicular grav fields could be brought into phase with the grav wave, thus eliminating the interference effect between impellers and the wave. More, the new fields would stabilize a vessel relative to the grav wave, allowing a transition into it which eliminated the traditional dangers grav shear presented to the ship's physical structure. In effect, the alterations she made to Fleetwing to produce her "alpha nodes" provided the ship with gigantic, immaterial sails: circular, plate-like gravity bands over two hundred kilometers in diameter. Coupled with her grav wave detector to plot and "read" grav waves, they would permit a starship to literally "set her sails" and use the focused radiation hurtling along hyper-space's naturally occurring grav waves to derive incredible accelerations.
Not only that, but the interface between sail and natural grav wave produced an eddy of preposterously high energy levels which could be "siphoned off" to power the starship. Effectively, once a starship "set sail" it drew sufficient power to maintain and trim its sails and also for every other energy requirement and could thus shut down its onboard power plants until the time came to leave hyper-space. A Warshawski Sail hypership thus had no need for reaction mass, required very little fuel mass, and could sustain high rates of acceleration indefinitely, which meant that the velocity loss associated with "cracking the wall" between hyper bands could be regained and that use of the upper bands was no longer impractical.
This last point was a crucial factor in attaining higher interstellar transit times. The maximum safe velocity in any hyper band remained .6 c, but the higher bands, with their closer point-to-point congruencies, added a significant multiplier to the FTL equivalent of that velocity. Prior to the Warshawski Sail, not only had dimension shear made translating into the upper bands dangerous, but the successive velocity losses had made it highly uneconomical for any reaction drive ship. Now the lost velocity could be rapidly regained and the higher, "faster" bands could be used to sustain a much higher average velocity. As a result, the dreaded grav wave became the path to ever more efficient hyper travel, and captains who had previously avoided them in terror now used their new instrumentation to find them and cruised on standard impeller drive between them.
Of course, there wasn't always a grav wave going the direction a starship needed, but with the grav detector to keep a ship clear of naturally occurring grav waves impeller drive could, at last, be used in hyper-space. In addition, it was possible for a Warshawski Sail ship to "reach" across a wave (which might be thought of as sailing with a "quartering breeze") at angles of up to about 60 before the sails began losing drive and up to approximately 85 before all drive was lost. By the same token, a hypership could sail "close-hauled," or into a grav wave, at approach angles of 45. At angles above 45, it was necessary to "tack into the wave," which naturally meant that return passages would be slower than outgoing passages through the same region of prevailing grav waves. Thus the old "windjammer" technology of Earth's seas had reemerged in the interstellar age, transmuted into the intricacies of hyper-space and FTL travel. By 1750 pd, however, sail tuners had been upgraded to a point which permitted the "grab factor" of a sail to be manipulated with far more sophistication than Dr. Warshawski's original technology had permitted. Indeed, it became possible to create a negative grab factor which, in effect, permitted a starship to sail directly "into the wind," although with a marginally greater danger of sail failure.