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The impeller drive is a fictional spaceship drive in the Honorverse, a series of military science fiction novels written by David Weber. The main sublight propulsion system of starships, it is based on gravity manipulation. It is a form of reactionless propulsion and is limited to sublight speed (like a "slow" warp drive). The actual mechanics have been compared to a surfer riding a wave.
When a ship activates its impellers, two focused gravitational bands appear, one above the ship and one below. The bands are very thin top-to-bottom, but their width and length are many times that of the ship. They are sloped such that they are much closer together at the stern than the bow of the vessel; hence, they are often collectively referred to as "the wedge". The gravitational distortion the bands create is so strong that no material, not even photons (as from a bomb pumped laser or graser), can pass through them. Thus, ships are immune to attack from above or below.
As the wedge's stress bands block light and most sensors, this can be used to advantage. Eventually, when it became apparent that a sensor capable of penetrating a wedge might become available, a wedge design with double-layered stress bands was developed. All military vessels use this approach in the time of the series.
Theoretically, an impeller-drive ship could accelerate instantly to the speed of light. In practice, acceleration is limited by several factors, including the strength of shielding used in deflecting space-borne dust particles and micrometeroids, and the amount of acceleration force experienced within the ship not cancelled by the ship's inertial compensators. A Light Attack Craft equipped with the latest in military-grade impellers and inertial compensators can achieve an acceleration of approximately 600g (5.88 km/s²). Larger military vessels have reduced acceleration relative to their mass; the largest military vessels have a maximum acceleration of approximately 500g. Civilian vessels, usually built with a focus on cost instead of maximum performance, tend to have less powerful impellers and inertial compensators, which limit their maximum acceleration to 150-250g in most cases.
To reduce wear on the drive systems — as well as provide a safety margin in case of inertial compensator failure — most ships will not use more than 80% of maximum acceleration except in battles or emergencies. The physics of the compensator produce the interesting side effect that adding mass to a ship does not slow it down, unless the mass is being towed (by tractor beams) beyond the aft limit of the wedge.
Any object colliding with an impeller band is destroyed by gravitational shearing, similarly to objects passing the limit horizon of a black hole. If two vessels' wedges ever overlap, both vessels are destroyed as their wedges both release their energy instantly. Warships in formation must stay far apart for safety.
The wedge is projected by nodes arranged in two rings at the stern and bow of a ship. On many warships, each ring contains eight alpha nodes and sixteen beta nodes. Larger ships have larger nodes, not more of them. Both types of node provide power to the wedge; the larger but less efficient alphas (three times the size, six times the power consumption, twice the thrust) are needed because only they can project Warshawski sails for use in hyperspace.
Freighters often have only alpha nodes, as they do not need high sublight performance. Sublight vessels have only beta nodes. The latest generation of Manticoran light attack craft use "beta-squared" technology, and have only eight large beta nodes in each ring. The "beta-squared" impeller nodes are beginning to see service in the Manticoran Medusa-II class of SD(P)s and in the Grayson Harrington-II class of SD(P)s.
On civilian ships, each ring is operated and maintained from one engine room. To contain damage, military vessels divide each engine room into four or eight sections.
Loss of nodes to battle damage affects a ship's acceleration capability more severely if the damaged nodes are in the same ring. For example, losing twelve nodes from the forward ring is worse than losing six from each. For civilian ships, loss of one entire ring will prevent the operation of the other. This is a side effect of their single-layered stress bands.
Anti-ship missiles also use impellers; nothing else provides it sufficient acceleration. Missiles are built to handle huge acceleration and their drives are powered to suicidal levels, with capital ship missiles capable of accelerations of upwards of 95,000 gravities. This is what causes missiles to have very limited powered-flight endurance. Point-defense missiles have no warheads, relying instead on overlapping their wedges with those of the incoming missiles, causing instant destruction of both.
In the fictional Honorverse, a series of military science fiction novels written by David Weber, prolong is a fictional genetic engineering process for life extension. It was developed on Beowulf two or three generations prior to the time of most of the stories (the early 41st century), but is available in almost all advanced star nations, and widely used in several. By Honor Harrington's time, prolong has only been recently introduced in less developed areas, such as pre-alliance Grayson and the Talbott Cluster.
Prolong slows aging and sufficiently so that it appears to be fully arrested for a long period. The 'freeze' does not last forever, but a prolong-affected lifespan is 200-300 years. Aging is 'frozen' at a younger point depending on the treatment version available when the patient was young enough (but not too old) to be treated (meaning that the older characters in the series look older). For instance, first-generation Prolong recipients will settle at a physical age in their mid-forties for an extended time, while third-generation Prolong recipients will appear to be in their late twenties. While the Prolong Treatment 'freezes' its recipient visually at a reduced age, it also prolongs development. Many individuals (eg, Honor Harrington and Queen Elizabeth of Manticore, both 3rd Generation Prolong) complaining that it took them many years to break out of their awkward adolescent stage.