Impeller drive
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The impeller drive is a fictional technological device in the Honorverse. The main sublight propulsion system of starships, it is based on manipulation of gravity. It appears to be a form of reactionless propulsion 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 bands of focused gravitons 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 back than the front; hence, they are often collectively referred to as "the wedge". The gravitational distortion the bands create is so strong that no known force, not even light, 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 technology 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 inertial force on a ship's crew negated by its 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 corresponding drops in 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 performance, tend to have less powerful impellers and inertial compensators, which limit their maximum acceleration from anywhere from 150-250g.
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% acceleration except in battles or emergencies. The physics of the compensator produce the interesting side effect that adding mass to a ship will 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, almost as if it had collided with a black hole. If two vessels' wedges ever overlap, both vessels are destroyed. Warships in formation must stay far apart for safety.
The wedge is projected by nodes arranged in two rings around the ship: one near the bow and the other near the stern. On most 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.
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 appears to affect a ship's acceleration capability more severely if all destroyed 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. It can be assumed from this that, on warships, each ring projects one layer of each half of the wedge.
Anti-ship missiles also use impellers; nothing else is fast enough. 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 a powered-flight endurance of about three minutes. Point-defense missiles have no warheads, relying instead on overlapping their wedges with those of the incoming missiles.
