The Kang-Smith Jump Drive.
Extra solar system human space travel became feasible when the Kang-Smith jump drive was developed at the University of Colorado at Boulder, USA in 2237. The KS drive allowed ships to disappear in one location and reappear up to three light years distant, instantaneously. Later improvements to the KS drive allowed jumps of up to seven light years.
The jump drive, as it is called, is powered by a battery system, the Twitch Battery, which discharges all of its energy in one giant release. The battery is then recharged via the ship's fusion engines. Initial battery/engine combinations took up to three days to recharge, limiting the speed of a ship to approximately six light years a week. Later developments allowed two battery sets to be carried, and recharged in only two days, enabling a jump per day, or two in rapid succession. When combined with the longer jump distances, of seven light years, this improved the speed of a jump ship from six light years per week to 49 light years per week.
The first KS ships had no maneuvering thrusters at all; being served by dropships for all inter-system movement. By the year 2340 improvements in technology had provided for maneuvering thrusters to be added to ships. This caused a balancing act in design between longer and faster jumps or intra-system maneuverability. Warships were generally designed to be maneuverable, with less capacity for rapid jumps, while merchant ships had minimal, or no, maneuvering capability in order to maximize their long distance carrying capacity.
The propulsion systems for a KS ship are, therefore, composed of four systems. The first is the Kang-Smith jump drive. The second is the fusion reactor which both charges the batteries and provides ship power. The third is the twitch batteries, between the fusion reactor and the KS drive. The final component is the maneuvering thrusters.
The improvements in jump distance and speed allowed for efficient administration of far flung empires and were the key to the birth and survival of interstellar governments. The Empire of Man could never have existed with the slower, original, speed of faster than light travel.
The KS drive creates a sphere around the core of the drive, transporting anything within that sphere to the new location, and leaving anything outside of it behind. The larger the KS drive, the larger the sphere but also the greater the fusion/battery requirements to power it. There is no theoretical limit to the size of a vessel that can be created, and if it is not fitted with maneuvering thrusters, the mass is largely irrelevant as the mass within the sphere is not a factor, only the size of the sphere itself. One result of this is that most merchant vessels are large spheres, since there is no need to move within a system.
The result of these facts is that merchant vessels tend to be larger than warships. Some merchant vessels are over a million tons and can carry dozens of heavy dropships. Most warships, due to the need to maneuver in combat, are much smaller, with the largest dreadnoughts being around 250,000 tons and most ships being under 100,000 tons.
All KSV's (Kang-Smith Vessels) are designed to carry at least one dropship, whether they can maneuver or not. No KSV is capable of entering atmosphere.