15.7 Sub-Light Drive

Each unit of sub-light drive costs 3 BURPS. It is used for maneuvering in combat and for short journeys (such as within a system), since travelling sub-light avoids the time it takes to enter and leave warp. It is likely that you will design some ships without warp drives for reasons of economy and efficiency, relying entirely on sub-light drives. Sub-light speed is calculated from the percentage of ship made up of sub-light drives in a manner similar to the warp speed calculation described above, except that sub-light speed is 4 times the square root of the percentage of the ship's mass devoted to sub-light drives, rather than 2.

Travelling in a System

Note that you do not need warp drive to move to the star of the system you are in - you can do that using sub-light drive. Likewise you can move to a near or far sister star in a binary or multiple star system without warp drive (although it may be a very slow trip - on the order of 50 orbits, typically.) You cannot move between star systems using sub-light drive.

With regard to in-system movement, a ship can move 1.5 orbits per turn per point of sub-light speed it possesses. For example, a spaceship with a sublight speed of 10 could move 15 orbits in one turn, or one orbit in about 7% of a turn.

The distance between a world and its moons is .01 orbits.

Lifting off and landing
There is no need to worry about these details, unless you are trying to calculate the time a ship will take in advance.

A ship performing orders at a world or moon takes some time launching or landing. A ship can be in high orbit, low orbit, or on the surface of a world.

A ship arrives at or leaves a world from high orbit. It takes 1% of a turn to travel from high orbit to low orbit, where ship combat occurs, world surveys are performed, and the like. In order to buy or sell cargo or load civilian units, the ship must land, which takes 1% of a turn from low orbit. Likewise, it takes 1% of a turn to launch from the surface to low orbit, and 1% to travel from low to high orbit. 

               high orbit (departure and arrival point for ships)
                                 ^
                                 |  1% turn travel time
                                 v
               low orbit (ship combat, world surveys, etc)
                                 ^
                                 |  1% turn travel time
                                 v
               surface (buy world, sell, load military or civilian units, etc)
Hence, when calculating times taken for round trading trips, don't forget to include 2% of a turn to travel from high orbit to the surface and 2% of a turn to launch and reach high orbit again, as well as the 10% of a turn it takes to trade.

Sub-light Drive and Combat

As mentioned above, sub-light drives determine a ship's maneuverability in combat, since combat cannot occur in warp. Ships with little sub-light drive are almost stationary in combat situations, and are therefore very easy to hit, and the more sub-light drive a ship has, the harder it is to hit. A spaceship's dodge chance, like its sub-light speed, is determined by the percentage of the spaceship that is sub-light drive. As a rule of thumb, a spaceship that is 20% sub-light drive is quite dodgy, a ship that is 50% drive is very dodgy, and one that is 80% or more drive is unbelievably dodgy. The following table indicates the modifier of an attack's chance to hit for various percentages of drive. A modifier of 200 means that the attack has double the normal chance of hitting, while 20 indicates that the attack has only one-fifth the chance to hit.

Dodge Percentage versus Percentage of Ship Devoted To Sub-Light Drive 

Drive %    1       5       20      35      50      80
Dodge %    200     100     50      35      25      10