Thruster Module
This article details the existence and use of a Thruster Module
The Essentials
A Thruster Module (Formerly known as a Hypercoil Thruster) is required to give a ship propulsion, and subsequently, the ability to move. Thrust used to only impact a ship's acceleration, however as of 0.19538 it also determines the max speed of a ship (ranging from half the server speed to three times the server speed, which can be further increased with the help of a defensive Overdrive effect). Even if a core on its own doesn't provide any thrust, a ship made of a single core and a couple blocks is faster than an astronaut (in vanilla and most server configurations) while reasonably maneuverable. However if a player wants anything to move at a decent speed then additional Thruster Modules will be required.
Starmade's thrust system has diminishing returns. In other words, the mother thrusters you have, the less each thruster block adds to your thrust. In practice, this makes ships slower and less maneuverable as their mass increases.
Module Use
Thrusters are very commonly used for decorative purposes, either in large groups or scattered around individually. Thruster Module do not gain any additional bonuses from being adjacent to another Thruster Module, and so may be placed freely without any constraints. Often times it's seen as a good idea to have small groups of thrusters separated around a ship, so that it's harder to target them down and shut down a ship's movement. While it may be difficult with the many other modules in place, it's certainly recommended for absolute safety.
Not only that, but considering the amount of thruster modules often needed, while uncommon, they can be used as a "system shield" to layer over other more important/more expensive systems so as to keep them safer, possibly increasing a ship's combat endurance. This ultimately depends on the ship and it's role, but for say a brawler where speed is not exactly important, it could be a good idea to use Thruster Modules to protect the more defensive/tanky modules.
Thrust carries between entities. You may have a separate entity made up entirely of Thruster Modules, and dock it to a much larger ship, giving the parent ship the thrust from the child ship (you however have to "inherit thrusters from docks", this option is available under the Structure/Thrust menu). Your total thrust will then be calculated as if all your Thruster Modules were placed on the main entity rather than divided between several docked entities.
Formulas
The total thrust of a ship is determined by its number of Thruster Modules only (including the Thruster Modules placed on docked entities if you inherited thrusters from docks). The formula is as follows:
Thrust = 0.75 * (5.5 * NumberOfThrusterModules)0.87
Therefore, the required number of Thruster Modules to have a given Thrust score is:
NumberOfThrusterModules = (Thrust / 0.75)1/0.87 / 5.5
The maximal speed is determined by the thrust to mass ratio of a ship and the server speed setting (this speed may then be further increased with an Overdrive effect):
MaxSpeed = (max(Thrust / Mass, 2.5) + 0.5) * ServerSpeed
The thrusters' power consumption per second is linear in the number of Thruster Modules:
EnergyConsumption = 100 * NumberOfThrusterModules / 3