Quasi-Newtonian Propulsion

Quasi-newtonian propulsion (QNP) is a system for balancing ship hulls by which larger hulls require more engines to move at any given speed. This is typically done with an Engines Per Move value set on the hulls in the game's data files. It is called "quasi-Newtonian" because in a true Newtonian propulsion system, acceleration would be equal to thrust divided by mass, but since Space Empires does not typically measure ship acceleration, that value is instead replaced with speed in the equation.

Space Empires I
Space Empires I did not have ship components, so QNP could not exist in this game.

Space Empires II
In Space Empires II, starting with the Cruiser, ships required more than one engine in order to get each movement point, but cruisers could also install twice as many engines as smaller ships, for the same top speed. Larger ships such as Battleships required even more engines per move, and also were more limited in terms of top speed.

Space Empires III
Space Empires III kept the same basic mechanic from SE2: bigger ships require more engines in order to get a movement point.

Space Empires IV
Space Empires IV actually got rid of QNP in the stock data files, with all ships requiring exactly one engine per move. However it also added a new feature for modders: engines were no longer limited to generating only a single point of thrust. This was taken advantage of by mods such as Carrier Battles, Adamant, and Dark Nova, where there were multiple sizes or classes of engines, trading off fuel efficiency for speed, and the like. Many of these mods also eliminated the engine caps on hulls, so instead of being limited in how many engines you could install on a ship, you could put as many as you wanted up to the hull's tonnage limit. Some mods even made a little minigame out of the tradeoffs between fast ships and long-ranged ships, by removing supply storage abilities from engines, so that adding more engines would make your ship faster but shorter ranged (due to the increased fuel usage).

There was however a limitation: due to a single byte being used to store a ship design's thrust value, if a ship installed engines giving it more than 255 thrust points, the thrust value would wrap around to zero and the ship would become very slow or even immobile! So modders needed to be careful when designing their QNP systems in SE4, to make sure that ships could not realistically install more than 255 thrust points worth of engines.

Space Empires V
QNP in Space Empires V is basically identical to the way it worked in SE4, the main difference being that the 255 thrust point limit was removed.