when the thrust of a rocket or an engine is compared to the weight, it is called the thrust-to-weight ratio. the number that comes from this comparison does not have any units, because it is a ratio. a ratio in this case means that the thrust of the engine (in newtons) is divided by the weight (in newtons). the purpose of this comparison is to show how well the engine or vehicle performs, for example how much acceleration. it is a number that can be used to compare various types of motors like airplane motors, jet engines, rocket engines, or car engines.