What is a rubber engine?
A rubber engine is not to be confused with a rubber motor (a rubber band which is twisted in order to store energy which can be released when the rubber band untwists. This type of engine is good for short energy bursts in small model airplanes).

A rubber engine is an engine deck (tables of engine data) which can be scaled according to your needs. In a way, you stretch one parameter of the data like a rubber band, and all other parameters change accordingly. In most cases, you adjust thrust to your needs and the scaled engine deck will produce mass, size and fuel consumption for the most likely actual engine with the given thrust. This is very helpful in the initial stages of engine design when you search for the smallest possible airframe to fulfil given mission requirements.