How is the efficiency of a rocket launch calculated when the weight keeps changing with fuel being spent + stages detaching?

Question

I assume it has to be a really complex calculation, how is the weight accounted for when the target number keeps changing constantly so dramatically?

In: 5

Answer 1

The simplest is to think of the useful energy output: the increase in gravitational potential energy and kinetic energy of the payload. The total input chemical energy is far higher since most of that energy is used to transport the fuel etc.

Since the input energy is dependent on the exhaust gas velocity and flow rate (as well as number of stages etc) efficiency is limited by mechanical design and chemistry.