In Sect. 1.3.1, we found out that there are limits to the obtainable payload mass because of the finite structural mass. This limit is crucial for the construction of a rocket. If a rocket is to reach the low Earth orbit, a propulsion demand of about 9 km s−1 has to be taken into account (see end of Sect. 6.4.7), which is at the limit of feasibility for today’s chemical propulsions. If a higher payload ratio beyond 3% is required, or the S/C needs to leave the gravitational field of the Earth requiring a higher propulsion demand, one has to take measures to increase the rocket efficiency.