> Structural weight is mostly a function of the surface area and fuel capacity and payload are a function of the volume.
Agree with this.
> If you make the rocket bigger, it's payload mass fraction increases.
Don't agree with this. The way I see it, as a rocket gets bigger, and stays in one piece, its payload mass fraction decreases. The inability to jettison heavy engines and structural components means those things become part of the 'mass fraction' that ends up in orbit. And the bigger you make those things, more and more of your mass fraction is being committed to carrying stuff and less is available for non-rocket payload. Hence its a decreasing mass fraction in terms of amount of payload.
Its fun to play around with this in Kerbal but its pretty much a hard limit.
Things that can affect the mass fraction in a positive way are things like more ISP in the engines. When you look at the evolution of the Falcon 9, the increase in mass to LEO was a result of two things, one upgrading the efficiency of the Merlin 1D engine and two developing a way to put more propellant and oxidizer in the same volume by super chilling it.
Agree with this.
> If you make the rocket bigger, it's payload mass fraction increases.
Don't agree with this. The way I see it, as a rocket gets bigger, and stays in one piece, its payload mass fraction decreases. The inability to jettison heavy engines and structural components means those things become part of the 'mass fraction' that ends up in orbit. And the bigger you make those things, more and more of your mass fraction is being committed to carrying stuff and less is available for non-rocket payload. Hence its a decreasing mass fraction in terms of amount of payload.
Its fun to play around with this in Kerbal but its pretty much a hard limit.
Things that can affect the mass fraction in a positive way are things like more ISP in the engines. When you look at the evolution of the Falcon 9, the increase in mass to LEO was a result of two things, one upgrading the efficiency of the Merlin 1D engine and two developing a way to put more propellant and oxidizer in the same volume by super chilling it.