Structural weight is not your enemy
In most cases, a reusable SSTO loses some margin in what it can lift to orbit compared to a standard single-use rocket. Industry however knows of more powerful engine configurations that can eliminate loss of that margin. Thanks to modern advances, we can build our reusable SSTO.
We also argue that, if you think of the versatile vehicle – that keeps our crew alive and transports it to deep space and returns it from planetary surfaces – as the most important part of the payload to lift to orbit, than the rocket equation would suddenly make a lot more sense. The structural mass of an SSTO no longer is your enemy, it becomes your useful payload. You now can focus on making it more robust and durable in an iterative design cycle.
Versatility as your design philosophy
An SSTO to our design can function as a habitat, a temporary space station, an artificial gravity research station, a planetary lander and a planetary ascender for cargo and crew. Today these capabilities are spread out over a multitude of vehicles, specialised to the task but needing a separate rocket to lift them into orbit. Our vehicle combines all of those functions. It dedicates a larger portion of its mass to build in the required safety, durability margins and margins needed to safely support a crew while taking advantage of decades of advances in building light weight and thermally stable high performance space structures.
Once you get it into orbit, it still has enough internal volume to be outfitted with large inflatable volumes, science gear or any type of mission hardware needed. Commercial space is actually asking for more destinations to send their hardware or crew and the hardware of their clients to as the crew and payload opportunities on the International Space Station (ISS) are saturated.
Micrometer thick films
We also need to thank miniaturization of bulky electronics, sensors, science packages, avionics, solar power systems and even some advanced propulsion systems, that has reduced very heavy support systems of the useful payload to mere micrometer thick films.
This allows us to shrink and even decimate the mass that used to be required for all these subsystems and their radiation shielding. And now, with the advent of the multi launch of cheaper small sats, we no longer need to orbit very heavy satellites, to deliver a cargo with the same profit margins. All these advances combined, we get the same capability in a lighter package or more capability added for the same structural mass.
A marriage of capabilities
We have finally reached the point where we no longer have to compromise between the capability that an SSTO would be able to lift, and the actual mass to orbit. And we have figured out how to do this with mere chemical propulsion.