The 21% empty weight fraction and the attending ~10% structural fraction for a 1500 to 2000 psf vehicle are more than a factor of 2 away from being rational.
This is a difficulty that I have run into with a lot of the people "designing" ALSR cars. Stresses are factors of 4 to 6 greater than any wheel driven car, 10X for Mach 2. Think about that factor of 10 very carefully. The design database of supersonic-at-sea-level capable aircraft and missiles provides plenty of structural weight and design guidance that simply cannot be ignored.
If anyone wants to pencil out something like this, a 1000 mph FIA record requires a 35 to 40% structural mass fraction with advanced materials, straight load paths, and good structural design. Poor design or heavy materials can raise this to 60 to 75%. Note that the "structural" mass fraction does not include the engine.
For reference, the structural fraction of our current project is nearly 60% due to the use of mild steel and a far-from-optimum structural layout. Thrust SSC was about 50%. 10% is irrational.
