Don't need to learn it second hand since I had first hand experience with the Shuttle. Additionally, I know many of the lecturers in the course.
The IUS motors were intentionally undersized so that there never would be overburn. Each IUS stage burn was followed by an IUS RCS burn to make up the shortfall and provide the exact impulse required.
Something like Sea Dragon, but smaller doesn't need a gantry or carrier aircraft, and can pop out of the water like a cork. But Musk was forced off the coast due to range safety, and he probably has had a belly full of at-sea logistics.