A plane that will be able to fly anywhere in the world within 4 hours
Honest question: Can someone explain why a Mach 5 air-breathing engine is important for reaching low-Earth orbit (~Mach 25)?  I still don't understand after watching the video.I recall from a video interview that Elon Musk is skeptical of the utility of air-breathing engines for economic access to LEO since, as he put it, the atmosphere is "thick as soup" and you basically want to get out of it ASAP before you get up to orbital velocity.  Breathing air up to Mach 5 allows you to save some weight, but you have to bring enough liquid O2 for the rest of your acceleration from Mach 5 to Mach 25.  So is the idea that this modest savings in weight is all that's needed for a single-stage to orbit?