NASA moves a step closer to supersonic passenger flights

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10. The real problem is the energy expenditure. For sure, materials and engines...

...are so much better than even a couple decades ago; the new jet engines seem like reverse-engineered UFO tech compared with what we had in the '80s. But physics plays some cruel tricks on engineers, especially when we want to pack a couple hundred people into an aircraft that can travel at that kind of speed. Physics is already playing cruel tricks on emissions and fuel consumption right now. Making jet engines and aircraft more fuel-efficient with lower emissions is a major engineering challenge even at Mach 0.85, let alone Mach 4.

It makes one wonder whether we will eventually end up with nuclear-powered planes. However, that didn't work out so well the last time we tried it (both the US and USSR).