[V_Terranova_Jr]

> I wonder what the situation will be in terms of fuel efficiency. Obviously it will take more energy per second to push through the atmosphere, but that may be defrayed by not having to hold the airplane up as long.

The relevant metric is mass of fuel burned per passenger seat-distance. In American units, this would be (lbs. fuel)/(pax-seat mi.). This measure allows direct comparison of differently sized airplanes with different design ranges and cruise speeds.

Take a look at Figure 1.2.7 in this study a Boeing-led team performed for NASA: https://ntrs.nasa.gov/citations/20100030607 . There were plenty of study contracts awarded under this project - Lockheed Martin did a good study as well, but the Boeing one was the first I found.

The dual/tri class band includes the (lbs. fuel)/(pax-seat mi.) for a high-efficiency large subsonic transport. Call it about ~0.1 for this aircraft type. Note the target of the study, which requires state-of-the-art technology or beyond, is 0.3 for a low-boom supersonic cruiser.

Even taking credit for efficiencies beyond Concorde technology levels, and cruising slower than Concorde, it's still ~3X the fuel burn per pax seat-mile compared to a modern high-efficiency subsonic transport. So reduced flight time is more than offset by the energy expended to fly fast and carrying fewer people in a given flight. The picture will be uglier still for supersonics compared to target efficiencies for next-generation subsonics.