Commercial supersonic flight will not return in the foreseeable future because the costs simply exceed the benefits. Every study made in the past 20 years indicates that airline passengers are not willing to pay the fare premiums that would support development (probably a $30 billion, 20-year project) and sale (at close to a half-billion dollars per aircraft) of a commercial SST.
Boeing, Douglas, NASA, and Airbus have all concluded, at least somewhat independently, that a 300 passenger SST with 5,000-6,000 nautical mile range is the most promising candidate. Boeing and Lockheed proposed such aircraft as early as the 1960s, although ranges then were lower as transpacific travel wasn't as important. Boeing and NASA tried again several years ago on the HSCT. Douglas was an early participant. Airbus has floated its "Alliance" concept. None of them have gone anywhere.
On the propulsion side, supersonic engine technology has not advanced as much as subsonic, for two reasons. First, there hasn't been any demand on the commercial side since the Olympus 593. Military turbofans, the only supersonic engines currently being built, typically run in the high-Mach range for a just few minutes at a time. Even "supercruisers" usually operate below Mach 1.5, which requires relatively simple engine intakes and modest modifications. Building an engine that cruises continuously at Mach 2 or 2.4 for 4-5 hours is a very different challenge.
Second, a lot of the efficiency gains on the subsonic side in the past 40 years have come from simply upping the bypass ratio on each successive engine generation. On subsonic turbofans, raising the bypass ratio lowers fuel consumption and decreases noise. The JT8Ds that powered the 727 ran at 2:1, CF6 is the 5:1 range, the GE90 is at about 9:1, and the GENX will probably have 10-11:1.
Manipulating the bypass ratio isn't an option for sustained supersonic flight, which by its nature requires very high exhaust velocities and therefore very low bypass ratios (zero, or a perfect turbojet, is optimum for supersonic flight but is terribly noisy and inefficient in the subsonic regime). Engine noise remains an extremely difficult challenge and was NASA's main stumbling block on the HSCT. The silencers needed for each HSCT engine were the size of RVs and weighed several tons, but still could not clear the proposed Phase IV
There have been attempts to develop a variable-cycle engine that can change its bypass ratio in flight, but current designs are heavy, complex, and expensive, and so far haven't shown enough promise to make the concept feasible.
Even if all these problems are resolved, specific fuel consumption still will not drop very much. GDB can tell you that Rolls-Royce engineers still
do not expect to beat the Olympus 593's efficiency on a next-generation SST engine, especially with all the design compromises needed to meet noise regulations. We all know how fuel-thirsty Concorde was, so this does not bode well.
Aerodynamic developments would provide some efficiency gains compared to Concorde, but again, not enough to radically change the picture. Wing designs that deliver quiet takeoffs (relatively low sweep with blunt, rounded leading edges and thick, high-camber cross-sections) are not ideal for supersonic cruise; delta wings that do well at Mach 2 are notoriously inefficient at 160 knots. Flaps and slats require a horizontal tail or large canards to offset the pitch effects. Boom-reducing aerodynamic modifications also tend to degrade aerodynamic performance. It's a constant tradeoff.
IMHO, the most promising area for research is weight reduction. Concorde could fly 100 pax about 4,000 miles. A 767-300 with the same takeoff weight can carry almost twice as many people almost twice as far. Composites and other new materials could substantially increase structural efficiency, but skin panels and many structural components also have to be able to withstand 250 degree heat soak for several hours. Any faster than Mach 2.2 and aluminum and most composites become unsuitable; then you have to go to titanium.
I should add that these arguments don't apply to a small corporate SST. I think it's quite possible that a 10-passenger SSBJ could fly within 10 or 20 years. An SSBJ can milk the super-premium end of the aircraft market -- business travelers and the mega-rich who will pay whatever it takes to be there yesterday.
On the other hand, airlines that introduce SSTs risk diverting high-margin subsonic first-class passengers onto lower-margin SSTs. There's no economic case there. Otherwise, the airlines would be clamoring for an SST from Boeing or Airbus. They're not. As I've said before, that's probably the most conclusive evidence that a next-generation SST will not appear for a long time.
Keynes is dead and we are living in his long run.