The conventional wisdom is to avoid this range, and either stay subsonic or go all the way to Mach 2+, because you sacrifice some efficiency the moment you cross Mach 1:
- Sharp nose and supersonic airfoils result in lower L/D across the envelope.
- Thin airfoils add weight and require more fuselage space for fuel tanks.
On the other hand, staying at low Mach avoids a few other penalties:
- The 12:1 slenderness of modern widebodies translates well to a low-Mach SST, allowing for a similar cabin layout. This is critical.
- Skin temperature allows the plane to be built with the same resins as subsonic airliners.
- Less COL shift, higher AR wings, reasonable bypass engines make it easier to meet TO lift and thrust requirements.
- Speculatively, cruise altitude might be kept at FL450 to avoid stricter pressurization rules.
What I'm not sure is which set of factors matters more. It's difficult to glean from existing designs, as they're not really comparable between one another.
The Aerion's wing should behave well on landings, and it claims only a 22% fuel penalty for going supersonic. But its slender nose and long tail leave just 18% of its length for the cabin, so it can't be used as an airliner template.
This is probably caused by its role as a status symbol and regulatory weight limits; actual SST managed a proper length cabin, and the 2707 was even planned as a widebody. Thin wings have are also featured in some subsonic proposals, so they're not a total efficiency killer.
I'd imagine a Mach 1.x airliner to be a 779-sized twin, with 788-like seating capacity, area-shaped akin to the Sonic Cruiser, an ogive nose (F-16 style), Aerion-like trapezoid SNLF wings, nacelle engines.
The question is, could a low-Mach SST, if sonic boom was publicly accepted, be closer in efficiency to subsonics than to historic high-Mach designs?
To narrow down the scope, let's explicitly exclude:
- Whether there is any market for such an aircraft.
- Whether Mach 1.x is fast enough to make a difference.
- Engines, as it's a major subject in itself. Just assume a rubber engine matching a loose trend line of TSFC~=0.4*Mach.max.
Leaving in scope the technical subject of the efficiency loss imposed by the aerodynamics of supersonic flight. It comes down to how good a L/D can high-tech airfoils like SNLF and area-ruled widebody provide, with how little compromise on capacity, in the range of Mach 1.1 to 2. Fuel per seat-mile being the figure of merit.
To help set a baseline, I've plotted a mix of historic and calculated numbers for known aircraft:
To put the question in more formal terms, I'm looking to estimate the seat-miles/kg function's behavior for an optimal design in the low-supersonic range.
The red line assumes the worst-case scenario and is based on just stuffing seats into the AS2. The green line represents a best-case scenario based on claims with the Sonic Cruiser as a baseline. The truth is probably in between.