I'm wondering if this range of speeds is actually a better technical design point for SST than the traditional Mach 2 target.
The conventional wisdom is to avoid this range, and either stay subsonic or go all the way, as you sacrifice cruise efficiency the moment you "cross the barrier":
- 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:
- 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
- The 12:1 slenderness of modern widebodies translates well to a low-Mach SST, allowing for a similar cabin layout
- Speculatively, cruise could 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 is the most interesting, being likely to fly, with a wing that 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 perhaps they're not a total efficiency killer.
To give it some shape, I'd imagine an optimal design 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. But that's by no means the goal, just one possible option.
So my question is, could a low-Mach SST, if sonic boom was publicly accepted, be considerably closer in efficiency to subsonics than to historic high-Mach designs? Clearly it's not a yes-no question. What I'm trying to get a handle on is where specifically on the way between M0.85 and M2 most of the design and fuel efficiency loss happens.
To narrow down the scope, let's explicitly exclude:
- Whether there is any market for such an aircraft.
- Whether Mach 1.2-1.4 is fast enough.
- Engines, as it's a major subject in itself. Just assume a rubber engine matching the trend line (TSFC=0.35*Ma).
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.
Considered separately, though, one would result in an empty glider and the other in a box with wings. To combine them, I think fuel per seat-mile is the best figure of merit, as it covers the technical compromise, but avoids the economics. I'm fine with partial answers, as long as they are relevant to that practical optimization.
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 absolute 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. If this still doesn't focus the question enough, please feel free to suggest changes in comments.