[V_Terranova_Jr]

> No, it's exceptionally bad to fly close to Mach 1 and you do better if you fly faster up to Mach 3 or so.

Where do you get this from? I suspect you are talking about the Breguet range parameter, which is (M / SFC * L / D), where M is the cruise Mach number, SFC is fuel consumption per unit thrust, and L/D is aerodynamic efficiency.

Cruise L/D drops monotonically with Mach number until it asymptotes out to hypersonic waverider type figures (the well-known Kuchemann curve fit for supersonic L/D and the asymptoting to waverider values is illustrated at: https://aerospaceweb.org/design/waverider/design.shtml ).

SFC for gas turbines is extremely sensitive to cycle parameters and internal temperature limits, but generally well-designed gas turbine propulsion systems are going to beat ramjets until they hit thermal limits. Today's gas turbine technology is far more capable than the J-58 technology of the the A-12/SR-71 era. That said, SFC is still going to increase monotonically with cruise Mach number until you asymptote out to flatter slopes of ramjet and scramjet curves. (Edit: see the Isp illustration at: https://en.wikipedia.org/wiki/Specific_impulse ; Isp is the inverse of SFC).

So you are banking on the increase in M more than offsetting the increases in SFC and decreases of L/D. This is not going to support the case for increased efficiency in high supersonic (M >= 3) cruise. You could make the argument going full-on hypersonic waverider could "make sense" since the range cost functions plateau out, but that is still going to have a strictly lower overall range parameter than a supersonic vehicle. Kuchemann addresses this topic in his well-known text, "The Aerodynamic Design of Aircraft".

A naive Breguet range parameter perspective also fails to consider airframe mass penalties associated with flying faster (roughly, aircraft acquisition costs trend with aircraft unfueled weight), or that high aerodynamic efficiency is often at extreme odds with volumetric suitability for passenger & cargo carriage. Mass penalties will come from variable geometry, thermal management, Cg management, degree of compromise in structural efficiency for aerodynamic efficiency, etc. Let's say you manage to keep Breguet range parameter really high and fuel burn per seat-mile is acceptable. All the complexity and increased airframe mass of the faster aircraft is still going to make recurring costs to operate & maintain the aircraft really high.

The above also neglects boom, which you ideally want to minimize to maximize overland flight potential for your supersonic aircraft.

As per a previous comment I made: https://news.ycombinator.com/item?id=27386119 , if you're going to go in on supersonic civil flight, you likely want to be below Mach 2. Mach 1.4 - 1.8 might be the sweet spot when you consider all factors, assuming it makes any sense at all economically.

For the majority of air travel, we would be better served by making the airport experience much more streamlined and less shitty. And I say this as one who loves and works in high-speed flight.