KJ_Lesnick, quoted: "From what I remember the Tu-144 prototype and Tu-144 production models both required afterburner for Mach-2 flight and as a result had a short range.
Right. To get the advertised range, something like 90% of the payload weight had to be taken off. It might do about 3000 mile then and at high altitude. Also, there was a description---I think in
Aviation Week---that passengers had to yell at each other ;D

:

over the engine's exhaust thunder.
KJ: "At least the Tu-144 production model could do Mach 1.6 without afterburners."
I didn't know it was that fast on just dry thrust. Bigger engines, I suppose.
KJ: "The prototype Tu-144 was shorter, featured a simple double-delta wing, a 5-abreast fuselage
(thicker than the Concorde)..."
I did see that on drawings of the plane. No area ruling. The engines had to be big to overcome this inefficiency.
KJ: "...and two paired engines so close to each other that it looks like it's a single four engine box from some angles.
Yeah, sort of like a B-70. This could have made the wing like a sonic-boom-rider like the B-70.
KJ: "The production Tu-144 was longer, featured a more complicated delta-wing design and a retractable canard with slat and flap to allow the elevons to droop as flaps."
This improved aerodynamic qualities on landings very favorably. Pilots were impressed. Before then, landings required care.
KJ: "Engines were spaced further apart but in rectangular-structures like before with two engines each."
Probably reduced drag as well. Wheels were still in the nacelles between the engines and I think they were artificially cooled against aerodynamic heating.
KJ: "It's the Tu-144D that could do Mach 2 performance without afterburner --...The difference was the engines. Although I'm not sure about the specifics of that. I've been told
-The engine was a turbojet, unlike the turbofans used on the previous designs.
-The plane used the NK-321 Turbofan
-The plane used a variable-cycle engine which was believed to be beyond the Russians capability in 1970 when they began development by US estimates.
I'm not sure which."
I only read one reference to those engines(many years ago) and it did indicated the pressure ratio and thrust was high enough to allow Mach 2 speeds without afterburner. I would logically think it was one of the newest military-style engines that became available then. NASA did some SST experiments(briefly) with the biggest Russian engines from a bomber and then the Tu-144 was capable Mach 2.4.