A jet-turbine can use a bypass to exchange some of the speed of its exhaust for a greater mass of exhaust moving at a slower speed. This is desirable as matchin

A jet-turbine can use a bypass to exchange some of the speed of its exhaust for a greater mass of exhaust moving at a slower speed. This is desirable as matching speed of the exhaust to the airspeed increases propulsive efficiency. A modern jetliner flies relatively slowly to its (un-bypassed) jet exhaust speed, therefore it will slow it down a lot and move a large mass of air - this is a high-bypass turbofan. A jet flying at supersonic speeds obviously has less need to slow down the exhaust, therefore such a high bypass ratio would actually be counterproductive. Despite having zero bypass the Olympus engines in the Concorde were the most efficient heat engine of their day, achieving 43% efficiency [0]. Their propulsive efficiency was in the 50%s (compared to the 60%s for a B747-100) [1]. Despite this going at supersonic speeds does require an awful lot of energy over subsonic speeds so even with its high efficiencies it burnt a lot more fuel than an equivalent subsonic flight. Even if Boom could produce the most efficient engines seen, it would still require a lot more energy for the total flight, and therefore would have worse total fuel consumption. [0] https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 [1] https://www.nap.edu/read/23490/chapter/6#37 https://www.nap.edu/read/23490/chapter/6#37