At supersonic speeds the shape and dimensions of the fuselage have a strong effect on the aircraft drag. Supersonic wave drag increases quickly as the fuselage volume increases and the fineness ratio is reduced. For this reason, the cabin diameter is kept as small as possible and the cabin length increased.
The figure below shows a Aerospatiale design for the fuselage of a Mach 2.0 transport (Avion de Transport Supersonique Futur, ATSF).
Note that the diameter and seat layout is similar to the MD-80, but the fuselage is much longer. The Concorde diameter of 113 inches is very small because of the strong impact of fuselage diameter on wave drag.
The requirement for a high overall fineness ratio is reflected in the fuselage geometries shown below.
For comparison, a Boeing design for a high speed civil transport is shown below.
Note that the Boeing design has a fuselage whose diameter varies over the cabin section. This is done to reduce the interference wave drag between wing and fuselage. This was not done on the Concorde as it was felt that the increase in production costs would be too high. Indeed the variable cross-section introduces many difficulties and affects the seating arrangement as shown below.
The supersonic business jet represents a somewhat less ambitious entry into commercial supersonic flight. Since supersonic wave drag depends on volume, the motivation for a smaller cabin cross-section is greater, and high fineness ratios are required. The drawings below illustrate the fuselage and cabin design for a supersonic business jet by Reno Aeronautical Corporation.
