Time is money, as the saying goes. If time is worth enough money then supersonic flight makes sense.
Supersonic flight will always be more expensive than subsonic flight. We have already seen that the CDp is higher in supersonic flight than in subsonic flight. Therefore, more fuel will always be required for supersonic flight than for subsonic flight.
However, a well designed supersonic aircraft has commercial potential. Several are currently on the drawing boards, along with supersonic business jets. Whether the future holds many or a few supersonic airliners depends entirely on the demands of the traveling public. It is certainly well within the ability of aeronautical engineers to build a safe and reasonably efficient supersonic aircraft.
In this section we will consider some of the challenges and known solutions to sustained flight in the speed range from about Mach 2 to Mach 5.
The normal shock wave which formed in Transonic flight moves back to the trailing edge of the wing as the aircraft reaches the speed of sound. In this position it posses relatively little problem. Once the aircraft accelerates beyond the speed of sound the shock wave will will remain attached to the trailing edge of the wing, but will sweep back becoming a weaker Oblique Shock Wave. We will explore the Oblique shock wave further below.
The Oblique shock wave has the characteristic of slowing the airflow which passes through it, but not necessarily to subsonic speed. IE, the airflow behind a normal shock wave is always subsonic, but the airflow behind an Oblique Shock Wave may still be supersonic.
The Oblique shock wave is not really much of a problem for supersonic design. It does represent a certain amount of drag, since energy goes into it's formation, but very little can be done about that.
The primary drag problem in supersonic flight comes from the bow wave which forms ahead of the wing. The bow wave causes an area of very high pressure to form just in front of the wing. This causes a large increase in pressure drag.
There are two ways for the designer to deal with the Bow wave:
Swept Wings in Supersonic Flight
If a wing is placed behind the shock wave as shown above, then the air flowing at right angles over that wing will be subsonic, even though the aircraft is flying faster than the speed of sound. Therefore, a subsonic airfoil, with round leading edges can be used without creating a bow wave.
Required Sweep Angle For Supersonic Flight
In order for the above procedure to work the wings must be swept back more than the mach angle. IE, to fly at mach 2 the wings should be swept at least 30 degrees, to fly even faster will require more sweep. However, as long as the designer can accommodate this requirement the wing will react as though it is in subsonic flight. Therefore, the designer will be able to use a conventional airfoil with a round leading edge. The air will flow smoothly around this wing and no bow wave will form. This is the design strategy of choice for most modern supersonic designs.
The primary advantage of using the swept wing, with a round leading edge airfoil is that low speed characteristics, including stall, will be good and therefore takeoff and landing should be less of a problem.
The disadvantages of the highly swept wing include a very large stalling angle of attack which will require unusually nose high attitudes for landing and also excessive lateral stability making control difficult.
It is important to note that the supersonic airfoil designs, with very sharp leading edges are generally only applicable as an alternative to swept wings. In other words both sweep and sharp leading edge airfoils would not normally be applied to the same aircraft.
Supersonic airfoils are used extensively on devices such as missiles, because subsonic performance is not important for such devices. However, for passenger carrying aircraft subsonic takeoff and landing performance makes conventional airfoils, with round noses more suitable. This could possibly change in the future if very high speed aircraft designers are combined with vectored thrust VTOL capabilities. Generally aircraft designers concede that straight wings with supersonic airfoils are superior above Mach 2. However, they have inherently high stall speeds and therefore require very long runways.