World's Fastest Airliner -- Solution

World's Fastest Airliner

Topics in aerodynamics seem to be amenable to empiricism more than theory -- wind-tunnels more than calculus. The puzzle asks, "What remedies for the sonic boom would you propose that might enable the world's fastest airliner to fly at supersonic speeds over inhabited areas?" Without access to fast flying machines for experiments aloft, solvers will need to rely on intuition for evaluating propositions for sonic boom abatements. Intuition, in this case, won't always work...

{1} Airspeed: Faster or Slower?

Something of a surprise here: Flying slower does not help (unless, of course, the airspeed is reduced to below the speed of sound -- so much for the world's fastest airliner). Counter-intuitive as it may be, flying faster is the way to go.

For an illustration, consider the SR-71 Blackbird, arguably the fastest airplane in the world. Since the middle sixties, the SR-71 had been flying at over Mach 3.0 as depicted in this sketch...

...in which you will notice how its shock cone diverges at 20^o, thus wrapped in an expanding circle closer to the flight path than that for Concorde at 30^o. That may serve as a rationale for why the "boom signature" for the SR-71 at Mach 3 was less harmful than that of Concorde at Mach 2 by a factor of two. For sure, but there's more to the story...

{2} Fuselage: Lengthen or Shorten?

As indicated in the sketch above, the fuselage of the SR-71 is half that of Concorde. At Mach 2, the duration of its N-wave would be 45 ms. With its speed increased to Mach 3, the SR-71 generates an N-wave of 30 ms vs 90 ms for Concorde.

Some solvers will infer that 'destructive interference' may have existed between the over-pressure boom and under-pressure boom in rapid succession, partially canceling each other out. Perhaps such a phenomenon augurs in favor of supersonic business jets in the future.

Experiments in 2006 with an ingenious aircraft called quiet spike, which can telescope its fuselage length in flight, provided evidence to the contrary -- that a longer airframe 'smears out' the N-wave and reduces its harmful effect. Go figure.

{3} Wingspan: Longer or Shorter?

Longer, maybe. One proposal is a tailless flying wing. The concept is to put the whole aircraft inside the wing, eliminating the fuselage and its N-wave altogether. Solvers must wait for experimental results. If ever there are any experiments..

{4} Shape of the Aircraft: Redesign?

To address this alternative, the most ambitious experimental programs to date is the two-year Shaped Sonic Boom Demonstration, which concluded in 2003 with the so-called 'quiet supersonic platform'. The official photographs are available here.

Mach 3 SST: A Proposal

With Carbon Footprint asserting primacy in so many realms of life, the aviation world has necessarily shifted its emphasis toward new realities, as explored in Green Flight. When -- rather whether -- there will be a renaissance of the SST is much in doubt. Nevertheless, it is fun to think about what a Mach 3 SST might look like. The sketch above is one such concept.

For commercial success, the Mach 3 SST needs to carry a much larger passenger load than Concorde -- 256 being a likely minimum.
For reaching its cruise altitude of 80,000 feet at 5,000 fpm rate-of-climb, the Mach 3 SST takes full thrust from its six J-58 turbojets.
For cruising, the Mach 3 SST calls upon on its two ram-jet engines, which are able to develop maximum thrust only at supersonic speeds.
For minimizing ancillary shockwaves, the Mach 3 SST airframe is rudderless, and its fly-by-wire system controls yaw by split ailerons and differential thrust.
For sonic-boom abatement, the Mach 3 SST has a 400-ft fuselage in an empirically optimized shape producing an attenuated N-wave spread over 120 ms.