aerospaceamerica.aiaa.org | MARCH 2020 | 37Exploiting the Mach cutoff
Aerion also is not waiting for the X-59 results, but it
has gone in an entirely different direction on the
noise question. Part of its business case will involve
fl ying over land at a top speed of Mach 0.95, but the
company also wants to convince the FAA and inter-
national regulators that an AS2 can fl y faster than
Mach 1 without creating a sonic boom that can be
heard on the ground. How? By taking advantage of
a phenomenon called the Mach cutoff.
Aerion’s Gene Holloway, the company’s chief
sustainability offi cer, explains the strategy like this:
The cutoff point refers to the fact that, under the
right combination of wind, altitude and speed, no
sonic boom would be heard on the ground. By the
numbers, the speed of sound — or Mach 1 — decreas-
es as altitude increases, up until about 37,000 feet. If
a plane is fl ying at 30,000 feet at 1,091 kph, which is
Mach 1 at that altitude, then it won’t create a sonic
boom at sea level because the plane is not exceeding
1,225 kph — the speed of sound at sea level.
With the Mach cutoff concept, an airplane could
fl y at 30,000 feet at Mach 1.1, or 1,201 kph, and still
not generate a sonic boom on the ground, assum-
ing the ground is at sea level. By monitoring the
temperature, tailwinds and headwinds, and the
altitude of ground level along its fl ight path, an
AS2 could fl y at just above Mach 1 without creating
sonic booms along the ground, Holloway says. It
would sound like a subsonic commercial plane
fl ying overhead.
“ Yo u’r e not going to have this major disruptive
sound like a Concorde or a military jet; nothing like
a space shuttle and certainly not like a SpaceX
booster coming back,” he explains.
Pilots fl ying over land under the Mach cutoff
concept would want to avoid cruising at Mach 1
exactly, where drag is maximized, and aerodynam-
ics are unsteady. The further away from Mach 1,
faster or slower, the lower the drag. Over the ocean,
AS2 will cruise at Mach 1.4, Holloway says.Competing with today’s air travel
Business travelers and the super-rich will be the fi rst
benefi ciaries of their aircraft, but the supersonic
pioneers see themselves as laying the foundation to
someday bring supersonic fl ight to all of us.
Wilding predicts that supersonic aircraft will
rival conventional aircraft on cost of travel and
environmental sustainability.
“ We think that’s several generations of airplane
and engine away, but we do think that’s feasible
within our lifetime,” he told an audience at SciTech.
As for the competing strategies of Aerion and
Boom, Wilding says there’s another way to look at it.
“ We’re all helping each other indirectly or di-
rectly. We’re pushing the idea forward. We’re push-
ing the mainstream acceptance of this forward.
We’re training people. We’re helping to infl uence
organizations like AIAA to give more space for these
discussions,” he says. “ We’re big fans of everybody
working in this space.” ★The hangar at Boom
Supersonic headquarters
in Colorado.
Boom