Earlier this year, United Airlines made headlines,
signing a purchase agreement with Boom Supersonic to buy 15 of their future supersonic jet airliners. In what could be a bold return of supersonic
travel, Boom is looking to develop the next generation of supersonic planes. Building on the legacy of the iconic Concorde,
and attempting to address the problems that led to the Concorde's final flight on October
24th 2006. The Concorde was a marvel of engineering. Capable of flying from London to New York
in under 3 hours. So fast that passengers, thanks to the 5 hour
time zone difference, had to rewind their clocks two hours on arrival, arriving earlier
than they left. Flying across the Atlantic in the Concorde
was super easy, barely an inconvenience, so why did it stop being produced? The Concorde failed for several reasons. The Concorde had over 100 purchase agreements
with major airlines of the day, including United Airlines, but soaring production costs
forced everyone but British Airways and Air France to cancel their orders. Then, cramped and limited seating along with
rising fuel prices made turning a profit for British Airways and Air France difficult. Causing ticket prices as high as eleven thousand
dollars. Why fly on the cramped Concorde at that price
when you could purchase a first class reclining seat that only took 4 hours longer at a fraction
of the price? At that price, the Concorde was always going
to struggle to fill seats, and that just got harder after the unfortunate air crash of
Air France 4560 in 2000. These economic problems were exacerbated further
by the extremely limited routes available to the Concorde. Supersonic travel was banned over the United
States and Europe, excluding the plane from some of the world’s most valuable airline
routes. The plane’s range was also limited, preventing
it from capitalizing on potentially lucrative transpacific routes. In it’s 27 years of operation, the Concordes
only regularly scheduled routes were across the Atlantic between New York, and London
and Paris. In order for the next generation of supersonic
planes to succeed, they need to address these problems, and today we are going to see how
Boom is looking to prevail where Concorde couldn’t. First off, those pesky sonic booms. Anything travelling over the speeds of sound
is going to cause a sonic boom. That’s just a fact of nature. This became a major issue in the 1960s, when
hearing sonic booms was not a terribly rare occurrence. With Cold War fear at an all time high, the
Air Force regularly conducted military drills near cities, and in the fall of 1965, public
acceptance of this nuisance was becoming a serious political issue, forcing the US Government
to form the National Sonic Boom Evaluation Office. This group then began taking measurements
of sonic booms with different planes, atmospheric conditions, and flight altitudes to gauge
public acceptance. They recorded boom signatures with pressure
gauges and created these sonic boom signatures. Here’s three signatures, one for the F-104,
one for the B-58 and one for the XB-70. We see a peak in pressure caused by the sonic
boom followed by a drop in pressure, and a final jump in pressure caused by the tail
of the aircraft. Interestingly we can see that the signature
for the XB-70 is longer than the others, and that’s because the plane itself is longer. [3] The studies uncovered a huge amount about
how supersonic waves travel through the atmosphere, and honestly the entire study needs a video
in itself, because it’s incredibly complicated and rich in history. But for now, the important thing is that people
absolutely hated these sonic booms. The key word that emerged from the 6,375 interviews
conducted was “startling” and people did not get used to them; the annoyance only got
worse over time. Many complained about physical damage to their
house as the pressure waves could physically rattle windows. NASA is planning to revisit these tests in
2022 with the X-59 which has been designed with decades of research to minimize the pressure
associated with sonic booms. With highly swept delta wings, a blunt duck-billed
nose and a very narrow fuselage that narrows over the wings to conform to the area rule
[4]. With these design considerations the X-59
is expected to have a boom with a perceived decibel level of about 75. [5] Which the NASA team equates to the sound
of distant thunder. They plan to begin testing public acceptance
of this again in 2022, but critically Overture, Boom’s concept commercial plane, features
few if any of these distinctive low boom designs and will likely have a boom just as loud as
the Concorde which registered at a perceived decibel level of 105. That blunt nose increases drag and reduces
fuel efficiency, while a narrowing fuselage reduces space for passengers. Sacrificing range and passenger capacity for
the sake of quiter sonic booms is not an option for a commercial transport plane. And judging by the routes advertised on Boom’s
website, this is a problem they are perfectly aware of and are instead focusing on increasing
range and opening up those valuable transpacific routes. So, how are they going to manage that? Thankfully the airline industry has been heavily
focused on increasing fuel efficiency and range in the past 5 decades since the Concorde
was designed, and the team at Boom will be able to take advantage of many of these advancements. The Concorde was primarily constructed from
Aluminium. Overture will primarily be constructed from
carbon fibre composites, similar to the Boeing 787, which allowed Boeing to reduce weight
and drag while creating extremely thin and perfectly sculpted wings. Bending aluminium into the perfect shape without
creating stress concentrations is difficult, with composites we have much more freedom
in design. Carbon Fibre also comes with the advantage
of lower thermal expansion. The aluminium structure of the Concorde expanded
by 0.3 metres at cruise, which had to be accounted for with expansion joints in the fuselage
and hydraulic piping and slack in critical wiring The Overture for the most part shares a very
similar design to the Concorde. A design that focused on minimizing supersonic
drag, while being capable of providing enough lift at low speed flight for take-offs and
landings. Let’s compare the top view of both the Concorde
and the Overture. The first similarity we can immediately spot
are the ogival delta wings. This is a form of compound delta wing where
we have two sections to the wing, with the forward section having a higher sweep angle
than the rear section. The ogival delta wing of the Concorde is a
variation of the compound delta, with an ogee curve connecting each section. Going from concave to convex curves smoothly. This shape was used to optimize for both low
speed subsonic flight and high speed supersonic flight. Creating a wing that can minimize drag at
supersonic speeds was vital, but these short wingspan wings create issues when taking off
or landing, as the plane cannot generate enough lift at slow speeds. The ogival delta wing of the Concorde helped
alleviate this by generating vortex lift. Where at high angles of attack, separated
air flow would roll over the wings to form two stable cone shaped vortices where air
speeds were high and air pressure was low. Effectively creating lift. This is why we see the Concorde landing with
its nose extremely high, which would have completely blinded the pilot's view of the
ground. The engineers of the Concorde solved this
by creating a nose that could actually angle downwards on command, to allow the pilots
to see the runways when landing. The mechanics of this system of course increased
the weight of the plane, required routine maintenance, increased operating costs and
the mechanism itself would have increased the overall price of the aircraft. The Overture will not feature this droop snoot,
but instead use a system of cameras and screens to allow the pilots to keep their eyes on
the runway even when their cockpit windows show nothing but sky. The engineers at boom have a massive leg up
in engineering tools over their counterparts at concorde, who would have had to design
the concorde with pencils and paper while testing it in physical wind tunnels. The computational tools we have today are
absolutely mind blowingly advanced. Just take a look at this simulation performed
by NASA supercomputers modelling airflow around the landing gear of a plane. This particular simulation was done to investigate
ways to improve noise generated from landing gear and we can see this massive vortice rolling
off the landing gear doors. Telling us that we can improve noise generated
by focusing on airflow around these doors. In the past engineers would have had to use
low resolution physical visualisation techniques to piece together ways to reduce noise and
create model after model to test different designs. Today we can make the models in a computer
and test them on a computer, allowing incredibly fast turn over times and vastly improved quantitative
measurements. Tools like this will be used by Boom to land
on the optimized airframe and carbon composites will allow them to manufacture those optimized
shapes with ease. On the surface, the wings of the overture
are remarkably similar to the Concordes by all accounts, which makes sense, they worked
very well, but in order to increase range they can’t just be as good, they need to
be better. It’s clear that Overture have made efforts
to make the wings thinner while extending the delta wing further up the aircraft, seamlessly
integrating into the plane's sharp nose. This will all help minimize wave drag. Wave drag occurs as shock waves form over
the plane. These shock waves act like huge invisible
air brakes, slowing down passing air significantly and increasing drag. Area ruling is one of the key methods to reduce
it. The area rule is a weirdly simple rule in
aerodynamics. It tells us that the ideal distribution of
cross-sectional area along the length of a supersonic plane should look something like
this. That includes the fuselage, wings, engines
and every other part of the plane. That’s why we often see waisted fuselages
that narrow to compensate for the increase in cross sectional area caused by the wings. Planes never conform perfectly to the area
rule. We need wings and engines to fly, and narrowing
the fuselage over the wings isn’t really an option for a commercial airliner that needs
to carry people. However, carbon fibre composites do give designers
much more freedom in shaping the aircraft to conform to the area rule, and Overture
will likely benefit from these advances. Overture will benefit from other advances
in material science that have enabled the creation of incredibly efficient engines like
the GEnX. Advances in aerodynamic modeling, metallurgy
and 3D printing have made today's jet engines vastly more efficient than the engines of
the the 1960s and 70s, but it’s important to note one major advance that supersonic
planes cannot take advantage of, high bypass ratios. Bypass ratios have continually grown over
the past 5 decades and this has been one of, if not the primary driver for increased efficiency,
and supersonic planes cannot take advantage of it. [1a] Supersonic jet engines need to be pure jets,
avoiding the high bypass ratios of traditional airliners. The large diameter of these engines would
cause an immense amount of drag, and the large quantity of relatively slow moving air coming
from the bypass would actually just slow a supersonic jet down. Instead the engines of the Concorde, and Booms
test planes use pure jets, with no bypass. These engines have not benefited from the
past 5 decades of research that high bypass ratio planes have, and for Boom to succeed
they are going to have to develop an entirely new engine. Boom’s flight demonstrator aircraft, the
XB-1, which is set to begin testing in 2022, is using the GE J85, an engine that has been
in use since the 1950s. This simply will not do for the final passenger
aircraft, and this year Boom announced a collaboration
with Rolls Royce to develop a propulsion system for use with their aircraft. [8] Engine development takes years if not decades
of development, so it will be interesting to see what Rolls Royce can come up with before
Boom’s advertised first flight date of 2026. It should be noted that the primary way Overture
is going to reduce fuel consumption is by slowing down.Overture is aiming for a cruise
speed of 1.7 mach [9], whereas the Concorde cruised at mach 2. Even with all of these potential fuel saving
technologies Boom rather confusingly advertises the range of the Overture at 7866 kilometres,
just over 500 kilometres over Concordes. One of the shorter routes over the Pacific
is Tokyo to Seattle at 7651 kilometres. I’m not sure whether Boom is factoring in
the additional fuel capacity required by regulators to ensure planes have enough in reserve to
deal with emergencies, but this is cutting things extremely close for even the shortest
pacific route, and it most certainly is not enough for the
Sydney to Los Angeles flight advertised on Boom’s website. There is no doubt in my mind that we can build
a more efficient supersonic passenger jet today, but I am skeptical we can build one
that will succeed economically. Boom appears to be trying very hard to obfuscate
the limitations of supersonic travel, and are even advertising net zero carbon nonsense
about the plane being able to use sustainable biofuel, which is not only not commercially
available at scale, but is vastly more expensive than kerosene. No flight operator is going to use biofuel
on a plane that will already be struggling with ticket prices. The average price of a round trip ticket from
London to New York was 12,000 dollars. It exclusively catered to the ultra-wealthy
who were happy to pay an insane premium to save 2 hours of their day. A narrow market. A product most of us will only ever consider
as a once in a lifetime novelty, and that’s not a sustainable business model for airlines. Without a doubt Boom can build a prototype
worthy of being the Concorde’s successor, that improves on all aspects of supersonic
passenger travel, but whether they can build it at an affordable price point and get fuel
consumption down low enough for it to be commercially viable is another challenge entirely, one
I’m not confident they can overcome. Overcoming challenges to me is the joy of
engineering. That dopamine rush of solving a problem you
have been working on for days if not weeks. That feeling is what draws most engineers
into their career, and perhaps you have someone in your family that gets that same joy, and
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