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.
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Or if you are looking for something else to watch right now you could watch our last video
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