We are often told how loud super sonic booms are, but rarely actually get the chance to
hear it for ourselves.
One of the rare timas where a boom was heard in an American city occurred in 2010 when
a small floatplane entered restricted airspace over Seattle when, former US President, Barack
Obama was visiting.
Twin F-15 Eagles were scrambled to intercept the floatplane and in the process broke the
sound barrier over the greater Seattle area.
The low flying aircraft created a boom strong enough to shake homes and was heard as far
as south Washington state and caused a huge surge in calls into local emergency services
centres.
This phenomenon of sonic booms is one of the key limiting factors that caused the Concorde
to be an economic failure, but high flying aircraft don’t create near as loud booms
as the sound energy is dissipated as it travels through the air.
The other limiting factor, preventing the Concorde from achieving economic success was
it’s fuel economy.
When comparing it to one of it’s major competitors, the Boeing 747, it consumed about 50% more
fuel, carried a quarter the amount of passengers, and had about half the maximum range.The range
was so poor, the plane was basically limited to transatlantic flights as it couldn’t
cross the Pacific Ocean and and was banned from flying over land, as a result to those
pesky sonic booms.
Although the entire airplane was treated like first class, flight tickets were still very
high compared to competitor first class offerings, and it was difficult to find enough people
to pay those prices regularly.
Although the Concorde was, for all intents and purposes, a safe and successful aircraft
for the entirety of its 27 year career, there were more than a handful of reasons the aircraft
was officially retired in 2003.
Growing pressure from the British and French governments, concerns about fuel costs, the
lack of route options and the simple shortage of mass market potential simply killed this
iconic aircraft.
For the next generation of supersonic aircraft to succeed they are going to have to overcome
these challenges and this exactly what Boom Technology plans to do.
Boom is a startup company aiming to create a 45-passenger civilian supersonic transport
aircraft to fly up to Mach 2.2 at a price comparable to a business class ticket on a
regular airline.The laws concerning supersonic flight over land are still in place, so their
plans are currently focusing on creating economically viable transoceanic flights.
Like a London to New York flight that would take just 3 hours 15 minutes with a price
of $2,500.
A San Francisco to Tokyo flight that would take just 5 and a half hours and cost $3,250
each way or an LA to Sydney route with a flight time of 6 hours and 45 minutes at $3,500 each
way.
These are still extremely expensive flights, but they open up the possibility of getting
a early morning flight in LA, arrive in Tokyo for a business meeting over Sushi and being
back in time to tuck your kids into bed.
Time is money for many people.
Boom are currently developing two aircraft.
An all-premium class airliner and a geometrically similar two seater jet called the XB-1.
The XB-1 will be a 1/3rd scale demonstration aircraft, intended to be a test bed and proof
of concept for the larger airliner
The team, don’t just have the advantage of hindsight guiding their design.
When the Concorde was being developed computers weren’t really used for design work.
Everything was done on paper and tested in wind tunnels.
Today the engineers at Boom have incredible design software allowing them to quickly develop
prototype designs and have powerful computational analysis software that can test the designs
the same day, without pouring money into building a scaled prototype and paying for wind tunnel
hours.
This allows them to tweak the design to the finest detail to create the most efficient
aircraft possible.
What once took days and thousands of dollars to test, takes a single engineer a couple
of hours.
This coupled with carbon fibre reinforced plastics is allowing the design of the Boom
Supersonic airliner to be lightweight, be shaped perfectly and perform well at the high
temperatures of supersonic flight.
So beyond these advantages, what are the differences between the Concorde and Boom?
Boom believe that a 30% increase in fuel efficiency would reduce operating costs sufficiently
to allow a viable business model.
The Concorde consumed 16.7 litres of fuel every 100 kilometres per passenger, but the
Concorde had an average passenger capacity of about 100 with 25 rows with 4 seats in
each.
Boom plan to have just 45 passengers each getting their own window seat and aisle access.
Creating a much more enjoyable experience over the cramped interior of the Concorde,
but it also makes that target of a 30% decrease in fuel consumption per passenger much more
difficult.
Let’s look at some of the design features they are introducing to achieve this goal.
Both planes feature an extremely thin delta wing, named after the triangular greek letter
delta.
This design feature is essential to allowing the wing to function at both subsonic and
supersonic speeds.
If you watched my video about Why Plane Wings are Angled Backwards, you will understand
why sweeping the wing is incredibly important for planes that travel close to the speed
of sound, but it’s important for planes breaking the speed of sound too.
As an object approaches the speed of sound it experiences a sharp increase in coefficient
of drag and as explained in my Greatest Innovations in Formula One video the drag force on an
object increases with the square of the velocity, so creating a streamlined aircraft is incredibly
important for supersonic aircraft, especially when fuel efficiency is paramount.
Intuitively we want to minimise the cross-sectional area of the aircraft to reduce drag, but we
also want to minimise the changes in cross sectional area along the length of the plane
to reduce the wave drag.
If we compare two shapes with the same maximum cross-sectional area, but one is smoothed
and other has a sudden change in cross sectional area, we can see the later has a much high
coefficient of drag as it enters supersonic speed.
So the plane's fuselage is narrowed where where it meets the wing, as the wing increases
the cross sectional area of the plane at that location.
This is called the Area-Rule.
Sweeping the wing backwards decreases the coefficient of drag significantly at transonic
and supersonic speeds too and the Delta wing allows for a sweep angle of up to 70 degrees,
while also allowing for a very thin wing, thanks to the large chord and short wingspan.
The Delta wing performs well at low speeds too, as a result of unique swirling vortices
that form on the uppers surface of the wing.
On a traditional aircraft's wing a swirling vortex is formed only at the wing tips.
On a delta wing they form on nearly the entire wing surface and produces a considerable amount
of lift.
This is particularly apparent on damp days where the vortices can be seen forming on
the upper surface of the wing, due to the water vapour condensing in the low pressure
vortex
The Delta wing gains additional lift, particularly on landing, as a result of the ground effect
where the downwash of the air between the wing and the ground creates a cushion of air.
But generating lift using these technique needs a large angle of attack, and The Concorde’s
angle of attack on landing was so high that the nose of the plane needed to be tilted
downwards to allow the pilots to see what they were doing.
I imagine Boom are just employing a camera to help the pilots see on landing.
Boom also uses a chined fuselage, which helps maintain the centre of lift as the plane gains
speed.
As a supersonic plane gains speed, the the centre of lift tends to move backwards, creating
issues for the balance and control of the aircraft.
A chine is a ridge that extends from the wing, you can see a similar shape on the SR-71.
This structure generates more lift at supersonic speeds than subsonic and thus helps hold the
centre of lift location.
The engines may be the most interesting part of the design, with a variable geometry inlet.
At supersonic speed the air is efficiently slowed to the ideal subsonic speed for the
engine, with digitally-controlled movable surfaces precisely shaping shock waves to
achieve ideal compression at a range of speeds and flight conditions.
The Concorde used Turbojet engines, which were superior for supersonic flight over turbofan
engines, as they have a much smaller frontal area reducing drag.
The Russian supersonic jet Airliner Tu-144 initially used turbofan engines, but later
changed to turbojet and gained a significant increase in efficiency.
But todays engines are much more efficient and Boom can reach their performance targets
with a medium bypass turbofan engine, which also reduces the noise on takeoff and landing.
An important trait to allow the plane to land in many airports.
Boom isn’t just some concept plane like the Aurora D8, shown previously.
It is an actual plane in development and the company closed a $33 million dollar round
of investment last March .If the flight tests of the 1/3rd scale XB-1
next year prove successful, we could be in for a reemergence of civilian supersonic flight
in the near future.
Thanks for watching.
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