How It Works-Amazing Vehicles

Shattering Mach 1

Supersonic aerodynamics are much more complex than subsonic

aerodynamics for a variet y of reasons, the foremost being

breaking through the transonic envelope (around Mach 0.85-1.2).

This is because to pass through this speed range supersonic jets

require several times greater thrust to counteract the extreme

drag, a factor that raises two key issues: shock waves and heat.

Shockwaves come from the passage of air (with positive,

negative or normal pressures) around the fuselage, with each part

of the aircraft affecting its progress. As such, while air is bent

around the thin fuselage with minimal effect, as it reaches the

wings – a huge change in the cross-sectional area of the jet – it

causes shock waves along the plane’s body. The resulting waves

formed at these points bleed away a considerable amount of

energy, and create a very powerful form of drag called wave drag.

To mitigate this, any supersonic jet design must allow for a

smooth-as-possible change in cross-sectional areas, with the

wings fl uidly cur ving out from the fuselage.

Heat is the other big concern. Sustained supersonic fl ight – as a

by-product of the drag it generates – causes all of its materials to

experience rapid and prolonged heat, with individual parts

sometimes reaching in excess of 300 degrees Celsius (572 degrees

Fahrenheit). As such, conventional subsonic materials like

duraluminium (or dural) are infeasible for a supersonic jet, as they

experience plastic deformation at high temperatures. To counter

this, harder, heat-resistant materials such as titanium and

stainless steel are called for. However, in many cases these can

push up the overall weight of the aircraft, so reaching a workable

compromise between heat resistance and weight is the key.

There is far more to creating a supersonic aircraft than

simply strapping larger engines to a subsonic fuselage...

Sonic boom science

Sonic booms are caused as, when an object

passes through the air, it generates a series

of pressure waves. These pressure waves

travel at the speed of sound and increase in

compaction the closer the object is to Mach 1

• approximately 1,225 kilometres (761 miles)
  per hour. When an object is travelling at the
  speed of sound (ie Mach 1), however, the
  sound waves become so compressed that
  they form a single shockwave, which for

aircraft, is then shaped into a Mach cone.

The Mach cone has a region of high pressure

at its tip – before the nose of the aircraft –

and a negative pressure at its tail, w ith air

pressure behind the cone normal. As the

aeroplane passes through these var y ing

areas of pressure, the sudden changes

create t wo distinctive ‘booms’: one for the

high-to-low pressure shift and another for

the low-to-normal transition.

What are sonic booms and how are they generated?

Streams of dye are used to show the f low of water over the surface

of a supersonic jet. The f low of water over the surface of the f uselage

indicates what the airf low would be like over a f ull-si zed aircraf t

This shows the airf low over a supersonic jet’s surface (including turbulence over the

w ing). The colour of the lines shows the air speed from red (fastest) to blue (slowest).

In addition, the f uselage colour indicates its temperature, from blue (coolest) to red

(hottest). Supersonic jet f uselages can be heated to over 100 ̊C (212 ̊F) by air friction

2x © SPL

DID YOU KNOW? The speed of sound in air is approximately 1,225km/h (761mph)