What is Mach 1?- An introduction to supersonic flight

"The physical characteristics of subsonic flow and supersonic flow are totally different- a contrast as striking as that between day and night"- John D.Anderson

What is Mach 1

Mach 1 is essentially the speed of sound but this itself is changes and therefore the definition of mach 1 becomes open ended and subject to other variables. Even through the earths atmosphere the speed of sound changes so mach 1 is defined as:

'the ratio of the speed of a body to the speed of sound in the surrounding medium. It is often used with a numeral (as Mach 1, Mach 2, etc.) to indicate the speed of sound, twice the speed of sound, etc.'

It is specifically defined as a ratio is due to the fact that the speed of sound varies from one location to the next, due to differences in temperature and air density.

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Now there are some key differentiations to make:

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-Subsonic flight: This is what we typically see in commercial airlines and most current flight. Subsonic flight means that the plane is travelling at speeds below mach 1

-Transonic flight: This is when there is local flow over some parts of the aircraft which exceed mach 1 but a significant part of the plane is still below mach 1. For example, some parts of the wing may exceed mach 1 due to aerodynamic properties

-Supersonic flight: This refers to flight consistently above mach 1 and below mach 5. The fastest manned airplane in service was the SR-71 Blackbird, which had a top speed 3540 km/h (Mach 3+)

-Hypersonic flight: This sort speed is very much still new to engineers and the huge array of implications in travelling 5 or more times the speed of sound are excessively complicated with current knowledge and technology.

Nevertheless, there have been various attempts at hypersonic flight but these have mainly been experimental.

At these speeds, not only does the air friction induce heat, but conventional technology and engines are no longer sufficient. I will look at the science and implications behind hypersonic flight in a future blog.

Currently, the fastest manned hypersonic aircraft is the NASA X-15, which was a hypersonic rocket-powered aircraft. It reached a top speed of 7,274 km/h (Mach 6)

The fastest unmanned hypersonic aircraft was the experimental NASA X-43. This miniature plane was only 3.65 m long with a wingspan of 1.5 m. It had a top speed of 10,617 km (Mach 9.6)

While these are the current records set by aircraft, it is worth noting that rockets have achieved speeds significantly higher than this. The escape velocity from earths atmosphere is 40,270 km/h; 25,020 mph, but rockets such as the Atlas V broke the speed record for fastest rocket launch when the rocket carrying NASA's New Horizons spacecraft hit 47,000 miles per hour; 75200 km/h (over mach 100 relative to sea level at 20 degrees celsius) .I will look at space travel and rockets in a future blog.

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High speed civil transport (HSCT)

The Anglo- French Concorde is one of the first examples where supersonic flight has been used purely for civilian as oppose to military reasons. The design specifications of this project called for a plane that could:

-cruise at mach 2.4

-range of 5000 miles

-carry 300 passengers

Achieving all the above simultaneously would be an extreme design challenge especially due to the severe effect on airplane performance associated with supersonic speeds: There would most notably be a sharp decrease in Lift to drag ratio thus resulting in a shortfall in range. As a result of this fundamental issue, the observation can be made that given all conditions are constant, the range of a plane travelling at supersonic speeds is significantly smaller than at subsonic speeds.

In addition to this, to be a viable option, the engine of the plane is also to produce the lowest possible thrust-specific fuel consumption while at the same time producing an acceptable and environmentally friendly amount of pollutants

Furthermore, during take off and landing, the engine noise has to be acceptable and their is a certain degree of social responsibility in this aspect of design. However, when designing supersonic planes this is a problem as even at low speeds, exhaust jet velocities are large and therefore noisy. Consequently, during its time in service, the Concorde was only permitted to take off and land at certain times during the day.

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To conclude, in this post I have barely scratched the surface of this vast field of supersonic flight and it is a subject I will continue to explore in future blogs, more closely looking at specific aspects of travel beyond Mach 1.

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Sources used:

-https://www.livescience.com/39829-fastest-military-airplanes.html

-'Introduction To Flight' - John D. Anderson