# What is Mach? When is it used, how useful is it? Comparative

What is Mach? In the simplest of terms, the Mach number is a representation of one’s speed in relation to the speed of sound. In other words, it is a ratio of the velocity of an object when compared to the speed of sound within that same environment.

Because the speed of sound changes depending on the medium through which it travels, the temperature of that medium, and many other variables, the Mach number is not a definite number, but a ratio used for relative comparison for any object in motion.

## What is Mach?

The Mach number is the ratio of flow velocity after a certain limit of the sound’s speed. In simple words, it is the ratio of the speed of a body to the speed of sound in the surrounding medium.

## Mach Number Formula

The formula of Mach Number is:

M=u/c

Where,

• The Mach number is M
• Based on the limits the local flow velocity is u
• The speed of sound in that medium is c

We can say the speed of sound can be equated to Mach 1 speed. Thus, Mach 0.75 will be 75% of the speed of sound that is also called subsonic, and Mach 1.65 will be 65% faster than the speed of sound, which is also called supersonic.

The Mach number due to the local speed of sound is dependent on the surrounding mediums in specific temperature and pressure. Flow can be determined as an incompressible flow with the help of the Mach number. The medium can either be a liquid or a gas.

The medium can be flowing, whereas the boundary may be stable, or the boundary may be travelling in a medium that is at rest. The medium and boundary both may be travelling with a certain speed, but their velocities concerning each other matter.

The medium may be channelled through several devices such as wind tunnels or may be immersed in the medium. The Mach number is termed as a dimensionless number because it is a ratio of two speeds.

An Austrian philosopher and physicist Ernst Mach is after whom the Mach number is named. Due to it being a dimensionless quantity and not a measurable unit, the number is put after the term Mach for instance, Mach 4 and not 4 Mach.

## Mach Number Classification

Speed below the speed of sound is termed as subsonic whereas speed above the speed of sound is termed as supersonic, although scientists practising aerodynamics most of the time use these terms to describe a particular range of Mach values.

### Subsonic

Commercial aircraft with aerodynamic features such as rounded nose and leading edges. The Mach is below 0.8.

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### Transonic

Aircraft that are built with swept wings. The Mach value is between 0.8-2.1.

### Supersonic

The aircraft created to go supersonic have a definite design, it has the complete movement of the canards, thin aerofoil sections, and sharp edges. The Mach levels are between 1.2 and 5.0.

### Hypersonic

These planes have several distinctive features, such as cooled nickel-titanium skin and small wings. The Mach values are between 5.0 and 10.0. The U.S. plane X-15 created the world record of flying at Mach 6.72.

### High-Hypersonic

The Mach levels are between 10.0 -25.0. When flying at such huge speeds, thermal controls becomes an integral portion of the design. The hotness of the surface must be considered beforehand.

### Re-entry speeds

The Mach speed is above 25.0. This does not require wings and a blunt design.

## Why the Name Mach?

Ernst Mach was an influential scientist in the nineteenth century. He conducted important work in the area of shock waves, and the behavior of light in different media. Additionally, he published important information about his observations of objects traveling at supersonic speeds.

Swiss aeronautical engineer, Jakob Ackeret, one of the most influential aviation experts of the 20th century, proposed the use of Mach’s name as the measurement of the Mach number. This was done as a tribute to Ernst Mach’s contribution to aeronautics.

## When Is It Used, How Useful Is It?

Though we can use the Mach number to measure the speed of any object or vehicle through any medium, we typically use it in aviation for vehicles traveling through the air. This is because it is more useful to measure a vehicle’s speed in units that are both accurate and comprehensible.

Vehicles in aviation can more easily approach speeds that we can measure in terms of the speed of sound. Miles-per-hour does not translate very well to our intuition of travel through air.

Land vehicles do not regularly approach these speeds, so we use a lower unit of speed (miles, kilometers, etc.). Famous exceptions to this include vehicles that are built to break land speed records. These vehicles regularly approach and surpass Mach 1.

Nautical vehicles calculate their speed using knots. This is useful in the Ocean where a vehicle’s surroundings are in constant motion. Due to the resistance and friction inherent in water travel, these vehicles do not approach Mach 1.

The Mach number cannot be used in space. Since sound does not travel through space, it is impossible to calculate the Mach number.

## How to Use Mach Number Correctly

It is important to understand that the Mach number is a relative measure of speed. It is not an absolute unit of velocity or distance.

When calculating the Mach number, make sure you use the speed of sound within a similar medium. For example, though an object might travel at 500 miles per hour on Earth and Mars, the Mach number in each location will be different.

It is also best to use the Mach number for objects or vehicles that travel at speeds that make sense to translate into a Mach number. It is not very helpful to describe the speed of a dog in terms of Mach, for instance.

When using the Mach number, it is important to remember that the number follows the unit of measure. (Mach 1, not 1 Mach).

## Supersonic And Hypersonic Speeds

That covers Mach 1, but what about the other Mach numbers? Put simply, Mach 2 means that an aircraft is traveling twice the speed of sound. Mache 3 means an aircraft is traveling three times the speed of sound, and so on. Remember that like Mach 1 these other Mach speeds are relative, they reflect two or three times the speed of sound in those specific conditions.

In general, the Mach number of an aircraft can be determined by dividing the velocity of the craft by the speed of sound at the given altitude of the craft.

While anything below Mach 1 is considered to be Subsonic, aircraft that fly at approximately Mach 1 are considered to be Transonic craft. This covers craft moving at 0.8 to 1.3 Mach.

The compression effect of sound comes into consideration around Mach 1, and because of the high drag that is created by the compression Transonic aircraft typically have swept wings to deal with the drag.

From 1.3 Mach to 5.0 Mach exists the Supersonic range of speed, air behaves quite differently above Mach 1, so planes that fly at Supersonic speeds must be extremely aerodynamic with sharp edges and moveable canards.

Continuous shockwaves will be generated by planes moving at Supersonic speeds, necessitating their slick design. Planes that move at Supersonic speeds must also be designed with aerodynamic heating in mind.

The first-ever Supersonic aircraft was the Bell X-1, an experimental rocket-powered plane. Most aircraft that will exceed Mach 1 will be either experimental or military craft. There is seldom a need for a civilian craft to go Mach 1 or higher, although there have been attempts at making Supersonic passenger travel a real thing.

The Concorde was a supersonic passenger plane, but ultimately it was decided that the operational cost did not justify its use and the project was shelved.

The fighter jets that are capable of moving at Supersonic speeds, such as the F-15 Eagle or the F-35 Lightning II, must make concessions in their design so that they can still move and fight adequately at lower speeds. “True” Supersonic jets (jets designed for the main purpose of moving at Supersonic speeds) include the aforementioned Concorde and the SR-71 Blackbird.

The range of speed that exists from Mach 5 to Mach 10 is considered to be hypersonic. At Hypersonic speeds, some of a plane’s energy will actually excite the chemical bonds that make up molecules of oxygen and nitrogen in the air. This means that planes designed to move this fast require that the chemistry of the air be taken into account when designing them.

Hypersonic aircraft include the X-15 experimental research craft and the Boeing X-37 Orbital Test Vehicle. The X-15 set the record for the highest speed ever achieved by a manned aircraft at almost 7,237 kilometers an hour (4,520 miles an hour) in 1967, piloted by William J. Knight.

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Meanwhile, the X-37 is an experimental vehicle used by Boeing to test reusable space technologies and conduct experiments in space which can then be returned to Earth.

Any speed above Mach 10 is considered to be in the High-Hypersonic range. At these speeds, the aerodynamic form of a vehicle takes a bit of a backseat to thermal control, as the sheer friction in the atmosphere can heat up a craft to extreme temperatures. High-Hypersonic designs are usually much blunter than regular Hypersonic or Supersonic designs.

## What Mach Is the Speed of Light?

The speed of light is a universal constant. This means that although light may appear to move slower, or propagate more slowly through different states of matter, it does not. It is always traveling at the same speed of 670,616,629 miles per hour, or c.

Using our equation to calculate the Mach number of the speed of light, we get Mach 874,030.49.

As we can see, using Mach to calculate anything relative to the speed of light is beyond reasonable comprehension or comparison. We usually only use the speed of light with anything related to stellar and interstellar sciences and subatomic colliders.

## Comparable Speeds of Popular Objects

### Commercial Airliner

The Boeing 747 can reach a cruise speed of 559 miles per hour, which is equal to Mach 0.85.

### Space Shuttle

During takeoff, the space shuttle needed to reach a speed of 18,000 miles per hour in order to enter a stable orbit around the Earth. This is equivalent to Mach 23.4.

It began reentry into Earth’s atmosphere at an altitude of 400,000 feet. It normally reentered at speeds over 19,000 miles per hour, or Mach 25.

### Bullets

The maximum speed (muzzle velocity) an average rifle bullet can reach is around 1,700 miles per hour. Equivalent to Mach 2.2.

### Whips

The whip was one of the first man-made objects to break the sound barrier. The end of the whip can reach speeds in excess of 1,434 miles per hour, or Mach 1.8. The classic crack you hear while using a whip is actually a miniature sonic boom produced by the whip itself breaking the sound barrier.

## FAQs

### What’s the highest Mach speed?

The Guinness World Records awarded NASA’s X-43A Scramjet a new world speed record for jet-engine aircraft, Mach 9.6, or about 7,000 miles per hour. The X-43A set new standards and set its own world record on its third and final flight on November 16, 2004.

### Does pressure affect Mach Number?

As the Mach number increases, so does the intensity of the shock wave, and the Mach cone becomes narrower and narrower. As the fluid flow crosses the shock wave, it slows down and increases in temperature, pressure, and density.

### Does temperature affect Mach Number?

The warmer the air, the lower the density, the slower the speed of sound, and the faster the speed on the ground to achieve the same Mach number.

### Can anything go Mach 10?

On November 16, 2004, NASA made history by launching the first-ever air-breathing hypersonic aircraft, the X-43A, into the atmosphere and reaching Mach 10 speeds. Separated from the booster, the X-43A used scramjet propulsion to accelerate to about 110,000 feet at nearly 10 times the speed of sound (7000 MPH).

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