# How fast is the Mach speed in hypersonic speed? How much is it converted into kilometers per hour?

2021-04-07 | Flower planter on Star Sea Road Original |

Mach is a common speed unit, which is often expressed in the performance parameters of aircraft. For example, the performance parameters of the SR-71 Blackbird reconnaissance aircraft once equipped in the United States are:

Maximum speed Mach 3.28 3,492km/h

Climb rate≥60m/s

Practical ceiling: 24,285 meters

Maximum ceiling: 25,900 meters

Maximum range: 5,400 kilometers

Push-to-weight ratio: 0.382:1

I believe that friends who are familiar with aircraft performance will be amazed by these data, because the speed of Mach 3.28 is still a very high speed even now. For example, the top speed of F22 is also Mach 1.95 public data, actual data is top secret.The maximum speed is unknown, but it is absolutely impossible to exceed the speed of SR-71.

Mach is a speed unit. Mach 1 is double the speed of sound, but you must have found a problem. The maximum speed of SR-71 is Mach 3.28 3,492km/h. There seems to be some problem. The speed of sound is 340 km/s.Based on this calculation, the maximum speed is only Mach 2.85. Where is Mach 3.28? Is the Wiki data wrong?

In fact, the Wiki data is not wrong. Mach is just a concept of multiples of the speed of sound, and the speed of sound will change!

Mach and the famous Ernst Mach

Ernst Mach is an Austrian-Czech experimental physicist. His research field is the law of light propagation and supersonic phenomena. In 1887, he published on the sound effect of supersonic projectile motion namely sonic boomHe deduced and experimentally verified the existence of a conical shock wave at the tip of a high-speed projectile, which is a shock wave, and the ratio of fluid velocity to local sound velocity vp/vs, which is now called the Mach number.

Ernst Mach

The supersonic field of Mach’s research is almost the only way for modern aircraft. Fighter designers in World War II discovered that when the speed of the fighter's propeller tip is close to the speed of sound, the efficiency drops sharply, and the flat wing has great resistance at high speeds.Wait, so to enter the supersonic field in the future, there must be revolutionary changes, so later jet engines and swept wings appeared!

The speed of sound transmission in the medium is the speed of sound. It does not have an exact number and will change with the density of the medium. For example, the speed of sound waves in steel is 5900 m/s longitudinal waves, and the speed in copper materials4900 m/s, the longitudinal wave speed in water is 1497 m/s 25°C, distilled water, and the speed in sea water is slightly higher at 1531 m/s.

The speed of propagation in the air will decrease with the increase in altitude. For example, the speed of sound at sea level is about 340/s, but at the ceiling of the SR-71 Blackbird reconnaissance plane at 24,285 meters, the speed of sound is 295 meters/s.Therefore, using this as a standard, the calculation is 3,492 kilometers per hour, which is exactly equal to 3.28 Mach.

A bright and dark wake phenomenon occurs behind the tail nozzle of jet engines or rockets and supersonic ramjets, called Mach rings. The great god Ernst Mach was the first to explain this phenomenon, soNamed Mach Ring in memory of him.

When the Pratt & Whitney J58 engine used by the SR-71 Blackbird reconnaissance aircraft turned on the afterburner, the high-speed and high-temperature exhaust gas formed a clear Mach ring

Mach ring principle

The flow field in the wake of the engine is very complicated. When the high-temperature and high-pressure wake is discharged into the atmosphere from the tail nozzle, it will experience a sharp change in local density. The exhaust will go through an expansion process, and the atmospheric pressure will compress the wake.Therefore, inclined shock waves will be formed, and will alternate repeatedly to form a continuous light and dark wake.

Modern hypersonic aircraft are mainly divided into two categories, one is the gliding hypersonic aircraft, and the other is the aircraft propelled by hypersonic engines. The former is propelled by rocket engines to high altitude and uses its potential energy to glide at hypersonic speed.This kind of aircraft, its aerodynamic shape determines its flight characteristics.

There are two types of hypersonic engines, one is sub-combustion stamping, and the other is super-combustion stamping. The airflow of the former is compressed and passes through the combustion chamber at subsonic speed. Its combustion characteristics are relatively easy to control. Generally, the current stampingMost of the engines are sub-fuel stamping.

Another type of scrambling is the compressed airflow passing through the combustion chamber at supersonic speed. The speed of the airflow in the combustion chamber exceeds the speed of sound, and the residence time is extremely short. For example, the difficulty is equivalent to that of igniting a typhoon in a level 12 typhoon.A match must be burned stably, but the difficulty of stabilizing combustion in the combustion chamber of a transcendent ram is obviously much more difficult than this.

Of course, there is a more promising type of hypersonic engine that does not belong to the stamping type, but the pulse detonation type. This type of engine does not use compressed gas, but is achieved through a mixture of gas and fuel explosion.

Future aerospace aircraft engines are either turbojet + ram, or knock engines. Rocket engines need to carry fuel and oxidizer at the same time, which takes up a lot of space, while ram and pulse detonation engines can use the oxygen in the atmosphere, butIt seems that no matter what kind of difficulty is extremely high, so it has not been fully realized until now.

The spacecraft returns to the earth and the meteoroids pass through the atmosphere. Many books will tell you that the reason for this is the high temperature generated by the friction between the spacecraft and the atmosphere. Of course, this explanation is not entirely wrong!

The temperature distribution around the fuselage during supersonic flight of SR-71

But the main source of heat is the hypersonic shock wave heating, because when the object is moving at a speed far exceeding the speed of sound, the air molecules cannot "notify" the surrounding air molecules in time, so they will be squeezed together to form a compression surface.

This compression surface will spread in a roughly inversely proportional manner to form a shock wave. When it is in contact with a moving object directly in front, high temperature will be transmitted to the object to form a high temperature and high pressure area, and some aircraft will use the wing to carry the shock wave.Forming a wave rider, the lift at this time does not come from the so-called angle of attack and Bernoulli effect, but a shock wave.

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