In the previous explanations, we looked at the evolution of aircraft reciprocating engines and propellers.
This time, let's take a look at the birth of the jet engine, which has become the mainstream aircraft engine in modern times.
The beginnings of the jet engine
The concept of the jet engine itself was proposed by an Englishman in 1791, but it took a considerable amount of time before it was actually realized.
Even the laboratory-level prototypes were only operational by Norwegians in 1903, about 100 years after the principle was proposed.
However, as the military importance of aircraft increased after 1900, it began to be seriously researched as a next-generation power source.
The countries that were particularly enthusiastic about this research were Britain and Germany, which began serious work on it in the 1930s.
What's interesting here is that, although not entirely accurate, in various contexts...In Britain, centrifugal jet engines were dominant, while in Germany, axial-flow jet engines became the norm.
Here, I'll briefly explain the compressor (turbine, or windmill), which is essential not only for jet engines but also for superchargers.
There are various types of compressors.This article introduces centrifugal and axial compressors, which are the mainstream types of compressors used in jet engines and superchargers.
Centrifugal compressor
Even today, centrifugal compressors remain the mainstream for automotive cooling pumps and superchargers.
To briefly explain centrifugal compressors, firstThe shape of a compressor is a disc with several walls standing on it, and these walls act as blades.
This is called an impeller.
Since it's difficult to understand from a picture, I've included a real-life example for reference.
This is a coolant pump for a car engine.
Naturally, this impeller alone cannot compress air, so it is placed inside a spiral-shaped case like the one shown below.
That covers the structure; next, I will explain the principle of how fluids such as air are compressed.
in this wayBecause the fluid is pushed out in a circumferential direction relative to the direction of suction, it is called a centrifugal compressor.
The air gains kinetic energy from the rotation of the impeller (windmill), so the pressure at the outlet becomes higher than that at the inlet.
This principle is used to compress the air.
Interestingly, if you don't rotate the impeller (windmill) and instead introduce high-pressure air from the outlet, the impeller (windmill) will rotate and the reduced-pressure air will exit from the inlet.
To simplifyAn impeller is a device that converts rotational energy into fluid pressure when it rotates, and conversely, it converts pressure into rotational energy when high-pressure air is introduced.
Next, let's briefly explain axial-flow compressors.
axial compressor
Axial flow compressorThe principle is very simple, and to put it somewhat bluntly, a fan becomes an axial-flow compressor.
To briefly explain the structure, it's essentially the same as an airplane propeller, consisting of several diagonally angled plates attached in a circular pattern.
You can imagine it as a propeller with thinner blades and a larger number of blades.
The operating principle of this compressor is also extremely simple, just like a propeller. (V represents the flow velocity (speed of the air)).
Because the direction of suction and the direction of air compression are the same, this type of airflow system is called an axial flow system.
This tooJust like with centrifugal compressors, rotating an axial-flow compressor compresses the air. Conversely, blowing high-pressure air into an axial-flow compressor causes the blades to rotate, generating rotational energy.
Basically, these two types of compressors are the main ones used in transportation equipment.
Centrifugal compressor vs. axial flow compressor
Here's a brief overview of the advantages and disadvantages of centrifugal and axial flow dynamometers.
The advantages of centrifugal pumps are as follows:
• Even with miniaturization, efficiency does not decrease significantly.
• The structure is simple and has few parts. The important impeller can also be manufactured relatively easily.
- It's somewhat resistant to foreign object contamination.
- It can handle a wide range of flow rates for a given impeller rotation speed.
- Compared to axial flow systems, it has a lower flow rate per unit frontal area.
- Increasing the compression force requires multiple stages, but the structure is more complex compared to axial flow systems.
To increase compression force in a single stage, the only options are to increase the frontal area or increase the impeller's rotation speed.
Increasing the impeller's rotation speed concentrates centrifugal force on a single point on the impeller shaft, making it prone to breakage.
And so on.
This is, unintentionally, the opposite of axial flow.
Summary
Centrifugal compressors maintain their efficiency even when miniaturized, but it is difficult to increase their size to further enhance compression force.
Axial flow compressors lose efficiency when miniaturized. However, they are easier to make larger and multi-stage to increase compression force.
Therefore, when a relatively small size and low to medium output are required, centrifugal compressors are common. Moreover, they are inexpensive.
On the other hand, for relatively large-scale applications requiring medium to ultra-high output, axial-flow compressors are more common.
This concludes the general explanation of the compressor, the heart of a jet engine.
And then, finally, the jet engine was invented.
The birth of the jet engine
Earlier, I explained that Germany and Britain played a central role in the development of jet engines in the 1930s.
Germany was the first to put it into practical use, by just a hair's breadth.
First, let me introduce German jet engines.
The birth of the axial-flow jet engine (Germany)
In 1939, the renowned German aircraft company Junkers decided to embark on the full-scale development of the jet engine.
Instead of the relatively simpler centrifugal jet engine, we decided to take the more challenging path of developing an axial-flow jet engine, with future high-power output in mind.
As you may have noticed, the compressor of a jet engine isSince it uses almost the same technology as a supercharger for a reciprocating engine, development was led by Franz, a supercharger expert at Junkers.
Interestingly, around the same time, BMW also began developing axial-flow jet engines.
BMW is originally an aircraft engine manufacturer. I believe BMW is an abbreviation for Bavarian Motor Works.
They were completed around the same time, and mass production began in early 1944.
The jet engine was a magnificent 8-stage compression engine, as shown in the following diagram (Junkers Jumo 004).
Despite numerous challenges, we managed to reach mass production. In particular, we struggled with the development of the shaft and compressor due to the high durability and operating speed of the axial flow compressor used to extract power from the high-temperature exhaust gases.
In terms of performance, it had an air compression ratio of 3.14 and a thrust of 910 kg (at 8700 rpm).
Here, thrust can be simply understood as the force that, if you were to operate a jet engine vertically, would generate just enough force to lift a 910kg weight.
This engine is quite powerful because it maintains this power throughout the entire operation.
BMW also completed a jet engine (BMW 003) with a similar structure and performance around the same time.
Despite various political interferences, this engine was eventually deployed in combat, with two units (1820 kg thrust) mounted on the wings of the Messerschmitt Me262 (Schwalbe means swallow).
Although it had some drawbacks inherent to jet engines, it achieved its initial goals of high-altitude and high-speed flight to an astonishing degree.
Its high-altitude performance exceeded 12900m, and its top speed was 869km/h, surpassing all reciprocating engine aircraft of the time.
The commander of the jet fighter squadron at the time, Major General Adolf Galland, reportedly said of his first flight, "It felt like an angel was pushing me forward" (I don't know if that's true or not).
However, it was already too late, and Germany lost the war.
Next, let's look at the UK.
The birth of the centrifugal jet engine (England)
Like Germany, Britain also began working on jet engine development quite early on.
hereFrank Whitney, a very famous engineer (often called the father of the jet engine)Although they had been working on it since the 1930s and understood the potential of axial flow systems, they stubbornly insisted that centrifugal systems were necessary for their early realization.
After that, they made contact with the automobile manufacturer Rover (the company that developed the famous Mini Cooper), andIt was developed in collaboration with Mr. Hooker, an expert in superchargers for reciprocating engines, just like in Germany.
Due to various circumstances, it's difficult to determine who first succeeded in commercializing a centrifugal jet engine, as there's too much conflicting information. However, at least one of the earliest commercially viable centrifugal jet engines was the Halford H1 from de Havilland (later the developer and manufacturer of the Comet) (first flight in March 1943).
Similar to Germany, we faced considerable challenges with the axial-flow compressor used to extract the rotational force from the shaft on the exhaust side, but we eventually managed to bring it to mass production.
Although I couldn't find detailed engine data, the air compression ratio was probably around 3.3 and the thrust was around 900 kg, which is comparable to German engines.
Compared to German axial-flow engines, it had a shorter front-to-back length but a larger frontal area. This was a major obstacle for later centrifugal jet engines, as frontal area significantly impacts flight drag.
These were immediately installed by embedding them in both wings of the Gloucester Meteor.
This one also had performance very similar to the German model, with high-altitude performance at around 13000m and a top speed of around 900km/h.
It was deployed in July 1944 and was a prized jet fighter, so it was mainly used for the air defense of mainland Britain, and therefore saw very little actual combat.
However, it seems that they occasionally shot down V1 missiles (pulse jet engine aircraft bombs) flying in from the continent.
It became quite widespread after World War II and played an active role in the Korean War.
This marks the beginning of the jet engine era.
Summary of the early days of jet engines
In this way, axial-flow and centrifugal jet engines were realized almost simultaneously, at just the right time.
Let's summarize the early jet engines here.
Since the diagram is simple, let's look at an axial-flow jet engine.
At that time, reciprocating engines evolved not only in the engine itself, but also in peripheral components such as superchargers and propellers, which underwent explosive advancements.
The evolution of superchargers, in particular, has been tremendous.I started thinking that maybe it would be better to just burn the compressed air from the windmill directly in the reciprocating engine, rather than having to feed it into the engine.
Yes, this is a jet engine (both axial and centrifugal).
until nowThe reciprocating part is gone, and the combustion chamber is attached to the supercharger—a complete reversal of roles.That happened.
This is truly a paradigm shift.
Although there were various difficult technical challenges, we managed to overcome them by trying.
Moreover, while thrust was previously generated by attaching a propeller to a reciprocating engine, jet engines generate thrust by directly releasing the force of volume expansion.
This is amazing.The then-state-of-the-art reciprocating engine and propeller became obsolete in an instant; it's no exaggeration to say that a revolution had occurred.
Incidentally, this type of jet engine is correctly called a turbojet engine.
The amazing and challenging aspects of jet engines
This jet engine overcame two limitations at once: the altitude limit imposed by thin air and the speed limit imposed by propellers.
moreoverIn a 4-stroke reciprocating engine, combustion energy can only be extracted during half a rotation of the crankshaft.(The rest are rotating due to inertia)Because jet engines are constantly burning, the amount of energy they can extract per unit of time is significantly higher.
Furthermore, reciprocating engines convert reciprocating motion into rotational motion using pistons, connecting rods, and crankshafts.Jet engines are mechanically efficient because they operate using rotational motion from the start.(Of course, the rotational speed cannot be used as is, so it is reduced using gears or other means.)
It's constantly burning while it's running.
This dramatically improved the altitude, speed, and overall performance of the aircraft. It was a technological revolution.
Just when Japan was about to gain an advantage in the war, it ended with them giving up.
This technological innovation evolved and was realized in less than 10 years, including the conceptualization period. While the reciprocating engine was undergoing a dinosaur-like evolution, at some point it transformed into the jet engine, and the reciprocating part disappeared.
It is truly terrifying.
However, jet engines have drawbacks as well as advantages.
A typical example would be...It has extremely poor fuel efficiency (like it's leaking fuel), extremely poor responsiveness, and is incredibly difficult to manufacture.
Even if you only consider the act of makingThe material is constantly exposed to high temperatures, and the rotation speed of the shaft is also quite high.Because it's expensive, it requires advanced materials and rare resources.A very high level of precision is required for the machining accuracy of the parts.
However, the Korean War (1951) broke out immediately after the war, and during that period, the jet-powered aircraft industry rapidly advanced.
It was around that time that the world's first four-engine jet-powered passenger aircraft, developed by the British Empire as part of its revitalization efforts, made its maiden flight. That was in 1949.
Yes, this is the future Comet.
It was against this historical backdrop that the four-engine jet-engine passenger aircraft, the Comet, was about to make its debut.
After all, the latest military aircraft at the time had an altitude limit of 12000m, and the Comet was roughly the same.
Its cruising speed was 750 km/h, which was roughly the same as the top speed of the most advanced propeller-driven fighter planes of the time.
A major national undertaking for the British Empire was about to begin.
with thisThe difficulty of this world lies in the fact that even if a high-altitude flight engine is completed, it doesn't necessarily mean that it can take to the skies.
It's hard to compare it to in modern terms, but it's like having a passenger plane that can cruise at Mach 2.0, while the latest fighter jets have a top speed of around Mach 2.0 (current passenger planes cruise at around Mach 0, which is about 900 km/h at sea level). We might even be able to travel to space more casually.
Next time, we'll introduce an overview of the modern evolution of reciprocating engines, which are no longer the main engines used in aircraft, and the mechanism of superchargers from World War II.
To those who found this article helpful in understanding design:
Since we're on the subject, I'd like to recommend a book that's essential for mechanical design.
To be honest, the content is extremely unhelpful, but it can be used like a dictionary when you forget the details. If you read this article, you should be able to understand the content and use it effectively. It also includes commonly used standards, making it quite useful.
If you don't already own one, I highly recommend getting one, even though it's a bit pricey. However, new ones are expensive, so if you're considering buying a used one, I strongly recommend checking that the surface roughness conforms to the new JIS standard.
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