In the previous article, I explained the design philosophy for machinery like pressurized chambers, where a malfunction would immediately have a serious impact on safety.
This time, we'll take a more concrete look at the difficulties involved in designing a massive pressurized chamber like the Comet.
The design and manufacture of this pressurized chamber remains an extremely difficult task, both in the 1950s when the Comet was developed and in modern times.
Even today, it is likely that only a very limited number of companies are capable of designing and manufacturing such a product.
This machine is the result of steady progress and the transmission of technology within a limited number of companies and organizations.
So, let's take Comet as an example to see just how difficult it actually is.
General design conditions for the Comet's pressurized chamber
Let's consider the general requirements necessary for actually designing a pressurized chamber.
First, the conditions are roughly as follows.
1. Maintain a constant pressure in an environment that continuously changes between atmospheric pressure (1 atm) at ground level, 20°C, and pressure below 0.1 atm at an altitude of 12000m, and around -40°C.
2. Install windows or entrances in the pressurized chamber.
3. Unlike military aircraft (which typically carry around 10 people), it should be large enough to accommodate around 50 people, including pilots and passengers.
3. The power source (engine) must function normally under all conditions. In other words, it must always function normally, regardless of whether the engine is at low power during startup or at maximum power during high-speed flight.
4. Make it as lightweight as possible while meeting all the requirements.
Those are harsh conditions.
First, let me briefly explain the loads placed on the pressurized chamber.
Consider the load on the pressurized chamber.
First, the pressure inside the pressurized chamber is determined by setting it to half the difference between the ground pressure of 1 atm (100 kPa) and the operating maximum altitude of 12000 m (0.2 atm (20 kPa)), which is 0.6 atm (60 kPa).
The reason is that the pressure at ground level is 100kPa, which is higher than the 60kPa inside the pressurized chamber, resulting in a difference of +40kPa.
At an altitude of 12000m, the pressure in the pressurized chamber is 60kPa, while atmospheric pressure is 20kPa, resulting in a -40kPa pressure. Therefore, the load on the entire pressurized chamber is roughly the same at both altitudes, and the pressure is set to 60kPa.
This process repeats itself every time the aircraft flies.
In short, as explained in "Materials Mechanics for Beginners 22: Fatigue Fracture," the average pressure (average load) is 0 kPa (equilibrium at an altitude of 6000 m), and the repeated pressure (amplitude load) becomes ±40 kPa.
40kPa is quite a large number. To put it in terms that's easier to understand, it's like repeatedly pushing and pulling a board with an area of $1m²$ with a force of 4000kg.
To put it simply, a load of 4 tons is applied.
Moreover, since it's a passenger cabin and the pressurized chamber is large enough to accommodate 50 people, the surface area (or area) is quite large, so the total load applied is naturally large (pressure is the load per unit area).
In other words, it can potentially cause fatigue failure due to repeated pressure.
Another reason for maintaining a pressurized chamber at 0.6 atm is that while humans may feel slightly breathless at 0.6 atm, it does not impair their ability to perform vital functions.
This represents the basic load on the pressurized chamber.
This allows them to decide on the materials to be used in the pressurized chamber, and Comet is doing something quite remarkable in many ways.
For the Comet, it appears they selected ultra-super duralumin, a high-grade aluminum alloy in the 7000 series, as the material for the pressurized chamber to meet condition 4's requirement of being lightweight.
This is where the difficulty lies. The tensile strength of high-grade steel, a representative of iron, is around 700 MPa, while even the highest-grade duralumin has a tensile strength of only about 400 MPa, meaning its strength is less than 6% of that.
Even though aluminum has a specific gravity of 2.8 and iron has a specific gravity of 7.8, making it lighter, using a material with less than 6% of the strength for the same volume in a pressurized chamber (pressure vessel) is extremely risky.
It's like trying to build a submarine out of aluminum alloy. Something like that would collapse and sink immediately.
Normally, for safety reasons, pressure vessels (pressurized chambers) are made of steel.
The author is too afraid to have the courage to use aluminum alloy in such a crucial pressurized cabin.
Moreover, even though aluminum alloys have high tensile strength, their elastic range is shorter than that of steel, so their fatigue limit is not particularly high.
For details on tensile strength, see below.
Details on fatigue limits:
moreoverAirplanes need to be as light and compact as possible to reduce flight drag, so the thickness of the pressurized cabin walls cannot be made very large. It is likely that the design is quite compromised in terms of strength.
Next, let's consider the structure of doors and windows, which are essential for passenger aircraft.
Consider the necessary window and door structures for an airplane.
Next, we need to install the windows and doorways specified in condition 2.
According to Pascal's principle, pressure acts evenly everywhere, ensuring that no weak points are overlooked.
In other words, joints like windows and doors are quite dangerous.
Therefore, submarines basically have no windows, and the number and size of openings and closing parts, such as lids, are kept to the bare minimum.
Aside from fighter jets, military aircraft operated by multiple people rarely have windows other than in the pilot's cabin.
However, since it's for passengers, it needs a certain number and size of windows and doors.
I'll only show you the windows.
I think everyone would agree that a simple window installation would look like the following diagram.
So, it will break no matter whether you install it from the outside or the passenger compartment side.
Even if the window area is only $0.25 m²$ (approximately 50cm x 50cm), the load it will bear is ±1000 kg. That's about 1 ton.
If something like that gets caught on a window, it will break immediately in the direction it's being pulled.
Details of fracture caused by bending stress in a single blast can be found here.
Therefore, it has the following structure, which is extremely costly and difficult to manufacture.
Furthermore, the difference in altitude causes the windows to fog up, so an anti-fog gas (probably nitrogen gas) must be sealed inside the windows.
Since bolts or rivets wouldn't work for joining them, I think they'll be glued.
You can probably tell just by looking at it that it's a lot of work. And since it's a passenger plane, there are dozens of these windows. That's why it's heavy and complex, and to make the overall weight down, the thickness of the pressurized cabin walls is likely to be compromised and made thinner.
Moreover, the rubber used for the seals must withstand temperatures ranging from around 20°C at an altitude of 0m to around -40°C at an altitude of 12000m, making it an extremely expensive and difficult-to-produce rubber.
Furthermore, this structure makes it impossible to replace or maintain the windows, so it needs to remain in perfect condition without any maintenance for the entire lifespan of the aircraft, which makes the design and manufacturing process extremely difficult.
Anyways,Even something as simple as a window is incredibly difficult. I don't even want to think about doors.
We've touched on some of the major design challenges of pressurized chambers, but in reality, their manufacture is also extremely difficult.
Next, let's consider the manufacturing method.
Considering how to build a massive pressurized chamber.
Next, manufacturing a pressurized chamber large enough to accommodate 50 people, as per condition 3, is extremely difficult.
First of all, a pressurized chamber is like a sealed container, and it cannot be sealed unless you put a lid on it.
Furthermore, it is virtually impossible to produce a container large enough to accommodate 50 people all at once.
Moreover, while ultra-super duralumin in the 7000 series has the physical properties to be welded (melting materials at high temperatures to join them together), it is difficult to weld.
While welding is technically possible, the parts are so large and precise that the heat generated during welding can deform them or alter their strength.
It was quite a hopeless situation, but as this was a project that would determine Britain's revolution, there was a genius involved.
First, the pressurized chamber was divided into sections and then joined together with epoxy adhesive (Britain had excellent adhesive technology at the time) to complete the tank.
De Havilland, the development and manufacturing company, was particularly confident because they had produced wooden Mosquito fighter planes during the war using adhesives.
It looks easy at first glance, but it's incredibly difficult.
If the divided pressurized chambers are large and numerous, manufacturing tolerances can cause the surfaces where the parts meet to become misaligned, reducing the bonding area and thus lowering the strength.
I don't know how many divisions it was actually divided into, but even if it was divided into nine, a considerable amount of error would accumulate.
For details on the accumulation of tolerances, click here.
Furthermore, the adhesive is problematic because it's difficult to tell if there's enough adhesive on all surfaces to ensure a secure bond.
To put it bluntly, the testing process will become considerably more complicated.
In the worst-case scenario, there are non-destructive testing methods using magnetism or ultrasound, but even then, guaranteeing accuracy is difficult.
Even the slightest defect in the adhesive will be noticed by air pressure, causing leaks or abnormal pressure that can lead to damage.
Since the bonding surface is large, I think they've made grooves for adhesive to accumulate vertically and horizontally, slightly offset from each other on the bonding parts, to ensure a secure bond.
This way, the number of secure bonding surfaces is increased, maintaining strength and airtightness.
Such littleConsideration is something that is difficult to acquire in school or academics (it's not about matching things with formulas or numbers). It's a kind of consideration that you tend to forget unless you gain various experiences.
Not only adhesives like this,Even in engines, my area of expertise, the adhesion properties of liquid gaskets and the like are still considered cutting-edge research topics, demonstrating just how difficult they are.
Let's consider the following condition: the pressurized cabin maintains a constant pressure regardless of the aircraft's condition.
Regardless of the aircraft's condition, the pressurized cabin must maintain a constant pressure.
The final challenge is ensuring that the power source (engine) operates correctly under any conditions.
In other words, whether the engine is idling while the aircraft is parked or at full power while the engine is accelerating, the pressurized cabin must always be maintained at a constant pressure (0.6 atm).
Well, this isn't that difficult; you just need to borrow some power from the engine, connect it to a generator to produce electricity, and then use that electricity to power the wind turbine and valves.
However, it's highly questionable how reliable and dependable the electrical systems were at that time (around 1950).
Furthermore, since I think the only batteries available were likely lead-acid batteries, it must have been quite heavy.
Ultimately, the system becomes heavier, and the resulting strain is then placed on the thickness of the pressurized chamber walls, which are further burdened.
While it's not impossible to adjust the air compressor and valves using a completely mechanical system, even a rough estimate from my own experience would reveal it to be quite complex, so I think it was probably unrealistic even back then.
It seems difficult to control it mechanically without using a fairly complex transmission and multiple clutches.
This should at least give us some hope for the pressurized chamber.
The impressive features of the pressurized cabins that were put into practical use at the time (around 1945) and the Comet aircraft.
Only the United States was able to mass-produce such complex technology during World War II.
Yes, that famous B-29. Frankly speaking, it's on the level of an out-of-place artifact.
I don't know if it's true or not, but apparently, during the war, when B-29s were occasionally shot down over Japan and the crew members who fell to the ground were wearing short sleeves, the Japanese misunderstood and tried to spread the idea that "America is so desperate that they can't even afford to put flight suits on their crews," as a way to boost their morale.
Back then, the B-29s cruised at an altitude of 10000 meters, and the crew compartments were all pressurized and equipped with air conditioning, making them comfortable enough for the crew to wear short sleeves.
Also, according to rumors, there may or may not have been a vending machine selling bottled Coca-Cola in the Kinai region.
Meanwhile, Japan, which was then our country, was also working on the development of pressurized cabins.
However, since they were struggling to develop engines for high-altitude flight in the first place, the priority for pressurized cabins was not very high, and they couldn't put much effort into developing them.
Nevertheless, the army was developing pressurized cabins, but these were for single-seat high-altitude interceptor fighters (Ki-94II), so they were quite small.
It seems they had quite a bit of trouble. They lacked the technology to maintain airtightness, so they supplied excessive amounts of compressed air assuming leaks would occur (which was very wasteful), oil for lubricating the machinery got into the airtight chamber, and since they couldn't build an air conditioner, they had to carry oxygen tanks, among other things.
However, it seems that just as they were finally getting things sorted out and ready to put it into practice, the war ended.
The young engineer in charge of this pressurized chamber was Mr. Hasegawa, whom those familiar with cars might know. After the war, he was a great man who developed the Publica, Japan's first full-fledged monocoque frame automobile, at Toyota.
Even this is considered an out-of-place artifact; the Comet is truly amazing.Building a massive pressurized cabin for 50 passengers just a few years after the B-29 is quite a crazy idea.
The B-29 is also impressive, but its crew consists of only about 10 people (I think it was 11?), and they are basically all trained military personnel (whether volunteered or compulsory).
CometAside from a few professionals, everyone else is a customer. You have to consider that they might not even be listening properly to the explanations, let alone providing proper training.
Ensuring the safety of military personnel is already quite difficult, so doing it for profit in the private sector requires a significantly higher level of safety technology (especially in the case of transportation equipment).
Incidentally, when it comes to rockets, the difference in technological level between unmanned and manned rockets is like the difference between the moon and a turtle; they are, frankly, completely different things.
Hopefully, this has given you an idea of just how long ago it was in 1952 when the Comet began commercial flights carrying passengers in whatever form that may have been.
Next time, I'll finally tell you the story of how Comet began commercial flights through the skies.
The amazing and challenging aspects of the purely domestically produced jet passenger aircraft, the MRJ.
Incidentally, at the time of writing, a certain domestic passenger aircraft project has fallen through. It ended after test flights (it's amazing that they even managed to conduct test flights).
This makes sense to me; for example, even if high-profile components like engines and wings can be domestically produced, it doesn't mean that an airplane can be built.
A wealth of know-how is needed for parts that are so subtle you might not even notice they're there, and that's where we often stumble (like the windows on the Comet).
These kinds of components are often overlooked during the planning stage, and when you actually start working on them, you find that unexpected parts are incredibly difficult, and you often hit a wall at a point where it's too late to go back.
For example, parts we often overlook, such as windows, doors, and chairs, or even bolts, might be considered special.
If America and Europe can do it, then Japan, with its high industrial capacity, might think it can do it too if it just tries a little harder, but that's a huge mistake. Anything that has never been done before, no matter how easy or feasible it may seem, is inherently difficult.
This kind of thing happens all the time; any company or organization, blinded by prestige and profit, overlooks the real technical challenges and ends up pursuing reckless goals and plans.
The author is not dismissing such a wonderful challenge at all.For engineers, one of their important roles is to maintain a calm perspective, identify truly difficult technical challenges, and report them.
SoIf you're going to take on a challenge, it's important to plan with the understanding that you'll stumble, and to persevere without giving up—that's what's crucial in the art of skill (even though it's difficult).
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.
Comment:
Comment list (2)
Nice to meet you.
While the information about pressurization is correct, there are some points that concern me, which is why I am contacting you.
Regarding pressurization, there is a regulation that states that, in principle, no pressure is applied at ground level (the same as atmospheric pressure).
In Japan, this is stipulated in the Civil Aviation Bureau's Airworthiness Examination Guidelines, while in the United States, it is stipulated in the FAR (Flight Arrangement Regulations).
The reason is that if the pressure inside the aircraft is high, the doors will not open, making escape impossible.
There are two main mechanisms to prevent pressurization: one that detects the presence of ground level and either (1) opens a valve to release the pressure, or (2) prevents the inflow of pressurized air.
In passenger aircraft, comfort is essential, and air conditioning (engine bleed air) is absolutely necessary, so a system combining the two is the mainstream.
This is a digression, but bleed air is a tricky issue, especially during takeoff when engine power is needed, as it reduces that power. (Though I shouldn't be saying this to engine engineers.)
Therefore, we limit the level of extraction or adjust the performance to match the state when extraction is occurring.
Now, to the main point: the way pressurization is applied has a significant impact on the human body. During ascent, the cabin altitude is 500 ft/min, and during descent, it's 300 ft/min, with a cabin altitude of 2000-8000 ft (this varies depending on the flight altitude). In terms of aircraft strength, ascent is not much of a problem (if pressurization is insufficient, the amount of air added can be increased), but during descent, if the descent rate is fast, the amount of pressurization released may not be sufficient, potentially resulting in negative pressure compared to atmospheric pressure. Releasing too quickly can cause ear pain, or in the worst case, decompression sickness. Of course, warnings will be issued so you will notice.
Structurally, it's a transition from tension to compression.
One of the pressurization-related accidents was due to an inadequate design in the repair method for the pressure bulkheads of Japan Airlines aircraft.
In fighter jets, the cabin pressure is kept at around 0.5 to minimize the impact on the crew and the aircraft's structure in the event of a hit.
I apologize for the lengthy and rambling message.
Taneki-sama
Thank you for your comment. (I apologize for the very late reply.)
informative.
In the article, I explained the concept of pressurization in a very simplified way because I wanted you to understand it, but with modern technology, I think sensors are installed in various places inside and outside the aircraft, allowing for much more precise control.
Also, as you mentioned, the air conditioning system, including the pressurization, places a considerable load on the system, and I felt it was quite a tricky one to deal with, even in a car.
Finally, if possible, I would appreciate it if you could provide specific details about the issue regarding the inadequate design of the repair method for the pressure bulkhead of the JAL aircraft.