In the previous article, we explored the concept behind the development of the Comet and imagined what the actual design process would be like.
Regardless of the internal circumstances, the fact that Comet was completed means that all the design drawings must have been released and prototypes built.
Every single component of the prototype Comet is checked and verified through some kind of test or examination.
This time, let's imagine the testing and verification process using that prototype.
For reference, here is a general overview of the typical product and machine development process.
Before we imagine the ground tests and prototypes used during the development of the Comet, let's first explain the general testing and evaluation process in product and machine development, which has remained unchanged throughout history and across cultures.
Basic principles of testing in product and machine development
In a typical company or organization, the blueprints are officially issued only after undergoing the rigorous checks and reviews described in the previous article.
Based on officially issued drawings, prototypes are manufactured at the company's own factory or affiliated companies.
Various tests and trials are conducted on the prototypes that have been manufactured to confirm whether the product or machine functions as intended in the design.
そのTesting of machinery and products can be broadly classified into three categories: functional testing, performance testing, and durability testing.
Although there is some overlap in time, the order of these three exams/tests is as shown in the following diagram.
By now you might be wondering, "What are function, performance, and durability?" So, I'll give a brief explanation.
Since we're on the subject, let's explain using the pressurized cabin, which was the cause of the Comet crash.
First, let me explain the functions.
Functional testing, testing
First, the word "function" refers to the way something works.
I think this is probably hard to understand and won't make sense, so let me explain using a pressurized cabin (since this is a Comet article) as an example."The function of a pressurized chamber, as the name suggests, is to apply a certain amount of pressure to a sealed space.".
Applying a certain amount of pressure inside this container is what makes it work.
In other words, At this stage, the only thing to consider is whether a certain pressure can be applied to a sealed space, regardless of the pressure (e.g., XX atm).Let's focus on that.
Furthermore, considering the components of a pressurized chamber, it basically consists of a sealed container, an air compressor for pressurization, and a pressure regulating valve.
Considering each function
The function of a sealed container is to contain the liquid, gas, etc. inside the container.
- The function of a pressurized air compressor is to be able to compress air.
The function of a pressure regulating valve is to release gas or liquid from inside a container.
And so on.
At this stageWe don't focus on numbers at all; we simply test the functionality, that is, whether the product works properly.
These types of verification tests are called functional tests or simply tests.
Next, let's look at performance tests and evaluations.
Performance testing, testing
The term "performance" refers to the ability or properties of something to function.
This is also a bit confusing, like yesterday, so I'll explain it in the pressurized chamber.
I just explained the function of the pressurized chamber.
Performance is a term that indicates the level of capability a function possesses.
In other words, in a pressurized cabin, for example,"Maintaining a constant pressure of 0.6 [atm] inside the pressurized chamber"—this 0.6 [atm] part represents the performance, and this number is called the performance value.
If we consider the performance of the pressurized chamber components in the same way as its function (this is just an example):
• The sealed container must have an internal pressure of 0.6 [atm] and be free from leaks of liquid or gas.
- The air compressor must take in XXX [L] of air per minute.
- The pressure relief valve must be able to discharge YYY[L] of air per minute.
And so on.You can generally think of performance as being represented by numbers (although there are exceptions).
Tests and examinations that confirm how well the pressurized chamber and its components function and what capacity they exhibit are called performance tests or tests.
Therefore, once we've confirmed through functional testing that the prototype works correctly, we can finally proceed to verifying its performance.
In modern times, there aren't that many things that are that new.Often, we check the functionality and performance at the same time.
Next, I'll introduce the final durability test.
Durability test, test
The word "durability," as the characters suggest, refers to how much something can withstand.
Actually, strictly speaking, durability is part of performance, but it's an important factor for a machine, so it should be considered separately from performance.That's the case for most people.
Let's consider this specifically in the context of a pressurized chamber, for example."The pressurized chamber must not break even when subjected to a load of 0.02 [atm] to 1 [atm] change in external pressure."And so on.
If we consider it in terms of the components of the pressurized chamber, as before...
The container must withstand an external pressure of 0.02 to 1 atm for XXXXX hours.
- The air compressor must not break down within YYYYY [hours] of operating time.
- The air control valve must not break down within ZZZZZ [hours] of operation.
In short, durability refers to how much load a machine can withstand, or how many hours and how many times it can withstand a given load.
Tests that verify durability in this manner are called durability tests.
ObviouslyThis durability test is meaningless unless it confirms that the machine can perform its intended functions and performance as designed.
Therefore, it is usually the final stage of an exam or test.
Up to this point, we've focused on functionality, performance, and durability, but let me introduce one more important perspective.
Individual and finished product testing.
Let's consider machines for a moment. Basically, machines are composed of two or more parts (although ornaments can sometimes be made from a single part).
A massive machine like Comet is probably composed of tens of thousands to hundreds of thousands of parts.
そのWhen you perform the tests and examinations (function, performance, durability) described earlier on a complex machine as a finished product, it becomes difficult to determine the cause of any problems that arise.
For example, it becomes unclear which part is the problem, whether it's its function, performance, or durability.
This would make development extremely inefficient.
thereIn typical development, testing is not performed on the finished product immediately, but rather on a certain number of components at a time.
To put it in extreme terms, if a company has no know-how or experience, it will start by testing even a single bolt, and this type of testing is called unit testing or testing.
It's unthinkable to start with a single bolt, but when considering a pressurized chamber, instead of immediately testing the entire chamber, you would break it down into individual tests, such as testing a sealed container, testing an air compressor, and testing a pressure regulating valve.
Only after testing each individual component is complete is the entire pressurized chamber tested; this is called the final product test or test.
このThe fact that each component has been tested individually does not mean that final product testing is unnecessary. It is absolutely necessary because you never know what will happen when all the components are assembled.
To digress slightly, it goes without saying that being able to test a larger number of components at a time is more efficient and allows for faster development.
To put it extremely, the ultimate ideal would be to be able to perform all tests on the finished product in a single run, but in reality, that's quite difficult.
So how can we enable testing with a larger number of components? That depends on how much expertise a company has in small components, and how well they understand the physical phenomena involved.
このThe extent to which a company has accumulated know-how is a measure of its true capabilities that cannot be measured solely by economics.
Furthermore, with the accumulation of know-how regarding physical phenomena and components, there are now tests that can be simulated on a computer without actually conducting tests (this is called CAE, which seems all-encompassing but is not in reality).
HoweverEven today, there are countless unknown physical phenomena, so I think the day when everything can be simulated is still a long way off.
To summarize, in product development, testing is conducted using the following diagram.
Based on what we've discussed so far, let's take a closer look at the Comet's ground tests, focusing on the pressurized chamber.
A review of the problems encountered during the pressurized chamber testing in the development of the Comet rocket.
To reiterate, the problems with the pressurized chamber testing during the development of the Comet were as follows:
1. There was no test facility large enough to accommodate the entire aircraft.The pressurized chamber was divided and subjected to durability testing.(One of the main reasons was that the pressurized chamber was constructed by dividing the engine into parts, creating them, and then gluing them together.)
2. DividedTo determine how much load it would take for the pressurized chamber to fail (one-shot failure), a test was conducted applying a load twice the load experienced during flight.
3,A fatigue test was conducted by repeatedly applying loads to the divided pressurized chamber used in the second test.
4. The fatigue test involves repeatedly applying a load due to a pressure difference, a pressure of 0.6 atm (60 kPa).As a precaution, we applied double the pressure, 1.2 atm (120 kPa), every 1000 cycles.
5. To apply repeated loads to a pressurized chamberUsing compressed airThe pressurized chamber was repeatedly subjected to loads.
Please refer to this for details.
I actually have many thoughts on this, but this time I'd like to look at points 3 and 4 from a different perspective than before.
Comet's pressurized chamber single-shot failure test (limit load test)
Previously, I explained the difference between single-shot failure and fatigue failure, which are fundamental concepts in fracture.
The problem of using the same parts for testing, given that the modes of failure differ, was also explained.
This time, we'll explain how a true one-shot destructive test is conducted and what significance it holds.
First, the one-shot destructive test (limit load test) is a type of durability test explained above, which checks how much load it can withstand and at what load it will break.
Throughout history and across culturesA typical single-shot destructive test (limit load test) applies a load until the test actually fails, even if it has withstood the design estimate.
Because it's to find out the actual limits of that part.
このIf you don't know the actual limit value, you can't know how much safety margin there is when that part is actually used.
The value obtained by dividing the load at which the component actually breaks by the load assumed in the design is called the safety factor, and in the case of aircraft, it is often set between 1.0 and 1.2, although this varies depending on the part.
During development, Comet did not undergo a single-shot destructive test to destroy any components, resulting in the impossible situation where this safety factor is unknown.are doing.
Looking again at the example of Japan's Zero fighter, which was a successful development, it's obvious that even the Zero underwent a test to destroy it in one hit (maximum load).
In the case of the Zero fighter, the first prototype was tested by attaching weights to the parts being tested and increasing the weight until it actually broke (apparently, Mitsubishi at the time customarily referred to this prototype as "Unit 0").
Naturally, the safety factor is determined by verifying the location where the failure actually occurred and dividing the design load by the actual weight of the broken object.
This safety factor determines the various flight limits of the Zero fighter, such as the maximum G-force during aerial combat maneuvers and the maximum dive speed of xxx km/h.
This know-how regarding the Zero fighter was acquired at the very cost of the precious lives of test pilots.
If you're interested, I recommend reading the book I'm suggesting; I think you'll find it very interesting.
Conversely, unless these conditions are decided, it is absolutely impossible for permission to fly, even for testing, to be granted.
At CometIt's essentially the same as not conducting a single-shot destructive test, resulting in the abnormal situation where the actual limits of the aircraft's performance were unknown during development.
In other words, under normal development conditions, the Comet shouldn't even have been able to conduct test flights.
Furthermore, this single-test also includes testing of certain important functions and performance characteristics.
そのFunction and performance refer to the ability of a component to fail in a way that does not immediately endanger the crew when it is subjected to excessive load during flight.
Simply put, the design ensures that when an overload occurs, a part that won't immediately cause mid-air disintegration will break, allowing the crew to notice the failure and take emergency evasive action (fail-safe design, functional safety).
In the one-shot destruction test of the Comet's pressurized chamber, the test was not conducted until it actually failed, resulting in the unthinkable mistake of "not knowing where it would break under overload until a crash occurred."
If the tests had been properly conducted up to the point where it would break at this stage, stress concentration should have occurred at the corners of the window and the holes in the antenna, causing it to break.
Naturally, this is not the kind of damage that allows for emergency evasive action as explained earlier (the pressurized chamber is sliced in half), so it's only natural to assume that some kind of countermeasure will be implemented.
Even if development proceeds without taking countermeasures due to lack of time, lack of budget, etc.If the Comet's pressurized chamber's one-shot rupture test (limit load test) had been conducted correctly, many people would have known that a rupture in the pressurized chamber would lead to mid-air disintegration.
That way, when a crash occurs...At the time of the first accident, it should have been possible to recognize that "the pressurized chamber may have disintegrated in mid-air" as a possible cause.
Common sense dictates that if there is even a slight possibility of the pressurized chamber overloading and immediately causing mid-air disintegration, flight clearance will not be reissued, and a proper investigation into the cause will be conducted (in the first accident, it was determined that there was no problem with the Comet).
Click here for the explanation of the cause of the Comet plane crash.
that wayThe probability of a series of crashes like that occurring again is significantly reduced.
In other words, This means that during the development of the Comet, the functionality, performance, and durability—the very essence of a single-shot destructive test—were not properly confirmed at all.
It's hard to believe that such a level of understanding was lacking, even before 1950 (it was certainly possible in pre-war Japan).
alsoIt is inevitable to conclude that there were serious flaws in the development system, organization, and management, just as there were during the design phase.
In the next installment, we will introduce the Comet's ground testing, from fatigue failure to test flights.
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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