Thinking While Looking at the Real Things:
The Start of Independent Research and Development

1X, Honda’s first independently developed engine using ceramics

“Let’s start by making an actual engine, and we will take it from there.”
A team of young engineers, with an average age of 26, were assigned to start Honda’s aircraft engine research. None of them were gas turbine engine specialists, let alone experienced in the field of aircraft engines. Normally, a team like that would start by modeling an existing engine or looking for a joint research partner in order to gain some knowledge. However, in spring 1986, this team of young engineers began by drawing up layout plans and initiated research on their own.
As a late entrant to the market, Honda had to create an airplane which would offer new value to customers, and Honda wanted to make its airplane affordable to more customers. To this end, it was important for the aircraft engine to be fuel-efficient, yet affordable.
So, the team decide to take three measures.
First, was to use precision casting as much as possible for parts with complex shapes to minimize the need for costly high-precision machining.
Second, was to use state-of-the-art ceramics (fine ceramics), a heat-resistant material, for certain areas of the gas turbine engine which would be exposed to high temperature. At the time, fine ceramics was rarely used for gas turbine engines, which generate rotational kinetic energy using the high-temperature gas produced by fuel combustion. In order to achieve high power output with this compact engine, it would be necessary to boost the turbine’s intake temperature. However, adding some sort of cooling system to prevent heat issues would only further complicate the design and increase costs.
The third measure was to adopt an advanced turboprop (ATP) engine equipped with contra-rotating propellers, which was only beginning to attract attention as a leading-edge technology with high propulsion efficiency.
The first engine prototype was codenamed “1.0X” (“1.0” for the “first created,” and “X” for “unknown”) and designed only with a gas generator section, since the main focus of its development was simply to confirm the feasibility of using ceramics.
To start a project with no prior experience, the team proceeded energetically with the necessary tasks in parallel, including engine layout, component design and deciding on analysis methods and test requirements. Furthermore, in addition to the introduction of specialized equipment and facilities such as aircraft engine test cells, the development team studied parts manufacturing methods and acquired new machining equipment.

After solving a slew of problems large and small, when the test engine was successfully brought up to idle speed for the first time – and the sound of the jet engine echoed through the bench room – every member of the team felt a sense of realization and excitement from that fact that they were working on aircraft engines.
However, this engine had a weakness: its main shaft was difficult to balance. On top of its cantilevered bearing structure, the coaxial design of the engine’s main shaft was determined by the perpendicularity of the mating surfaces of its two rotors. Because of such design, the engine had to be carefully assembled while making precise adjustments down to the one-micron level, rubbing the mating surface of the two rotors with an oilstone one stroke at a time. Otherwise, shaft vibrations were so severe that the engine was not usable.
In the end, the further development of this prototype engine was discontinued because the turbines’ ceramic blades were damaged repeatedly, and the engine couldn’t be spun up to its rated speed.

Further Pursuit of Originality

Advanced turboprop engine 2X with dual planetary gear
reversing mechanism

By the middle of the second year of research, in 1987, the team decided to give up on ceramics and adopted more common metals for high-temperature engine parts. Also, in pursuit of the ultra-compact size, the team adopted a high-revolution gas generator and a spiral combustor based on its own spiral theory.
As to ATP design, while other companies were adopting various designs such as one with two turbine shafts, Honda adopted a coaxial contra-rotating mechanism with planetary gears, which was one of the unique features of the Honda engine design. This engine, codenamed 2.0X, was truly advanced, and no other manufacturer had realized it at the time.
The adoption of metal turbines enabled more stable gas generator operation, and the team was able to move on to the ATP testing phase. However, there were still a number of challenges that needed to be addressed, including thermal deformation of components exposed to high-temperature such as the non-axisymmetric combustors, controlling of contra-rotating propellers with planetary gears, and preventing and managing foreign object damage (FOD) on carbon fiber propeller blades.

FAA Discontinues ATP Standardization

Honda wasn’t the only company researching ATP engines. Airlines that had experienced the oil crisis in the 1970s were strongly demanding aircraft engine makers to improve fuel economy, which drew attention to the ATP system equipped with a contra-rotating propeller that had high potential to improve fuel economy. It became an industry-wide trend to pursue ATP technology.
Although fuel economy was indeed improved, the shock waves generated by the dual rows of propellers interfered with each other and caused a tremendous noise when the aircraft engine was in operation. Moreover, due to the significantly higher rotational speed of the ATP engine’s uncontained propellers compared to a conventional turboprop engine, if the propeller should ever break, there was a risk that it could break through the fuselage of the aircraft. While ATP engines had advantages, it also had significant disadvantages. It was a technology fraught with challenges needing to be addressed toward the realization.
As the impact of the oil crisis subsided and fuel prices declined, the Federal Aviation Administration (FAA) made the decision to discontinue its initiative to standardize ATP technologies. This meant that even if engine manufacturers managed to develop viable ATP engines, they could never be released to the market. The team members, who by then had their hearts set on leading the world in ATP development, were dumbstruck by the FAA’s decision and the sudden loss of their target.

Going Back to the Starting Point After Many Struggles

Aft-fan engine that uses a fan instead of a propeller to
generate propulsion

A little over a year later, in 1989, the team adopted an “aft-fan” engine design, with the fan mounted at the back of the engine. The aft-fan engine rotates a cast metal fan using planetary gears, instead of propellers, to generate thrust. The switch from the contra-rotating propellers to a fan would cause a decline in the overall efficiency of the engine, thus it was designed to compensate the decline by adopting a multi-stage compressor and increasing the pressure ratio. However, this engine turned out to be much heavier and more complicated than originally envisioned.
To learn from the failures, the team paused in their quest for uniqueness, and thought through the reasons why their unique measures didn’t work. There was no doubt that the team took on very difficult challenges involving highly sophisticated technologies, but more than that, they realized that the main reason for the failures was a lack of fundamental technologies, as well as knowledge about technological requirements unique to aircraft engines.
After much deliberation, in late 1990, the team approved the proposal made by the team’s young engineer leader to shift their focus and work on a turbofan engine, which was already widely adopted in the industry, to solidify and enhance the foundation for aircraft engine technologies. This decision reflected the team’s determination not to make major design changes ever again.
One day, during a test where engine speed was gradually being increased, a loud blast suddenly echoed through the building that housed the bench room, followed by an unsettling silence. The team members rushed into the bench room to find that the engine had split in half. The cast centrifugal compressor near the center of the engine had shattered, destroying the engine. Further investigation identified that a casting defect caused the rupture of the centrifugal compressor. This event impressed upon the team how critical the reliability of all rotating parts was for aircraft engines.
Although this aft-fan engine managed to reach the targeted output of 600 horsepower for the first time among all prototypes the team had tested, other aspects of the engine, including weight, fuel economy and reliability, turned out to be far from satisfactory. In the end, all three unique measures the team tried out – adoption of fine ceramics, contra-rotating propellers and precision cast components – didn’t work out.

Achieving the Rated RPM For the First Time and Conducting
the First Flight Test

The team began working on a new engine under the code name of HFX-01. The target for thrust was set at 1,800 pounds, which was the thrust for engines used for the smallest class of business jets.
Although the turbofan was already a common type for aircraft engines, it was new to Honda. Starting from virtually no experience, the prototyping division determinedly strived to acquire cutting-edge technologies in the industry, such as the machining of large titanium rotors with an outer diameter of 400 to 600 mm, and high-precision machining to achieve tolerances of 10 μ or less for an axis nearly one meter long. As a result of the team’s hard work, all parts were ready and the engine assembly finally began. Surprisingly, within a month from the start of assembly, the engine reached its rated speed. This success was a result of the dedicated work of team members and all others involved.
In October 1994, the first outdoor test of the HFX-01 was conducted on an open test bench at the Honda R&D Takasu Proving Center in Hokkaido, the northernmost main island of Japan, and the team members were deeply impressed by the jet engine sound echoing across the Hokkaido sky.
Then, in December 1995, the first flight test was conducted over the Mojave Desert in California,U.S.A. with the HFX-01 engines mounted on the sides of the fuselage near the front of a Boeing 727. Through this flight testing, a wide range of basic operating data of the aircraft engine was gathered, greatly inspiring not only the team members who were at the test site, but the entire development team.

Honda’s first turbofan engine, the HFX-01, on its first flight test

Taking on the Challenge to
Further Improve Engine Performance

Originally, research on aircraft engines and airframes was being conducted separately within HGF. However, in 1998, a new project started with the HondaJet proof-of-concept (POC) aircraft equipped with Honda-made engines.
Although the HFX-01 had already achieved performance comparable to other turbofan engines that existed at the time, that alone didn’t provide justification for Honda to produce it independently. Honda thus decided to aim for another leap forward and create an engine that would be worth proposing to the world.
The project team set clear targets for the new engine, which was codenamed HF118. The targets were to improve thrust-to-weight ratio and cruising fuel economy, both critical factors for aircraft engines, by 20% and 10%, respectively, compared to existing engines. Additionally, although no emissions regulations existed for this class of engines, Honda, which once led the automotive world with its CVCC engine, set targets that would enable Honda aircraft engines to comply with potential future emissions regulations. The reliability target was set with “an in-flight engine shutdown probability of less than once in 14 years of uninterrupted, 24 hour-a-day flight and operation.” The team also strived to reduce the cost of the HF118 engine by significantly reducing the number of parts, by minimizing the number of rotors through the improvement in the efficiency of individual elements and by drastically simplifying the engine structure.

Adopting the Best Possible Technologies,
Including Application of Automotive Technologies

Once these targets were set, the team members came up with a variety of ideas, adopting several to pursue: a centrifugal compressor that delivered the world’s highest pressure ratio and efficiency; a pulsation- and surge-free low-pressure compressor system with no variable mechanism; carbon-composite, lightweight, lower cost fan stator blades with an original structure; and a combustor that featured 90,000 diagonal holes to realize efficient cooling and reduced emissions.
Other technologies adopted for the HF118 included: a high-pressure turbine that featured a nozzle with a minimal number of blades to prevent blade resonance within the operating range that could destroy the turbine; a single-stage low-pressure turbine made possible by high-load aerodynamic technology; and a high-precision supercritical shaft that suppressed vibration that could be caused by an imbalance of the rotating components with alignment effect, improving quietness inside the cabin. The supercritical shaft got its name as it was designed to be used beyond the primary resonance point, one of the critical points of vibration.
Moreover, the team adopted some new materials, such as high-strength heat-resistant forged material for the high-pressure turbine disk and titanium alloy with excellent low-cycle fatigue characteristics for the centrifugal compressor. The HF118 was fully loaded with the best technologies the team could possibly adopt.

The HF118 was designed to improve thrust compared to
existing engines
It was developed with the goals of low cost, low fuel
consumption, and low emissions.

Among a number of new technologies adopted for the HF118, it was worthy of note that the fuel control system featured the FADEC (Full Authority Digital Engine Control) system where automotive technologies were applied, which was an approach unique to Honda. When the HF118 research began, the technology to mount a bare chip directly to a ceramic substrate was only just emerging as an automotive technology. Using this technology, Honda was able to reduce the size and weight of the ECU to one-tenth and one-eighth, respectively, compared to the ECU produced with electronic components certified for aircraft use, reducing its weight from 8 kg to 1 kg. This also made it possible to integrate the ECU with the fuel pump, thus eliminating wiring between the units and achieving an exceptional reduction in weight and the number of parts.
Moreover, to increase the efficiency of each component, Honda independently developed software to simulate airflow within the engine, which was still only in its research stage at the time. Especially with a centrifugal compressor, the use of this software for design enabled the team to successfully improve the efficiency of the compressor to the world’s top level. Generally, the rotating parts of aviation engines have a limited lifespan^*1 due to exposure to high stresses. If rotating parts break due to fatigue failure, as was the case with the aft-fan engine bench test mentioned earlier, it could lead to an accident involving damage to the aircraft; therefore, lifespan prediction of rotating parts is a critical factor for the reliability of the engine. The team applied Weibull analysis^*2 to predict the lifespan of the rotating parts of the HF118, ensuring a high level of reliability.

• Limited lifespan of the parts refers to the number of hours of use or the number of uses after which the parts must be replaced as part of maintenance.
• Weibull analysis is an analytical method that uses “the Weibull distribution”, a probability distribution used to assess reliability, to determine the number of operations (lifespan) with which the probability of fatigue failure of the parts is less than a certain level

HF118 Engine Features

Low-pressure compressor/High Pressure Centrifugal Compressor

Combustor

Adopting the Best Possible Technologies,
Including Application of Automotive Technologies

At the time, Honda kept this aircraft engine project confidential, and thus could not outsource production of parts to external aircraft engine parts manufacturers. Therefore, Honda decided to conduct in-house production of a significant number of prototypes of aircraft engine parts. It had been Honda’s tradition to make things if they don’t already exist. In some cases, the team asked prototype makers of automotive parts to develop aircraft engine parts under the guise of automotive parts.
Core components of aircraft engines, namely centrifugal compressors, low-pressure compressors and fans are machined from forged titanium alloy blocks. Considering the balance between performance and cost, how those parts are machined is an extremely important matter. For example, point milling, a machining method that uses the tip of a cutting tool, is most often used because the use of the tip of a cutting tool makes it possible to machine more complex shapes.
However, this method requires more machining time and cost; therefore, Honda engineers took on challenges to realize flank milling, which uses the side of a cutting tool. The word “flank” means the side of something. This method requires less machining time, but because of the use of the side of the cutting tool, the milling work necessarily became more linear, making it difficult to machine parts with complex shapes.
Not willing to give up, the team conducted aerodynamic simulations of those core components featuring shapes that could be achieved by the flank milling method. Design and production engineers worked together and repeated calculations to refine shapes based on predicting the performance of each component. As a result, the team successfully designed core components that can be manufactured using the flank milling method, while satisfying the performance requirements. This enabled significant improvement of the balance between performance and cost (production time).