Courtesy of Pratt & Whitney
Although we may think of the jet engine as a modern-day invention, Hero of Alexandria, a first-century-AD mathematician, invented the aeolipile. Not having a practical application, it was little more than a curiosity.
In the 13th century, the true inventors of the rocket were the Chinese. It was used for fireworks and to propel weapons, but it was not further utilized for hundreds of years. The turbine engine was first patented in 1791 with the intended use being the horseless carriage.
Other work was done, but it was not until 1928 when Frank Whittle, a 21-year-old Royal Air Force cadet, published a thesis proposing aircraft jet propulsion. In 1941, Whittle’s engine was first used to power an airplane. The adoption of the jet engine quickly challenged the metallurgy of the day to deal with the unique material issues of such a high-speed and high-temperature device.
The basic operation of the jet engine involves vanes that move the air to propel the blades. The compressor and turbine sections of the engine contain alternating rows of vanes and blades. As the air is compressed, its temperature rises. This air is then mixed with fuel and burned to further raise the temperature. It is this high-energy gas that is used to propel the first-stage, high-pressure turbine (HPT) blades. The efficiency and performance of jet engines is determined by the ability to achieve very high gas temperatures.
Developing the materials able to handle these higher temperatures is always the goal of the aerospace-engine designers. The fan blades, which can be more than 50 inches long and rotate at several thousand revolutions per minute, are typically made from titanium for its light weight and strength. Remember that these fan blades also need to be designed to survive the impact of a bird.
The compressor section can see temperatures over 1000°F, and the materials must be designed to live in this environment. Typically, an iron- or nickel-based superalloy is used here. The keys to material survival in this part of the engine are high elevated-temperature strength, the ability to resist creep and crack growth, oxidation resistance and erosion prevention.
The combustion chamber is a huge challenge for materials, which see temperatures of 1800°F or higher. Materials used here are typically nickel- and cobalt-based superalloys. In addition to the high-temperature alloys, the blades are often directionally solidified, which results in the grains aligning along the length of the blade. Alternatively, single-crystal blades may be used. These are the ultimate because there are no grain boundaries to weaken the structure.
Ceramic coatings are often used in both the combustion chamber and the turbine. In 2008, researchers at Ohio State University were developing the technology to coat the turbine blades with zirconium dioxide (synthetic diamonds) to fight high-temperature corrosion. The benefit of the zirconia is that it chemically converts sand and other corrosive material into a new protective coating.
Because of the intense heat seen by the turbine, each blade contains “labyrinthine airways.” Cool air from the compressor passes through these channels, which allows the turbine to survive in gas streams with temperatures higher than the melting point of the alloy!
Recent events have seen jet engines disabled by bird strikes. In addition to the US Airways flight that crash landed in the Hudson River, a 757 in England hit a bird on takeoff and lost an engine. In Rome, a 737 hit a flock of birds during landing. Back in 1995, an Air Force plane crashed – killing all 24 on board – after it collided with geese. Since 1999, at least 65 planes have had to make emergency landings after hitting birds.
Jet-engine designs are rigorously tested to be able to withstand bird strikes. Some things are simply more than what we can expect from the limits inherent in metals and materials. For this reason, the FAA is testing radar – calibrated to detect birds – that can inform pilots where birds are. The Navy and Air Force currently utilize this type of radar.
Specialized materials, many of which are thermally processed, are used in the design of today’s jet engines. The challenges of design engineers also include criteria such as noise reduction and fuel efficiency. Tomorrow’s materials may be birthed from the mother of the invention needed to improve efficiency or safety.IH
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