With oil reserves being consumed at an unprecedented pace as global demand rises and prices on a consistently upward trend, short of a new power source being invented, fuel will remain as the single largest operating cost in aviation.
To deliver a more efficient aircraft, engineers typically focus on three key areas: improving aerodynamic profile and wing structure, mass reduction, and an engine design that can each deliver increased fuel efficiency.
However, aerodynamic efficiency and mass reduction are areas necessarily constrained by the current limits of technical engineering ability and material properties. Put simply, there is little potential for drastic improvements in these areas in the near future when taking into account current capabilities.
In recent years, the simplest route to achieving improved fuel efficiency has traditionally come from a combination of low drag co-efficient profiles and engines that burn fuel at a higher temperature to maximize economy. But the industry has hit something of a wall; the scope for simply raising fuel-burning temperatures or cooling engines is now relatively limited, with the hybrid metal-based alloys currently deployed in fixed-cycle engines operating at the upper limits of their temperature threshold of 2200°F. For both commercial and military jets, the current generation of engine designs has ceased to deliver impressive results, with any improvements being incremental in nature.
The problem facing the industry is how to continue to offer improvements in engine operation when designs are already working to their limits.
To design engines capable of exceeding these limits, there are two approaches currently being explored by the heavily staffed R&D departments of the global aerospace engine manufacturers. The first depends on further cooling the air inside the engine, which demands an entirely new, revolutionary engine architecture.
Today’s fixed-cycle variants will eventually be replaced by variable cycle designs, which use sophisticated air flow control systems to create regulated temperature pockets and are predicted to deliver improved efficiencies across the entire flight envelope.
The second approach is to develop new materials for use within engines, ones able to withstand temperatures exceeding the current limit of metal alloys. The most viable new material for this is ceramic matrix composites (CMCs), which promise to deliver unprecedented resistance to temperatures far exceeding current levels.
Indeed, CMCs are already being introduced and have been utilized within the CFM LEAP high-bypass turbofan engine, currently being developed in a joint venture between GE and Snecma. The preliminary statistics are highly encouraging, with anticipated increased efficiency which will see fuel consumption reduced by 16%.
But this is only the leading edge, and the full extent of the advantages CMCs will offer is yet to be explored. As with the early stages of any material development with potential application in the aerospace industry, rigorous testing is required.
Testing times and conditions
When testing a new generation of materials to gather robust data, unforeseen challenges always arise and a new methodology is sometimes required. This has certainly been the case in the early stages of CMC development, and their unique resistance to sustained, intense heat brought up several interesting complications.
The first major obstacle was equipment; put simply, when looking to test material at 2700°F and beyond, there was no commercially available furnace that could reach, let alone maintain, the required temperature levels. Element Materials Technology designed and built a furnace from the ground up using a machinable monolithic ceramic block, which was fashioned into a chamber, before being exposed to heat and hardened into shape.
Then there were other seemingly minor but still critical obstacles the testing team had to overcome, such as how to hold a sample in place in the new furnace when every existing technique could not withstand the pressure and heat required throughout the testing process. A tool for holding samples in place in the new furnace had to be designed and fabricated from scratch.
The single biggest challenge encountered, though, was when attempting to measure the temperature of the sample. Gauges that operate by making physical contact with samples were unsuitable, as the equipment typically used ignited a reaction at higher temperatures, causing a test failure. The standard alternative has been to use thermocouple probes, which can detect sample temperatures with precision from a microscopic distance, so that no contact is made. But this proved ineffective in delivering the data needed at these unprecedented temperature levels.
Element again had to turn to brand new methods to surmount this problem, devising a treated liquid ceramic coating to create a new generation of probes. These could touch the sample without a reaction, ensuring a complete set of test results at temperatures up to 2700°F and even delivering a higher fidelity of test results at lower temperatures than those obtained using contemporary techniques. But this method, as with any, has its limits. With CMCs expected to be able to effectively withstand temperatures in excess of 3000°F, Element is already exploring future techniques to enable testing in even more extreme conditions.
Despite having successfully designed, built, and tested the necessary equipment, challenges continued to arise even before the actual materials testing. Primarily, there was the question of what size and shape samples would best enable testing of the heat and corrosion resistance, mechanical properties, strength tension, compression, and durability of CMCs. The materials themselves are also tremendously expensive—currently around five times the cost of metals.
The industry is still in the early stages of understanding CMCs. However, thanks to the unique nature of the material and the evolving research and methodology required to test it, such challenges are only the beginning. There is a high degree of confidence that any obstacles can and will be overcome and that CMCs are already worth the significant investment. The impressive results released from the LEAP high-bypass turbofan engine test, which depended in part on the CMCs used in its construction, are just a preliminary indication of what this technology can achieve for the aviation industry.
Propelled forward: The future
If aerodynamic efficiency and mass reduction cannot be relied on to deliver the efficiencies required, then the holy grail for developers is an engine that combines the increased burning temperature of engines using CMCs with the cooling properties delivered by the adaptive air flow systems of variable cycle designs.
A viable engine is still a few years away from development, but initial research into such propulsion systems is under way. GE recently announced dramatic results from its engine core test with the U.S. Air Force Research Laboratory’s Adaptive Versatile Engine Technology program. The new engine core achieved a 25% improvement in fuel efficiency, a 30% increase in operating range, and a 5-10% improvement in thrust compared to today's fixed-cycle engines. The combination of CMCs with an adaptive low-pressure spool allowed the engine to generate the highest combination of compressor and turbine temperatures in aviation history, ensuring fuel was burned with far greater efficiency than ever before.
As the aerospace industry becomes more familiar with CMCs and continues to develop test methodology, these figures will no doubt be surpassed. Once manufacturing techniques have been perfected and the material becomes ubiquitous, economies of scale will ensure that costs are reduced, paving the way for faster, cheaper innovation.
David Podrug, Advanced Materials Business Manager, Element Materials Technology, wrote this article for Aerospace Engineering.