Download PDF Summer Bridge Issue on Aeronautics June 26, 2020 Volume 50 Issue 2 The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come. Aeropropulsion: Advances, Opportunities, and Challenges Friday, June 26, 2020 Author: Alan H. Epstein This is an exciting time of both opportunities and challenges for civil aeronautics. Opportunities include dramatic reductions in aircraft noise and the development of drones of all sizes and shapes, air taxis, and low-boom supersonic travel. Challenges include the need for new technologies and materials to improve efficiency, noise, engine-airframe integration, and, importantly, environmental concerns, especially those associated with climate change. Underlying all is aviation’s imperative of increased safety. Propulsion technology is one key to both the opportunities and the concerns. Background The span of aeronautical system size and capability is enormous. Air vehicles now range from 1 kg drones that fly for a few minutes to 500,000 kg airliners that travel halfway around the planet. Figure 1 The power and energy needed to enable these capabilities vary widely (figure 1), as do the technologies that propel them. Small autonomous aerial vehicles are electric and need only a few hundred watts of power. Small helicopters such as an R22 require about 100 kW, similar to an automobile, and are powered by internal combustion engines. A large airliner needs hundreds of MW, a power level now feasible only with gas turbines. Vehicle energy needs vary even more: a small drone may fly on flashlight batteries, an airliner may take off with 10,000 times more energy than a Tesla S car. Propulsion opportunities and challenges exist across this size range. Modern Jet Engines Most of the world’s aerospace industry’s $800 billion-per-year manufacturing and maintenance economic activity is associated with jet-powered commercial aviation. Engines represent about 20 percent of the value of a new airplane and account for about 14 percent of total operating costs of short-range airlines and up to 28 percent for long-range airlines. Performance, life, and safety improvements in airliner jet engines depend on massive investments in technology coupled with increasingly sophisticated design, as well as painstaking attention to detail and to system integration. Practical improvement requires reduced aerodynamic losses, increases in compressor pressure ratio, and higher turbine inlet temperature. Engine efficiency, reliability, and weight are major determinants of airliner capability and cost. There is as much still to be gained in jet propulsion as has been achieved in the last 70 years of engine development. This will require investments in advanced technology, materials, and design. Engine Efficiency and Reliability Since the introduction of turbojet-powered airliners in the late 1950s, engine efficiency and reliability have increased severalfold. In today’s airliners, powered by simple Brayton cycle engines driving ducted fans, thermal efficiency at cruise (the conversion of fuel chemical energy into shaft work) has increased from about 20 percent to 55 percent, and propulsive efficiency (defined here as the conversion of shaft work to work useful to propel the aircraft) has increased from about 40 percent to over 70 percent. Overall efficiency has about doubled, to 40 percent. The shaft power needed to drive fans or propellers can be generated in many ways. Simple Brayton cycles have proven the most efficient and economical, although more complex cycles have been and are still extensively studied. The theoretical efficiency of an ideal Brayton cycle at airliner cruising altitudes is over 80 percent, but the industry consensus is that overall efficiencies approaching 60 percent may be possible (Epstein 2014). One measure of reliability, the in-flight shutdown rate, has improved by a factor of 1,000 from the early 1960s. At current levels of 0.5 engine shutdown per million flight hours, most commercial pilots will never need to shut down an engine. Engine Weight Engine efficiency and maintenance cost can be traded against engine weight. Reducing engine weight or fuel burn means an airplane can be lighter and therefore less expensive to produce and operate, or it can fly farther at the same weight thereby opening up new routes. The power-to-weight ratio of engines has only doubled, because economics have favored improved efficiency, reliability, and life over weight. As engine efficiency has improved, the hot parts have shrunk (higher efficiency and higher pressures reduce the hot flow areas needed) but the cooler fan-related parts have increased in size, generating weight challenges. Recent Advances There has been much progress in fundamental understanding and in design tools for structural modeling, turbomachinery aerodynamics, combustion, heat transfer, fracture mechanics, and materials. Combined with new structural materials and coatings, advanced controls, and manufacturing technologies, as well as design innovations such as active clearance control, laminar flow airfoils, and 99+ percent efficient gear systems, the result is quieter, lighter, and more efficient and reliable engines. With advances in enabling technologies, designers decreased the pressure ratio across the engine fan to reduce engine exhaust velocity and improve propulsive efficiency. Airplane thrust needs and engine thrust capability both vary with flight speed and altitude, by a factor of 3–6, but not necessarily together. Since the thrust of the fan—which provides about 90 percent of the engine’s thrust—is the product of the fan’s exhaust velocity and air mass flow, the fan size must be increased to maintain thrust as its pressure ratio is decreased. Engine designs are closely tailored for each application to minimize airline total cost. Decreased fan pressure ratio more closely matches engine capability to aircraft needs. One result is that engines are no longer sized by takeoff requirements, as for previous generations, but for thrust at top of climb. Figure 2 The economics of long-range routes favors fuel consumption over weight and manufacturing and maintenance costs. Large aircraft are optimized for longer routes so engine cruise efficiency typically increases with engine size (figure 2). Opportunities Noise Reduction Aircraft noise is both an environmental impact and a major impediment to the expansion of air transportation: communities typically resist new airports, runways, and flight paths because of noise concerns. Technology advances have reduced the airliner noise “footprint” area on the ground by a factor of 10 since 1960 (Spakovszky 2019). Historically, the engine exhaust was the predominant noise source. As fan size has increased, the resulting reduction in exhaust velocity has greatly reduced jet exhaust noise. Airliner noise now comes as much from the airframe as it does from the engine, and the fan is the predominant engine noise source on takeoff and landing. Studies suggest that with a focused research investment over the next decade on airframe and engine noise–related technologies, a virtually silent (i.e., quieter than the urban ambient) large airliner may be feasible with competitive economics (Spakovszky 2019). Supersonic Commercial Flight Supersonic commercial flight was a major goal of the 1960s, but the Boeing 2707 was cancelled because of emerging environmental and economic concerns. Production of the rival British-French Concorde was limited to just 16 aircraft, which were exempted from environmental regulations but not the FAA’s ban on civil overland supersonic flight. Current environmental regulations do not apply to supersonic aircraft, but an international regulatory structure is needed before most enterprises will invest the billions necessary to design and certify a new large aircraft. Acoustic theories developed in the 1980s and ’90s suggest that supersonic aircraft can be shaped to reduce the sonic boom reaching the ground. The NASA X-59 aircraft in development is designed to validate those theories and to generate data on community boom acceptance to inform rulemaking (Kamlet 2019). Whatever the finding on boom noise, profound challenges still exist for landing and takeoff noise and for emissions, especially CO2, which will be several times greater per passenger-kilometer than for subsonic jets. All things considered, supersonic commercial flight remains a very hard problem. Vertical Takeoff and Landing and Urban Air Mobility Perhaps the most exciting nearer-term opportunities are vertical takeoff and landing (VTOL) small drones of all sizes and shapes and larger passenger-carrying air taxis, also called personal air vehicles or urban air mobility (UAM). Inventors worked on flying cars for 100 years without notable success, stymied by issues with control and reliability. Such vehicles are now feasible thanks to (i) progress in autonomy, navigation, and control (which are beyond the scope of this article) and (ii) innovations in propulsion. Modern gas turbines are sufficiently light and reliable, but much too expensive for UAM applications. Electric propulsion might offer the potential for high reliability at lower cost. The short range needed for UAM reduces the importance of the very low energy density of batteries compared to jet fuel. From a propulsion point of view, such vehicles are technically feasible in the next decade, although the economics of these systems have yet to be established. Electric Propulsion Electric propulsion presents exciting possibilities. For airliners, the nearest-term possibility is a mild hybrid, with an electric motor reducing the engine power needed for a few minutes near the top of climb, helping during transients, and improving engine optimization. A 25 MW takeoff-power single-aisle aircraft engine produces about 7 MW at top of climb, and preliminary studies suggest that a 1–2 MW motor could help resize the engine and other aircraft subsystems to save 2–6 percent in fuel and 1–2 percent in energy. This level of savings, however, requires several hundred percent improvements in the weight of current electric motors, drives, and batteries. An aircraft that uses this electric power and energy storage for other functions might improve the attractiveness of these mild hybrid systems (Lents and Hardin 2019). Multirotor electric propulsion for VTOL offers the advantage of greatly reduced mechanical complexity compared to traditional helicopters, although more power is needed for the same vehicle footprint. The energy density of rechargeable, high-discharge-rate batteries has more than doubled over the last two decades, and significant progress is expected, driven by land transport applications. Packaged battery energy density is now approaching the level that enables battery-powered UAM vehicles at shorter ranges. (Airliner applications are discussed below.) There is also new emphasis on developing lightweight, high-efficiency electric motor technology and power system electronics. The efficiency of current aeronautical engines in the size range of up to 1,000 kW is only 15–30 percent, so battery-powered electric drives are more attractive at this size than for higher-power airline applications (figure 2). Challenges Design Validation and Certification New engines, which consist of 20,000–40,000 parts, are complex devices to engineer. A new 30,000 lb thrust class engine for a single-aisle aircraft currently requires about 50 months and $1 billion to certify, with the cost roughly evenly split between personnel and the hardware and its testing. A twin-aisle aircraft engine, with three times the thrust, costs about twice that. Benefits in fuel burn and economics of at least 8–10 percent are typically needed to justify the required investment. Ideally, only the testing needed to validate the design and certify the engine would be done. Currently, however, first principles analyses do not yield solutions of sufficient accuracy to meet design requirements in many areas, so empirical methods backed by more extensive testing are used. Engine testing needs and development cost could be significantly reduced with improved modeling tools, especially for combustor emissions, noise, aeromechanics, compressor stability, and part life. The large number of parts in an engine and variations introduced in manufacturing and operation have also generated a need for tools capable of moving from deterministic design practices toward probabilistic approaches. New Technology and Materials New technology and materials are needed to advance the state of the art (Epstein 2014). Better thermal efficiency requires structural materials and coatings with improved temperature capabilities and chemical resistance for both compressor and turbine parts. After decades of development, 1300°C-capable ceramic matrix composites are just entering service; research is now focused on 1500°C-capable ceramics. Titanium-aluminide was introduced in the past decade to reduce weight. The safety and maintenance implications of replacing ductal metals with low-fracture-toughness materials still represents a major challenge. Also, some advanced materials have proven susceptible to corrosion from atmospheric pollution such that the overhaul life of an engine operated in hot areas with poor air quality can be half that of an engine operated in cooler, cleaner air. The introduction of new material families often requires new manufacturing techniques. In this and other ways, manufacturing technology has a large role to play in both enabling design innovation and reducing cost. For example, the turbine airflow area per unit thrust has decreased by an order of magnitude since 1970, so fabricating increasingly smaller parts at high precision is an ongoing need. Advanced additive manufacturing is increasingly important as precision, internal flaws, and surface finish improve. Engine-Airframe Integration Engine-airframe integration is of growing importance as engine diameter grows. The current practice of wing-mounted engine pylons started in the early 1950s with the Boeing B-47. The pylons decoupled the wing aerodynamics from the propulsion system to a degree, but this isolation eroded as fan size increased. Engine nacelle diameter now approaches that of the fuselage because improving propulsive efficiency requires increasing fan area. Further area increases could come from either larger fans or more of them. Increasing fan diameter requires technologies that decrease the weight and drag penalties of nacelles and their installation. Another approach is to keep the fan and nacelle diameter small but to increase their number. The maintenance economics that favor two-engine aircraft must be addressed. Other concepts under study more tightly integrate the propulsion system with the airframe to reduce energy needs. Some, like the so-called D8 “double-bubble,” pass the fuselage boundary layer through the fans to improve the propulsive efficiency for the same fan area (Uranga et al. 2017). Such highly integrated configurations that depart from the traditional tube and wing architectures raise a new realm of technical challenges. The Climate Change Challenge and Propulsion Aviation’s most pressing challenge is climate change. Emissions CO2 is the principal anthropogenic driver of climate change. Aviation produces about 2 percent of the world’s man-made CO2, far less than ground transportation, and modern aircraft require less energy and produce less CO2 per passenger-kilometer than do cars and trains in the United States. Nevertheless, both the political threat and the need to act are very real. A decade ago, in response to this concern, aviation leaders pledged to reduce aviation’s CO2 to half that of 2005 by the year 2050. Since air travel is growing by 4–5 percent a year, this is a significant challenge. CO2 reduction requires some combination of reducing both the energy needed for flight and the net carbon associated with that energy. If aviation grows as projected, halving its CO2 by 2050 will require a fourfold reduction, far more than can be expected from aircraft energy-saving technologies. Reducing the carbon intensity of aviation energy must therefore be a major focus of research and development. More than 95 percent of aviation’s CO2 is emitted by airliners capable of flying 70 or more passengers thousands of kilometers, and large twin-aisle airliners, which typically fly average stage lengths of 6,500 km, account for over 50 percent (figure 3). Thus to achieve the desired global CO2 reduction, technology is needed for large airliners flying long distances rather than for general aviation or small regional aircraft. Figure 3 The challenge of low-carbon energy for aviation can be considered in two parts: the source of the energy and the manner in which it is stored on the aircraft. Two choices are electric energy stored in batteries and liquid fuels stored in tanks. Battery Power Given that jet fuel has an energy density of 12,000 Wh/kg and the latest long-range aircraft have 55 percent cruise thermal efficiency, the net energy density is 6,600 Wh/kg. Current automotive battery packs are in the 120–180 Wh/kg range, less than 3 percent that of jet fuel. There is no known battery technology that can power large electric airliners. While there are speculations on electric aircraft configurations that reduce fuel burn, none have been proposed that could not be implemented with fueled engines. There are also significant weight and reliability challenges with multimegawatt electric drives. Together, these imply that battery power will not be capable of halving aviation’s CO2 by 2050. Furthermore, even if there were a scientific breakthrough that enabled a battery-powered airliner, an electric airliner would not reduce aviation’s CO2 because new aircraft engines produce less CO2 per unit of energy than does the US electric grid (figure 2; Epstein and O’Flarity 2019). Of course, both should improve over time. Liquid Fuel Alternatives The only path forward with the technical potential to reduce aviation’s CO2 by factors of 2 to 4 by 2050 is improved aircraft and engines powered by fuel that does not release net CO2 into the atmosphere. One such fuel is hydrogen, which was first considered for military aircraft propulsion in the 1950s. It proved a poor fuel for high-speed aircraft because its low density and thus large tank volume increases aircraft weight and drag. There are also profound safety challenges for cryogenic aircraft fuels. The most promising fuel candidates are liquid hydrocarbons known as sustainable alternative jet fuels (SAJFs). They are sustainable because they can save 80 percent or more on CO2 and not adversely affect food and water supplies; alternative means they are compatible with existing aircraft and fuel infrastructure. Several SAJFs have been certified for use and are in limited commercial service (TRB 2019). SAJFs can be produced by many processes. Those certified to date include biofuels, Fischer-Tropsch fuels manufactured from CO and H2, and the conversion of alcohol to jet fuel. Much needs to be done to expand their availability. The principal challenges are economic, including capital and feedstock cost. Policy can play a role in shaping aviation’s energy supply, but that is beyond the scope of this discussion. Conclusion Civil aircraft propulsion faces important opportunities and challenges. Opportunities include the improvement of overall propulsion efficiency from 40 percent to 60 percent, elimination of aircraft noise, and realization of urban air mobility vehicles. Each application is rich in technical opportunities. Looming over the opportunities is aviation’s need to address the challenges of climate change. Given the expected growth rate of civil aviation, the only technically viable approach for reducing aviation CO2 by a factor of 4 is a switch to a low-carbon energy source, sustainable alternative jet fuels. References EIA [United States Energy Information Agency]. 2018. Electric Power Annual. Washington. Online at https://www.eia.gov/electricity/annual/pdf/epa.pdf. Epstein AH. 2014. Aeropropulsion for commercial aircraft in the 21st century and research directions needed. AIAA Journal 52(5):901–11. Epstein AH, O’Flarity SM. 2019. Considerations for reducing aviation’s CO2 with aircraft electric propulsion. AIAA Journal of Propulsion and Power 35(3):572–82. Kamlet M. 2019. NASA supersonic testing: Streets, suburbs, and sonic booms. Journal of the Acoustical Society of America 146:2753. Lents CE, Hardin LW. 2019. Fuel burn and energy consumption reductions of a single-aisle class parallel hybrid propulsion system. AIAA Propulsion and Energy Forum, Aug 19–22, Indianapolis. Spakovszky ZS. 2019. Advanced low-noise aircraft configurations and their assessment: Past, present, and future. CEAS Aeronautical Journal 10:137–57. TRB [Transportation Research Board]. 2019. Factsheet: Sustainable alternative jet fuels and emissions reduction. Online at http://onlinepubs.trb.org/onlinepubs/acrp/acrp_wod_ 41Factshe et.pdf. Uranga A, Drela M, Greitzer EM, Hall DK, Titchener NA, Lieu MK, Siu NM, Casses C, Huang AC, Gatlin GM, Hannon JA. 2017. Boundary layer ingestion benefit of the D8 transport aircraft. AIAA Journal 55(11):3693–708. Yutko BM, Hansman RJ. 2011. Approaches to representing aircraft fuel efficiency performance for the purpose of a commercial aircraft certification standard. MIT International Center for Air Transportation Rpt. No. ICAT-2011-05.  Also important for overall aircraft capability are the weight of the structure that supports the engine and the fuel that powers it.  About 90 percent of the CO2 is emitted on routes longer than 750 km. About the Author:Alan Epstein (NAE) is the RC Maclaurin Professor emeritus at the Massachusetts Institute of Technology and former vice president of technology and environment at Pratt & Whitney.