Michael T. Tong

Performance and Weight Estimates for an Advanced Open Rotor Engine

NASA’s Environmentally Responsible Aviation Project and Subsonic Fixed Wing Project are focused on developing concepts and technologies which may enable dramatic reductions to the environmental impact of future generation subsonic aircraft. The open rotor concept (also historically referred to an unducted fan or advanced turboprop) may allow for the achievement of this objective by reducing engine fuel consumption. To evaluate the potential impact of open rotor engines, cycle modeling and engine weight estimation capabilities have been developed. The initial development of the cycle modeling capabilities in the Numerical Propulsion System Simulation (NPSS) tool was presented in a previous paper. Following that initial development, further advancements have been made to the cycle modeling and weight estimation capabilities for open rotor engines and are presented in this paper. The developed modeling capabilities are used to predict the performance of an advanced open rotor concept using modern counter-rotating propeller designs. Finally, performance and weight estimates for this engine are presented and compared to results from a previous NASA study of advanced geared and direct-drive turbofans.

Initial Assessment of Open Rotor Propulsion Applied to an Advanced Single-Aisle Aircraft

Application of high speed, advanced turboprops, or “propfans,” to subsonic transport aircraft received significant attention and research in the 1970s and 1980s when fuel efficiency was the driving focus of aeronautical research. Recent volatility in fuel prices and concern for aviation’s environmental impact have renewed interest in unducted, open rotor propulsion, and revived research by NASA and a number of engine manufacturers. Unfortunately, in the two decades that have passed since open rotor concepts were thoroughly investigated, NASA has lost experience and expertise in this technology area. This paper describes initial efforts to re-establish NASA’s capability to assess aircraft designs with open rotor propulsion. Specifically, methodologies for aircraft-level sizing, performance analysis, and system-level noise analysis are described. Propulsion modeling techniques have been described in a previous paper. Initial results from application of these methods to an advanced single-aisle aircraft using open rotor engines based on historical blade designs are presented. These results indicate open rotor engines have the potential to provide large reductions in fuel consumption and emissions. Initial noise analysis indicates that current noise regulations can be met with old blade designs and modern, noiseoptimized blade designs are expected to result in even lower noise levels. Although an initial capability has been established and initial results obtained, additional development work is necessary to make NASA’s open rotor system analysis capability on par with existing turbofan analysis capabilities.

Sensitivity of Mission Energy Consumption to Turboelectric Distributed Propulsion Design Assumptions on the N3-X Hybrid Wing Body Aircraft

In a previous study by the authors it was shown that the N3-X, a 300 passenger hybrid wing body (HWB) aircraft with a turboelectric distributed propulsion (TeDP) system, was able to meet the NASA Subsonic Fixed Wing (SFW) project goal for N+3 generation aircraft of at least a 60% reduction in total energy consumption as compared to the best in class current generation aircraft. This previous study combined technology assumptions that represented the highest anticipated values that could be matured to technology readiness level (TRL) 4-6 by 2030. This paper presents the results of a sensitivity analysis of the total mission energy consumption to reductions in each key technology assumption. Of the parameters examined, the mission total energy consumption was most sensitive to changes to total pressure loss in the propulsor inlet. The baseline inlet internal pressure loss is assumed to be an optimistic 0.5%. An inlet pressure loss of 3% increases the total energy consumption 9%. However changes to reduce inlet pressure loss can result in additional distortion to the fan which can reduce fan efficiency or vice versa. It is very important that the inlet and fan be analyzed and optimized as a single unit. The turboshaft hot section is assumed to be made of ceramic matrix composite (CMC) with a 3000 F maximum material temperature. Reducing the maximum material temperature to 2700 F increases the mission energy consumption by only 1.5%. Thus achieving a 3000 F temperature in CMCs is important but not central to achieving the energy consumption objective of the N3-X/TeDP. A key parameter in the efficiency of superconducting motors and generators is the size of the superconducting filaments in the stator. The size of the superconducting filaments in the baseline model is assumed to be 10 microns. A 40 micron filament, which represents current technology, results in a 200% increase in AC losses in the motor and generator stators. This analysis shows that for a system with 40 micron filaments the higher stator losses plus the added weight and power of larger cryocoolers results in a 4% increase in mission energy consumption. If liquid hydrogen is used to cool the superconductors the 40 micron fibers results in a 200% increase in hydrogen required for cooling. Each pound of hydrogen used as fuel displaces 3 pounds of jet fuel. For the N3-X on the reference mission the additional hydrogen due to the increase stator losses reduces the total fuel weight 10%. The lighter fuel load and attendant vehicle resizing reduces the total energy consumption more than the higher stator losses increase it. As a result with hydrogen cooling there is a slight reduction in mission energy consumption with increasing stator losses. This counter intuitive result highlights the need to consider the full system impact of changes rather than just at the component or subsystem level.

Engine Conceptual Design Studies for a Hybrid Wing Body Aircraft

ABSTRACT Worldwide concerns of air quality and climate change have made environmental protection one of the most critical issues in aviation today. NASA’s current Fundamental Aeronautics research program is directed at three generations of aircraft in the near, mid and far term, with initial operating capability around 2015, 2020, and 2030, respectively. Each generation has associated goals for fuel burn, NO x , noise, and field-length reductions relative to today’s aircrafts. The research for the 2020 generation is directed at enabling a hybrid wing body (HWB) aircraft to meet NASA’s aggressive technology goals. This paper presents the conceptual cycle and mechanical designs of the two engine concepts, podded and embedded systems, which were proposed for a HWB cargo freighter. They are expected to offer significant benefits in noise reductions without compromising the fuel burn. Keywords: hybrid wing body, fuel burn, noise, emissions INTRODUCTION More passengers and cargo are moved by air today than ever before, because of the global economy and worldwide connectivity. Over the next 15 to 20 years, the volume of air traffic is expected to at least double (for passenger traffic) or even triple (for cargo traffic) [1 and 2]. This robust growth rate causes growing concerns about the contribution that aircraft emissions will have on local air quality and global climate change. Chemical emissions of concern consist of anything that affects local air quality, global climate, or atmospheric ozone, including CO

Mission Analysis and Aircraft Sizing of a Hybrid-Electric Regional Aircraft

The purpose of this study was to explore advanced airframe and propulsion technologies for a small regional transport aircraft concept (approximately 50 passengers), with the goal of creating a conceptual design that delivers significant cost and performance advantages over current aircraft in that class. In turn, this could encourage airlines to open up new markets, reestablish service at smaller airports, and increase mobility and connectivity for all passengers. To meet these study goals, hybrid-electric propulsion was analyzed as the primary enabling technology. The advanced regional aircraft is analyzed with four levels of electrification, 0 percent electric with 100 percent conventional, 25 percent electric with 75 percent conventional, 50 percent electric with 50 percent conventional, and 75 percent electric with 25 percent conventional for comparison purposes. Engine models were developed to represent projected future turboprop engine performance with advanced technology and estimates of the engine weights and flowpath dimensions were developed. A low-order multi-disciplinary optimization (MDO) environment was created that could capture the unique features of parallel hybrid-electric aircraft. It is determined that at the size and range of the advanced turboprop: The battery specific energy must be 750 watt-hours per kilogram or greater for the total energy to be less than for a conventional aircraft. A hybrid vehicle would likely not be economically feasible with a battery specific energy of 500 or 750 watt-hours per kilogram based on the higher gross weight, operating empty weight, and energy costs compared to a conventional turboprop. The battery specific energy would need to reach 1000 watt-hours per kilogram by 2030 to make the electrification of its propulsion an economically feasible option. A shorter range and/or an altered propulsion-airframe integration could provide more favorable results.

Advanced Single-Aisle Transport Propulsion Design Options Revisited

Future propulsion options for advanced single-aisle transports have been investigated in a number of previous studies by the authors. These studies have examined the system level characteristics of aircraft incorporating ultra-high bypass ratio (UHB) turbofans (direct drive and geared) and open rotor engines. During the course of these prior studies, a number of potential refinements and enhancements to the analysis methodology and assumptions were identified. This paper revisits a previously conducted UHB turbofan fan pressure ratio trade study using updated analysis methodology and assumptions. The changes in propulsion, airframe, and noise modeling are described and discussed. The impacts of these changes are then examined by comparison to the previously reported results. The changes incorporated have decreased the optimum fan pressure ratio for minimum fuel consumption and reduced the engine design trade-offs between minimizing noise and minimizing fuel consumption. Nacelle drag and engine weight are found to be key drivers in determining the optimum fan pressure ratio from a fuel efficiency perspective. The revised noise analysis results in the study aircraft being 2 to 4 EPNdB (cumulative) quieter due to a variety of reasons explained in the paper. With equal core technology assumed, the geared engine architecture is found to be as good as or better than the direct drive architecture for most parameters investigated. However, the engine ultimately selected for a future advanced single-aisle aircraft will depend on factors beyond those considered here.

Turboelectric distributed propulsion benefits on the N3-X vehicle

Purpose – The purpose of this article is to present a summary of recent study results on a turboelectric distributed propulsion vehicle concept named N3-X. Design/methodology/approach – The turboelectric distributed propulsion system uses multiple electric motor-driven propulsors that are distributed on an aircraft. The power to drive these electric propulsors is generated by separately located gas turbine-driven electric generators on the airframe. To estimate the benefits associated with this new propulsion concept, a system analysis was performed on a hybrid-wing-body transport configuration to determine fuel burn (or energy usage), community noise and emissions reductions. Findings – N3-X would be able to reduce energy consumption by 70-72 per cent compared to a reference vehicle, a Boeing 777-200LR, flying the same mission. Predictions for landing and take-off NOX are estimated to be 85 per cent less than the Tier 6-CAEP/6 standard. Two variants of the N3-X vehicle were examined for certification noi...

A Probabilistic Assessment of NASA Ultra-Efficient Engine Technologies for a Large Subsonic Transport

NASA’s Ultra Efficient Engine Technology (UEET) program features advanced aeropropulsion technologies that include highly loaded turbomachinery, an advanced low-NOx combustor, high-temperature materials, intelligent propulsion controls, aspirated seal technology, and an advanced computational fluid dynamics (CFD) design tool to help reduce airplane drag. A probabilistic system assessment is performed to evaluate the impact of these technologies on aircraft fuel burn and NOx reductions. A 300-passenger aircraft, with two 396-kN thrust (85,000-pound) engines is chosen for the study. The results show that a large subsonic aircraft equipped with the UEET technologies has a very high probability of meeting the UEET Program goals for fuel-burn (or equivalent CO2 ) reduction (−15% from the baseline) and LTO (landing and takeoff) NOx reductions (−70% relative to the 1996 International Civil Aviation Organization rule). These results are used to provide guidance for developing a robust UEET technology portfolio, and to prioritize the most promising technologies required to achieve UEET program goals for the fuel-burn and NOx reductions.Copyright

Analysis of Turbofan Design Options for an Advanced Single-Aisle Transport Aircraft

The desire for higher engine efficiency has resulted in the evolution of aircraft gas turbine engines from turbojets, to low bypass ratio, first generation turbofans, to todays high bypass ratio turbofans. It is possible that future designs will continue this trend, leading to very-high or ultra-high bypass ratio (UHB) engines. Although increased bypass ratio has clear benefits in terms of propulsion system metrics such as specific fuel consumption, these benefits may not translate into aircraft system level benefits due to integration penalties. In this study, the design trade space for advanced turbofan engines applied to a single-aisle transport (737/A320 class aircraft) is explored. The benefits of increased bypass ratio and associated enabling technologies such as geared fan drive are found to depend on the primary metrics of interest. For example, bypass ratios at which fuel consumption is minimized may not require geared fan technology. However, geared fan drive does enable higher bypass ratio designs which result in lower noise. Regardless of the engine architecture chosen, the results of this study indicate the potential for the advanced aircraft to realize substantial improvements in fuel efficiency, emissions, and noise compared to the current vehicles in this size class.

Adaptive Engine Technologies for Aviation CO2 Emissions Reduction

NASA/TM—2006-214392 1 CO2. It should be noted that NOX generation is a function of the combustor design, and may or may not decrease with decreased fuel burn. This paper contains a brief description of some representative adaptive turbine engine technologies, followed by a description of the methodology used to assess their benefit for emissions reduction. A table is presented showing potential emissions reduction for each as a function of vehicle class. II. Adaptive Engine Technologies There are numerous technologies that are under development to adaptively modify turbine engine performance. These adaptive technologies can lead to improved engine component efficiency and/or reduced weight, both resulting in overall fuel burn reduction. As a rule of thumb, for a large subsonic aircraft a 1000 pound reduction in weight yields a 0.5-0.7% reduction in jet fuel consumed. For carbon based fuels, there is a 1:1 relationship between the amount of fuel burned and the amount of CO2 generated. Alternative fuels were not considered. The primary classes of adaptive technologies are flow control, structural control, combustion control, and also enabling technologies that are applicable to each. Representative technologies from each of these classes are briefly described below. A. Flow Control Flow control technologies directly manipulate air flow through or around a specific engine component. The manipulation is enacted by actively injecting or extracting air, by inserting small mechanical protuberances into the flow, or by using plasma actuators. Injected air can be supplied by bleed from a rear compressor stage, or by forming “synthetic” jets from a local cavity with an oscillating membrane that cyclically entrains and discharges air. Air injection is then used to energize low momentum regions within the main flow. The protuberances can be actively inserted and retracted based on flow conditions, or they can be designed to passively react to the flow; in both cases the intent is to influence boundary layer separation. Plasma actuators employ electrical actuation rather than pneumatic. Subsonic inlet flow control can be used, for example, to maintain performance, engine stability, and engine durability under a variety of flow conditions. This leads to shorter and more conformal inlet designs with reduced weight. Since for a large subsonic aircraft engine, inlets can contribute about 10% of the engine system weight, this reduction can be substantial. For supersonic inlets, bleed drag is often found to be the most significant component of inlet drag at cruise. Therefore, eliminating the bleed drag and the weight and complexity of the bleed system is a major thrust in modern flow control for supersonic inlet design. Similarly, fan flow control enables the fan to operate with poorer quality flow from the inlet. This can allow a tighter integration of the inlet and fan, providing good performance over a wide range of operating conditions with a shorter, lighter inlet. Flow control can be used to improve compressor performance by sensing pressure disturbances preceding flow separation, then energizing the air ahead of the separation line. Flow can be controlled through the airfoil to improve flow quality, and in the end-wall region to enable safe compressor operation at reduced stall margins. Both offer the potential to increase aerodynamic loading per blade without reducing aerodynamic efficiency, and thus offer the promise of reducing the number of airfoils (and therefore compressor weight) needed to achieve a given pressure ratio. 6,7 Reduced stall margins can also enable compressor operation closer to the peak efficiency operating point. For a large subsonic aircraft engine, compressor stages can be 15% of the engine’s weight, and a 1% improvement in high-pressure compressor efficiency can lead to 2% reductions in fuel burn. Flow control can be used to cool structures as well, such as closed-loop cooling control for turbine blades. By sensing hot-spots as they occur and only cooling as necessary, the total mass of bleed air can be reduced. Bleeding air from the compressor directly reduces the percentage of inlet air available for combustion, so bleed air reduction translates directly into propulsion efficiency improvement. B. Structural Control Actively controlling the clearances between rotating blades and shrouds directly improves fan, compressor, and turbine efficiency by reducing leakage through the clearances at each stage. Current engines are designed with sufficient clearance to minimize rubbing during flight. Typically these clearances are sized to prevent rubbing during take-off, and are thus larger than necessary during cruise. Excess clearance allows leakage through the gap, diverting air away from its intended path through the core or bypass ducts. Current open-loop clearance control systems use compressor and/or fan bleed air to cool the case during cruise and therefore close the gap. Closed-loop clearance control promises finer control of the gap while preventing rub-induced component degradation. For a NASA/TM—2006-214392 2 large subsonic aircraft engine, each 10 mils of excess clearance increases specific fuel consumption by roughly 1%. This will require an increase in exhaust gas temperature margins by about 10 °C, in order to maintain the same engine thrust level. The ability to maintain tight clearances can provide both a substantial fuel-burn reduction and increased engine life. These closed-loop active clearance control systems require robust, accurate and precise sensors and actuators. High-temperature, high-loading magnetic bearings and self-tuning vibration absorbers for engine blades can be used to adaptively control structural vibrations. Conical magnetic bearings can also be used for active compressor stall control. Prime reliant magnetic bearings can eliminate the need for existing oil systems, reducing the weight of engine peripherals. However, weight penalty can be large if auxiliary bearings are needed to handle blade-out load and as safety backup, in addition to the weight of the electrical power and control systems required for operation. Variable-area fan nozzles have been considered to enable low fan-pressure-ratio, high bypass-ratio thermodynamic cycles that operate well during both low speed operation (take-off and landing) and high speed cruise. These cycles improve propulsion efficiency, and therefore reduce fuel burn and emissions, although their benefits diminish with increasing fan pressure-ratio. Shape memory alloys have been investigated to provide up to 20% nozzle area variability, and are substantially lighter than conventional hydraulic actuators. C. Combustion Control Combustion control technologies are being developed to both enable lean-burning combustors and to directly control the local combustion process thus providing more uniformly efficient burning. A new generation of leanburning combustors is being developed to reduce emissions, but they are more susceptible to combustion instability and flame-out. Active combustion control provides closed-loop, dynamic control of fuel injection, fuel air mixing, and fuel source staging to disrupt the coupling between the combustion process and combustor acoustics leading to instabilities. Pressure sensors are used to monitor the combustor acoustics, and control laws are used to dynamically modulate high-response-rate actuators in the fuel line. To achieve uniform burning, sensor arrays determine the planar cross-sectional temperature distribution to drive actuators in individual fuel injectors. The larger the number of fuel injectors, the finer the control of the spatial distribution. “Pattern factor” control is also being investigated to produce spatially uniform combustion, eliminating hot and cold spots that generate NOX and CO2 emissions, respectively. Sensors determine either the local temperature distribution across a cross-section of the combustor, or sense emissions directly for use in closed-loop fuel injector control. D. Enabling Technologies Adaptive control can be either active or passive. Passive techniques include self-triggered mechanisms such as thermally-triggered shape memory alloys or microstructures triggering flow disturbances after a specific velocity has been reached. Active techniques require at a minimum a sensor, control logic, and an actuator. To achieve these, some subset of sensors, electronics, materials, actuators, wireless communications, power generation, and control logic are required. These technologies do not reduce emissions on their own, but they are critical for the practical embodiment of the aforementioned flow, structural, and combustion control technologies that directly reduce emissions. Specific sensors of use for adaptive engine components include: temperature and pressure sensors (both static and dynamic), surface and gas; mass flow, surface strain, and blade tip clearance sensors. Applications exist for each of these sensors throughout the engine, including the hot sections of the turbine and nozzle. In addition, specialized sensors for the combustor include fuel flow, chemical species, and temperature sensors that can withstand high temperatures (typically 1000 °C) and can operate in the presence of by-products from burning jet fuel. Not only the sensors need to operate at elevated temperatures; each sensor system typically includes processing electronics, and weight is reduced (hence fuel-burn reduced) by using wireless communications and locally-scavenged power. Actuators are needed for flow control in the inlet, fan, compressor, and turbine; clearance control in the compressor and turbine; and for fuel modulation. Desirable actuator characteristics include fast response times, low weight and bulk, and reliable operation in the engine environment. Active materials such as piezoelectric and shape memory alloys can be used as both actuators and sensors, including in the hot sections.