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FUNDAMENTAL AERONAUTICS - RESEARCH OPPORTUNITIES - SUPERSONIC
NASA loads-comparison test with 6 component force/moment balance and 1.7% high-speed research model 5 mounted in the 10x10 Supersonic Wind Tunnel at Glenn Research Center

The Supersonics Project is a broad-based effort designed to develop knowledge, capabilities, and technologies supporting all vehicles that fly in the supersonic speed regime. From this broad spectrum, the project has selected two classes of supersonic vehicle challenges. The first is eliminating the efficiency, environmental, and performance barriers to practical supersonic cruise vehicles. The second involves the project team working on the supersonic flight challenges of the High Mass Mars Entry System (HMMES) in partnership with the Hypersonics Project team.

The Supersonics Project has identified a set of key technical challenges that are barriers to success for these two vehicle types. These barriers are:

  • Efficiency challenges, including supersonic cruise efficiency, and lightweight, durable airframes and engines for supersonic cruise temperatures
  • Environmental challenges, including airport noise reduction, sonic boom modeling, and high-altitude emissions reduction
  • Performance challenges, including Aero-Propulso-Servo-Elastic (APSE) analysis and design
  • Entry, descent, and landing challenges, including supersonic entry deceleration
  • Multidisciplinary design, analysis, and optimization challenges
Understanding and exploiting the interactions of all these supersonic technology challenges is the key to the creation of practical designs.

Supersonic Project Key Research Areas:

Supersonic Cruise Efficiency

Propulsion Efficiency

Airframe Aerodynamic Efficiency

Lightweight, Durable Airframes and Engines for Supersonic Cruise Temperatures

Airport Noise Reduction

Sonic Boom Modeling

High-Altitude Emissions Research

Entry, Descent, and Landing Challenges: Supersonic Entry Deceleration

Multidisciplinary Design, Analysis, and Optimization

 

Supersonic Cruise Efficiency

In order to achieve mission viability, supersonic cruise vehiclesboth military and civilneed to achieve unprecedented levels of cruise efficiency. This efficiency must be preserved while providing excellent performance in the transonic and takeoff/landing flight regimes. This is especially true for transport-class aircraft because operating economics are more critical, and because such aircraft may be constrained to fly a larger portion of a typical flight at off-design conditions. There are two principal elements to supersonic cruise efficiencypropulsion efficiency and airframe aerodynamic efficiencyand these must be treated as an integrated challenge.

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Propulsion Efficiency

In order to achieve low-thrust-specific fuel consumption (TSFC) or high propulsion efficiency, the inlet, fan, core engine, bypass duct, and nozzle must be designed and optimized as an integrated system and installed in an optimal way on the airframe. Inlet-spillage drag, boundary-layer drag, inlet-bypass-bleed drag, and nozzle-aft-body drag need to be accounted for in determining the overall cruise efficiency. The inlet must achieve high-pressure recovery while maintaining good operability and stability. The engine cycle and the core engine must be designed to produce low-specific fuel consumption and high-specific thrust throughout the aircraft's operating envelope. Since a major portion of the flight of a supersonic aircraft occurs at higher altitudes than the subsonic aircraft, a substantial gain may be realized by utilizing a variable cycle engine (VCE) system. A VCE system enables the core-engine-bypass ratio to change during flight from the higher bypass ratio flow required during takeoff to a lower bypass ratio needed for improved performance at supersonic cruise. The higher bypass ratio flow required during takeoff enables the meeting of future takeoff noise regulations with a lower jet noise nozzle that must achieve a high thrust coefficient and efficiency. The propulsion control system must maintain optimum performance and stable operation at all subsonic, transonic, and supersonic operation conditions, and during transients.

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Airframe Aerodynamic Efficiency

Aerodynamic design for high-cruise lift-to-drag ratio and efficient performance at off-design conditions (transonic and low speed) must be achieved in concert with environmental and performance constraints. Integrated multi-point airframe designincluding propulsion installation effectsmust be applied, incorporating a broader range of design constraints. Drag reduction techniques and technologies must be explored and integrated into the overall airframe design. Advances in understanding of the complex surface and off-body flows for this vehicle type are required to achieve these unprecedented levels of aerodynamic efficiency. Detailed experimental databases of steady and unsteady flow quantities will be required to enable advances in computational fluid dynamics analysis and design methods. It is expected that novel and innovative approaches to concept design will be required. In addition, techniques such as advanced active flow control and aircraft morphing should be among the tools used to aid in optimal multi-point operations.

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Lightweight, Durable Airframes and Engines for Supersonic Cruise Temperatures

The airframe and propulsion system components must be lightweight while retaining appropriate durability and damage tolerance. The airframe life requirements for civil aircraft, combined with designs that incorporate slender fuselages and thin wings, indicate that airframe durability and damage tolerance must be studied in conjunction with lightweight material systems and structural configurations. Advanced airframe materials must be incorporated into innovative, light, adaptive structural concepts, optimized with the aid of advanced computational structural analysis tools.

Gas turbine engines will provide thrust during extended supersonic flight. Because of the extended supersonic cruise time with very high inlet air temperature, engine components will experience long duration operation at high operating temperatures throughout the cruise portion of the mission. Innovative and advanced cooling approaches will be required in addition to improved materials and coatings to ensure the life and durability of the hot section components of the engine. Improved materials with thermal barrier coating (TBC) and environmental coatings will be required for the combustor liners and turbine vanes and blades, and turbine and compressor disks will require improved materials. Creep and thermal mechanical fatigue properties of these hot section components must exceed those in subsonic engines. Combustor exit temperatures during cruise will result in turbine inlet temperatures about 300 degrees F higher than current subsonic engines. Significant weight reduction could be possible by introducing ultra-lightweight sandwich composite material/structural concepts for engine case and duct components. Making these case and duct components more multi-functional could enable other system-level benefits; for example, incorporating passive or active noise, thermal, or flow control features directly into case and duct structures could eliminate non-structural components that would otherwise be required, thereby enabling systems with reduced complexity and fewer parts.

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Airport Noise Reduction

International regulations regarding allowable aircraft noise are in effect for the commercial jet fleet and, in all probability, will be enforced for supersonic civil aircraft. These regulations pertain to the airport as well as surrounding communities. Significant noise reductions must be obtained in order to achieve supersonic aircraft that are as quiet as subsonic aircraft operating at the same airport. For propulsion noise, current subsonic aircraft meet requirements through the use of high-bypass turbofan designs. But high-bypass engine systems do not lend themselves to a supersonic cruise regime, due to large installed drag penalties. The supersonic aircraft requires the characteristics of a large bypass engine in the airport/community regions and the characteristics of the turbojet during the supersonic portions of the flight.

The challenge is to develop a combined cycle or variable-bypass engine cycle that retains the characteristics required for low noise at takeoff and landing and the high efficiency required for the supersonic portion of the flight. It is expected that these cycles will require variable geometry inlet and exhausts, and probably push the limits of acceptable noise. These two aspects of their anticipated behavior dictate the need for noise prediction methods that are robust enough to handle non-conventional geometries, including embedded engines, and accurate enough that the aircraft can be designed to a very close tolerance of the actual noise level with little margin for error. Current empirical prediction methods are ill suited for this task. Better tools for unique geometry nozzles are those that have an input that is Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD), and that rely on theoretically based acoustic analogies. A still better result would be achieved through directly simulated jets, in which full, unsteady Navier-Stokes equations are computed to the lowest resolutions. Recent advances in jet control actuators also need to be explored to understand how jets make noise and how to reduce the noise by manipulating the unsteady jet.

In addition to noise improvements via engine design, other noise-reduction concepts should be investigated for important engine and airframe sources. Innovative low-noise nozzles and propulsion integration concepts must be studied. Improvements in low-speed aerodynamic performance can also result in reduced airport community noise.

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Sonic Boom Modeling

Successful supersonic civil aircraft must be capable of supersonic flight over land. In order to achieve this end result, questions associated with sonic boom noise must be addressed. Existing knowledge is based primarily on experiments conducted during the 1960s as part of the United States Supersonic Transport (SST) program and the Concorde development program. It was concluded that high-amplitude sonic booms (Concorde ~2psf) were clearly unacceptable to a large segment of the population, and overland flight was prohibited. As a result, there has been little effort devoted to the study of sonic boom impactexcept for the military context of prediction of booms within training areas.

The ability to model sonic boom propagation from the aircraft to the ground will be a necessity, with all relevant physical phenomena included in such models. This will enable the accurate prediction of sonic boom levels on the ground under realistic atmospheric conditions and for all flight conditions. In addition, in order to fully understand human reaction to sonic booms, it is necessary to be able to predict the transmission of booms into buildings. Both interior noise levels and structural vibration are of interest since both are important characteristics of the indoor environment.

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High-Altitude Emissions Research

Supersonic aircraft cruise more efficiently at higher altitudes than subsonic aircraft. For lower-speed supersonic vehicles (~Mach 1.5 to 2.5) typical of a future commercial aircraft, flight will occur near 50,000 feet, or in the stratospheric ozone layer. Increased amounts of aircraft emissions, nitrogen oxides (NOx), carbon dioxide, water vapor, and particulates will be released into the stratosphere. Concerns are that these cruise emissions may cause ozone destruction and possibly influence other factors affecting the global climate. Continuing research is critical to establishing a fundamental understanding of combustion chemistry, liquid fuel atomization and vaporization, turbulence-chemistry interaction, fuel-air mixing, and particulate formation and transport that will allow the development of new combustor designs to minimize these harmful cruise emissions. (It should also be emphasized that emissions in the airport community are local air quality concerns as well.) The key challenge will be to achieve cruise emission levels lower than those currently targeted for commercial subsonic aircraft.

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Entry, Descent, and Landing Challenges: Supersonic Entry Deceleration

Technologies must be developed for the design and analysis of a new class of decelerators for large, high-speed planetary entry vehicles. These decelerators are needed to permit the safe entry and landing of substantially larger craft on Mars than those flown previously, with potential applications for human-scale missions as well as those of robotic spacecraft to other planets, or on sample-return missions back to Earth. New computational tools will be developed for calculating the dynamics and aerodynamics of the deployment and inflation of decelerators, as well as the static and dynamic stability of decelerators in their fully deployed state. Performance predictions of the vehicle/decelerator combination will be critical to ensuring mission success, and the calculated trajectory of the system during entry will be key to minimizing landing dispersion and increasing confidence in reaching the targeted landing area. High-speed, high-resolution experimental techniques for measuring the deployment and stability of decelerators will be developed, both for CFD validation and for supplementing the computational performance predictions. Investigations will be conducted into novel approaches that use rocket propulsion for slowing an entry vehicle through the supersonic speed regime in a planetary atmosphere.

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Multidisciplinary Design, Analysis, and Optimization

A wide variety of work is required that focuses on the combined supersonic propulsion system, airframe system, and overall vehicle. Challenges encompass the development of improved system-level analysis and design capabilities, the integration of discipline-level tools into vehicle-level analysis models, and system-level validation testing leading to building a multidisciplinary physics-based predictive design, analysis, and optimization system.

Some near-term efforts to meet these challenging objectives include:

  • Improved levels of propulsion performance predictions in several technical disciplines, such as the performance, weight, noise, and thermal management for supersonic applications, thus reducing overall propulsion system performance uncertainty levels.
  • The ability to simulate advanced propulsion concepts, such as a variable cycle engine (VCE) including the conceptual design of the integrated components of a VCE system (inlet, fan, core engine, low-pressure turbine, bypass duct with the bypass valve/exit guide vane, mixer/ejector, and nozzle).
  • Improved integration of existing airframe tools, such as the design capability to seamlessly integrate both performance and aerodynamic analysis codes. Emphasis will be placed on geometry tools and the integration of lower-fidelity analysis codes with higher-order CFD gridding tools and flow solvers.
  • Framework development (pursued jointly with the Subsonic Fixed Wing Project) involving improved flexibility and throughput of design frameworks and the development of tools for rapidly linking analysis tools into these frameworks. A key area of work is the development of guidelines for determining the level of analysis sophistication that is required to represent specific disciplines in the design process.

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