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February 28, 2013

Superconducting Distributed propulsion - many small engines that are integrated with the airframe for radically different airplanes

Advances in computational and experimental tools along with new technologies in materials, structures, and aircraft controls, etc. are enabling a high degree of integration of the airframe and propulsion system in aircraft design. The National Aeronautics and Space Administration (NASA) has been investigating a number of revolutionary distributed propulsion vehicle concepts to increase aircraft performance. The concept of distributed propulsion is to fully integrate a propulsion system within an airframe such that the aircraft takes full synergistic benefits of coupling of airframe aerodynamics and the propulsion thrust stream by distributing thrust using many propulsors on the airframe. Some of the concepts are based on the use of distributed jet flaps, distributed small multiple engines, gasdriven multi-fans, mechanically driven multifans, cross-flow fans, and electric fans driven by turboelectric generators. This paper describes some early concepts of the distributed propulsion vehicles and the current turboelectric distributed propulsion (TeDP) vehicle concepts being studied under the NASA’s Subsonic Fixed Wing (SFW) Project to drastically reduce aircraft related fuel burn, emissions, and noise by the year 2030 to 2035.

Other aspects of this work were covered in 2009

All Superconducting motors could be three times smaller

Next Generation More-Electric Aircraft: A Potential Application for HTS Superconductors





Superconducting generators have already been demonstrated to exhibit power densities in the range of turbine engines thus validating the feasibility of future ultra lightweight machines for airborne applications.


An integrated electromagnetic/thermal model of superconducting rotating machines has been developed to be integrated into aeropropulsion system design/analysis tools developed at Georgia Institute of Technology and NASA Glenn Research Center. The model is composed of an analytical electromagnetic model and a lump-parameter thermal mode.

A superconducting engine design was patterned after that of the General Electric T700 turboshaft engine with a single spool compressor having multiple axial stages followed by a single centrifugal stage. The major technology in this propulsion system is obviously with the superconducting generator. The generator is designed using the methodology outlined in this paper, and the result is truly remarkable. The diameter of the generator at 10.24 inches is half that of the maximum engine diameter, and the light weight of the fully superconducting generator yields a power to weight ratio of 40 HP/lb (66 kW/kg). The generator rotates at engine rotational speed resulting in reduced torque and very light weight (335 lb each generator, with each turbine engine at 894 lb)

A fully superconducting motor outside diameter at 7.24 inches is an excellent match with the hub diameter of the fan exit, and the light weight of the motors is based on a power to weight ratio of 24.6 HP/lb (40 kW/kg), a lower power density that the generators. Each motor weighs 110 lb, and with cables included, the total turboelectric propulsion system weighs slightly more than 5100 lbs.

Detailed design studies for HTS propulsion motors supported by experimental validation have convinced us that superconducting rotating machines today can achieve power densities comparable with that of turbine engines (3-8 kW/kg).

This remarkable achievement, however, is still not enough for deployment into commercial aircraft. Electrically-propelled airliner aircraft would become feasible when power densities approach 25 kW/kg for motors and 50 kW/kg for generators, which appears to be achievable with fully superconducting machines (both inductor and armature).

The design examples of HTS motor-drive aircraft that were studied indicate that turbo-electric propulsion using superconducting machines can substantially contribute towards achieving the aggressive goals set for overall fuel efficiency. This is primarily due to the separation of power generation devices and propulsors, which offers an unprecedented level of design freedom facilitating the integration of short take-off capabilities into aerodynamically efficient body shapes (i.e, very high lift/drag ratio).

A development roadmap includes:

• Develop and demonstrate fully superconducting rotating machines in the range of 25-40 kW/kg for motors, and 40-80 kW/kg for high rotation speed generators (up to 15,000 RPM)

• Develop low AC loss HTS conductors (less than 10 W/Am @ 500Hz, equivalent to 10 µm filament) for fully superconducting machines

• Develop cryocoolers capable of 30% of carnot efficiency and weighing less than 3 kg/kW-input (or alternative lightweight refrigeration schemes)

• Refine the physics-based models for superconducting machines and ancillaries to continue exploration of aircraft design space and alternative concepts

Short Overview of superconductivity by EADS (Airbus) CTO in 2012.

Superconductor current applications
• Highly efficient and compact electric motors (expected to reach 30 kW/kg and more than 99% efficiency)

Superconductor potential use in aeronautic
• For a future electrically powered aircraft, superconductivity can be used for:
• High efficiency and power density cables
• High efficiency and power density electric motor

The sizing of a high speed, high power density, high temperature superconducting (HTS) electric generator is discussed here. Previous work discussed the advantages of a homopolar inductor alternator (HIA) machine topology for the high speed, high power density application - i.e., the enhanced magneto-motive force (MMF) capability of the HTS coil combined with high rotor tip velocity and a liquid cooled `air gap' wound armature. In this work we present the sizing/scaling of a family of machines based on this topology. The goal of this exercise is to obtain power entitlement and power density of the machine for a given physical size within mechanical, thermal and electrical constraints. A prototype machine was designed and tested validating the assumptions used in this sizing/scaling model. Effects of some key design changes are also discussed. Power densities in the range of 4.2-8.8 kW/kg can be obtained depending on the rotor material, and HTS wire, for 3-5MW rating. Comparison is made with high speed permanent magnet (PM) machines indicating a significant weight reduction - at least 500 kg for a 5MW machine or 1000 kg for a 15.6MW machine.

A large HTS bearing is being tested for stabilizing a 600 kg rotor of a 5 kWh/250 kW flywheel system.

Jet Flap

The jet flap is a concept where a high-velocity thin jet sheet emanates from a tangential slot at or near the wing trailing edge and provides spanwise thrust for cruise and supercirculation for high lift around the whole wing section during take-off and landing. The Hunting H.126 aircraft was built and flown in the 1960’s at lift coefficient CL = 7.5 and maximum operationally usable CL = 5.5. To enable such high lift, the engine diverted almost 60% of its thrust across its wing trailing edge to achieve very high lift capability.

Distributed Multi-Fans Driven by Few Engine Cores

Distributed propulsion employing multiple propulsors driven by a few fuel-efficient engine cores has been studied and is being pursued under NASA’s SFW N+3 project

* Gas-Driven Multi-Fans
* Gear-driven Multi-Fans
* Electrically Driven Multi-Fans

The following possible benefits of distributed propulsion concepts have been identified through various studies mentioned in previous section:

• Reduction in fuel consumption by ingesting the thick boundary layer flow and filling in the wake generated by the airframe with the distributed engine thrust stream.

• Spanwise high lift via high-aspect-ratio trailing-edge nozzles for vectored thrust providing powered lift, boundary layer control, and/or supercirculation around the wing, all of which enable short take-off capability.

• Better integration of the propulsion system with the airframe for reduction in noise to the surrounding community through airframe shielding.

• Reduction in aircraft propulsion installation weight through inlet/nozzle/wing structure integration.

• Elimination of aircraft control surfaces through differential and vectoring thrust for pitch, roll, and yaw moments.

• High production rates and easy replacement of engines or propulsors that are small and light.


Next Generation More-Electric Aircraft: A Potential Application for HTS
Superconductors (14 pages, 2008]
Fully superconducting machines have the
potential to be 3 times lighter.

Sustainability in the aviation industry calls for aircraft that are significantly quieter and more fuel efficient than today’s fleet. Achieving this will require revolutionary new concepts, in particular, electric propulsion. Superconducting machines offer the only viable path to achieve the power densities needed in airborne applications. This paper outlines the main issues involved in using superconductors for aeropropulsion. We review the work done under a 5-year program to investigate the feasibility of superconducting electric propulsion, and to integrate, for the first time, the multiple disciplines and areas of expertise needed to design electric aircraft. It is shown that superconductivity is clearly the enabling technology for the more efficient turbo-electric aircraft of the future.

Here is a propulsion system design that uses advanced superconducting, cryogenically cooled electric generators and motors to drive a multitude of low noise electric fans. The obvious break-through that must be achieved for this to happen is a marked increase in the power to weight ratio of electric generators and motors

Present-day high bypass turbofans

The bypass ratio (BPR), defined as the ratio of the mass flow rate of the stream passing outside the core divided by that of the stream flowing through the core, plays a key design parameter of the engine. A higher BPR, in general, yields lower exhaust speed, which serves to reduce fuel consumption and engine noise at the cost of an increase in weight and fan diameter

* Turbofans can be very compact with specific power in the range of 3-8 kW/kg.
* Recent engines such as the GE90 turbofan exhibit a BPR of 9:1.

The Case for Electric Propulsion

Torque and speed are coupled in turbofans, limiting any potential efficiency gain through speed control. Fig. 5.b illustrates a notional example of how HTS motor
technology can help relax this coupling. The electric propulsion scheme opens
up the aircraft design space to many new possibilities in which major leaps can be made towards achieving the performance goals. Decoupling torque and speed would lead to very valuable control flexibility to enable a more favorable trade between on-design and off-design performance. In addition, this architecture is intrinsically compatible with the emerging concept of “distributed propulsion” that produces thrust by means of multiple small propulsors or engines embedded on the wing or fuselage. This arrangement is anticipated to surpass other distributed propulsion concepts in many aspects. Such a system is feasible only if electrical motors can be of about the same size or better than aero turbines. Conventional motors exhibit a specific power up to 0.5 kW/kg. Superconductors can raise the specific power limits.


















Cryocoolers

Off-the-shelf cryocoolers exhibit efficiencies of about 10- 15% of Carnot efficiency, which correspond to about 70W/W at 30 K. The lightest cryocoolers today weigh about 5 lb/HPinput (or 3 kg/kW-input). This is just for the cold head portion, the associated compressors and ancillaries represent an overhead of about 5 times that weight. The use of packaged turbocompressors may reduce this overhead significantly, and coupled with the development of much lighter cold heads, it may be possible to reach the target of 3 kg/kW-input as overall specific weight for cryocoolers (2030-2035)

Superconducting Generators



LEI is developing a 3MVA/15,000 RPM generator.

General Electric used a bulk piece of magnetic material at the rotor magnetized by a stationary superconducting coil. This configuration provides a very robust rotor able to spin at high RPM. The flux distribution is not optimal but the high rotation
speed brings the power density to an impressive 7 kW/kg.

Superconducting motor for a Cessna has been made:

Total length 160 mm
External diameter 220 mm
Number of poles 8
Rotation speed 2700 RPM
Power 160 kW
Total mass (including conduction cooling apparatus) 30 kg
Power density 5 kW/kg
Heat load of superconducting part < 10W Operating temperature 30 K The turbine engines in a typical small business jet are about 1.5 MW. The concept described above is modular, and more HTS coils/YBCO plates can be stacked axially to increase power. The power density of this system was estimated to be 6.6 kW/kg, comparable to that of state-of-the-art turbines. A case study of an unmanned aircraft, fully electric, able to fly and loiter for up to 14 days without refueling or returning to base. For maximum efficiency, the superconducting motor for the propulsor needs to be both extremely light and compact, but also have very low losses. We chose a lead-less axial flux configuration (allowing for higher trapped flux for compactness). The design concept, described is projected to achieve an impressive power density of 7.4 kW/kg using conventional HTS materials available today.

















Superconducting Jetplane Design



A study is now being conducted to design short-field regional subsonic transport aircraft having a full payload of nominally 100 passengers. These aircraft are for the N+2 time frame, and the study has been extended to include a design having a
superconducting electric propulsion system (for possible N+3 introduction).

Superconducting generator is designed using the methodology outlined in this paper, and the result is truly remarkable. The diameter of the generator at 10.24 inches is half that of the maximum engine diameter, and the light weight of the fully superconducting generator yields a power to weight ratio of 40 HP/lb (66 kW/kg). The generator rotates at engine rotational speed resulting in reduced torque and very light weight (335 lb each generator, with each turbine engine at 894 lb).

Five fans per wing are installed above the wing with the exhaust nozzle near the trailing edge.

The fully superconducting motor outside diameter at 7.24 inches is an excellent match with the hub diameter of the fan exit, and the light weight of the motors is based on a power to weight ratio of 24.6 HP/lb (40 kW/kg), a lower power density that the generators. Each motor weighs 110 lb, and with cables included, the total
turboelectric propulsion system weighs slightly more than 5100 lbs.

The gross weight of the electric powered aircraft is approximately 5% lower than the turbofan powered aircraft primarily due to a reduction in the propulsion system weight.



A development roadmap includes:
• Develop and demonstrate fully superconducting rotating machines in the range of 25-40 kW/kg for motors, and 40-80 kW/kg for high rotation speed generators (up to 15,000 RPM)
• Develop low AC loss HTS conductors (<10 W/Am @ 500Hz, equivalent to 10 μm filament) for fully superconducting machines • Develop cryocoolers capable of 30% of carnot efficiency and weighing less than 3 kg/kW-input (or alternative lightweight refrigeration schemes) • Refine the physics-based models for superconducting machines and ancillaries to continue exploration of aircraft design space and alternative concepts FURTHER READING Compact superconducting power systems for airborne applications (3 pages)

A major issue with superconducting wire has been overcome with the recent introduction of the YBCO coated conductor. The latest 2G power cables can conduct up to 10 times the amount of power comparable copper cables manage.
* MEGAWATT AIRBORNE GENERATOR
* GYROTRON MAGNET
* COMPACT POWER CABLES
by using a high-temperature superconductor system (HTS) instead of copper wire, transmission power densities could be increased three- to ten-fold, and the system heat loss and weight could be reduced by 10-15 kW and 1500-3000 lbs., respectively.


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