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.
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)
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 (