NASAfacts

National Aeronautics and Space Administration

HEATheR's high-efficiency megawatt motor (HEMM) uses 

superconducting coils to carry large currents with almost no 

energy loss.

Megawatt-level electrified aircraft propulsion (EAP) systems reduce emissions and expand air travel, but they also generate a lot of 

heat. EAP concepts like these will need lightweight thermal management systems that do not produce drag.

HEATheR Activity

High-power, megawatt- (MW-) class electric aircraft 

propulsion (EAP) concepts have performance 

benefits for aircraft but produce large amounts 

of waste heat that must be rejected. How do we 

achieve the performance and operational cost 

benefits that EAP offers while minimizing the 

mass, drag, and power penalties of an associated 

thermal management system? Technology being 

developed as part of the High-Efficiency Electrified 

Aircraft Thermal Research (HEATheR) activity 

under NASA’s Convergent Aeronautics Solutions 

project seeks to address this challenge. MW 

electrical power systems produce a large 

amount of waste heat with relatively low rejection 

temperatures (<200 °C), requiring large, heavy 

thermal management systems that often produce 

additional drag.

The HEATheR solution is a two-pronged approach: 

first, minimize thermal load by building an innovative 

power system with four times lower losses; and 

second, manage the remaining waste heat through 

local air cooling or direct outer mold line (OML) 

cooling. 

Losses will be reduced by eliminating half of the 

conversion steps and associated components 

and by making low-loss components, including 

a high-efficiency MW AC-to-AC converter and a 

high-efficiency MW motor (HEMM). Heat losses for 

a state-of-the-art power system are 20%, whereas 

anticipated heat losses for the HEATheR powertrain 

are just 5%. 

Aircraft with MW electric power manage waste heat 

by moving it from the source to a heat exchanger, 

using a pumped fluid. The heat is rejected into 

the fuel and/or into the airstream through the 

heat exchanger with a penalty of additional mass 

and drag. HEATheR’s approach is to reduce the 

complexity of the thermal management system by 

locally rejecting heat through the skin of the 

aircraft, minimizing mass and drag penalty. This 

approach is called OML cooling.

Electric or Hybrid

Urban Air Mobility & Regional Air Mobility

1 to 19 passengers

≈1 MW power

≈200 kW heat

Hybrid or Partial Turboelectric

Regional Turboprops & Turbofan

20 to 150 passengers

1 to 5 MW power

200 kW to 1 MW heat 600 kW to 6 MW heat

Hybrid, Partial or Full Turboelectric

Single Aisle

150 or more passengers

3 to 30 MW power
National Aeronautics and Space Administration

Glenn Research Center

21000 Brookpark Road

Cleveland, OH 44135

www.nasa.gov

NP-2021-01-097-GRC

HEATheR Accomplishments

• Methodologies were developed to integrate power and 

thermal system modeling and sizing into the systems 

analyses of three NASA EAP concepts: a tilt-wing urban air 

mobility (UAM) reference concept, the fixed-wing SingleAisle Turboelectric Aircraft with an Aft Boundary-Layer 

Propulsor (STARC–ABL) concept, and the regional Parallel 

Electric–Gas Architecture with Synergistic Utilization 

Scheme (PEGASUS) concept. These methodologies 

showed the potential for a 15% decrease in fuel burn for 

the turboelectric tilt-wing concept, a 3% decrease in fuel 

burn for the STARC–ABL concept, and a 3% decrease in 

total energy for the PEGASUS concept. 

• The feasibility of OML cooling of electric components 

for conventional takeoff and landing EAP vehicles was 

established using computational fluid dynamics heat 

transfer modeling, heat pipes, and an oscillating heat pipe 

heat spreader.

• The HEATheR converter: Architecture for a 1.4-MW 

converter was designed, and a 72-kW version was built 

and tested for two critical feasibility items: interleaving and 

multilevel topologies for the converter power stage.

• The HEMM: A 1.4-MW high-efficiency motor prototype 

was designed, and feasibility was established for the 

superconducting coil pack, the slotless stator, and part of 

the integrated cryocooler.

HEATheR Benefits

• Tools and methods developed under HEATheR can be 

used for evaluation of new EAP and UAM concepts.

• Technology successfully developed through the HEATheR 

project could enable certain highly distributed or tilt-wing 

electric aircraft concepts and improve performance and 

operational costs for any MW-level electric aircraft.

Powertrain Goal:

Minimize Thermal Load

Thermal Goal:

Use Local Passive 

Thermal Management

State of the art: 20% heat loss

Compared with fluid cooling and air 

cooling, direct heat removal via outer 

mold line (OML) cooling has no substantial 

penalty of added mass or drag.

HEATheR: 5% heat loss

AC circuit

breakers AC to AC

1.5% 2% 1.5%

Generator Motor

Generator

DC circuit

breakers DC to AC

Motor

AC to DC

5% 5% 5% 5%

Dedicated OML cooling 

patches located near 

powertrain components 

and low structural load 

concentration zones

High-efficiency megawatt 

motor (HEMM)