Single-aisle Turboelectric Aircraft with Aft Boundary-Layer Propulsion
A key priority for NASA Aeronautics is investing in electrified aircraft propulsion (EAP) technologies for their potential to reduce fuel consumption, emissions, and noise for commercial airplanes. Achieving these benefits could be a key step towards environmental sustainability for the entire aviation industry.
While electric propulsion may be the next revolution in air transportation, development is limited by current battery technology and electric charging infrastructure. Today’s batteries and maintenance systems are simply not sophisticated enough to sustain commercial all-electric aircraft. Many electric aircraft concepts currently being studied rely on future innovations to solve these problems and could be waiting decades for a suitable power system.
The STARC-ABL concept under development by ASAB aims to bridge the gap between current jet fuel-powered aircraft and future all-electric vehicles. A joint project between ASAB and other NASA branches, the STARC-ABL is a 150-passenger class commercial transport with a traditional “tube-and-wing” shape. The plane relies on turboelectric propulsion, meaning that it uses electric motors powered by onboard gas turbines.
The aircraft’s two traditional jet engines are mounted under the wings, which also contain electric generators. Electrical power is sent to the tail of the aircraft, where an all-electric propulsor takes advantage of an aerodynamic benefit known as boundary layer ingestion (BLI). The idea behind BLI is to reduce drag caused by slow-moving air that collects around the plane. In the STARC-ABL, this slow moving air flows through the tail-mounted engine and is sped up then released behind the airplane to reduce drag and improve fuel efficiency.
The STARC-ABL is a promising concept that could reduce emissions and airline ticket prices through lower fuel consumption. The aircraft has many similarities to current, conventional designs, which reduces risks and maintains the high level of safety enjoyed by today’s air travelers. The project’s completion will benefit not only aircraft manufacturers and airlines but also the flying public by outlining a path to more efficient and environmentally friendly aircraft.
Safe2Ditch is an autonomous crash management system designed to run on a small processor onboard unmanned aerial vehicles (UAVs). Designed to be inexpensive and light, the technology takes advantage of components that are already part of many small UAV systems such as visual spectrum cameras and commercial off-the-shelf (COTS) autopilots.
The system’s sole mission is to get the vehicle safely to the ground in the event of an unexpected critical flight issue. Safe2Ditch uses its intelligent algorithms, knowledge of the local area, and knowledge of the disabled vehicle’s remaining controls to select and steer to a crash location that minimizes risk to people and property. As the disabled vehicle approaches the intended landing site, Safe2Ditch uses machine vision to inspect the site to ensure that it is clear, as expected. Most importantly, Safe2Ditch does this autonomously—without any input from a pilot or ground operation.
Highly capable small UAVs provide substantial business opportunity, especially in suburban areas. Prominent businesses have proposed using UAVs to deliver goods and packages. Other potential uses include spotting wildfires and locating people during search and rescue missions. UAVs could also support law enforcement, including protecting endangered animals from poaching.
Current flight regulations require that each unmanned aircraft be monitored by a safety pilot. In case of emergency, the human pilots must be able to physically see the UAV they are flying in order to guide the aircraft to a safe landing spot. Human pilot interaction is cost-prohibitive for large-scale commercial applications and limits the use of these UAVs to line-of-site (LOS) operation.
Developing an autonomous emergency landing system like Safe2Ditch may help extend the use of small UAVs to beyond visual line-of-sight (BVLOS) operations by providing reliable emergency decision and control to replace the current need for a human pilot. The technology may help open the door for greater commercial use of UAVs by ensuring the safety of people and property on the ground, especially in populated areas.
Fostering Ultra Efficient, Low-Emitting Aviation Power (FUELEAP)
NASA has supported the development of fuel cell electric power systems for decades, a history dating back to Project Gemini in the 1960s. Continuing the legacy, NASA Langley’s ASAB branch recently led research to power future electric aircraft using a hybrid-electric fuel cell power system under the Fostering Ultra Efficient, Low-Emitting Aviation Power (FUELEAP) project.
Rather than relying solely on traditional jet fuels, electric fuel cells combine hydrogen and oxygen through a catalyst to generate electricity. Typically, hydrogen fuel is stored as a cryogenic liquid or high-pressure gas. However, the complex, heavy, and expensive storage tanks and plumbing required to store the fuel are not practical for smaller airplanes.
The FUELEAP project investigated a hybrid-electric, fuel cell power system that could create enough power for airborne electric propulsion yet be lightweight and efficient enough for use in small aircraft. The technology gathers hydrogen for the fuel cell by “reforming” traditional hydrocarbon fuels (diesel, gasoline, etc.) that could be stored on the aircraft without specialized storage systems or additional fueling infrastructure on the ground.
FUELEAP combined the technical advancements in Solid Oxide Fuel Cells (SOFC), high-yield fuel reformers, and hybrid-electric aircraft architectures to develop an integrated power system. The system was estimated to produce electricity from hydrocarbon fuels at twice the efficiency efficiently as typical airplane engines in a similar power class. The power system’s feasibility was studied using the X-57 “Maxwell” Mod II aircraft as a baseline.
A FUELEAP power system would generate energy more efficiently than the fuel burned in a standard piston engine, consequently saving fuel and reducing emissions. In addition, since the power system uses the same types of fuels as used in today’s aircraft and automobiles, it would require little to no additional fueling infrastructure. Meaning, this new technology could be adopted immediately by existing airports and would not require any expensive new facilities or equipment.
NASA’s X-57 Maxwell is an all-electric experimental aircraft designed to demonstrate multiple leading-edge technologies. The goal of the X-57 is to demonstrate that an all-electric airplane can be more efficient, quieter and more environmentally friendly than airplanes powered by traditional gas piston-engines.
The X-57 will demonstrate the use of a high-power distributed electric propulsion system for use on an aircraft, including a 460 volt battery to power 14 motors and propellers. In addition, the X-57 will demonstrate that vehicle cruise efficiency can be optimized by integrating the versatility and efficiency of electric propulsion into the vehicle design.
The X-57 began as a gas-powered Tecnam P2006T General Aviation aircraft in a phase known as Modification I. The wing, which is being reduced to 42% of the original size to significantly reduce drag, will feature wing-tip propellers to reduce the wing-tip vortex at cruise. At low-speeds, the distributed propellers nearly double the wing lift, allowing the X-57 to land as slowly as the original Tecnam P2006T. A test program is planned through a series of modifications, to allow researchers to take a step-by-step approach to demonstrate the technical improvements.
When complete, the X-57 Maxwell test program hopes to have demonstrated how to safely operate an all-electric, zero-emissions aircraft, including its battery and power distribution systems. That knowledge will be helpful to future engineers interested in designing all-electric air vehicles that might be used for everything from urban air mobility to moving passengers and cargo between nearby cities.
In collaboration between all four NASA flight research centers and Lockheed Martin, the X-59 QueSST is set to become the first low-boom supersonic plane.
Stemming from low-boom flight research that had its roots in ASAB for decades, NASA Langley and SACD has been instrumental in the development of the X-59. The ASAB supersonics team has been involved in the development of both the low boom shaping tools and methods being used to design the X-59 aircraft as well as having the lead role in the various concept studies conducted over the past few years that lead to the award of the X-59 contract with Lockheed Martin. This ASAB lead research was instrumental in establishing the confidence needed in the design to secure Agency support for this manned X-plane.
The FAA currently prohibits supersonic flight over land due to the loud boom caused by the shockwaves produced when the vehicle flies faster than sound. The engineers of ASAB, other NASA centers, and Lockheed have designed X-59 to divert the shockwaves, softening the boom. Instead, only a light thump would reach civilians below.
The first flights above civilians will occur in early 2023. Select communities will assess the noise level caused by the X-59 QueSST flying overhead. Then the results of this community testing will be presented to the FAA and ICAO in the hopes of changing regulation prohibiting supersonic flight over land.
X-59 QueSST will seat one pilot in its 94-foot body, cruising at 55,000 feet at Mach 1.4 (940mph). ASAB is also developing low-boom supersonic passenger jet concepts, guiding the future of aviation by bringing commercial supersonic flight into reality.
Parallel Electric-Gas Architecture with Synergistic Utilization Scheme (PEGASUS) Concept
In the last decade, airlines have shifted their focus from capturing market share to consolidating operations along the most profitable routes. Airlines have also increased the average load factor of flights and have shifted from under-50 seat regional aircraft toward 70-90 seat aircraft, which tend to have superior operating economics.
This new airline operations paradigm has contributed to the significant loss of connectivity for many regional airports. If an under-50 passenger regional aircraft could be operated economically relative to larger aircraft, it could potentially encourage airlines to open up new markets, reestablish service at smaller airports, and increase mobility and connectivity for all passengers.
The Parallel Electric-Gas Architecture with Synergistic Utilization Scheme (PEGASUS) concept is a novel hybrid electric regional aircraft that could reduce operational costs by lowering the amount of energy required to complete a given mission. The PEGASUS concept was designed to satisfy both hybrid (gas and electric) and purely electric missions. PEGASUS’s hybrid electric and electric propulsors are located strategically to provide increased aerodynamic benefits. PEGASUS uses parallel hybrid electric propulsors at the wingtips to decrease downwash effects, reducing the energy needed to maintain flight. Two electric propulsors providing additional thrust for takeoff and climb are located inboard on the wing. These propulsors are capable of folding mid-flight to decrease windmilling effects during cruise. Recent research suggests that adding a final electric propulsor to the tail of the aircraft will provide a benefit due to boundary layer ingestion.
The total energy needed to complete a mission decreases significantly when the propulsors are arranged on the airframe to provide an aerodynamic and propulsive benefit. Utilizing the size and placement flexibility of electric motors in the design of an electric or hybrid electric aircraft yields substantial energy cost savings, and could allow regional aircraft to compete economically with larger aircraft.