For NASA's aeronautical innovators, when it comes to designing the next generation of passenger-carrying airplanes, you can think of it as being about four E's: Environment, efficiency, electrification, and economy.
Like a set of Russian matryoshka nesting dolls, they fit within each other to provide a whole idea, one that especially resonates with what Earth Day is all about - working toward a cleaner environment at a time of global concern over climate change.
"Conceptually, it's really quite simple," said Robert Pearce, NASA's associate administrator for aeronautics.
"In order to lessen our impact on the environment we must increase aircraft efficiency in every way we can, integrate electrification to aid or replace current propulsion methods, and do it all in a way to benefit the economy," Pearce said.
To be clear, we're not talking here about coming up with a future airliner that flies faster than sound, or a smaller personal air taxi or package delivery aircraft of the type that will be part of Advanced Air Mobility. NASA already has resources dedicated to that.
Instead, the focus is on a future airliner that might carry 150-175 passengers, flies at subsonic speeds and could supplement or replace aircraft such as the Boeing 737 or Airbus 320 in the 2030 timeframe.
More specifically, starting with the environment - keep that vision of nesting Russian dolls handy for the next few sentences - the goal is to make aviation sustainable.
To make aviation sustainable you must reduce aviation's impact on climate change.
To reduce aviation's impact on climate change you must reduce greenhouse emissions.
To reduce greenhouse emissions - carbon dioxide being the biggest contributor - you must reduce the amount of fuel burned.
To reduce fuel burn, you must make the aircraft design more efficient. It must move through the air easier, possibly use electricity to augment or power the propulsion system, and it must be as lightweight as is safely practical.
As a result, NASA is focusing on four technologies to help deal with those efficiency challenges related to aerodynamics, propulsion and weight.
"These are technologies that will build from the foundation laid during previous NASA projects such as the Environmentally Responsible Aviation project and studies on future aircraft designs that we called N+3," said James Kenyon, NASA's manager for the Advanced Air Vehicle Program.
1. Electrified Aircraft Propulsion
Electrification in aviation is all about how you manage to propel your airplane forward so you can reduce the amount of fuel burned but still get the desired power during every phase of flight - from taxi, to takeoff, to cruise, to landing and taxi again.
"At the large aircraft level, maybe it's not fully electric. But if I can use electricity to help me out with certain parts of the flight envelope, I can design my engine differently and make it more efficient overall," Kenyon said.
This can mean an all-electric airplane in which electric motors turn propellers or fan blades to generate thrust. Such a capability could enable all sorts of new ways airplanes could be designed, either by modifying current airplanes or coming up with new configurations.
NASA's work on the all-electric X-57 Maxwell provides a glimpse of what might be possible.
Another configuration is a hybrid set up where both conventional jet engines and electricity are used to turn the fans during flight. The jet engines also can power generators to directly supply electricity to the electric motors, or to charge batteries for the electric motors to use later.
"Our plans are to test increasingly more powerful electric systems, up to one megawatt of power, first in a laboratory on the ground, and then later in flight on a testbed aircraft yet to be selected," said Fay Collier, NASA's director for flight strategy in the Integrated Aviation Systems Program.
2. Small Core Gas Turbine
Another way to get more fuel efficiency out of an engine is to change its configuration in terms of how air flows through it and at what pressures and temperatures.
For years, jet engines of the type seen on big commercial airliners have become more efficient by changing the amount of air flowing through the hot jet core of the engine vs. flowing around, or bypassing, the core through its fan blades - something called the bypass ratio.
In general, the higher the bypass ratio the more efficient the engine can be at generating thrust. But there is a limit - or at least there has been a limit - as to how big you can make that bypass ratio.
That's because the engine - core and fan blades - must be contained in a housing, or nacelle. This is a safety feature to contain and minimize any danger that might arise should an engine catastrophically fail in flight.
The problem is the nacelle of an engine hanging off the wing of an airliner can only be so big in diameter before it starts dragging on the ground. A minimum clearance is required, and you can only make the landing gear so long before it weighs too much or takes up too much room when stowed.
So, if you can't make the overall engine wider in diameter, yet you want to increase the bypass ratio so more air flows around the core, then the solution is to make the core smaller in diameter. This is one of the goals of the small gas turbine research effort.
The research will take advantage of earlier work with exotic metals, ceramics, and unique internal geometries to manage the increased temperatures and pressures that are a natural result of managing combustion in tighter quarters.
3. Transonic Truss-Braced Wing
Tackling the challenge of increasing the aerodynamic efficiency of an airplane moving through the air will be researched through continued studies of the Transonic Truss-Braced Wing (TTBW) aircraft concept.
One of the designs that came out of earlier research projects into future aircraft designs, the TTBW is essentially a classic tube and wing airplane but with a wing that is extremely long and thin. So long and thin, in fact, that it needs a little help on both sides of the fuselage to hold it up.
Such a wing stretched out to the proper length - known as a high-aspect ratio wing - generally creates the same amount of lift as the thicker, shorter wings you see on airliners today, but does so with much less drag.
"You could get some of the benefits of the thin wing without the truss, but the truss allows us to really extend the wing out to maximize its benefits," Kenyon said. "We can even fold up the wing tips so airport gates don't need to be rearranged."
Although other revolutionary aircraft designs have been studied - such as the Double Bubble and Blended Wing Body - the TTBW technology shows the most promise for being ready the soonest.
"We think the TTBW design and associated technology could be ready for manufacturers and airlines to consider using within the 10-year-future timeframe we're looking at, while the others might need another five to 10 years," Kenyon said.
4. High Rate Composites
Composite materials have been used in aerospace settings for decades. They can be fabricated into complex shapes, are structurally stronger and weigh much less than the same parts made from metal. They also last longer and are easier to repair when damaged.
But there remain opportunities to increase use composites in aviation, especially in the construction of big airplanes. Although the industry has made progress - fifty percent of Boeing's 787 Dreamliner is made of composite material - much work still needs to be done.
Two challenges related to a significantly increased use of composites need to be overcome.
The first has to do with reducing the time it takes go from concept, through design, fabrication, testing and then certification of materials by federal regulators charged with ensuring public safety.
The second has to do with increasing the rate at which composite parts - especially larger structural components - can be manufactured.
NASA's recently completed Advanced Composites Project addressed the first challenge.
"The project attacked that and put into place a lot of tools. From design methods to better modeling capabilities, inspection methods, and processes for automating parts of the fabrication that allow us to reduce the time to certify," Kenyon said.
To address the second challenge, NASA is planning a new technical effort focused on tackling the barriers for manufacturing composites at a high rate.
"What we need to address now is coming up with ideas for how composites can be manufactured in a way that is reliable, repeatable and results in a quality product that can be routinely certified as safe," Kenyon said.
Environmental and Economic Benefits
As plans for conducting research related to these four technologies continues to be made and executed, some might ask why is NASA doing this?
The answer is that all these efforts are part of NASA Aeronautics' Strategic Implementation Plan, which was developed through listening to the needs of other government agencies, industry, academia and other stakeholders in the future of aviation.
And the incentive for doing this work goes well beyond the sincere desire to help the planet's environment.
"We can invest in the things that are for the greater good, but we don't build, produce, or operate commercial airplanes. We just develop technologies so that industry can competitively bring these to market as desired," Kenyon said.
The good news is that the same set of technologies that can reduce carbon emissions are those that reduce fuel burn, which in turn reduce operating costs for the airlines. And if these new airplanes are attractive to the airlines, then manufacturers will want to build them, improving their bottom line as well.
"This all lines up our incentives so we can all work together in terms of something that is good for the climate, for sustainability, is something good for the market, and helps the U.S. maintain its role as a world leader in aviation," Kenyon said
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