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The 747 is out. The green airplanes are coming.

After more than 50 years of production, the last 747 is taking to the skies. boeing delivered the last 747 ever built to Atlas Air on Tuesday. Aviation enthusiast John Travolta was there and said the plane was the “the most well thought out and safest aircraft ever built.” Richard Branson said:farewell to a wondrous beast” in an interview with Reuters, lamenting the high fuel costs of transatlantic jumbo jet flights. Airlines have taken a similar stance, slowing 747 sales reflected increased demand for smaller, more fuel-efficient aircraft. In fact, sustainability is also on Boeing’s mind.

Air travel is a massive contributor to climate change and is becoming increasingly popular. Flying accounts up to 4 percent of greenhouse gas emissions in the United States, and as more people fly, the United Nations predicts that carbon dioxide emissions from aircraft will triple by 2050. A transatlantic flight produces about a ton of CO2 per passenger, which is equivalent to about half of the carbon footprint a person would produce by eating food for a year. The Boeing 747, which seats over 500 people, is the third largest aircraft in the skyso you can imagine the environmental cost of maintaining the so-called “Queen of Heavens” to fly.

This isn’t the end of the 747—existing planes could stay in the air for decades—but it’s a pivotal time for the future of aviation. A couple of weeks before the big 747 goodbye, Boeing and NASA announced an important partnership, the Sustainable Flight Demonstrator project, to produce an extravagant-looking single-aisle aircraft that promises to dramatically reduce fuel consumption for commercial aircraft. The new aircraft looks like a giant glider with long, thin wings supported by diagonal struts to reduce drag. It’s called the reinforced transonic wing concept, and if widely adopted, it could transform sustainable air travel as we know it.

Unlike cars, you can’t just bolt a battery onto a plane and make it electric. (Make an electric vehicle it’s more complicated than that, but you get the idea.) Improvements to airplanes happen in small increments over the decades. Typically, a single-digit reduction in an aircraft’s fuel burn would be significant. Boeing says innovations in the new truss braced wing concept will amount to a 30 percent reduction. This is exactly the kind of leap NASA wanted from the Sustainable Flight Demonstrator project, which Boeing won.

“If you think or have the perception that aviation has not worked on sustainability or respect for the environment, this is a bad perception because each generation of aircraft that has come out has been 15, 20, 25 percent better than the one it replaces “What we’re trying to do now is skip a generation,” Rich Wahls, mission integration manager at NASA’s National Sustainable Flight Partnership, told Recode.

The big idea behind the reinforced transonic wing concept is an upgrade of the aircraft configuration or aircraft architecture. Unlike the low-wing design that dominates the commercial aircraft configuration today, Boeing’s new design has wings that extend over the top of the aircraft’s tubular body. This reduces drag, but also allows for a wider variety of propulsion systems, from larger jet engines to exposed propellers. It’s also fast. The “transonic” part of the concept name refers to its ability to fly just shy of the speed of soundor about 600 miles per hour.

NASA likes this idea so much is investing $425 million in the project under a funded space law agreement. Boeing and other partners will add another $300 million. Once Boeing builds a full-scale demonstration aircraft, NASA says it will complete testing in the late 2020s, and hopefully the public could see new technologies in commercial aircraft in the 2030s.

Conceptual drawings of three experimental aircraft in flight.

Here’s a sample plate of new aircraft configurations NASA is exploring. NASA

If you squint, though, the new transonic reinforced wing concept looks a lot like the commercial aircraft you see on the runways today. That’s not a bad thing. For one thing, it’s not a radical redesign, unlike, say, the very strange looking mixed wing X-48 – which could scare passengers. The similar design also has some advantages for the manufacturing process. But ultimately, the new aircraft configuration alone won’t make these next-generation aircraft any greener, according to Brent Cobleigh, project manager for NASA’s Sustainable Flight Demonstrator project.

“Lighter materials, better aerodynamics, better propulsion systems, more direct operations,” Cobleigh said, “you need all of these together to get as much efficiency as possible, to have maximum impact.”

Because, again, it’s really hard to make airplanes more efficient. And the aircraft configuration is only one piece of the puzzle. More efficient propulsion systems and cleaner jet fuel are the other two moving parts that need to fit together. Further down, we’ll see designs for hybrid propulsion systems that use both jet fuel and batteries to power an aircraft. All-electric aircraft they are already taking off, although it will be decades before we see large battery-powered passenger planes. In the near term, hydrogen increasingly looks like a viable substitute for the fossil fuels we currently put in aircraft. Rolls-Royce and easyJet successfully tested a hydrogen jet enginethe first in the world, just a few months ago.

What we will see before these big advances are more incremental improvements. Just a couple of weeks before the NASA-Boeing announcement, for example, Rolls-Royce showed off a new UltraFan propulsion system for planes, which offers a 25% increase in efficiency and can run on 100% sustainable aviation fuel, or SAF, which is a biofuel derived from waste material. Even if it is not a conventional fossil fuel, SAF still spits carbon in the atmosphere, and it is also in short supply. Some commercial flights already use SAF blended with conventional and United jet fuel did a demo last year with a flight from Chicago to Washington, DC, powered 100% by SAF. An innovation like SAF is certainly a step in the right direction — what you might call an evolutionary change — but it’s not what’s needed to make air travel as green as possible.

“The revolutionary change would be to change the energy source, for example, if you switch to hydrogen or hydrogen fuel cells,” explained Marty Bradley, an educator and sustainable education consultant who worked at Boeing when the company was exploring early truss-braced wing concepts. “It would be that big leap.”

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Two transonic braced-wing airplanes in flight. Boeing