Most of the pricey machines NASA outlined in a 2019 budget proposal this week — a 32-story mega-rocket, moon landers, and Mars rover — will be launched to Earth's orbit or beyond.
But aside from those big-ticket items, the space agency's budget also requests funding for a curious airplane that currently exists only in sketches and the minds of NASA engineers: The X-Plane.
This aircraft is designed to travel faster than the speed of sound, and to do so relatively quietly, without sending sonic boom sound waves into unsuspecting neighborhoods thousands of feet below. If it works, NASA's X-Plane could revolutionize flight, cutting flight times in half and changing the way we fly.
If NASA has its way, the first X-Plane — a prototype — will take flight for the first time in 2021, according to the budget.
"We’re heading into an exciting phase here — we have the funds to proceed forward," said NASA engineer David Richwine, in an interview.
Flight engineers already know how to build planes that can break the sound barrier — take the retired European craft the Concorde, a commercial jet that could hold over 100 passengers, as an example. But governments would not allow the Concorde to fly over land in order to protect their citizens from the sonic booms the plane emitted.
Beyond that, sonic booms also produce blasting bursts of sound waves that jolt buildings.
"If you’re not expecting them, they can be startling," said Richwine, who is the Deputy Project Manager of Technology for the X-Plane, known more formally as the Low Boom Demonstration project.
The X-Plane, unlike the Concorde, will be designed to produce low booms that will sound like "a thump or a heartbeat," said Richwine.
The exact design of the plane is yet to be determined, but it will look something like the Concorde, said Richwine, with a needle-like body and swept-back wings, as opposed to the look of a traditional airplane's wings.
To break the sound barrier without creating massive shockwaves, the plane will also need to change shape during flight — though quite subtly. Most of those in-flight changes will be imperceptible to passengers, however.
"A little tweak goes a long way when it's traveling 30,000 feet to the ground," said Rodney Bowersox, director of the Texas A&M National Aerothermochemistry Laboratory, in an interview. "It’s not like transformers where the plane is really going to change shape or anything."
According to Bowersox, the plane will likely need to subtly change its shape during various weather conditions — like hitting a storm front, for example — in order to keep the sonic booms quiet.
"The goal is to shape the airplane such that the shockwaves generated by the plane don’t coalesce into a large shockwave that hits the ground," Bowersox, who leads a team responsible for making recommendations to NASA about how the shape of the plane will need to change in flight, said in an interview.
"You want a series of smaller waves so it sounds more like a rumble and less like a boom."
This could mean that the body, or fuselage, of the plane may need to change its shape by just 1 percent while in flight. This could take the form of small bumps that would extend or retract on a portion of the plane's body or wings to deflect sound. Engineers will spend the next few years figuring out what will produce low booms, and what doesn't.
These subtle changes are necessary for NASA's purposes with the X-Plane.
"You have to be able to fly over land for this enterprise to be commercially viable," said Darren Hartl, an aerospace engineer at Texas A&M University and member of Bowersox' design team.
Hartl added that the Concorde's business model didn't work out "because you could only go from New York to Paris."
Once NASA gets its first X-Plane flying, it plans to test it by flying over government lands at first, but eventually the agency plans to fly the supersonic craft over real U.S. communities.
However, before any of that happens, NASA is going to have some hoops to jump through.
The Federal Aviation Administration will need to approve any plane that changes its shape, even slightly, in-flight. In all likelihood, that approval will take years, though exactly how long remains unknown.
"It is a long process for a new airplane to be put in production," said Hartl.
"As exciting as it is, when it comes time to put your aunts and mother on the plane it becomes a very conservative business."