Aerospace giant Boeing has announced it’s developing technologies that could result in a new “hypersonic” spy plane capable of flying five times the speed of sound.

But Boeing cautioned that any new Mach-5 spy plane could still be 10 or 20 years away. After spending decades and billions of dollars on hypersonic technologies, the aerospace sector is still struggling to make super-fast aircraft work.

Boeing scientist Kevin Bowcutt revealed the company’s latest hypersonics effort at an industry conference in Florida in mid-January. “We asked, ‘What is the most affordable way to do a reusable hypersonic demonstrator vehicle?’” Bowcutt said, according to Aerospace Daily, a trade publication. “And we did our own independent research looking at this question.”

The demonstrator vehicle, which would not be meant for day-to-day use, is still just an idea, Boeing spokeswoman Brianna Jackson told The Daily Beast. “Boeing is not currently developing a hypersonic airplane,” Jackson said. “However, we continue to conduct several studies around hypersonic technology. There will need to be further advances in several technology areas before an actual aircraft is feasible.”

Still, Boeing has gone as far as to draw up artwork depicting a possible Mach-5 spy plane. Bowcutt reportedly said that, in experimental form, the wedge-shaped vehicle would be roughly the size of an F-16 fighter jet—that is, around 50 feet from nose to tail. An operational version of the same vehicle would be much larger, with roughly the same dimensions as America’s last high-speed spy plane, the 107-foot-long SR-71 Blackbird, built by Lockheed Martin.

The twin-engine SR-71, which could fly faster than Mach 3, flew reconnaissance missions for the U.S. Air Force and CIA from 1964 to 1998. Satellites and stealthy drones replaced the expensive, hard-to-maintain Blackbirds, but never quite matched the plane’s capabilities.

Satellites are locked into fairly predictable orbits that leave gaps in their coverage. The latest drones, while stealthy, are subsonic and vulnerable to certain kinds of enemy countermeasures. In 2011, an Air Force RQ-170 stealth drone crashed on the border with Afghanistan after Iranian forces tracked the aircraft and potentially jammed its control signals.

As America’s rivals improved their defenses against stealthy aircraft, the Pentagon began mulling a return to Cold War-style fast spy planes that could simply outrun anything fired at them. In 2013 Lockheed unveiled its concept for a Mach-6 successor to the SR-71 that it called the SR-72. “The aircraft would be so fast, an adversary would have no time to react or hide,” the company claimed.

With the Pentagon’s encouragement and apparent assistance from the Defense Advanced Research Projects Agency, Lockheed has been working on the SR-72 since at least the 2013 announcement. DARPA failed to respond to written questions before this story’s deadline.

Separately, the Defense Department and aerospace firms have been working on hypersonic missiles and gliders. Space planes such as Boeing’s X-37B launch atop rockets, reach orbit, and eventually land like aircraft, reaching hypersonic speeds at multiple points in their flight profiles.

But engineers and scientists across the government and industry acknowledge the difficulty of designing a reliable, affordable, reusable hypersonic vehicle strictly for atmospheric flight. Mach-5 planes must withstand extreme temperatures. Equally challenging, the kind of engines that can most efficiently boost a vehicle to hypersonic speed are fundamentally different from the kinds of engines that sustain super-fast flight over long distances.

“The highly integrated nature of air-breathing hypersonic vehicles make them very difficult to properly design, and extreme heating from air friction requires hypersonic vehicles be made of very high-temperature materials and structures that are both light and durable,” Jackson said. “Integrating the engines and airframe in a manner that achieves high performance across a very large operating envelope exacerbates the design challenge.”

But there has been evidence of progress. In 2017 trade publication Aviation Week reported that a small-scale SR-72 prototype had been spotted flying over a Lockheed facility in California. At the same January conference where Bowcutt revealed Boeing’s new hypersonics effort, Lockheed vice president Jack O’Banion hinted, while discussing new digital design processes, that the SR-72 might already be flying in some form.

“Without the digital transformation, the aircraft you see there could not have been made,” O’Banion said, referring to an artist’s rendering of the SR-72. “In fact, five years ago, it could not have been made.”

Lockheed declined to respond to The Daily Beast’s specific questions about the SR-72. “A reusable hypersonic system is a far-term solution that will be made possible by the path-finding work we are doing today,” spokeswoman Melissa Dalton said. In its 2013 announcement, the company projected the SR-72 could be operational by 2030.

Jackson said Boeing expects a similar timetable for any hypersonic spy plane it develops. “While it would be premature to speculate precisely when hypersonic flight will be a reality, it is fair to say that it could be feasible looking 10 to 20 years into the future.”