Just a couple of decades ago, aviation had some rules. If you wanted to fly fast within the atmosphere, you used a jet engine. The champion here was the SR-71 Blackbird, designed by Lockheed Martin, but that topped out at Mach 3. If you wanted to go faster, you needed a rocket. But that also meant hauling your own oxygen and operating more like a spaceship than a plane. This was before NASA’s X-43A came along.

The X-43A was an unpiloted aircraft with just a 12-foot-long airframe that managed to fly at ten times the speed of sound in 2004. It was a result of Hyper-X, a roughly $230 million research initiative designed to prove that a radical new type of engine, a scramjet, could actually work outside of a lab. Before it, scientists had only crunched the numbers in computer simulations and wind tunnels.

The X-43A couldn’t take off on its own. A massive B-52B bomber would give it that initial push, carrying the X-43A up to about 40,000 feet. From there, it would drop the craft, which was strapped to the nose of a modified Pegasus rocket. The rocket would then fire, blasting the X-43A up to its test altitude.

The test wasn’t a runaway success. The first attempt in June 2001 actually went south after the booster failed. This forced the team to spend two years re-engineering their approach. They came back with a vengeance in 2004. In March, the craft hit a blistering Mach 6.8. Then, on November 16, 2004, a second vehicle screamed through the sky at an incredible Mach 9.6, or nearly 7,000 miles per hour, at an altitude of around 110,000 feet. The engine only burned for about ten seconds, but in that tiny window, it proved air-breathing hypersonic flight was possible.

Helping achieve those impossible speeds is a technology called scramjet, which basically stands for “supersonic-combustion ramjet.” Unlike a normal jet engine that uses fan blades to squish air down, a scramjet’s working principle is that it has no moving parts. Instead, it uses the sheer speed of the aircraft to compress incoming air. The mind-bending part is that the air stays supersonic throughout the entire engine as fuel is injected and burned. This is a major engineering challenge because you have to sustain a flame in an airflow moving faster than sound. It’s also why scramjets can’t work at low speeds and need a rocket to get them going fast enough to “turn on.”

But all this trouble isn’t for nothing, as the big advantage is that scramjets breathe oxygen from the atmosphere — unlike rockets that have to carry their own heavy oxidizer. This means they can be smaller, lighter, or carry more payload. It’s all really fascinating, but the Hyper-X program was never meant to produce a production aircraft — or even be an ongoing mission. Rather, it was scoped as a three-flight research project from the start. After those two successful flights in 2004, NASA had all the important data it needed and wrapped things up.

The Hyper-X program never fully died, though. The baton was passed to the U.S. Air Force, which was tasked with figuring out what came next. That next step turned out to be the record-breaking Boeing X-51 WaveRider, a direct successor that took what the X-43A started and ran with it. In 2013, the X-51 showed just how far the tech had come by achieving a scramjet-powered flight that lasted a whopping 210 seconds.

But the real story is the ripple effect that tiny craft had. The data pulled from its flights sort of became the playbook for all subsequent American hypersonic programs. Engineers learned some massive lessons, like the fact that the entire vehicle has to be designed as one piece with the engine, and they got a treasure trove of info on how to handle the heat of hypersonic flight.

Even today, that 20-year-old flight data is still the ‘answer key’ engineers use to double-check their modern computer simulations for designing new vehicles. It’s also what keeps the dream of flying a plane straight into orbit from being pure fantasy.