When Charles “Chuck” Yeager reached a speed of Mach 1.06 while flying the Bell X-1 Glamorous Glennis in 1947, he became the first man to fly faster than the speed of sound in controlled level flight. Specifying that he reached supersonic speed “in controlled level flight” might seem superfluous, but it’s actually a very important distinction. There had been several unconfirmed claims that aircraft had hit or even exceeded Mach 1 during the Second World War, but it had always been during a steep dive and generally resulted in the loss of the aircraft and its pilot. Yeager’s accomplishment wasn’t just going faster than sound, but doing it in a controlled and sustained flight that ended with a safe landing.

In that way, the current status of hypersonic flight is not entirely unlike that of supersonic flight prior to 1947. We have missiles which travel at or above Mach 5, the start of the hypersonic regime, and spacecraft returning from orbit such as the Space Shuttle can attain speeds as high as Mach 25 while diving through the atmosphere. But neither example meets that same requirement of “controlled level flight” that Yeager achieved 72 years ago. Until a vehicle can accelerate up to Mach 5, sustain that speed for a useful period of time, and then land intact (with or without a human occupant), we can’t say that we’ve truly mastered hypersonic flight.

So why, nearly a century after we broke the sound barrier, are we still without practical hypersonic aircraft? One of the biggest issues historically has been the material the vehicle is made out of. The Lockheed SR-71 “Blackbird” struggled with the intense heat generated by flying at Mach 3, which ultimately required it to be constructed from an expensive and temperamental combination of titanium and polymer composites. A craft which flies at Mach 5 or beyond is subjected to even harsher conditions, and it has taken decades for material science to rise to the challenge.

With modern composites and the benefit of advanced computer simulations, we’re closing in on solving the physical aspects of surviving sustained hypersonic flight. With the recent announcement that Russia has put their Avangard hypersonic glider into production, small scale vehicles traveling at high Mach numbers for extended periods of time are now a reality. Saying it’s a solved problem isn’t quite accurate; the American hypersonic glider program has been plagued with issues related to the vehicle coming apart under the stress of Mach 20 flight, which heats the craft’s surface to temperatures in excess of 1,900 C (~3,500 F). But we’re getting closer, and it’s no longer the insurmountable problem it seemed a few decades ago.

Today, the biggest remaining challenge is propelling a hypersonic vehicle in level flight for a useful period of time. The most promising solution is the scramjet, an engine that relies on the speed of the vehicle itself to compress incoming air for combustion. They’re mechanically very simple, and the physics behind it have been known since about the time Yeager was climbing into the cockpit of the X-1. Unfortunately the road towards constructing, much less testing, a full scale hypersonic scramjet aircraft has been a long and hard one.

A Tight Squeeze

In a conventional turbojet engine, an axial compressor is used to increase the pressure and temperature of ambient air as it enters the engine. This hot compressed air is then combined with atomized fuel and ignited in the combustion chamber, which causes it to expand and get even hotter. These hot gasses exit through the engine’s exhaust nozzle as a high velocity jet, but not before passing through a turbine which generates the power to run the compressor. It takes a delicate balance to get a turbojet engine running, and the multitude of rotors and stators which make up the compressor and turbine stages must be constructed to exacting specifications and of the highest strength materials. Turbojets are also limited to a maximum speed of around Mach 3; any faster and the engine simply can’t keep up with the pressure of the air entering the inlet.

In comparison, a scramjet engine in its most basic form doesn’t require any moving parts at all. Air moving through the engine still goes through the same three stages of compression, combustion, and expansion; but the difference is that the air entering the engine is moving so fast that the geometry of the inlet is enough to compress it to the point it’s ready for the combustion stage. With no compressor to power, the engine doesn’t need a turbine stage either, so the expanding gasses are free to leave the nozzle immediately. Since the air doesn’t need to be slowed down while moving through a scramjet, such engines are theoretically capable of operating at speeds up to Mach 24.

Like its supersonic counterpart the ramjet engine, scramjets are sometimes referred to as “flying stovepipes”, as they’re quite literally hollow tubes in which air and fuel are combined to produce thrust. It’s a design that’s so incredibly simple, at least in theory, that it almost seems too good to be true. So then why are we still struggling to develop a practical version?

Getting Up to Speed

The problem is that a scramjet engine doesn’t actually work until it’s physically moving at near hypersonic speeds. Any slower than Mach 4 or so, and the incoming air isn’t moving fast enough for it to become compressed inside the engine’s inlet. Accordingly, testing of scramjet engines thus far has been largely limited to mounting them to the front of conventional rockets in a one-time test that ends with the destruction of the engine. It’s a slow and expensive way to develop an engine, and has played a big part in holding practical scramjet development back.

So while scramjet technology was being studied as early as the 1950’s, it wasn’t until 1991 that one was successfully tested by the Soviet Union. Even then, it was a fairly limited proof of concept. It would be over a decade later, in 2004, that NASA really made serious headway towards a practical scramjet-powered vehicle with the X-43.

This unmanned aircraft was mated to a modified version of a Pegasus rocket and launched from the bottom of a B-52 bomber, much like commercial air launched orbital vehicles. Upon separation from the booster rocket, the X-43 fired its own scramjet engine for ten seconds to accelerate up to Mach 9.6. The program was a complete success, and the X-43 still holds the record as the fastest aircraft ever flown.

State of the Art

Even though its been fifteen years since the X-43 made its last flight, the cutting edge of hypersonic scramjet development really hasn’t progressed much. Plans by the United States to build an aircraft that combined the low-speed performance of a turbojet with the Mach 3+ capabilities of ramjet and scramjet engines were canceled in 2008; meaning testing still relies on complicated and expensive air launch programs.

In the United States, the direct successor to the X-43 program is the Boeing X-51 Waverider. Development on the X-51 started in 2005, just a year after the X-43 made its record breaking Mach 9.6 flight. In fact, the X-51 uses an engine that was originally intended for a later variant of the X-43 that was canceled in favor of developing a newer vehicle.

The X-51 first flew in 2010, but due to a number of subsequent failures it didn’t have a fully successful test until 2013. On that flight it was able to maintain a speed of Mach 5.1 until the engine’s fuel was depleted (approximately 210 seconds), after which the vehicle splashed down into the Pacific Ocean. It might not have flown faster than its predecessor, but the X-51 demonstrated it could fly for longer.

China is also reportedly working on several scramjet powered vehicles, potentially even a spacecraft which uses a hybrid rocket-scramjet propulsion system. Unfortunately there’s little public information about these programs, outside the handful of test flights that have been reported by Chinese media. Most recently Chinese media reported on the successful flight of the “Starry Sky-2”, generally believed to be analogous in design to the X-51, in August of 2018. Officials claim the vehicle attained a maximum speed of Mach 6, and flew under power for over 400 seconds. If these claims are accurate, it would have bested its American counterpart by a considerable margin.

The Future

For a hypersonic aircraft to be truly practical, it will need to be able to lift off under its own power and smoothly transition to its hypersonic engine while in the air. Lockheed Martin has proposed such a system, which they call the turbine-based combined cycle (TBCC), for their next-generation SR-72 reconnaissance aircraft. Comprised of a turbojet engine and ramjet which share a common inlet and exhaust nozzle, it’s an evolution of the concept used in the SR-71’s engines.

While it’s debatable if the SR-72 as envisioned will actually get built, Lockheed Martin has ready been pushing ahead with the TBCC engine technology as a stand-alone project. It’s even rumored that they have built and flown a small unmanned aircraft for flight testing. But even in the most optimistic of timelines, this research won’t produce a workable vehicle any earlier than the late 2020’s.

Excepting some military black project which the public doesn’t know about, a practical aircraft capable of reaching Mach 5+ under its own power by 2030 seems plausible. It took 44 years to go from the Wright Flyer to Glamorous Glennis, and it will be at least 80 years from that point until a practical hypersonic aircraft takes to the skies. Considering that we’re still tackling the finer points of practical supersonic aircraft and the relative complexity of the accomplishments, history will likely look back on this as a rational and necessary progression.