That scramjet engine would propel the X-24C at sustained speeds in excess of Mach 6, reaching intended peak speeds that were higher than Mach 8, or more than 6,130 miles per hour. The aircraft itself resembled the lifting body design leveraged by the Martin Marietta X-24A and B programs that tested unpowered reentry flight characteristics.

That scramjet propulsion concept is still widely considered to be the most likely candidate for hypersonic aircraft to this day. A normal jet engine uses a compressor section (fan blades) to compress air to be mixed with fuel and ignited, but at hypersonic velocities, that compressor becomes a hindrance, slowing airflow to subsonic speeds as it passes through. A scramjet, in its simplest form, uses the immense pressure created by air flowing into the engine at supersonic speeds to handle compression, making it a far more efficient means of propulsion at such high velocities. According to NASA, scramjets can be good for speeds up to “at least Mach 15” and NASA scramjets have been successfully flight tested at speeds as high as Mach 9.6 (in the X-43A experimental unmanned hypersonic aircraft).

Had the X-24C been successful in testing, America would likely have leveraged scramjet technology in more platforms. And, while hypothetical-history isn’t good for much more than conversation at dinner parties, it seems awfully likely that with continued testing in this realm, the U.S. wouldn’t now find itself lagging behind nations like Russia and China in fielding operational hypersonic weapons. China and Russia both have hypersonic weapons in service today, with America hoping to follow suit in 2022.

But to extend that theorizing beyond missile technology, Lockheed’s Skunk Works were legends in their own time in the 1960s and ’70s for fielding the most groundbreaking military aircraft the world had ever seen. If anyone could make a hypersonic aircraft leveraging a combination of conventional and scramjet propulsion work in the 1970s, the guys who were making rocket-assisted F-80 fighters in the ’50s and the space-scraping SR-71 in the ’60s probably could.

But by the end of 1977, the L-301 program and its notional X-24C were canceled. Lockheed’s approach to hypersonics, it seemed, wouldn’t be the next revolution in aviation technology. Instead, it was a different design floating around the very same Skunk Works offices that was about to change military aviation forever. But while hypersonic aircraft flying at the edge of space might have seemed awfully promising at the time, this other program started out a bit more… hopeless.

The Hopeless Diamond’s First Kill Was America’s Need for Hypersonic Aircraft

On December 1, 1977, less than two months after Lockheed’s L-301 program was shuttered, a technology demonstrator dubbed “Have Blue” took to the sky. The unusual aircraft was covered in hard angles and jagged edges, and by all accounts, was tough to fly. And it should have been, the aircraft was based a computer-generated design so aerodynamically compromised that engineers had taken to calling it the “hopeless diamond.”

Unlike other military aircraft of the day, Have Blue hadn’t been built for speed. The real goal of the platform was to defeat radar detection, and tough to fly or not, it did just that. Before long, Have Blue matured into the world’s first operational stealth aircraft, the F-117 Nighthawk.

The F-117 ushered in a revolution in military aviation that continues to this day. Now stealth, not speed or altitude, has become the preeminent capability all military aircraft aspire toward. All told, America has invested some $2 trillion into stealth jets since the days of Have Blue, with two stealth fighters (F-22 and F-35) and one stealth bomber (B-2) currently in service and another bomber (B-21) in development.

But it’s not just America who pivoted away from speed in favor of stealth. Russia’s 5th generation Sukhoi Su-57 is certainly quick, but would lose every time in a drag race with a 50+ year old MiG-25. Their next stealth fighter, the recently unveiled Checkmate, promises to be even slower. China’s J-20 and forthcoming J-31, both stealth fighters, aren’t slow either, but they’re not pushing the envelope in terms of speed. All of today’s stealth fighters, it’s worth noting, have operational ceilings of under 60,000 feet, more than four miles lower than the SR-71 once operated. Today’s 5th generation fighters are engineering marvels capable of incredible things, but with no pressing need to fly higher or faster, they simply prioritize other capabilities.

Today, every new military aircraft in development is expected to leverage some degree of stealth in its design, and it stands to reason that one day in the not-too-distant future, nations like the U.S. will even operate stealthy tankers, AWACs, and even cargo planes. But stealth doesn’t mean invisible, and it’s no guarantee of a mission’s success. As stealth technology becomes more pervasive, nations will once again be looking for ways to gain the tactical and strategic advantage over the competition.

So, who knows. Maybe the days of hypersonic aircraft aren’t over. Maybe, just maybe, they’re just beginning.

This article first appeared at Sandboxx News and is being reprinted due to reader interest.

Image: Reuters