The year was 1974, and Defense Research Projects Agency was becoming more and more interested in the idea that an aircraft, or a remotely piloted vehicle, could be almost totally invisible to enemy sensors. The aircraft that resulted was the F-117 Nighthawk, better known as the first stealth fighter. This is not that story.

This is all about the loser. The plane that could have been.

After fielding some secondary studies regarding the possibility of a jet that could pass completely unseen by enemy radars, the XST (eXperimental Survivable Tactical) program was formally launched and DARPA went searching for aircraft manufacturers to pursue the program’s goals.

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What DARPA was looking for in particular from the initial phase of the project was to answer two primary questions:

  • What were the signature limits that an aircraft would have to meet to be undetectable at an operational range from enemy sensors?
  • What could each aerospace company bring to the table when it came to designing and building an aircraft with the signatures levels established in question one?

Five major aerospace defense firms were approached to take part in the competition to build a scale model concept that would demonstrate a substantial reduction in radar cross-section for a tactical air vehicle. Originally, Lockheed was not even one of these companies. Word got around that the shadowy program was in the launch phase, and legendary Lockheed Skunk Works engineer Ben Rich went and persuaded the powers-that-be at the Pentagon to give him and his Skunk Works team a shot.

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He wanted in so badly that and he proposed competing for free, while all the other competitors would receive a paying contract for their work on the program.

Nowadays, it’s almost unbelievable to think that Lockheed’s the legendary and secretive Skunk Works division was not even on the Department of Defense’s list for the XST competition, but then again, the mid-1970s were a low time for Lockheed. Having not produced a fighter for more than a decade, their commercial aircraft division was also in deep public turmoil. The company was facing a possible firesale, after which its bones would be picked apart piece by piece.

Still, what many in the DoD did not know was that the Skunk Works built the first stealthy aircraft, the A-12 Oxcart and SR-71 Blackbirds, although its low-observable features were still highly classified at the time.

After starting with a half dozen competitors, the XST competition was narrowed down to consist of the aforementioned Lockheed Skunk Works team, a Northrop team and a McDonnell Douglas team that would later also drop out of the competition. Skunk Works, being accustomed to working on highly classified projects, had every element of the their XST team working together openly. Powerplant, flight controls, low observables, aerodynamics and so on, were all at the same collaborative design “table.”

Northrop, on the other hand, had built an almost firewall-like divide between the highly classified low observable folks and the less sensitive aircraft systems and design folks. This mistake would result in a very inefficient design process that would cost them later on in the competition, and would once again uphold Kelly Johnson’s management structure as superior to all others in the classified aircraft development business.

Team Skunk Works used a fairly obscure research paper from a Soviet scientist named Pyotr Ufimtsev to build a then cutting-edge computer program called “ECHO1,″ that could predict effects of radar waves on an object. This resulted in the famed “Hopeless Diamond” design, which was shaped like a rough-cut gem, and was fully faceted to reflect radar waves away from the transmitter/receiver from almost every direction with great efficiency.

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When it came to its radar signature, the ephemeral “Hopeless Diamond” was downright exciting. But when it came to its aerodynamics, it was a messy conundrum to say the least. Which was something that legendary high-performance aircraft designer Kelly Johnson, Ben Rich’s boss, was not too excited about.

Meanwhile, the Northrop team had been working closely with Hughes Radar Systems Group since early on for its XST contender. Hughes, the gold standard purveyor of American military sensors at the time, gave Northrop a deep theoretical understanding of how radars and infrared sensors detect targets, and what shapes were hard to detect under various conditions. With this in mind, and without Lockheed’s novel “ECHO1″ computer-based radar cross-section modeling program, Northrop’s design moved forward, albeit clumsily.

The aforementioned intense compartmentalization at Northrop, between the highly classified low observables team, the aircraft systems team, and the airframe design team, was proving to be almost impossible to work through. Some individuals who were active in Northrop’s XST program at the time have described this unsatisfactory arrangement as like trying to build the most advanced aircraft design in the world via playing a game of telephone.

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Nonetheless, leveraging their work with Hughes, the team began experimenting with different shapes and configurations, and in a learn-as-you-go creative process, a design began to materialize.

DARPA, by this time having realized the promise of low-observable technology, had upgraded the program from a theoretical design study to one that would provide a flyable prototype. With this in mind, the name of the program changed to eXperimental Survivable Testbed. A winner-take-all “pole off” showdown, in which scale models of both manufacturers unique designs would be evaluated mounted on a pole at a radar cross-section measurement range, was set for the summer of 1975.

Only the winner would get the chance to see their exotic design take flight as a real-life technology demonstrator.

Lockheed’s ”Hopeless Diamond” was tweaked a bit to better resemble a plausible aircraft. The whole design was still made up of a series of flat panels, or diamond-like facets, but its rear trailing edge would be notched in instead of shaped like one half of a diamond. It would also feature more highly swept wings, its inlets would be mounted behind both sides of the cockpit and the aircraft’s exhaust would exit through slits in the upper rear trailing edge of the fuselage to mask its infrared signature.

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Northrop’s design looked more like a plausible flying machine, with the cockpit set far forward and a large air inlet, covered by a fine mesh grill, was set high atop the fuselage. It didn’t feature a complex array of facets like Lockheed’s entry, rather it used smooth, broad surfaces and finely rounded edges to reflect radar energy, as well as a diamond-delta like wing platform. The aircraft’s exhaust were mounted deeply inward of the trailing edge, shrouded between the jet’s inward canted vertical tails.

Both designs were very impressive, to say the least, having achieved massive reductions in overall radar returns as well as dampening their theoretical infrared signature to a large degree. Northrop, not having the luxury of Lockheed’s “ECHO1″ program, and being handicapped by a fragmented design team, concentrated on making the aircraft as invisible as possible from its front and rear quadrants. Their thinking was that the most risk for a penetrating attack aircraft is posed when it is approaching and leaving the target area, so this is where their signature reduction goals were focused. The Northrop team accomplished this goal very well, but when the aircraft design was viewed by radar from the side hemisphere, the aircraft’s return “spiked” higher than Skunk Works’ Hopeless Diamond-based competitor.

The Northrop XST’s less competitive side-on radar signature seemed to be more of a result of the stiff compartmentalization within the Northrop design team than just their design philosophy alone, and it’s possible that with some tweaks the Northrop XST would have featured a lower overall radar cross-section than the Lockheed contender.

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Northrop’s XST design was also already optimized to have a lower cross section over a broader range of radar frequencies. Additionally, the argument was made that Northrop’s design would have provided better aerodynamic performance and airframe adaptability, as well as lower overall production risk than Lockheed’s wildly faceted design. In other words, there have been multiple voices, not just from within the Northrop camp, that think that the Northrop XST would have been a better choice than Lockheed’s design, especially considering how immature the designs, and their team’s accompanying low-observable knowledge bases really were at the time. Still, regardless of these opinions, Lockheed’s design best met the particular design goals laid out by the Pentagon, as such, there was no denying Lockheed won the pole off.

Interestingly, years after the XST competition concluded, Northrop’s non-faceted design philosophy seems much more ahead of its time than the Lockheed faceted approach, especially when you consider that second and third generation stealth aircraft and unmanned systems have much more in common with Northrop’s XST design than Lockheed’s XST design.

Still, both teams had solid manufacturing capabilities, competitive cost estimates, and aggressive timelines, so all things being fairly equal, Northrop’s slightly less stealthy pole model gave DARPA something to hang their final decision on, and the Lockheed Skunk Works design was chosen for flight testing. This action would result in the “Have Blue” technology demonstrators, then the “Senior Trend” project, which resulted in the YF-117 and eventually the famous F-117 Nighthawk stealth fighter as we know it today.

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The loss of the XST program did not mean the end for Northrop when it comes to low observable aircraft. Quite the contrary, in fact. The team regrouped and learned from its mistakes over the next few years, and eventually fielded the absolutely game-changing plane known as the Battlefield Surveillance Aircraft Experimental, otherwise known as the “Tacit Blue” technology demonstrator. This aircraft, aptly nicknamed “The Whale,” paved the way for Northrop to win the contract that led to the B-2 Spirit Bomber, build the YF-23 Advanced Tactical Fighter contender, and even the Global Hawk and X-47B unmanned aircraft testbeds as we know them today.

Strangely, the “Tacit Blue” concept would also indirectly lead to the Lockheed’s RQ-170 Sentinel, the General Atomics Avenger and and who knows how many other clandestine aircraft, both of the manned and unmanned variety, not to mention stealthy missile designs like the JASSM and JSOW. In fact, the rumored Northrop RQ-180 is supposedly basically the final implementation of the concept that “Tacit Blue” proved over thirty years ago.

Seeing how close the XST program decision was, and taking into account just how handicapped Northrop was by having two compartmentalized teams working on one integrated aircraft, as well as not having the help of the groundbreaking “ECHO1″ computer modeling program, one has to wonder just how successful their stealth aircraft could have been with more time to mature. I guess we’ll be left wondering what could have been if Northrop won the XST competition and became the first to market with a production level low observable combat aircraft.

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If this were the case, I have a feeling we would be seeing a YF-23-like design blasting off the runways at Nellis AFB during Red Flag sorties today instead of the F-22A Raptor.

Who knows, in an alternate reality where the Skunk Works’ “Hopeless Diamond” lost the XST competition, maybe Northrop’s version of the F-117 would still be in service instead of being largely locked away in their tomb-like hangars or being ripped to shreds and buried at Tonopah Test Range Airport, with just a handful kept flying for unknown purposes.

Then again, we got a pretty damn good bang for our buck out of Lockheed’s history making F-117A Nighthawk, so maybe we should just stop daydreaming.


Adapted form a previous piece posted at Aviationintel.com

Contact the author at Tyler@jalopnik.com.

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