The year was 1974 and DARPA was becoming more interested in the idea that an aircraft, or a remotely piloted vehicle, that could be almost totally invisible to enemy radar. After fielding some secondary studies regarding the matter the XST (eXperiimental Survivable Tactical) program was formally launched and DARPA went searching for aircraft manufacturers to pursue the program’s goals. Originally, Lockheed was not even approached to be part of the competition to build a scale model concept that would show substantial reduction in radar cross-section for a tactical air vehicle, but word got around that the shadowy program was in the launch phase and legendary Lockheed Skunk Works engineer Ben Rich went and persuaded the DoD to give him and the Skunk Works team a shot. Nowadays, it is almost unbelievable to think that the legendary Skunk Works was not even on the DoD’s list for the XST competition, but then again it was a low time for Lockheed, having not produced a fighter for over a decade and their commercial aircraft division was also in deep turmoil.
The XST competition consisted of the aforementioned Lockheed Skunk Works team, a Northrop tea, and a McDonnell Douglas team that would later drop out of the competition. The 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” if you will. Northrop on the other hand had built an almost firewall-like divide between the highly secretive low observable folks and the aircraft systems and design folks. This mistake would result in a very inefficient design process that would cost them later on in the competition.
Team Skunk Works used a fairly obscure research paper from a Soviet scientist to build a computer program called “ECHO1″ that could predict the radar cross section of a an object. This resulted in the famed “Hopeless Diamond” design, which was shaped like a diamond and fully faceted to reflect radar waves away from the transmitter/receiver from almost every direction efficiently. When it came to signature control the “Hopeless Diamond” was downright exciting, but when it came to aerodynamics it was a messy conundrum to say the least.
The Northrop team worked closely with Hughes Radar Systems Group early on for their XST contender. Hughes, the gold standard purveyor of American military sensors at the time, gave Northrop a deep theoretical understanding of how radars and infra-red sensors detect targets, and what shapes are 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, although clumsily. The aforementioned intense compartmentalization between the aerodynamics and aircraft systems team and the highly classified low observables team was proving to be almost impossible to worth through. Some individuals who were active in Northrop’s XST program at the time have described this unsatisfactory arrangement like trying to build the most advanced aircraft design in the world via playing a game of telephone. None the less, 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, having already realized the promise of this new 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, in doing so the program’s title was more defined but its acronym “XST” stayed the same. A “pole off” showdown, where scale models of both manufacturers’ unique designs would be evaluated at a radar cross-section measurement range, was set for the summer of 1975. 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. 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 did not feature a complex array of facets like Lockheed’s entry, instead it used smooth broad flat 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, 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 a 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 when you are 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” much higher than their 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 then just their design philosophy alone, and some say that with some tweaks the Northrop XST would have featured a lower overall radar cross-section than the Lockheed contender by a serious margin. 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. 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 accompanying knowledge bases, really were regarding both potential aircraft. Interestingly, years later, Northrop’s non-faceted design philosophy seems more ahead of its time than the Lockheed faceted approach, especially when you consider that second and third generation stealth aircraft and UAV’s have much more in common with Northrop’s design than Lockheed’s design!
None-the-less, 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 project “Senior Trend” which resulting in the YF-117 and eventually the famous F-117 Nighthawk Stealth Fighter as we know it today.
The loss of the XST program did not mean the end for Northrop when it comes to low observable aircraft, quite the contrary. The team regrouped and learned from its mistakes over the next few years and eventually fielded the absolutely game changing BSAX, otherwise known as the “Tacit Blue” technology demonstrator. This aircraft, aptly nicknamed “The Whale,” paved the way for the B-2 Spirit Bomber, the YF-23 Advanced Tactical Fighter contender (and the aircraft that should have won the ATF contest over the YF-22) and even the X-47B UCAV testbed as we know it today. Strangely, the ”Tacit Blue” concept would also eventually lead to the Lockheed Skunk Works RQ-170 Sentinel, and who knows how many other clandestine aircraft, both of the manned and unmanned variety. In fact the rumored RQ-180, although I highly doubt that is its true name, an aircraft that this website has predicted to have been in existence for years now, is 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 fighter could have been with more time to mature. Additionally, Northrop now has a solid record for being incredibly innovative even with fewer resources at their disposal then the competition when it comes to designing stealthy tactical aircraft (see the YF-23). I guess we will 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. 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 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 locked away in their tomb like hangars or being ripped to shreds and buried at Tonopah Test Range Airport…
…Then again we got a pretty damn good bang for our buck out of Lockheed’s history making F-117A Nighthawk so we really need to stop daydreaming!