Looking Back At The X-36

Since Boeing was announced as the manufacturer of the U.S. Air Force’s new crewed sixth-generation stealth combat jet, designated the F-47, last Friday, there has been no shortage of speculation about how the final aircraft might look. With the very limited imagery we have received so far, there are many more questions than answers, but there are certainly some interesting comparisons to be made — if not parallels — between the F-47 and earlier demonstrators and concepts, especially in the realm of tailless designs. One in particular, the Phantom Works X-36, has drawn perhaps the most interest since the F-47 announcement.

To catch up on everything we know about the F-47, as well as our initial analysis, click here and here.

As we noted at the time, the overall design of the F-47 fuselage appears very loosely reminiscent to Boeing’s X-45 UCAV demonstrators as well as the company’s highly stealthy and once secretive Bird of Prey demonstrator jet. There are certain parallels, too, with some of the Boeing concepts that emerged in the mid-1990s as part of NASA’s “Investigation Into The Impact Of Agility In Conceptual Fighter Design” study, which you can read more about here.

Above all else, in our initial analysis we pointed out the F-47’s superficial likeness to the X-36 Tailless Fighter Agility Research Aircraft, designed to be representative of a low-observable high-performance fighter. We were not alone. Many across the defense aerospace and social media space mentioned the similarities.

The uncrewed X-36 was developed in the mid-1990s by the McDonnell Douglas Phantom Works (now a part of Boeing), in collaboration with NASA.

The X-36 was a 28-percent scale representation of a theoretical advanced fighter aircraft configuration. The X-plane’s design did away with traditional tail surfaces and instead adopted canard foreplanes, combined with split ailerons on a lambda-like wing and an advanced thrust-vectoring engine nozzle for directional control. Since the X-36 was unstable in both the pitch and yaw axes, an advanced, digital fly-by-wire control system was used to ensure its stability.

Weighing around 1,250 pounds fully fueled, the X-36 was 19 feet long and three feet high with a wingspan of just over 10 feet. It was powered by a Williams International F112 turbofan generating around 700 pounds of thrust. Although two X-36s were completed, only one was ever flown.

The X-36 was flown remotely by a pilot in a ground station, aided by a video camera mounted in the nose of the aircraft. The pilot was provided with a standard fighter-type head-up display (HUD) and a moving-map representation of the aircraft’s position within the test range.

Ultimately, the X-36 completed 31 test flights at NASA’s Dryden (now Armstrong) Flight Research Center in Edwards, California, between May and November of 1997. According to NASA, over the course of 15 hours and 38 minutes of flight time, “The X-36 program met or exceeded all project goals.”

The X-plane’s agility was tested at high roll rates and low speed/high angles of attack and at high speed/low angles of attack. NASA determined that the aircraft “was very stable and maneuverable and handled very well at both ends of the speed envelope.”

The X-36 returned to the air for two more test flights in late 1998, after the Air Force Research Lab (AFRL) contracted Boeing to fly its Reconfigurable Control for Tailless Fighter Aircraft (RESTORE) software. This was to demonstrate how machine-learning software could compensate for in-flight damage or malfunction of flight control surfaces (flaps, ailerons, rudders, etc.).

Most intriguingly, in the context of current developments, is the fact that the X-36 was used to prove out tailless fighter design — the basic configuration concept that has very likely been selected for the F-47, though we can’t be sure about that until we see the rear portion of the finalized design.

Among the similarities between the two types, perhaps the most visible is the canard foreplane arrangement. At this point, it’s worth noting that the canards might still be absent from the final aircraft. You can read our full analysis of the canard question as it relates to the F-47 here.

While canards are not generally optimal for low-observability, there is a precedent for them appearing in advanced fighter designs, including stealth ones. They have also appeared before in NGAD-like concepts, especially from Boeing, as well as in the X-36 and even in earlier exploratory concepts for the Advanced Tactical Fighter (ATF), which gave birth to the F-22 Raptor.

There are also broad similarities between the canopy on the F-47 and the X-36. Although the X-36 was uncrewed, it was intended to replicate the flying qualities of a crewed fighter, so it featured a cockpit ‘shape.’ At the end of the test program, caricatures of the test pilots who flew the jet from the ground station appeared on the cockpit, applied in Magic Marker.

As might be expected, the F-47 has a relatively large, bubble canopy, providing the pilot with excellent vision. At this stage, we don’t know for sure if the F-47 will have one pilot or two, and the existing official renders don’t give an impression of the overall length of the cockpit. Tandem seats are certainly a possibility at this point, especially considering the drone controller role that the F-47 is expected to take on within the broader NGAD system of systems.

Then there is the F-47’s nose, which also has some distinct parallels with the X-36. In both cases, the nose is very broad (it’s especially wide in the F-47), with a shovel-like shape. In the F-47, this likely accommodates a very large radar array.

Overall, there are certainly some interesting visual similarities between the X-36 and the renderings that we have seen so far of the F-47. Again, this is with the proviso that these renders could be deliberately misleading as well as the fact that the design of the F-47 may yet change, perhaps even radically, before the final aircraft appears.

That being said, the X-36 was developed to explore the concept of a tailless combat aircraft design and, as a program that ended up in Boeing’s hands, it would be surprising if it hadn’t fed, at least in some way, into the company’s work on the NGAD crewed fighter.

While it’s long been understood that a tailless design offers significant advantages in terms of low observability, it comes at a price. Tailless aircraft are typically much less stable, with reduced maneuverability. This becomes a problem for a tactical aircraft that will be expected to operate over a very wide performance envelope. The addition of canards on the F-47 might be evidence of efforts to mitigate these deficiencies. Another option could be thrust-vectoring engines or a combination of both.

The X-36 is not the only tailless demonstrator or concept that may well have fed into the NGAD crewed fighter program, although its Boeing lineage is certainly notable.

There were also other important tailless fighter research programs, including the Lockheed Skunk Works X-44 Manta initiative, which doesn’t appear to have yielded any hardware — at least that we know about. Confusingly, the same X-plane designation was used for a different program, the X-44A, a flying-wing drone that was built by Skunk Works in 1999, which you can read more about here.

Otherwise, the X-44 Manta is understood to have been aimed at studying tailless crewed aircraft designs, also around the late 1990s. The notional aircraft would have used thrust vectoring as a primary flight control system, with the objective being to achieve a combination of speed, fuel efficiency, and maneuverability within a single design. Other objectives of the study included demonstrating simpler and cheaper forms of aircraft structures production. While we know little about the results of what remains a shadowy program, it’s not a stretch to imagine that, if it wasn’t canceled, it might have played a significant role in informing the design of the F-47 and the other NGAD crewed fighter designs.

While not a fighter with the extreme performance that goes along with it, the McDonnell Douglas/General Dynamics A-12 Avenger II stealth attack plane is another notable 1990s-era design that features a tailless configuration, albeit this had a triangular-shaped flying-wing type platform.

The aforementioned Bird of Prey, another Boeing product, is also worth looking at, as a largely tailless design, although it was provided with downswept wingtips and, at least for some of the time, a ventral vertical stabilizer. In fact, the renderings of the F-47 appear to indicate a significant degree of wing dihedral, a feature that is also very characteristic of the Bird of Prey. The technology demonstrator was flown clandestinely out of Area 51 in the 1990s and is understood to have yielded valuable information on various different technologies, as well as the nearly tailless configuration, including advanced stealth, new methods of manufacturing, and visual stealth. The Bird of Prey was only revealed to the public in 2002 and we may well yet learn more about its relevance to the NGAD crewed fighter program.

It’s also worth looking at the rumors around the so-called YF-24. Although the existence of this aircraft has never been confirmed, it’s speculated to have been a demonstrator for future combat aircraft designs, specifically tailless ones, and to have been a Boeing product. At one stage, an official U.S. Air Force pilot biography included a reference to them having flown an aircraft designated YF-24, although this was later redacted.

A Boeing design study, labeled MRF-24X, and reproduced below, shows a tailless fighter-like platform with a single engine and an X-32-style wing with significant anhedral. Presumably, like the X-36, it would have had an advanced thrust-vectoring engine nozzle to assist in directional control. Once again, however, we don’t know for sure what the YF-24 exactly looked like, if it did indeed exist at all.

Overall, it’s clear that tailless fighter design was a major future technology growth area all the way back in the 1990s, and there have likely been multiple other clandestine demonstrators that have helped prove its feasibility at Area 51. These would have been precursors to the two (and possibly more) demonstrators that were built and flown as part of the NGAD crewed fighter effort. There is also the Navy’s own F/A-XX NGAD initiative that likely has produced its own demonstrators.

While there is plenty more to discover about the F-47, as well as the secretive X-plane predecessors that fed into the NGAD crewed fighter program, it’s certainly interesting, at this point, to consider how it might have been influenced by some of these test programs that stretch back at least three decades. All in all, it seems highly probable that some of their results will have found their way into what is likely the Pentagon’s first true tailless fighters, although we still cannot confirm this at this time.

Contact the author: thomas@thewarzone.com