Designed as a revolutionary VTOL fighter, the Bell D-139 and its Vertiburners mainly served as a risky laboratory for vertical takeoff technology.
In summary
The Bell D-139 is one of those Cold War projects that never flew, but which says a lot about the ambitions and limitations of vertical takeoff aviation. Conceived in the mid-1950s as a daytime VTOL fighter for the US Air Force or US Navy, it was based on a bold concept: the Vertiburner, a system derived from a single turbojet engine whose gases were redirected and reactivated in vertical ducts to generate downward thrust. The objective was clear: to obtain a supersonic VTOL aircraft without adding more engines or sacrificing cruise performance. In practice, proof-of-concept tests revealed excessive complexity, poor efficiency, and high risks of hot gas recirculation and loss of control. The project was abandoned in favor of the Bell D-188, then the Bell D-188A, which featured more conventional solutions such as dedicated lift jets and tilt-jets. However, the D-139 played a discreet but real role in the evolution of vertical takeoff technology, highlighting what should no longer be done: relying on increased thrust systems that were too sophisticated and difficult to control in operation.
The context of Bell’s VTOL projects in the 1950s
In the mid-1950s, Bell Aircraft was at the heart of American technological excitement. The company had just emerged from the symbolic success of the X-1 and was already exploring the post-classic supersonic fighter era. The United States was obsessed with forward operating bases at the time: the ability to deploy interceptors or attack fighters without relying on large, vulnerable runways. This was the basis for numerous VTOL and V/STOL aircraft projects.
In this landscape, Bell multiplied its internal studies, grouped under “D-” designations. The D-118, D-139, and then the Bell D-188 formed a line of jet-powered VTOL concepts intended to offer performance comparable to that of a conventional supersonic fighter, while taking off vertically from small platforms. The D-139 was clearly identified as a “VTOL day fighter weapon system,” with the first design draft dating from around 1955.
The strategic logic was straightforward: bring the fighter closer to the front line, disperse the aircraft to survive a first Soviet strike, and avoid the vulnerability of large air bases. With this in mind, a VTOL program capable of fielding a supersonic vertical takeoff interceptor seemed attractive, despite the technical risks. Bell wanted to position itself as the champion of this niche, drawing on its experience with X-planes and its privileged relationships with the US Air Force and US Navy.
The D-139 would serve as the linchpin of this strategy. It had to demonstrate that a single turbojet engine, intelligently operated, could provide the thrust for both horizontal and vertical flight, without resorting to a forest of additional engines that were heavy and costly to maintain. This is where the idea of the Vertiburner came in.
The concept of the Bell D-139 Vertiburner as a VTOL fighter
The few illustrations and descriptions available show the Bell D-139 as a compact, single-seat fighter with swept wings, reminiscent in its proportions of certain interceptors of the time. A single main turbojet engine provides propulsion in horizontal flight, with a conventional side or ventral air intake depending on the version under study.
The originality lies in the lift architecture: instead of adding dedicated vertical engines, the D-139 uses internal ducts connected to the main engine. During vertical takeoff, part of the hot gas flow is diverted to downward-facing nozzles located in the fuselage and/or wings. These nozzles do not simply deflect the flow: they increase it, thanks to the Vertiburner system, to generate sufficient additional thrust to lift the aircraft off the ground.
This concept should make it possible to maintain a relatively slim airframe, with a fuel capacity comparable to that of a conventional fighter, while offering acceptable lift thrust. On paper, the aircraft promises transonic or even supersonic cruising speed, with a takeoff weight compatible with the turbojet engines available at the time. The goal is to obtain a VTOL fighter capable of intercepting high-speed bombers while operating from dispersed sites.
But the price to pay is obvious: the airframe becomes a maze of ducts, bypass flaps, valves, and steerable nozzles. The center of gravity, the center of vertical thrust, and the control moments had to be aligned with great precision, otherwise the transition phase would become uncontrollable. At the time, fly-by-wire systems and digital computers did not yet exist. The pilot’s hand and a few relatively rudimentary assistance systems were relied upon.
Vertiburner technology and thrust vectoring
The Vertiburner is the heart of the concept, and also its main weakness. Detailed technical documents remain scarce, as much of this work was recorded in internal reports or studies such as those by Bell Aerosystems on supersonic V/STOL fighters. But the clues point to a system similar to a bleed-air burner or ducted rocket, using air taken from the compressor or hot gases from the main turbojet engine, reinjected into vertical ducts where additional fuel is burned.
In other words, the Vertiburner is a form of internal vector thrust propulsion: rather than tilting the entire engine like a tilt-jet, the engine is kept fixed and the thrust is distributed through oriented outlets. The ducts are designed to draw in ambient air, mixed with engine gases, in order to increase the flow mass and thus the thrust for the same fuel flow. On paper, the efficiency is higher than that of a simple exhaust jet deflection.
In practice, several problems immediately arise. The first is fluidic complexity: the ducts must handle high-temperature flows, with significant pressure drops, flow separations, and the risk of hot gases being reingested by the engine air intakes. The second is thermal: the internal walls are subjected to very high thermal flows, without the space available for massive thermal protection.
Finally, fine modulation of vertical thrust is difficult. The turbojet flow rate, the diversion to the ducts, the additional combustion in the Vertiburners, and the roll, pitch, and yaw control via small nozzles or reaction nozzles must all be coordinated. This is already a delicate task with modern digitally controlled systems; in the 1950s, it was frankly ambitious.
Indirect feedback indicates that the solution did not deliver the expected performance. In forums and historical reviews, the D-139’s Vertiburner system is often cited as being as inconclusive as the XFV-12’s augmentor wing or the XV-4’s jet augmentor. Technically brilliant, but operationally fragile.
The technical and operational limitations of the Bell D-139
In a context where armies were eager to obtain real capabilities, the Bell D-139 accumulated a number of disadvantages. In terms of weight, the network of ducts, valves, and nozzles was heavy. Every kilogram devoted to vertical propulsion is one kilogram less for fuel, radar, or weapons. The theoretical advantage of a single engine is quickly reduced when the weight of the structure and associated systems is added.
Operationally, the philosophy raises questions. A VTOL fighter aircraft must be capable of taking off from rudimentary sites, which are sometimes damaged or dirty. However, a Vertiburner system blows hot jets of air in close proximity to the airframe and the ground. The risks of debris ingestion, coating deterioration, and thermal damage to the ground are high. On a smaller scale, we see issues that would later affect aircraft such as the Harrier and the F-35B, but without the modern design and piloting solutions.
Maintenance is another sticking point. A single, heavily used turbofan engine, which provides both horizontal and vertical thrust, becomes a critical point of failure. An engine failure during takeoff or vertical landing leaves very little margin for error. Added to this is the maintenance of the ducts and secondary combustion chambers, which are difficult to access.
In a budgetary environment where the Armed Forces are looking for credible short- or medium-term solutions, this type of configuration quickly appears to be a dead end. Decision-makers realize that other avenues—dedicated lift jets, tilt-jets, tilt-rotors—offer more realistic trajectories, even if they are not perfect. The Bell VTOL program based on the D-139 was therefore redirected towards a less exotic approach.
From Bell D-139 to Bell D-188 and D-188A: the industrial legacy
The failure of the Vertiburner did not spell the end of the VTOL concept at Bell, quite the contrary. The company took the Bell D-139 as a basis, eliminated the Vertiburner system and replaced it with separate lift engines: this was the genesis of the Bell D-188, then the Bell D-188A, often mistakenly referred to as the XF-109.
This shift is instructive. It effectively marks the abandonment of the illusion of a “magic” engine that could do everything. Bell agreed to pay the structural price of several turbojet engines, some dedicated to vertical flight, others to horizontal flight. This was heavier and more complex in terms of logistics, but more controllable in terms of aerodynamics and thermal performance.
The D-188A also remained a project with no operational outlet, but it provided building blocks that would feed into other work, up to the X-22 and, later, the studies that would lead to modern tiltrotors. The lineage is clear: without the D-139 and its Vertiburners, Bell’s learning curve on vertical takeoff technology would have been different.
From an industrial perspective, these projects also reflect the logic of the 1950s: multiply concepts, push studies very far on paper, sometimes to the point of building a model, then cut them short as soon as a major obstacle appears. The D-139 is typical of this phase: little known, poorly documented, but significant of an era when people did not hesitate to explore solutions that would be considered unreasonable today.
The real contribution of the Bell D-139 to modern VTOL technology
It would be easy to dismiss the Bell D-139 as a failed curiosity. But that would be to miss the point. The project made it possible to test—at least in theory and experimentally—the limits of an approach focused on increased thrust via a single engine. The problems encountered with Vertiburners showed very early on that thrust augmentation systems based on complex internal ducts and secondary combustion were extremely difficult to operate.
This lesson has been learned in other programs. The Ryan XV-5 Vertifan, the Vought XFV-12, and certain wing-blowing demonstrators have each confirmed in their own way that these solutions remain fragile, sensitive to flight conditions, and difficult to maintain. On the negative side, the D-139 helped refocus efforts on more robust VTOL architectures: tilt rotors, dedicated lift fans, or simpler combinations of lift and propulsion engines.
Today, Bell is exploring HSVTOL concepts combining tilt-rotor and jet propulsion, with speed targets in excess of 400 knots (approximately 740 km/h). These projects are based on a detailed understanding of airflow, thermal loads, and mass trade-offs that was not available in the 1950s. It would be difficult to claim that the D-139 is the direct ancestor of these aircraft, but it clearly belongs to the same story: that of a manufacturer obsessed with the ability to take off vertically without sacrificing cruise performance.
The D-139 reminds us of a reality that is often glossed over in technological narratives: most advanced ideas never result in an operational aircraft. But they leave their mark on calculation methods, test models, and engineers’ intuitions. And sometimes, it is these “failures” that prevent subsequent generations from wasting another ten years on dead ends that have already been explored.
Sources
Aerospace Projects Review, Volume 2, Number 3 – Bell VTOL fighter concepts (D-139 and D-188)
The Unwanted Blog – Bell D-139 VTOL fighter, May 2009
SecretProjects.co.uk – Bell Aircraft / Bell Aerospace “D-” designations, discussion on D-139 and Vertiburner
James A. O’Malley – The Design of a Supersonic V/STOL Fighter-bomber, Bell Aerosystems, 1961
NASA – V/STOL Concepts in the United States: Past, Present and Future
Various articles and historical summaries on Bell’s VTOL projects and the evolution of the D-188 / D-188A demonstrators
Live a unique fighter jet experience