Return of the High-Level Bomber         

Stephen Liddle looks into the surprisingly long story that led to the first flight of the most advanced heavy bomber the world has ever seen.

The first flight of a completely new combat aircraft – especially the manned variety – doesn’t happen often these days. If we’re considering US heavy bombers, then the most recent prior to this month would have been the Northrop B-2A Spirit, in 1989. Before that, the B-1 in 1974 was preceded by the B-58 and B-52, both in the 1950s. It doesn’t happen often, which makes the stunningly lit baby steps of the new Northrop Grumman B-21A quite exciting.

The product of the Advanced Technology Bomber project and code-named Senior Ice, the B-2 was revealed not long after the F-117 and the whole concept of stealth was officially acknowledged. Attention was focussed inevitably on the seeming magic of the ‘invisibility’ to radar, but Low Observability is a suit of techniques and technologies that improve survivability. By limiting the opponent’s ability to detect and engage, the freedom to operate is vastly increased. In the 1980s, with the Cold War proper still in full swing, it had been two decades since the favoured method of penetrating the other side’s formidable air defence systems had become the Hi-Lo-Hi mission. The route into the target was only considered viable if low-level, terrain-following tactics were adopted. Aside from enemy action, these were dangerous for the crews of the existing heavy bombers adapted to the new the regime. As a USAF B-52G crew member once told me, “In the Gulf, we crossed the border at 250 feet. The minimum ejection height was 300 feet.” At least he had an ejector seat; as is well known, the three rear crew members on the RAF’s V-Bombers had to resort to unlikely manual escape.

A bomber that need not concern itself (overly) with what the enemy may try and do to it could avoid these pitfalls, but also take advantage of appropriate physics too. If it could fly at an optimum (high) altitude, then range would significantly benefit, supporting the idea of global reach and the USAF bomber’s raison d’etre. If outrunning defending fighters through speed too was deemed less relevant, then the aerodynamic efficiency of a subsonic jet pushed the range still further. However, whereas the B-52 and Vulcan conforming to this philosophy had been able to be aerodynamically dominated designs, the very technologies that might just allow high-flying subsonic bombers in the later Cold War exerted the strongest influence on external shape. Altitude – defeating visual detection from the ground as well as making both radar targeting and subsequent engagement more difficult – had to argue its priority in the LO toolkit.

Things should have moved on in the four decades since the B-2 was conceived, and indeed they have. The B-21 is familiar, but it clearly isn’t the same. As someone with a professional interest in aerodynamics, but from outside of the defence aerospace industry, I’ve been amusing myself by attempting to unpick some of what I’ve seen in the few available images. I claim no more than that.

The outstanding, fundamental difference in configuration between the new B-21 and the preceding B-2A Spirit, shown even in the early Northrop Grumman renders c.2015, is the simpler trailing edge shape. In fact, the ‘W’ of the new aircraft as opposed to the ‘Saw tooth’ of the old, would have been familiar to the designers in the early 1980s too. While the B-2 started life as a high-altitude bomber, the thinking behind ATB from both the USAF and ATB bounced between a pure optimisation in favour of 60,000ft penetration, and retention of the in-vogue low-level strike emphasis. Was the plan a stealthy FB-111, or B-52? The Request for Proposals (RFP) that stimulated the first design studies required quantification of the ‘fallout’ capability of the High-level design to perform a low-level mission if required, but without changes to the design to help this. That would change in April 1981 with a Modification Request to add significant low-altitude capability, as a ‘…prudent hedge against an ever-changing and maturing radar threat operational throughout the Soviet Union.’[i] In other words, with Stealth completely unproven operationally and even had it been, its longevity open to question, could the USAF afford put all of its chips on black?

Northrop conceptual design, c.1979 (Griffin et al)

The low-level penetration mission immediately added about 10,000lb to the expected structural weight of the ‘paper’ high-level design. It is worth noting in the light of what came after, that aircraft cost has been strongly correlated to weight.

While the airframe gained weight to meet new strength requirements, in terms of both fatigue and ride quality aeroelastic effects were more emphasised. Engineering work showed that much of the energy was absorbed with the first wing bending mode. On the baseline design, the node line ran over the planned outboard control surfaces. At the same time, ways were sought to improve the balance between carried at the front and rear of the wing carry-through box; 70% and 30% respectively. The final design introduced an IB control surface array, with the mode line running between them and the OB set. This meant the aeroelastic bending could be actively controlled by out-of-phase actuation of the two sets. The main gust alleviation work was in the hands of a powerful central control surface, which worked with sensor systems to pitch the aircraft into the local gust vector and minimise its effect, via a, “very aggressive flight control system which is designed to provide significant improvements in ride quality and load alleviation during low level contour flying.”[ii] The result was the familiar shape seen today, which exists only to adapt the high-level B-2 to the punishing low-level environment. On the other hand, it was noted during an investigation into the aircraft’s response to lateral gusts, that the flying wing shape, “…is sufficiently small in the vertical dimension that it can be considered planar, and the lack of vertical surfaces, nacelles, or external stores greatly reduces its sensitivity to lateral gusts.” So, it wasn’t all bad.

What does this tell us? It isn’t as simple as saying the B-21 is high level only. While the B-2 planform changes were driven by the low-level mission, there were other ways to skin the cat and in the intervening four decades, both structures and control have advanced. The B-2 was off the scale in terms of the proportion of composite structure it used for the time it was designed; it doesn’t necessarily follow that the structural modes of the B-21 follow the same pattern as its forebear.

B-2 Planform changes through both development and addition of low level requirements, c.1983 (Griffin et al)

The B-2 and B-21 obviously also share the overall stealth strategy of sharp parallel edges at oblique angles to the flight path, together with smoothly curved surfaces. From a cruise efficiency perspective, sharp leading edges were not a positive feature due to the loss in forward suction and hence a poor lift/drag ratio of the aerofoils. In the early 1980s, a new subsonic aircraft not considering LO would have inevitably used a supercritical aerofoil shape, with a relatively large radius leading edge, thick forward region and cusped trailing edge for aft loading. A compromise between the aerodynamicists and LO engineers was evolved, after the latter group were able to show that retaining sharp edges on the central region and tips should prove sufficient. This is a very obvious feature of the B-2, once one has noticed it anyway. Interestingly, it is much less clear from the images seen to date, that it has been incorporated on the B-21. It is certainly more subtle, but the need to align a sharper leading edge to the oncoming, upwashing flow at the nose has again resulted in the characteristic ‘beak’ shape. Potentially, an example of technology moving on and the aerodynamic restriction being lifted by advances in LO and the ability to model its effect.

A B-2A in flight. Note the open drag rudder surfaces, the outboard-most trailing edge controls. The leading edge appears to change sweep angle at the tip and near the centre; in fact, this is due to the change in leading-edge radius from an aerodynamically favoured relatively large section for the mid section, to the sharp LO-biased shape at root and tip.

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One thing that certainly isn’t evident on the B-2 is the level of leading-edge droop outboard on the B-21. Where have we seen that before though? Looking back to the 1950s and the similarly sized, high-altitude V-Bombers, it is well known that the Vulcan didn’t work aerodynamically in its initial incarnation. The natural tendency of the inboard, leading part of a swept wing to load the outboard part behind it, caused significant suction peaks to be developed there. Progressive LE droop was the answer, being made to work really well by an understanding of the ‘peaky’ mechanism and specific sectional shaping. The supersonic expansion followed by an initial compression using Mach wave reflections rather than a shock wave, resulted in a much more efficient flow at high subsonic speed. This is a fundamental characteristic of the supercritical aerofoils used on the B-2 as well. The rival Victor used a philosophy of ‘constant critical Mach’; the intention was the wing would aerodynamically adapt itself along the span. The OB was drooped, thinned and of reduced sweep. It may not be simple to build, but progressive full span LE droop can be a significant aid to transonic cruise efficiency. Are we seeing the ability to combine 202x design and simulation capability across the fields of LO and aerodynamics, vs. 198x era work on the B-2? This is further visible evidence of a quicker or better L/D bomber, less compromised by LO, as engineering has advanced.

We’ve discussed one reason for the outboard leading edge droop: transonic cruise efficiency. That’s definitely not the only reason one would see this though. For the vast majority of practical aircraft, a vertical fin and rudder provide the ability to account for the usual adverse yaw effect in a turn. In 1920, Prandtl and his Gottingen colleagues developed lifting line theory to analyse three dimensional wings. Constant downwash from an elliptical span load gave min induced drag. However, in 1933 this was extended to solve for given mass; a bell curve was superior. This 1933 shape implies a switch from downwash to upwash and hence thrust outboard. By deploying lateral control surfaces in this region, a local increase in load (down aileron) creates a yawing moment into the turn (Proverse Yaw), without requiring a rudder input. In 2016, Bowers and co-workers published a NASA report[iii] suggesting that this bell span load, proverse yaw inducing model applied to soaring bird flight. Birds don’t need a vertical tail for coordinated turns either. All of this leads us back to a place where the subsonic cruise efficiency (minimum induced drag) and lateral-directional control response in the absence of a fin (for stealth) are both served by a bell-shaped span load. Note that R T Jones and the Hortens all feature in this story, but are not necessary to the narrative. Please don’t write in and complain!

The B-2 users split drag rudders for directional control when not in stealth mode, while relying on differential thrust and the remaining control surfaces when LO is vital. The ability to mix these controls (together with a neutral to unstable configuration) was not available to Northrop in the 1940s, when such devices were used on the XB-35 flying wing bomber project. As well as increasing the aircraft’s general control authority, the strategy would specifically push the crosswind landing limits and hence allow missions to be launched in a wider range of conditions. The B-21 seems to have dispensed with the split surfaces entirely, which may be an indication of more confidence in the modelling of the aerodynamic derivatives and flight control system performance during the engineering phase. It might also hint at greater authority being available from the engines, with the combination now being able to meet all specified landing requirements. Of course, there are many more flight-validated data points available to the designers of a flying wing these days.

Another pillar of the LO shaping strategy for the B-2 was the shielding of engine inlets and exhausts from below, together with the highly reflective engine compressors themselves. The ideal of an S-shaped duct from the upper surface of the wing was far easier said than done. As Hans Grellmann, responsible for the aerodynamic design of the aircraft described the situation, “In essence, two supercritical airfoils had to be designed in series. The first being the wing surface where the flow expands to reach supersonic speed and then is recompressed to subsonic speed before it enters the inlet. The second “airfoil” is the nacelle between the inlet lip and the exhaust exit. In this region, flow accelerates over the inlet cowl to supersonic speed, while recompression becomes part of the compression region extending out to the outboard wing.”[iv] The impact of the upper surface nacelle configuration would make itself apparent towards the end of the flight test programme, as late as June 1994. Whilst expanding the Mach limit of the envelope at low altitude, a Residual Pitch Oscillation (RPO) was identified. The test involved checking the aircraft’s response to random small control surface inputs; the expectation was that any resulting oscillations would be damped and eliminated within a specified period. In this case, the oscillations continued at a low level, while the engineers were able to identify the trace of upper surface shock waves over the nacelles and inboard trailing edge notch, also oscillating with time. These had coupled with the structural modes and ultimately kept the vibration going. As the situation was outside of operational requirements, the solution was a Mach overspeed warning in order to give the pilots time to correct.[v]

Computational mesh from analysis work associated with correcting the RPO problem with the B-2. Note that the shock was located somewhere over the nacelle and affected the trailing edge notch region. Both of these features are avoided on the B-21. ( Jacobson et al)

The B-21’s nacelles are notably lower profile than the B-2 equivalent geometry, while the inlet plane is further forward. By reducing the pressure recovery demand over the top by these two geometry changes, the aircraft would likely be less susceptible to shock induced separation, in the manner that caused the B-2 RPO problem. The simpler trailing edge geometry has a strong part to play in this too, while the simulation challenges identified after the B-2’s problems came to light have been tacked with thirty years of transonic CFD tool development. The new bomber was starting from a much better place.

If the B-2 benefitted from moving the inlet rearwards and even then, required a boundary layer bleed duct underneath to remove the low total pressure flow, how has the B-21 team managed to move their inlet plane forwards? The answer, one suspects, is the work conducted in the intervening period on supersonic diverterless inlets, as featured on the F-35. These devices use careful shaping of the surface ahead of the inlet, usually via a bump, to control the local flow direction in the boundary layer itself by introducing a compression. While the B-21 itself is subsonic, the local flow on top of the wing leading edge will be marginally supersonic in the cruise. Images of this region are unclear at best, but the challenge of providing attached flow and maximum pressure recovery in the LO-compliant S-duct diffuser ahead of the engine is severe. It is inconceivable that attention has not been paid to adequate boundary layer control by some method such as this.

A few thoughts then on the B-21, as revealed so far.

• The fundamental planform and strategy are reminiscent of the initial B-2 proposals from Northrop, as accepted by the USAF for early development.
• Many of the planform differences between B-21 and B-2 can explained by the low altitude requirements introduced in the final Advanced Technology Bomber specification.
• Backing out of the Hi-Lo-Hi design certainly reduces weight (cost) and would have avoided a number of specific aerodynamic issues that were difficult to predict (time and cost).
• The B-21 geometry is consistent with advances in both predictive tools and confidence gained from related LO platforms. The elimination of the split rudders and potential exploitation of proverse yaw is an example.

Overall, there is a fascinating interplay between the conflicting aerodynamic and LO optimum solutions. As the ability of computational tools – particularly computational fluid dynamics (CFD) – to predict the physical phenomena associated with transonic flight regimes has advanced, then designers can have more confidence in pushing the shapes towards stealth-biased solutions. An example would the complex intake and exhaust geometries, which must retain healthy aerodynamic performance in terms of pressure recovery and minimal losses. Conversely, as analytical tools for the assessment of LO performance without the need for physical testing have matured, then the aerodynamic solution space widens. The clear variation in section shape and obvious rear loaded geometry of the B-21’s wing lower surfaces, looks to be a more geometrically refined and optimised transonic shape than the B-2, superficially at least. Things have indeed moved on, as much as the 1979 Northrop proposal may remind us of what has been recently revealed.

It would be remiss of me not to mention the great help in accessing background material given by David Lednicer – it was very much appreciated. All misinterpretations of the dataset are entirely my fault alone, however.

-Stephen Liddle is a must follow on X Twitter and is currently preparing a book on the aerodynamic development of the V-Bombers, that he hopes will be published Q2 2024.

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[i] Griffin et al; B-2 Systems Engineering Case Study; Air Force Center for Systems Engineering; 2007

[ii] Crimaldi,  J.P., Britt R.T. and Rodden, W.P; Response of B-2 aircraft to nonuniform spanwise turbulence; AIAA Journal of Aircraft, Vol 30 No.5, Sept-Oct 1993

[iii] Bowers et al; On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds; NASA-TP-2016-219072, 2016

[iv] Grellmann, H.W.; B-2 Aerodynamic Design; AIAA 90-1802, AIAA Aerospace Engineering Conference and Exhibit, Los Angeles, CA, February 13-15^th 1990.

[v] Jacobson, S.B., Britt, R.T., Dreim, D.R. and Kelly, P.D.; Residual pitch oscillation (RPO) flight test and analysis on the B-2 bomber; AIAA paper AIAA-98-1805, 1998.