New Chinese design software identifies critical weaknesses in US B21 stealth

Chinese researchers say they have developed a powerful aerospace simulation tool that could reshape how advanced military aircraft are designed. Their initial findings suggest potential aerodynamic and stability limitations in publicly known configurations of the United States’ newest stealth bomber, the B-21 Raider.

The software, known as PADJ-X, was detailed last month in a peer-reviewed paper published in Acta Aeronautica et Astronautica Sinica.

The researchers describe it as an “all-in-one” design platform that integrates multiple engineering disciplines into a single optimization process, significantly reducing the computing power and time traditionally required to refine advanced aircraft designs.

According to the paper, PADJ-X is based on adjoint optimization technology, an algorithmic method that allows thousands of design parameters to be adjusted simultaneously.

This contrasts with conventional approaches that rely on repeated trial-and-error simulations, which are costly and often produce only incremental improvements.

To demonstrate the system’s capabilities, the research team, led by Huang Jiangtao of the China Aerodynamics Research and Development Centre, applied PADJ-X to conceptual configurations resembling the US Air Force’s B-21 stealth bomber, which is currently undergoing flight testing.

The B-21 is being developed by Northrop Grumman and is intended to become the backbone of the US long-range nuclear and conventional strike fleet.

Using 288 parameters in their simulations, the researchers reported that aerodynamic optimization increased the aircraft’s lift-to-drag ratio by about 15% and significantly reduced shock-wave effects.

They also said the pitching moment, a measure of longitudinal stability, improved from 0.07 to nearly zero, a change that would theoretically allow smoother, more stable flight with less need for constant control input.

A near-zero pitching moment indicates that an aircraft can maintain level flight more naturally, improving fuel efficiency and extending range, the researchers wrote.

Combined with increased lift and reduced wave drag, the optimized configuration suggested strong potential for long-endurance missions.

The researchers stressed that their findings were based on theoretical models and publicly inferred shapes rather than classified design data. Actual performance characteristics of the B-21 remain closely guarded by the US military.

PADJ-X integrates five major disciplines, aerodynamics, propulsion, electromagnetics, infrared signature, and sonic boom, into a single framework.

Huang said this allows engineers to balance competing design demands, such as minimizing radar visibility while maintaining aerodynamic efficiency and structural integrity.

Trade-offs often define stealth aircraft design. Flat, blended shapes reduce radar reflection but can increase drag, while thinner wings improve efficiency at the expense of strength.

Traditional optimization methods test these variables separately, but PADJ-X uses sensitivity-based calculations to derive optimal compromises across all disciplines at once, the paper said.

In addition to the B-21 analysis, the researchers applied PADJ-X to a configuration similar to the US Navy’s X-47B stealth drone, a classified program that was discontinued in 2015.

In that simulation, the software reduced drag by about 10% and lowered the forward radar cross-section, a key measure of detectability, by roughly tenfold, from 13.55 square meters (about 146 square feet) to 1.33 square meters (about 14 square feet).

The study also described simulations involving hypersonic vehicle inlets, reduced infrared signatures from engine nozzles, optimized stealth coatings to save weight, and lower sonic boom profiles for supersonic aircraft.

Adjoint optimization tools are not new. NASA began developing similar platforms in the 1990s, including the FUN3D system used on projects such as the X-59 low-boom demonstrator.

Germany and France have also built comparable tools. However, the Chinese researchers argue that those systems typically focus on fewer disciplines and often require manual adjustments when expanded.

If validated, PADJ-X could accelerate aircraft development, reduce reliance on wind tunnel testing, lower prototyping costs, and help future manned and unmanned aircraft achieve longer range and improved performance.

Whether its simulated findings translate into real-world operational advantages remains uncertain, particularly when applied to highly classified foreign systems.