Why Does Boeing's MQ-25 Prototype Look So Stealthy?

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A short teaser video posted by Boeing on Twitter showing the company-funded prototype of its offering for the U.S. Navy’s MQ-25 Stingray has raised questions about why its design for an unmanned carrier-based aerial refueling tanker should look so stealthy.

                                Credit: Boeing

 

The answer may lie in the origins of the Navy’s long-gestated requirement for the so-called Carrier-Based Aerial Refueling System (CBARS), but what is obvious is that Boeing’s design bears a resemblance to one of the key aircraft in the history of stealth - Northrop’s 1980s Tacit Blue demonstrator.

The first teaser image released in December immediately raised questions about where the inlet was located on Boeing’s MQ-25 design. Earlier artists’ impressions of its concept for a carrier-based unmanned aircraft had the inlet mounted above the fuselage, similar to General Atomics’ Avenger. But such an inlet was not apparent in that first, tantalizing head-on image.

Credit: Boeing

The inlet is no more apparent in the latest video. In fact, there doesn’t appear to be one! The only hint at its existence is a black line mid-way along the prototype’s upper fuselage stenciled with the words “Jet Intake  Danger.” This suggests Boeing’s MQ-25 has an inlet buried in its back and therefore completely flush - like that on the Tacit Blue.

Credit: Boeing

The Tacit Blue was built for DARPA and the U.S. Air Force in part to show that a stealth aircraft could be designed that had curved surfaces, and was not entirely composed of faceted panels like Lockheed’s Have Blue and F-117. The design techniques developed for Tacit Blue were applied by Northrop to the smoothly curved B-2 stealth bomber.

Tacit Blue’s top-mounted flush inlet may have been stealthy, but it was hard to start, the flight-test crew at one point parking a C-130 in front of the aircraft so that its propwash would help start the airflow into the buried engines. There was also some flow separation in the inlet duct.

Credit: U.S. Army Force Museum

On Tacit Blue the engines exhausted from a slot nozzle located between V-tails - which also seems to be the case with Boeing’s MQ-25. The Tacit Blue’s wing and tails were unswept - as they appear to be on Boeing’s unmanned aerial refueler. Boeing’s design also has a chine running around the perimeter of the fuselage - a familiar characteristic of stealth designs.

By why? Stealth is not one the Navy’s requirements for CBARS. It was, once, a requirement for the CBARS’ predecessor, the Unmanned Carrier-Launched Surveillance Strike (UCLASS) system. But after a lengthy and contentious debate, UCLASS was downgraded to an unmanned tanker and the need for stealth removed.

Northrop Grumman - which built and flew the tailless, flying-wing X-47B under the predecessor to UCLASS, the Naval Unmanned Combat Air System Demonstrator program - dropped out of the running for CBARS after seeing the final request for proposals (RFP). The lack of a requirement for a stealthy platform that could be developed from the X-47B may have been one of the reasons.

Boeing’s MQ-25 design may have acquired its stealth features when it was UCLASS, and retained its shape through the many study phases that led up to the final CBARS RFP. It may be a bet placed by Boeing that the Navy, once it gets the Stingray on its carrier decks, will want to evolve the aircraft from an unmanned tanker to a surveillance/strike asset that needs stealth.

Credit: Boeing

Another sign of stealth influence in the design’s origins that is visible in the video is the arrestor hook, which is enclosed behind a door when retracted. But the prototype seen being rolled out by Boeing’s Phantom Works in St. Louis does not have sawtooth edges on its gear doors or access panels, the existence of which is another typical signifier of stealth requirements.

Other design features of note. Boeing’s MQ-25 prototype has three air-data sensors on the nose - indicating triplex digital fly-by-wing flight control - but they are probes and not the flush sensors used on Northrop’s stealth-shaped X-47B. There is also what appears to be a camera under the flat nose, presumably to provide the ground operator a view ahead during takeoff and landing. Credit: Boeing 

The main gear retracts forward into the fuselage inboard of the wing roots and the wing fold is just visible - a seam and bulge on the upper surface, over what is presumably the hinge and actuator, some way out from the wing root. There is also a pronounced vertical join that runs around the fuselage forward of the wing leading edge - the reason for which is far from clear.

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