With the U.S. Air Force revealing concept art and a designation for its shadowy Northrop Grumman B-21 Long Range Strike-Bomber (LRS-B) last week, there are many details that we can now glean about the new warplane.
First and foremost, the new B-21 looks very similar to its B-2 Spirit predecessor. In fact, the new aircraft look startlingly similar to the original Advanced Strategic Penetration Aircraft (ASPA) and the later Advanced Technology Bomber concept from the 1980s that ultimately resulted in the B-2. But the Spirit was redesigned late in the game to operate at low altitudes after Dr. Paul Kaminski’s—current chairman of the Defense Science Board—Red Team cautioned that the B-2 might have to resort to low-level penetration as the Soviets built new, more capable radars—as legendary Aviation Week journalist Bill Sweetman points out in his book “ Inside the Stealth Bomber .” The redesign caused a decrease in range and payload, as well as a larger radar cross-section.
If the current B-21 design is truly representative of the direction the Air Force is taking, the new aircraft will take the B-2’s all-aspect stealth design to the next level. Particularly, the B-21’s low observable design will be more effective against low frequency radars operating in the UHF and VHF bands, which are increasingly coming into vogue as a means to counter stealth aircraft. Indeed, as then Air Force chief of staff Gen. Norton Schwartz told the House Armed Services Committee in 2012, even the B-2 is starting to lose its ability penetrate hostile airspace. “The technology on which they were designed with respect to signature management . . . is ‘80s vintage,” Schwartz told the committee, adding, “the reality is that the B-2 over time is going to become less survivable in contested airspace.”
The B-21 design—which is similar to the original high-attitude optimized B-2 design—is built to counter the low frequency radars that can detect and track tactical fighter-sized stealth aircraft. Unlike an F-22 or F-35, which are designed to operate in an environment where the enemy might be aware of their presence, the B-2 and B-21 are designed to avoid detection altogether. Basically, the B-21 (and B-2 to an extent)—with its large flying-wing design—reduces its low frequency radar cross-section to the point where it blends in with the background noise inherent to those UHF/VHF band systems. That’s similar in concept to how a submarine hides in the background noise of the ocean. But—like all stealth aircraft—it will not be invisible. Stealth is not a cloak of invisibility, after all. Stealth technology simply delays detection and tracking.
While the Air Force’s rendering of the B-21 gives us some clues as to the configuration of the new aircraft, most of its other parameters remain unknown. The B-21’s size and payload will largely be determined by whatever propulsion system is readily available to power it. Given that the LRS-B is slated to enter into service in the mid-2020s, the aircraft will necessarily have to use an existing engine design. Moreover, that engine must have a profile conducive to a stealth aircraft.
That would almost certainly rule out a commercial airliner engine derivative with a large bypass—such an engine would have an extremely large diameter even if it is highly efficient.
A more likely choice is a derivative of an existing military engine that is already in production. Possible choices could include unaugmented derivatives of the F-15 and F-16’s Pratt & Whitney F100 or General Electric F110. The F110, though an aged design, would give the LRS-B commonality with the Rockwell International B-1 Lancer and Northrop B-2 Spirit, both of which use engines from the same lineage. The B-1’s F101 was derived into the F110, which in turn was derived into the B-2’s F118 motors.
An F110 derivative does have its advantages, but the most likely candidate to power the LRS-B is an unaugmented version of the Pratt & Whitney F135, which in its current state offers roughly 28,000lbs of dry thrust. With some tweaks, such as an increased bypass ratio, a version of the F135 could probably produce more than 30,000lbs of thrust while potentially increasing fuel efficiency. With two such engines, an LRS-B would have less than the roughly 70,000lbs of thrust available to the B-2, but there are indications that the B-21 is smaller than the Spirit.
While the LRS-B might be provisioned to accommodate whatever engine ultimately comes to fruition from the Air Force’s adaptive-cycle engine program—variously called ADVENT, AETD and AETP—if the service is serious about an initial operational capability date around 2025, the new bomber will necessarily use an existing propulsion plant. It takes a long time and large sums of money to develop a new turbine engine. It’s also not an endeavor without risk—look no further than China’s frustrated efforts to develop an indigenous jet engine.
If one accepts the premise that the B-21 will be powered by twin unaugmented F135 engines, one can then assume that the new bomber will be larger than a Boeing F-15E Strike Eagle or General Dynamics F-111 but smaller than the B-1 or B-2. Given the types of threats from low frequency radars that are projected to be out there in the future and the limitations of current low observables materials, B-21’s subsonic flying wing design will be large enough to counter low frequency radars.