September 8, 2015
While not much has been revealed about the U.S. Air Force’s shadowy Long Range Strike-Bomber (LRS-B) program, there are many details we can ascertain about the new warplane from available information.
First and foremost, its 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, an unaugmented 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 all signs point to an aircraft that 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 vain efforts to develop an indigenous jet engine.
If one accepts the premise that the LRS-B 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 threat low frequency radars that are projected to be out there and the limitations of current low observables materials, one can also make the inference that the LRS-B will likely be a subsonic flying wing design.
A tactical fighter-sized stealth aircraft must be optimized to defeat higher-frequency bands such the C, X and Ku bands as a simple matter of physics, but a strategic bomber like the B-2 or LRS-B can and necessarily have to be larger to counter lower frequency radars. There is a “step change” in a stealth aircraft’s signature once the frequency wavelength exceeds a certain threshold and causes a resonant effect. Typically, that resonance occurs when a feature on an aircraft—such as a tail-fin—is less than eight times the size of a particular frequency wavelength. That means a bomber like the LRS-B has to have allowances for two feet or more of radar absorbent material coatings on every surface or the designers are forced to make trades as to which frequency bands they optimize the aircraft to operate in. As such, to defeat low frequency radars operating in the L, UHF and potentially the VHF bands (this is easier said than done—and could in fact be impossible), the LRS-B has to be a flying wing design.
Recent media reports suggest that the Air Force is also planning on building significant electronic attack capability into the LRS-B airframe. This is likely necessary to counter low frequency radars operating in the VHF band, which is near impossible to defeat with airframe shape and low observable materials alone. The fact is that despite the Air Force’s public narrative that aircraft like the F-35 can go into a high threat zone alone and unafraid, the services’ own experts at the Air Force Warfare Center recognize the value of jamming. Stealth and electronic attack always have a synergistic relationship because detection is about the signal to noise ratio. Low observables reduce the signal, while electronic attack increases the noise.
If the LRS-B is somewhat smaller than the B-2, the designers have to pick between range and payload. Former Air Combat Command commander Gen. William Fraser, a former B-52 pilot, told me few years ago when the program was in its infancy that “a combat radius of between 2,000 and 2,500nm is sufficient, which equals a 4,000-5,000 nm range. All points on earth are within about 1,800nm from the closest body of water.” Thus, one can assume that the LRS-B will have at least that much range with whatever space leftover being dedicated to its payload. The LRS-B doesn’t necessarily need to carry the same amount of weaponry as B-2, it just needs to carry the biggest available weapon—maybe just one GBU-57A/B Massive Ordnance Penetrator (MOP) instead of two.
In terms of avionics, the Air Force appears to be doing something smart. The aircraft will apparently use an open architecture computer system, which means that the LRS-B won’t be hamstrung with antiquated avionics and ponderous processes to integrate new weapons and hardware like the Lockheed Martin F-22, for example. The aircraft will also be nuclear-capable from the get go, though it won’t be certified to perform that mission until later. That’s not surprising—and had been reported as early as 2011 during Gen. Norton Schwartz’s tenure as Air Force chief of staff. The service also plans for the LRS-B to be optionally manned, however the chances of anyone seriously operating a minimum $550 million aircraft without a pilot onboard are laughably small.
The bottom line is that the LRS-B is shaping up to be exactly what we all expected it to be, but we’ll find out for sure once the Air Force lifts the veil of secrecy on its pricey new toy.
Dave Majumdar is the new Defense Editor for The National Interest. You can follow him on
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