Speed bump: USAF sets modest goals for new bomber

The USAF wants a new bomber by 2018, but says available technology will limit it to subsonic and manned. Is it aiming too low?

After studying options ranging from unmanned aircraft to hypersonic weapons, the US Air Force has decided its next-generation bomber will be subsonic and manned. Given the budget and time­scale pressures it is not a surprising decision, but the USAF still faces financial and technical challenges if it is to field a new long-range strike aircraft by 2018.

By setting relatively modest requirements, the USAF appears to want to minimise the risk and cost to maximise the chances its next bomber will be delivered on time, within budget and in sufficient numbers. The service wants to avoid repeating its experience with the Northrop Grumman B-2 stealth bomber, cut from 135 aircraft to 21 as requirements changed.

Industry is digesting the decision, and Lockheed Martin and Northrop have put substantial effort into studies of unmanned supersonic aircraft. Boeing has long favoured a subsonic solution, arguing that survivability is not improved unless speed is increased substantially, beyond Mach 3 and into the hypersonic regime.

The US Air Force has been studying its future long-range strike requirements since the early 1990s, and had evolved a three-phase strategy: modernise the existing bombers deploy an advanced next-generation bomber, possibly supersonic and unmanned, around 2025 and field a "revolutionary" long-range strike capability, potentially hypersonic, by 2035, when the 1960s-vintage Boeing B-52 is due to retire.

Diverse and ageing

But, under pressure from a Congress concerned that the ageing and diverse bomber fleet was losing its ability to penetrate hostile airspace (read China), the 2005 Quadrennial Defense Review directed the US Air Force to field an interim capability by 2018. That acceleration set the stage for the USAF to scale back its ambitions.

After a year-long analysis of alternatives (AoA) that studied a matrix of four options - subsonic and supersonic, manned and unmanned - the USAF revealed in May that its next-generation bomber would be subsonic and manned, with an unrefuelled radius in excess of 3,700km (2,000nm) and payload of 6,400-12,700kg (14,000-28,000lb) - relatively modest targets that have raised concerns it is aiming too low.

Clearly cost is a major consideration. Revealing the AoA results, Maj Gen Mark Matthews, director plans and programmes at Air Combat Command (ACC), said the analysis showed the "best value" for a new 2018-timeframe bomber to be manned and subsonic, "based on cost-benefit analysis" of the technologies needed to meet the performance requirements, particularly range and payload.

Technology availability is a major concern that is driving the USAF to subsonic and manned. For a 2018 initial operational capability, the technology used must be at a readiness level - a TRL - of 6 by 2009. TRL 6 requires prototype demonstration in a relevant environment, and essentially limits the next-generation bomber to off-the- shelf technology available now.

"The technology is not there to support a high-speed penetrating platform with any kind of endurance," says George Muellner, president, advanced systems, Boeing Integrated Defense Systems. "We do not have the engine designs. We do not have the materials to reduce radar cross-section at high speed." Existing radar absorbent coatings cannot withstand the heat generated by sustained supersonic or hypersonic flight, he says.

A driver for the bomber is persistence. "Targets will rapidly appear and then disappear," says Matthews. "We see the need to penetrate inside enemy airspace to persist, to stay there, to find and fix these targets so we can attack them in a short order of time." The technology available requires a 2018 bomber to be manned, for flexibility, and subsonic, for endurance, he says.

The ability to loiter unseen while the crew looks for concealed or relocated targets is a key element of the USAF's justification for a new bomber. "They want to put a penetrating aircraft in there and keep it there, and a subsonic aircraft can stay in the area," says Muellner.

"We can build a very survivable, low-RCS aircraft that can remain undetected and provide surprise," he adds.

While a hypersonic bomber could attack rapidly, it would be almost impossible to make stealthy. "In the China scenario, they could detect a hypersonic aircraft at long range, and the target may be gone when it gets there," says Muellner. And sheer speed poses problems for retargeting. "At Mach 8 there is a chance it is not pointed in the right direction. The conops [concept of operations] are challenging with hypersonics," he says.

The next-generation bomber will incorporate new technology, says Matthews. "We anticipate we will explore new advances in propulsion technology, as well as advanced weapons and sensors to integrate on the platform," he says, also citing advanced low-observable (LO) characteristics.

Taking the next step in stealth could be the biggest technological risk. The USA has made significant strides in stealth technology since the B-2, particularly in LO maintainability with the Lockheed F-22 and F-35, but these fighters are designed to evade higher-frequency fire control, or "shooter", radars more than lower-frequency, longer-range surveillance radars.

To penetrate and persist in the presence of integrated air defences "the next-generation bomber will have signature reduction well below the F-22 and F-35," says Muellner. He adds: "They are good at shooter frequencies, but not at low frequencies. The B-2 is good at low frequencies, but not at shooter frequencies. The next-generation bomber will be really good at all frequencies."

As well as passive measures to reduce signature, the aircraft could require active cancellation, which generates a signal equal in intensity but opposite in phase to the predicted radar reflection to cancel the return and reduce RCS. But the technique demands precise waveform and direction information, and active cancellation across a wide range of radars and aspects requires complex processing. Muellner says active cancellation technology has been in development for some time, but getting the technique to work consistently has proved difficult. "We have had success with active cancellation. Is it repeatable? No. Does it work over wide-enough frequencies? No."

Other advances could improve stealth, including thrust vectoring. While the flying-wing B-2 uses conventional flying controls, with wingtip drag rudders replacing the tail for yaw control, Boeing's X-45A unmanned combat air vehicle demonstrator, also stealthy and tailless, introduced yaw thrust-vectoring. Multi-axis thrust-vectoring, mechanical or fluidic, reduces the need to move control surfaces.

"We worry about very small movements of surfaces," says Muellner. "We have the technology to do thrust vectoring with very low signature, but only at low speed. None of them are afterburning." This could be another technological factor in favour of a subsonic bomber.

But a major driver of the decision to stay subsonic appears to be the absence of a propulsion system that can combine high speed with long endurance - at least by 2018. The US Air Force Research Laboratory (AFRL) has launched the Adaptive Versatile Engine Technology (ADVENT) programme to demonstrate a variable-bypass turbofan by 2012, but that is too late for the next bomber.

Out of synch

Based on normal development cycles, an ADVENT-influenced engine will not be ready for production until 2018, when the next-generation bomber is planned to become operational. "We are telling the air force it would only be ready to go into production in 2018. It's not lined up necessarily with long-range strike," says AFRL's Jeff Stricker, ADVENT programme manager.

Pratt & Whitney is preparing to offer a derivative of its F119, which powers the F-22, for the next-generation bomber, believing the US Air Force wants go off-the-self for the 2018 aircraft, potentially spiralling in more advanced technology later. "The F119 is the well-developed, off-the-shelf model from which we can spiral up," says Tom Farmer, president, military engines.

ACC's Matthews says the "very challenging fiscal environment" and the need for persistence drove the decision to stay with a manned bomber. Despite the USAF's commitment to unmanned systems, he says, "given the level of technology today" and the need to field a new bomber by 2018 "the technology requires this platform to be manned at this point" - a statement that leaves open the possibility the vehicle could become unmanned or optionally manned at a later stage.

Matthews highlights "the flexibility and adaptability" of having a crew in the cockpit to cope with a dynamic battle environment. Despite advances in network-centric capability, "given the nature of the environment and the level of technology we see in the 2018 timeframe, we haven't obviated yet the need to have the man in the cockpit".

ACC commander Gen Robert Keys has a more direct reason for a manned bomber: "If it is going to take nukes, someone is going to want me to have a man on board." But there is a price for making the bomber manned when persistence is the goal. "We can increase the payload/radius if it is unmanned," Muellner says, adding: "I think the next-generation bomber will likely be manned and unmanned, versions of the same platform."

This accords with the preference of US Air Force chief of staff Gen Michael Moseley for incremental development. "I am into A models," he said earlier this year. "We get beat up for adding requirements [and driving up cost]. We need to set minimum requirements for the aircraft and get iron on the ramp."

Whether the USAF will succeed in getting a next-generation bomber on the ramp by 2018 remains to be seen. The Air Force Requirements Oversight Council has given its go-ahead, but now the concept must fight its way through the Department of Defense's joint procedure for evaluating requirements and approving an acquisition programme.

US defence analyst Loren Thompson predicts the concept will not survive the competition for funding, not least among the air force's own priorities, which include two fighter production programmes and a new aerial-refuelling tanker. "It doesn't sound like an impressive or urgent capability that can speak to the ageing of the heavy bomber fleet. I have a feeling this programme isn't going to go anywhere," he says.

Thompson feels the US Air Force has yet to adequately address why it needs a new bomber by 2018, when 2025 would give more time to develop new technology. "It appears that scarce money, immature technology and a tight development schedule has pushed the air force into a not very attractive solution."

Related link:

• USAF rules out radical bomber

Source: Flight International