Northrop, U.S. Air Force Roll Out the B-21 Raider
PALMDALE, California—The U.S. Air Force and Northrop Grumman on Dec. 2 introduced to the world the next generation of stealth, a new, post-Cold War strategic bomber with a new focus on potential conflict in the vast ranges of the Pacific.
The B-21 Raider, rolled out at the company’s secretive Plant 42 complex about 34 years after a similar ceremony for its predecessor, the B-2, is an evolution for the company’s stealthy flying wing shape. The aircraft has a new generation of low-observable technology, a lighter paint scheme than the B-2 and what appears to be elongated wings and lower profile inlets, contributing to a level of stealth that Defense Secretary Lloyd Austin says will get through any enemy’s air defenses.
“Fifty years of advances in low-observable technology have gone into this aircraft, and even the most sophisticated air defense systems will struggle to detect a B-21 in the sky,” Austin said during an address outside a new, classified hangar built for the airframe.
The long-awaited ceremony comes about seven years after Northrop Grumman was awarded the Long-Range Standoff Bomber contract. Northrop CEO Kathy Warden says since that contract award the company has doubled its staff at Palmdale, with plans to keep hiring special access program-cleared workers as production ramps up. The bomber currently represents about 10% of the company’s income, which will stay stable into 2023 and then increase over time.
Warden appeared alongside Air Force Chief of Staff Gen. Charles Q. Brown Jr. and Air Force acquisition boss Andrew Hunter inside the hangar in front of the still-covered B-21 before the event. She said the bomber was the result of iterating on thousands of designs to get to an aircraft that met the service’s requirements.
Brown says the Air Force still is targeting a buy of 100 aircraft, with deliveries starting in the mid-2020s. It is slated to replace the B-2 and B-1B, serving as the “backbone” of the bomber fleet alongside modernized B-52s. During the national anthem at the ceremony, a B-52, B-1 and B-2 flew over the crowd, in that order.
The Air Force is counting on a quickly progressing test program as the service and Northrop Grumman increasingly use digital methods for test.
Hunter says the program still is on its cost target, pegged at approximately $600 million per copy. Keeping the price relatively low is a requirement for the program and is needed to get to the fleet size required for the Pentagon’s National Defense Strategy.
“What we’re seeing with the B-21 Raider is a fielding of a long-range, penetrating capability that will allow us to carry out our missions in the Indo-Pacific theater, and in all parts of the world where the Air Force needs to operate and provide the capability the joint force requires,” Hunter says.
Austin highlights the B-21’s increased, still-classified range. This allows it to strike anywhere in the world without needing to be forward based. It is built to be reliable and more easily upgradeable.
“The B-21 is the first strategic bomber in more than three decades. It is a testament to the department’s long-term commitment to building advanced capabilities that will fortify America’s ability to deter aggression today and into the future,” Austin says.
Northrop Grumman officials say the timing is right for the rollout, because the aircraft will soon begin testing outside of a hangar, including taxiing and powering on more systems ahead of first flight in 2023.
“We’re going to enter into a new phase of the program. We’re going to do flight test. We’re going to learn things in flight test,” Hunter says.
Deeper, Wider Body
The B-21 follows the B-2 in adopting an all-wing layout with large span for cruise efficiency. The planform provides a large internal volume without having to resort to high surface slopes that could reduce stealth. Minimizing observables, the all-wing planform also provides full controllability without vertical tails and allows both engine inlets and exhausts to be shielded from below.
But the B-21 has a deeper and wider body section, more highly integrated engines and (judging from earlier renditions and the lapel pin on the national anthem singer!) lacks the B-2’s distinctive sawtooth trailing edge. This makes it similar in planform to the Advanced Tactical Bomber (ATB) concepts of 1979-80 that preceded the B-2.
The design distributes lift and weight across the entire span for a lighter structure and provides a high lift-to-drag (L/D) ratio. More-slender outboard wing panels are a key contributor to L/D while providing aerodynamic balance. The sweep angle, which appears to be similar–but possibly slightly shallower than the B-2’s 33 deg.–appears to have been selected for good transonic performance.
The inlets appear to be near-conformal to the upper surface of the body and located farther forward than those of the B-2. They are likely designed to combine a low radar signature with high inlet recovery. The deeply recessed inlet appears to have a similar duct shaping to that of the B-2.
During the rollout there was an indication of a vertical vane within each inlet. Whether this is for low observable reasons or to split the inlet flow to feed pairs of engines on each side is unclear. The wing span and body shape suggest an aircraft wide enough for four engines.
The B-21’s configuration also may have been influenced by Northrop Grumman’s work on the Air Force Research Laboratory’s SensorCraft project of 20 years ago, which was aimed at developing technologies for future stealthy, high-altitude uncrewed surveillance platforms.
Although centered on high-altitude designs, this work – and possibly lessons learned developing the RQ-180 uncrewed aircraft system – may have helped Northrop Grumman devise new solutions to the aeroelastic issues encountered during design of the B-2. These problems – particularly at low altitude and high speed – forced the company to redesign the B-2 with a W-shaped trailing edge.
In the B-2, aeroelastic loads were aggravated by the control-surface motions required to dampen gust response at high speed and low level. In the original, simpler ATB design, the shape of the first wing bending mode was a curved line that bisected the wing almost exactly at the crank in the trailing edge. This was corrected by adding the W trailing edge, which provided space for inboard elevons.
The ATB-like B-21 planform is likely to rely on outboard central control surfaces (split rudders and elevons) that may have a center of pressure ahead of where the first bending mode bisects the wing. New control strategies and re-distribution of weight may have allowed Northrop Grumman to return to this simpler configuration without having to add extra controls inboard of the wing trailing-edge notch.
Although the new aircraft has a less pronounced, hawk-like profile than the B-2 when viewed from the side, the overall appearance indicates the B-21 designers followed the B-2 in using a laminar-flow airfoil shape that combined camber for aerodynamic performance with the sharp entry angle required for low radar cross-section.
To overcome the obvious disadvantage of embedding the inlets within the lifting flowfield above the wing, designers appear to have made extensive use of computational fluid dynamics to merge the varying supercritical airfoil surfaces ahead of the inlet and inside the nacelle itself.
The forward position of the inlet is a challenge because in transonic cruise flight it sits in a region of local supersonic flow. The flow expands to supersonic speed over the wing leading edge before recompressing to subsonic speed before entering the inlet.
Although difficult to confirm visually, the deeply recessed B-21 inlet appears to be shaped to accelerate flow supersonically over the upper cowl, before recompressing it to become part of the compression region extending out to the outboard wing. Like the B-2, the presence of the inlet appears to have a strong design influence on the design of the B-21 body as well as the wing upper surface beyond midspan.
What's New on the B-21?
The Dec. 2 rollout of the Northrop Grumman B-21 Raider provided an initial look at the design of the new stealth bomber. Defense Editor Steve Trimble compared this U.S. Air Force image of the B-21 to one taken from a similar angle of Northrop's B-2 Spirit and found four key differences: shallower, tear-shaped inlets; a narrower fuselage crown with redesigned cockpit windows; a wider, deeper fuselage relative to wing span; and two wheels on each main landing gear versus the B-2’s four.