In June of 1977, President Jimmy Carter cancelled the B-1A bomber program. Although much criticized at the time, one of the reasons for the cancellation was the need to concentrate the limited funds that were available on the development of air-launched cruise missiles. Another reason, unsaid at the time, was the desire to concentrate on the development of low observable aircraft that would be hard to detect by enemy radars and thus more likely to penetrate sophisticated defenses.
In August of 1975, the Defense Advanced Projects Agency (DARPA) had invited Lockheed, Boeing, and Northrop to submit preliminary engineering data for a low-observable aircraft. This competition was won by Lockheed,, who went on to produce the Have Blue flight demonstrator which later evolved into the F-117A Nighthawk.
In 1978, the Carter administration secretly authorized the start of a stealth bomber program, termed the Advanced Technology Bomber (ATB). A bomber was sought which could replace the recently-cancelled B-1A in Air Force future plans and, in addition, also replace the B-52, which was by that time nearly a quarter-century old. The requirements were set by the needs of the Cold War--this aircraft would use low-observable technology to make it possible to penetrate unobserved for hundreds of miles into Soviet airspace with a full load of nuclear weapons.
During its preliminary studies for the ATB project, Northrop revised its flying wing designs--the XB-35/YB-49 projects of the late 1940s. By the summer of 1979, the Northrop bomber began to emerge as a flying wing.
In September of 1980, the USAF issued a request for proposals for an Advanced Technology Bomber (ATB). Since the costs and technological challenges both promised to be steep, collaborative efforts between different aerospace companies were encouraged. Two groups of competitors appeared-- Lockheed teamed with Rockwell and Northrop teamed with both Boeing and Ling-Temco-Vought.
When Ronald Reagan became president on January 20, 1981, things changed and there was now funding available for several types of military projects, including a new strategic bomber. On October 2, 1981, President Reagan announced a Strategic Modernization Program (SMP), in which 100 B-1Bs were ordered. Another (at the time secret) part of the SMP was the commencement of work on the ATB. At the time, it was imagined that as many as 132 ATBs would be built. Only a few insiders were aware at the time that the USAF was working on two separate bombers, not just one.
The Northrop proposal was codenamed Senior Ice whereas the Lockheed proposal was named Senior Peg. The Senior Ice program was led by Welko E. Gasich, Senior Vice President for Advanced Projects at Northrop and by project Director Hal Markarian. Not much has been revealed about the Lockeed/Rockwell Senior Peg proposal, but it was generally assumed that their team would probably have the advantage in the competition, given Lockheed's recent successes in stealth technology and Rockwell's experience in building the most recent strategic bomber--the B-1. However, Northrop was announced as the winner of the ATB contract on October 20, 1981. The initial contract called for six flying aircraft and two static test aircraft, plus options for the production of 127 production bombers. Initial Operational Capability (IOC) was scheduled for 1987.
Northrop was responsible for the forward center body and cockpit, the leading and trailing edges, and the control surfaces, as well as for final assembly and the overall coordination of the project.. Boeing was given responsibility for the aft center section and the weapons pay, plus the outboard wing sections and the landing gear. Ling-Temco-Vought was given responsibility for the wing intermediate sections with their engine intakes and exhausts (LTV was subsequently acquired by Northrop Grumman). Final assembly would take place at Site 4, USAF Plant 42 at Palmdale, California.
The design that emerged was a pure flying wing, with no vertical rudders or fins. Viewed from directly above or below, the B-2 resembles nothing more than an oversized boomerang. It has twelve ruler-straight lines. The wing leading edges run straight from the extreme nose to the extreme tips of the wing. The centerbody is thicker than the rest of the wing, and contains the cockpit, the weapons bays, and the electronics. Immediately outboard of the centerbody in separate bulges are the engines, two engines on each side. The wingtips are not parallel with the airflow but are cut off at nearly right angles to the leading edges. Apart from the tips, the outer wings have no taper. The inner trailng edges have a jagged shape, jutting rearward toward the centerline. The edges form two groups of six exactly parallel lines. The forward leading edge of the wing is swept back at an angle of 33 degrees. The sweep angle was determined by the requirement that a high subsonic speed be achieved, as well as by the need to locate the aerodynamic center close to the center of gravity--given that the wing extends to the front of the aircraft, it must be swept back to place the center of lift near the center of gravity. The length of the centerbody section was determined by the requirement that it had to be deep enough to accommodate the cockpit and the weapons bays, and this meant that it had to be a minimum length to avoid excessive drag at high subsonic speeds. Outboard of the centerbody section, the dimensions of the chord were set by the need to integrate the engines and their low-observable inlet and exhaust systems in to the wing. The thick supercritical wing section accelerates the air to supersonic speeds over the wing.
Most of the surface of the B-2A is covered with with a special elastomeric material which is designed to maintain a uniform conductivity over the the surface to reduce reflections from things such as joints or seams. Those areas which cannot by their nature be totally stealthy in their design (such as air intakes) are coated by radar-absorbent material (RAM), the composition of which is highly classified. RAM is a multi-layer sprayed-on elastomer which consists of an active element which converts radar energy into heat. The basic idea is to pace a coating on the aircraft which is of the proper thickness so that the small reflection from the front face of the material is exactly cancelled out by a residual reflection from the rear face. A similar principle is used in coated optics for binoculars and telescopes to eliminate unwanted light reflections.
Nine large control surfaces occupy the entire trailing edge of the wing, apart from the exhaust area immediately behind the engines. There is a moveable control surface at the very end of the beavertail, which is designed to counter pitching moments caused by vertical gusts of air at low altitudes. The very outermost pair of moveable surfaces are known as brake-rudders, and will be described in the next paragraph. The remaining six control surfaces are used as elevons for pitch and roll control, although the outermost pair functions purely as ailerons at low speeds. The B-2 was originally designed with a pair of split flaps under the aft central fuselage, but wind tunnel tests indicated that they would probably not be needed and the flaps were bolted shut on the first test vehicle. Vestiges of the flaps remain on later aircraft, but the B-2 has a sufficiently large wing area that flaps are not needed for takeoffs or landings.
There is no vertical fin. Unlike conventional aircraft, the B-2A is directionally neutral in yaw, which means that there are no aerodynamic pressures which would force it back to forward flight were it to yaw either left or right. Yaw control is provided by a set of Northrop-patented brake-rudders on the outer trailing edges of the wing. The very outermost pair of surfaces can split apart horizontally, one side moving up and the other side down. They can operate symmetrically as speed brakes and asymmetrically as rudders, and they are known as brake-rudders. The brake-rudders are the primary means of yaw control, but because of the boundary layer over the wing the surfaces are ineffective until they have moved at least five degrees from their neutral positions. In normal flight, the brake-rudders are set at the "5 and 5" position, that is, they are ordinarily slightly displaced so that they can be immediately effective when control is needed. However, open rudders are incompatible with stealth (especially from the rear), so the brake-rudders have to be locked shut when the B-2A is going into a combat zone. It is believed that differential engine thrust is used for yaw control when the B-2A is in full stealth mode. but the real means of control is classified.
The aircraft is intrinsically unstable, but is rendered stable by a quadruplex fly-by-wire system. The flight control computer units were developed by General Electric. The B-2 is fitted with eight actuator remote terminals spread out along the wing span which receive their instructions from the GE flight control computer via a quadruplex digital databus.. The remote terminals issue analog commands to the actuators and control all of the necessary feedback loops. There are five groups of air data sensors located on the leading edges of the wing just ahead of the windshield. These provide input to the fly-by-wire system--the system conpares air pressures to determine the angle of attack and the amount of sideslip.
The B-2A is powered by four General Electric F118-GE-11 non-afterburning turbofans, each rated at 19,000 lb.s.t. The F118 engine was based on the F101-X, a fighter engine originally derived from the F101 engine that powered the B-1. Compared with the F101, the F101-X had a smaller low-pressure spool which reduced the bypass ratio from 2:1 to 0.87 to 1. A lower bypass ratio engine was selected because it would need a smaller exhaust and inlet system than that of a high bypass ratio engine. The engines are seated in pairs in engine bays buried inside the wings just outboard of the central fuselage.
The engine inlets are set far back from the leading edge, thus shielding them from radar emissions coming from below. The inlet region resembles two supercritical wing sections in series. The first is the area behind the leading edge, where the airflow accelerates to supersonic speed and is then recompressed to subsonic speed before being swallowed by the main inlet and the auxiliary boundary layer suppression scoop. The second supercritical section comprises the region from the inlet lip to the exhaust exist, where the flow is accelerated and recompressed once again.
The engine air intakes were a special challenge, since they are absolutely necessary but represent a ready source of unwanted radar reflections. One of the major problems is the danger that enemy radar signals might reach the whirling blades of the turbofan engines, producing reflections so intense that the enemy can not only readily detect the aircraft but can often even tell the type of aircraft that is present. Unlike in the F-117A, the intakes are not covered by radar absorptive grids. Instead, the engines are buried completely within the wing, and are fed by S-shaped inlet ducts which completely conceal the compressor faces from direct line-of-sight illumination by radar. RAM is used to to cover the duct walls to suppress any radar energy that could bounce off the duct walls to reach the engines. As additional insurance, the lips of the air intakes are serrated in order to scatter enemy radar emissions, preventing them from being returned to their source. For maintenance, the engines are accessed via hatches in the underside of the aircraft. These hatches have serrated edges to suppress unwanted radar reflections.
Early wind tunnel testing showed that there was a certain amount of flow separation inside the highly curved intake duct, leading to a loss of power at low speeds. To prevent this, a pair of retractable auxiliary inlet scoops were added to the upper wing surface, immediately above the engines. These open at low speeds to increase the air flow to the engines.
Ahead of each air intake inlet is a serrated slit-like auxiliary inlet which removes the turbulent boundary layer air, preventing it from entering the engine. This boundary layer air is then mixed with the exhaust to cool its temperature and reduce the infrared signature.
An equally-demanding challenge was presented by the engine exhausts. The B-2A engine exhausts are designed to minimize the infrared signature, making it more difficult for enemy infrared detection systems to pinpoint the aircraft. Since most long-range IR sensors such as the infrared search and track systems fitted to some fighters and the homing heads of IR-guided missiles operate by detecting the radiation from the hot gas and water vapor emitted by the engines, considerable work has gone into reducing the infrared signature of the B-2. One of these involves the cooling of the engine exhaust as quickly and efficiently as possible. The B-2 exhausts are built into the top of the wing, with the primary nozzles well ahead of the trailing edge. The engine exhausts feed into a pair of soft-lipped trenches, which flare outward. The engines are fitted with flow mixers that blend the hot core exhaust stream with the cold boundary layer air which is swallowed by the secondary inlets. The exhausts are wide and flat, so the perimeter of the plume is longer than the perimeter of a round exhaust stream, and mixing takes place more quickly. The interaction between the exhaust stream and the airflow over the aircraft, at each side of the exhaust "trench", creates a vortex which cools the exhaust still further.
The four-wheeled main landing gear members are located immediately outboard of the engines. They retract forward into wheels inside the wing and are covered by doors with serrated edges to suppress unwanted radar reflections. The two-wheeled nose gear retracts backward into a wheel well in the forward nose. It too is covered by a door with a serrated edge.
Most of the interior of the outer wing beyond the engines is taken up by fuel tanks. The underside of the fuselage between the engines is taken up by a pair of large weapons bays, one on each side of the fuselage centerline. The edges of the weapons bay covers are serrated so as to prevent unwanted radar reflections from their edges when they are closed. Each bay can carry either a Boeing-developed Advanced Rotary Launcher capable of carrying 8 2000-lb class weapons or, alternatively, two Bomb Rack Assembly units for carrying smaller conventional weapons.
The B-2A has a crew of two, seated side by side on individual ACES II ejection seats, with the pilot on the left and the mission commander on the right. They are seated well to the rear of rather large windshields which fit flush with the surface of the centerbody airframe. The crew gain access to the crew compartment via a ventral hatch. In an emergency, the crew members eject upward through frangible panels in the roof of the aircraft. The mission commander is responsible for navigation and weapons delivery, but either crew member has the ability to perform the complete mission. Each crew station has four cathode ray tube color displays, with a a data entry panel to the right and a set of throttles to the left. There is room for third crew station to the rear, but it is rarely used, even though a frangible panel is provided in the roof in case this extra crewmember has to eject.
The navigation system for the B-2A initially combined two types of units, either of which is capable of navigating the aircraft on its own. However, greater accuracy is obtained when they operate in conjuction. One is a Kearfott inertial measurement unit and the other is a Northrop NAS-26 astro-inertial unit. The NAS-26 was originally developed for the Snark long-range cruise missile and is based on a stabilized electro-optical telescope which locks onto a pre-selected star even in cloudy daylight. The observation port for the system is to the left of the windshield.
At shorter ranges, infrared radiation emitted by the skin of the aircraft itself is a problem. This unwanted infrared radiation can be generated by reflected sunlight as well as by friction with the air passing over the skin of the aircraft. Infrared-absorbent paints are commonly used to absorb infrared light from sunlight, preventing it from being reflected from the surfaces. This results in an aircraft with an overall gray color. Heat generated by air friction cannot be affected by an absorbing paint, but coatings have been developed with change the emissivity of the surface, causing the infrared emission to take place in bands which are strongly absorbed by the atmosphere, making it less likely that a nearby detection system can see it.
The B-2A is capable of midair refueling, via a receptacle for a refuelling boom that is installed in the upper fuselage behind the cockpit. In the interest of stealth, this receptacle rotates out of the way when not in use, presenting a smooth upper surface.
The B-2A has a GM-Hughes Electronics AN/APQ-181 radar. One might ask the obvious question about the wisdom of putting a radar transmitter on an aircraft which is designed to be invisible. However, the radar installation is designed to achieve Low Probability of Interception (LPI). LPI is achieved by using the least amount of energy required to detect and track the target, while encoding the signal to make it difficult for the enemy to distinguish it from random noise. There are two separate electronically-scanned antennae for this radar, one in each of the lower leading edges of the wings just outboard of the cockpit. There are 20 modes that the radar can use, including synthetic aperture radar mode and terrain following and terrain avoidance modes. There is a ground moving target indication mode that can detect vehicles on the ground and an air-to-air mode that can be used during inflight refueling. Obviously, many features of this radar suite are still highly classified.
The B-2A does not have any defensive armament, relying instead on a Defensive Management Subsystem (DMS) to provide protection against threats. Lockheed Martin, Raytheon, and Honeywell have all provided components for the system. Although details of the system are largely classified, the primary component is believed to be the Lockheed Martin AN/APR-50 (sometimes known as ZSR-63). The APR-50 is designed to detect, classify, identify, and locate any hostile system that emits radio-frequency radiation. The system receives inputs from antenna distributed across the airframe, and the electronics system performs automated signal processing and analysis and presents real-time updates to the crew.
The B-2 was first revealed to the public on November 22, 1988, when AV-1 (82-1066) was unveiled at Palmdale. At this time, the aircraft was still not ready for its first flight. The fact that the production and service-introduction schedules were not revealed led the media to suspect that the program was well behind schedule, which was indeed the case. The media speculation got even worse when the first flight of the AV-1 was delayed. At this time, Congress was reluctant to commit to any production until flight testing was well under way.
Taxi tests began on July 10, 1989. The B-2 finally made its first flight on July 17, 1989 from Palmdale, crewed by chief test pilot Bruce J. Hinds and Col. Richard Couch. It lasted 112 minutes and ended with a landing at Edwards AFB. After carrying out initial tests, AV-1 was used for radar cross section tests. In early 1993, AV-1 was placed in long term storage to await upgrading to full service configuration prior to joining the operational fleet.
The mass media was generally negative about the B-2 all during this period, and numerous "exposes" appeared in the press and on television claiming that there were safety problems with the aircraft and that the the cost of the plane would be so high that it could bankrupt the federal treasury. The second test aircraft (AV-2, 82-1067) flew for the first time on October 19, 1990 from Palmdale, landing at Edwards AFB. It was heavily instrumented and served as the loads test aircraft.
AV-3 (82-1068) took to the air for the first time on June 18, 1991. It was the first radar and navigation test aircraft.
AV-4 (82-1069) followed on April 17, 1992, and AV-5 (82-1070) on October 5, 1992. These two planes were used for avionics and weapons testing. The first bomb to be tested with the B-2 was a 2000-lb Mk 84, which was dropped from AV-4 on September 12, 1992. AV-5 was the intended for armament, climactic, and low-observability testing.
The last development aircraft, AV-6 (82-1071) flew on February 2, 1993. It was used for Tech order validation and fur further weapons and avionics testing. By January 1995, the six FSD B-2s had logged more than 2300 hours in the air in more than 490 flights. Most of the work was done by the 420th Flight Test Squadron at Edwards AFB.
The first production B-2 (AV-1007 (88-0328) was delivered to Whiteman AFB in Missouri in December of 1993. By that time, it was clear that the initial production run would be far short of the 132 initially planned. A military review initiated by Defense Secretary Richard Cheney announced that the total B-2 buy would be drastically cut to 75, with the production rate being only 12 aircraft per year by the mid-1990s. In the aftermath of the collapse of the Soviet Union and the Warsaw Pact, in October of 1991, Congress froze the production plan at only 16 aircraft. In January of 1992, President George Bush announced that the administration would seek funds for only five more B-2s, bring total production to only 21 planes, including the six test aircraft.
Northrop acquired Grumman in 1994, becoming Northrop Grumman.
In November of 1994, the Republican Party unexpectedly gained control of both houses of Congress, and there was hope that defense budgets might increase and that it might be possible to buy more B-2s. Northrop Grumman proposed that 20 more B-2s be built. However, the Clinton administration was not supportive of spending more money on B-2s nor was the Air Force, which had earmarked funds for bringing the AV-1 to operational status and to fund other programs.
Terrain-following certification flights were undertaken by AV-4 in September 1996. By January of 1997, the B-2 had reached limited operational capability. The first 10 production B-2s (AV-1007 through AV-1016) were delivered as Block 10 aircraft between December 1993 and the end of 1995. The Block 10s were primarily used as trainers for pilots and ground crews. They all had a takeoff weight limited to only 305,000 pounds, had no terrain-following radar and did not have the ability to deliver any precision weapons.
Three new aircraft (1017-1019) were delivered as Block 20 aircraft in 1996. The Block 20 aircraft operate at a higher takeoff weight of 336,500 pounds, is cleared for terrain-following operations, and the DMS is operational in bands 1 to 3. The Block 20 upgrade also includes a Global Positioning System (GPS) receiver with a special low-observable antenna. It replaces the astro-inertial unit for routine operations, although the AIU will be retained in use as an unjammable backup. Block 20 also has an improved environmental control system. The Block 20 aircraft could carry B61 nuclear weapons, cluster bombs, and could deliver some types of precision-guided weapons. Starting in mid-1996, the five newest Block 10 aircraft (1012-1016) went through an upgrade program to bring them up to Block 20 status.
The Block 30 is considered as the definitive operational standard. The Block 30 modifications include removal and replacement of all the aircraft's edges, including the leading edges and control surfaces. The RAM coating is of an improved type. Improved avionics software make it possible to achieve a terrain-following mode as low as 200 feet. The DMS system adds Band 4, allowing crews to re-plan their mission in flight. The Block 30 is integrated with the transportable Air Force Mission Support System (AFMSS) which is designed to integrate the B-2 with other USAF operations all throughout the world. This replaces the Strategic Mission Development and Planning System (SMDPS), which was intended for the nuclear mission only and was never intended to be used outside the B-2's main base. The last two B-2As were originally built to Block 30 standards, with the first Block 30 B-2A being delivered in August of 1997. Earlier Block 10 B-2s were traded back to Northrop Grumman for retrofit into Block 20 and Block 30 standards on a planned, staggered schedule. By the end of 2000, all 21 B-2s were brought up to Block 30 standards. There is currently no plans for any more B-2A construction, so the fleet will be limited to only 21 planes.
The 509th Bomb Wing at Whiteman AFB in Missouri was the first wing to receive the B-2. It became active on April 1, 1993 to receive the B-2. The 509th can trace its history back to the 509th Composite Group, which dropped the atomic bombs on Japan to end the Pacific War. The 509th had previously operated FB-111As at Pease AFB, New Hampshire, and had been inactivated when Pease was closed in 1988. The 509th (without equipment or personnel) then moved to Whiteman AFB, which was at that time also inactive. By the time that the 509th was resurrected at Whiteman, the USAF had replaced the term "Bombardment" with "Bomb". The operational units of the 509th BW were the 393rd Bomb Squadron and the 394th Combat Training Squadron. The first B-2A arrived on December 17, 1993. A 509th B-2 participated in a Red Flag exercise at Nellis AFB in Nevada on January 24, 1995, dropping two Mk. 84 bombs. The B-2s have been regular participants in Red Flag exercises ever since. On January 1, 1997, the USAF announced that the 509th's B-2As had reached a limited capacity to carry and release conventional weapons. On April 1 of that year, the USAF declared the 509th capable of handling both conventional and nuclear missions. On January 8, 1998, a second squadron, the 325th Bomb Squadron, was added to the 509th BW.
In May of 1996, the B-2 fleet was grounded for eight days because of cracks seen in one of the aircraft's tailpipe clamps. All of the tailpipe clamps were inspected, and those found to be faulty were replaced. In April of 1987, there was another grounding after an engine-shaft assembly broke during flight, and an investigation revealed that the housing of the shaft assembly had nearly undetectable cracks. The planes were returned to flight status in mid April.
The B-2A made its combat debut in Operation Allied Force, the NATO intervention in Kosovo. On March 24, 1999, two B-2As from the 509th BW at Whiteman AFB flew a 31-hour round trip mission against targets in Kosovo. This mission also marked the first use of the GBU-29/30 JDAM in combat.
82-1066/1071 Northrop B-2A Block 10 Spirit AV-1 through AV-6, c/n 1001/1006 1066 *Fatal Beauty*, upgraded as Block 30 in 2000. Now named *Spirit of America* 1067 upgraded as Block 30 1997. Named *Spirit of Arizona* 1068 *Shady Lady*/*The Ghost*, upgraded as Block 30 in 1999. Named *Spirit of New York* 1069 upgraded as Block 30 in 2000. Named *Spirit of Indiana*. 1070 *Fire and Ice*, upgraded as Block 30 in 1999. Named *Spirit of Ohio* 1071 *Black Widow*, upgraded as Block 30. Named *Spirit of Mississippi* 88-0328/0332 Northrop B-2A Block 10 Spirit AV-7/AV-11, c/n 1007/1011 0328 *Spirit of Texas*. Upgraded to Block 30 status in 1998 0329 *Spirit of Missouri*. Upgraded to Block 30 status in 1997 0330 *Spirit of California*. Upgraded to Block 30 status in 1998 0331 *Spirit of South Carolina*. Upgraded to Block 30 status in 1998. 0332 *Spirit of Washington*. Upgraded to Block 30 status in 1999. 89-0127/0129 Northrop B-2A Block 10 Spirit 0127 AV-12 c/n 1012 *Spirit of Kansas*. Upgraded to Block 20 1996, to Block 30 1998. 0128 AV-13 c/n 1013 *Spirit of Nebraska*. Upgraded to Block 20 1996, to Block 30 1999. 0129 AV-14 c/n 1014 *Spirit of Georgia*. Upgraded to Block 20 1997, to Block 30 1999. 90-0040/0041 Northrop B-2A Block 10 Spirit 0040 AV-15 c/n 1015 *Spirit of Alaska*. Upgraded to Block 20 1997, to Block 30 1999 0041 AV-16 c/n 1016 *Spirit of Hawaii*. Upgraded to Block 20 1996, to Block 30 1999. 92-0700 Northrop B-2A Block 20 Spirit 0700 AV-17 c/n 1017 *Spirit of Florida*. Modified to Block 30 1999 93-1085/1086 Northrop B-2A Block 20 Spirit 1085 AV-18 c/n 1018 *Spirit of Oklahoma*. Modified to Block 30 2000 1086 AV-19 c/n 1019 *Spirit of Kitty Hawk*. Modified to Block 30 2000 93-1087/1088 Northrop B-2A Block 30 Spirit 1087 AV-20 c/n 1020 *Spirit of Pennsylvania* 1088 AV-21 c/n 1021 *Spirit of Louisiana*
Engines: Four General Electric F118-GE-100 non-afterburning turbofans, each rated at 19,000 lb.s.t. Performance: Cruising speed Mach 0.85 at 36,000 feet, Mach 0.8 at sea level. Service ceiling: 50,000 feet plus. Maxmum unrefuelled range with 8 SRAM and 8 B82 weapons 5075 miles (with a 1000 nm low-altitude segment) or 7250 miles all at high level. Dimensions: Wingspan 172 feet, Length 69 feet, Height 17 feet, wing area 5140 square feet. Weights: 153,700 pounds empty, 336,500 pounds normal takeoff, 375,000 pounds maximum takeoff. Maximum internal fuel capacity 180,000 pounds. Weapons: Maximum payload of 50,000 pounds in two internal weapons bays.