Structure and Systems
The B-2 is the shape it is because Low-Observable, or "Stealth" technology was the main design driver. A stealth aircraft has to be stealthy in six disciplines: radar, infra-red, visual, acoustic, smoke and contrail. The main threat to aircraft is radar, and what makes stealth possible is that very large reductions in an aircraft's radar cross-section (RCS) can be achieved. The reason they are achievable is that conventional aircraft are almost ideal radar targets, with a lot of reflective surfaces.
There are some popular design features which are incompatible with stealth aircraft, such as engines in external pods, vertical stabilisers, slab-sided bodies and external stores. All these things came together to make the flying wing the best solution for both aerodynamic and structural reasons.
The B-2 as built is naturally unstable, with most of its mass aft of the centre of lift, thus making the digital flight control system (DFCS) the most critical subsystem on the aircraft. This is almost certainly a quadruplex fly-by-wire system. The DFCS controls the eight large flaps which comprise most of the B-2's trailing edge. These are:
- Two sets of combination drag rudders and speed brakes, one on either side, located furthest outboard near the wingtips
- Two three-part sets of elevons, one set on either side, located inboard of the drag rudders
- A "beaver tail" assembly, located farthest aft on the center line, which is called the Gust Load Alleviation System (GLAS)
The two drag rudder assemblies are essentially a four-part split-flap system whereby each drag rudder has an upper and lower appendage. If used independently they will turn the aircraft, or if used together they act as speed brakes. The six elevon flying control surfaces, used together or separately, are used as elevators and/or flaps. The GLAS is used primarily for pitch control.
The shape of the B-2 is quite unlike that of any other aircraft. Viewed from above it consists of twelve straight lines, eliminating as much as possible any chance of reflected radar energy being returned to the receiver. Viewed from the side, the aircraft is all curves, except that they are not constant curves; they continuously change radius. There is no abrupt distinction between body and wing. The dorsal hump rises smoothly from the top surface, but the underside swells gradually from the outermost trailing-edge kink to the centerline.
Making such structures in metal would be very difficult, but they can be made of carbon-fiber composites. This material can be made so that it absorbs radar energy. It can also be made into radar-absorbing structures, for example honeycomb.
In order to build the B-2, a new concept called Computer Integrated Manufacturing was used. This means that the definition of every component used in contruction is held in a computer database, and the same database is used to control machine tools and robots which actually make the aircraft.
The B-2 is the product of an astonishing blend of technologies. The result is a virtually undetectable aircraft, which should be able to penetrate the most heavily defended airspace and survive.
Crew
Two pilots.
Dimensions
| Length | : | 20.9m |
| Wingspan | : | 52.4m |
| Height | : | 5.1m |
| Wing area | : | 477.5m2 |
| Empty Weight | : | about 54500 kg |
| Operational Empty | : | 69705 kg |
| Normal T/O Weight | : | 152607 kg |
| Maximum T/O Weight | : | about 159000 kg |
| Max Payload | : | 22700 kg |
Performance
| Speed, 10800m | : | Mach 0.85 (900 km/hr) |
| Speed, sea level | : | Mach 0.8 (980 km/hr) |
| Range | : | Approx 9600 km, unrefulled |
| Ceiling | : | Above 15000m |
Powerplant
The B-2 has four General Electric F118-GE-100 engines, each rated at 78.47 kN. The F118 is derived from the F101 used by the B-1, and has a bypass ratio of 0.87:1 as opposed to 2:1 on the earlier engine. Although the F118 uses more fuel at subsonic speed, it requires less air than the F101 and the inlet is therefore smaller and simpler. Shielding the engine fan face from radar is vital to meet low-observable objectives.
The B-2's engine exhausts are built into the top of the wing, and are well ahead of the trailing edge. They lead into a pair of trenches which flare outwards. The key to reducing the IR signature is to ensure that the exhaust dissipates as soon as possible after leaving the aircraft. The hot exhaust gases are probably mixed with cold bypass air, and the trench probably creates a vortex which further promotes mixing.
B-2 fuel capacity is about 81000 kg, and the aircraft can be refulled in flight by the boom method.
Radar
The B-2's main sensor is the AN/APQ-181 radar. This has 21 separate modes. It has two electronically-scanned antennae, one on each side of the nosewheel bay. It has unspecified "low probability-of-intercept" (LPI) features, in both signal performance and operational techniques, making it less likely to show on an enemy's electronic surveillance equipment.
The APQ-181 operates in Ku-band (12-18 GHz), which is a rather higher frequency than the 10GHz (X-band) used by most airborne radars. Ku-band radars suffer more atmospheric absorption than X-band, and normally require more time and more power to scan a given volume. However, they have inherently higher resolution than X-band radars, and for a given antenna size, a Ku-band radar will have smaller sidelobes which will dissipate more quickly.
The AN/ALQ-181 has a synthetic-aperture mode, which allows it to paint a picture of the target and this allows the GPS-Aided Targetting System (GATS) to compute the local GPS bias, and thus enhance weapon accuracy.
In May 2003 Northrop was awarded a contract for the further development of the B-2's radar system. The radar modernization effort consists of replacing the B-2's current radar antenna with a Ku-band active electronically scanned array antenna. The new antenna resolves potential conflicts in radio frequency usage between the B-2 and commercial satellite systems that also use the Ku-band spectrum. Installation of the new antenna on the B-2 fleet is scheduled to be completed by the end of the decade.
On September 9th 2004 Northrop Grumman, with Raytheon as principal subcontractor, was awarded a $388 million contract as the next phase of the program to modernize the B-2's radar system, which will replace the current radar antenna with an actively electronically scanned array (AESA). The new radar will resolve conflicts in radio frequency usage between the B-2 and commercial systems, and allow future upgraded to improve performance. During the system development and demonstration (SDD) phase Northrop Grumman will integrate six new radar systems on B-2 aircraft for initial demonstration and operational training. Ultimately the new radar will be installed on all 21 aircraft.
On October 19th 2004 Raytheon announced that the new AESA antenna had successfully completed a production readiness review for the transmit/receive (T/R) module at the heart of the array. Completion of the readiness review allows the SDD phase to begin.
Navigation
The B-2 almost certainly uses Navstar/GPS as its primary navigation system. The APQ-181 must have a navigation mode, but this would not be used unless essential. There is probably an INS as a further backup.
Traditional terrain-following requires radar emissions, so some form of terrain-profile matching (as used in the Tornado GR.4) is probably employed.
Defensive Management Subsystem
The B-2's defensive management subsystem (DSM) is largely classified. Its most important component appears to be the Lockheed Martin AN/APR-50 (also known as the ZSR-63). The ZSR-63 replaced the ZSR-62, which was cancelled after unspecified problems were encountered during development.
The ZSR-63 is designed to detect, classify, identify and locate hostile systems that emit RF energy. The B-2 probably has the same level of capability as dedicated EW aircraft such as the EA-6B Prowler.
The ZSR-63 may be able to actively cancel radar returns, just as "white noise" is a sound silencer by being the precise opposite of received pulses. It is possible that this capability was included in the cancelled ZSR-62 set.
Configuration Blocks
The Air Force accepted delivery of production B-2s in three configuration blocks: 10, 20, and 30. Delivery comprised 6 test aircraft, 10 block 10 aircraft, 3 block 20, and 2 block 30.
Block 10 configured aircraft provided limited combat capability with no capability for terrain-following or launching conventional guided weapons. It was able to carry 2000-pound Mk84 conventional bombs or gravity nuclear weapons, however. The DMS only had a limited capability.
Block 20 configured aircraft had an interim capability to launch nuclear and conventional munitions, including the GAM guided munition. It was cleared for manual terrain-following down to 200m, and the DMS was operational in bands 1-3. It also had an improved environmental control system.
Block 30 configured aircraft are fully capable and meet the essential employment capabilities defined by the Air Force. The first fully configured Block 30 aircraft, (93-1087 Spirit of Pennsylvania), was delivered to the Air Force on 7th August 1997. Compared to the Block 20, the Block 30s have almost double the radar modes along with enhanced terrain-following capability (auto down to 60m) and the ability to deliver additional weapons, including the Joint Direct Attack Munition and the Joint Stand Off Weapon. Other features include incorporation of configuration changes needed to make B-2s conform to the approved radar signature (no seams in the leading edge); replacement of the aft decks; installation of remaining defensive avionics functions (band 4); and installation of a contrail management system. All surface coatings are removed and replaced with improved materials.
All B-2s (except AV-4) have brought up to the Block 30 configuration standard. This process began in July 1995 and was completed in June 2000. AV-4 will become a "Super Block 30" aircraft in 2002.
Upgrades
Datalink � Providing Line-of-Sight data for aircraft-to-aircraft, aircraft-to-C2, and aircraft-to-sensor connectivity, Link-16 is a combat force multiplier that provides military services with fully interoperable capabilities and greatly enhances tactical C3I mission effectiveness. Link-16 provides increased survivability, develops a real-time picture of the cambat theater, and enables aircraft to quickly share information on short notice.
EHF Communication � The command and control of nuclear forces requires a survivable communication system. To satisfy this requirement, USAF plans to deploy an advanced Extremely High Frequency (EHF) satellite communications constellation. This constellation will provide a survivable, high capability communication system. The B-2 will be fitted with an EHF communication capability satisfying DoD requirements.
Digital Engine Controller - The B-2's current analogue engine controllers are high failure items. New digital engine controllers will improve the B-2�s performance and increase reliability and maintainability.
Computers - With advances in computer technology and increased demands on the system, the B-2�s computers will need to be replaced. Although reliable, maintaining the present computers will become increasingly difficult and costly.
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