美国隐形轰炸机B-2

2024年2月8日发(作者：)

Northrop Grumman B-2 Spirit

Overview

The Northrop (later Northrop Grumman) B-2 Spirit, also known as the Stealth

Bomber, is an American heavy strategic bomber, featuring low observable stealth

technology designed for penetrating dense anti-aircraft defenses; it is a blended

wing body design with a crew of two. The bomber can deploy both conventional

and thermonuclear weapons, such as eighty 500 lb (230 kg)-class (Mk 82) JDAM

Global Positioning System-guided bombs, or sixteen 2,400 lb (1,100 kg) B83

nuclear bombs. The B-2 is the only known aircraft that can carry large

air-to-surface standoff weapons in a stealth configuration.

Development originally started under the "Advanced Technology Bomber" (ATB)

project during the Carter administration, and its performance was one of his

reasons for the cancellation of the supersonic B-1A bomber. ATB continued

during the Reagan administration, but worries about delays in its introduction led

to the reinstatement of the B-1 program as well. Program costs rose throughout

development. Designed and manufactured by Northrop Grumman, the cost of

each aircraft averaged US$737 million (in 1997 dollars). Total procurement costs

averaged $929 million per aircraft, which includes spare parts, equipment,

retrofitting, and software support. The total program cost including development,

engineering and testing, averaged $2.1 billion per aircraft in 1997.

Because of its considerable capital and operating costs, the project was

controversial in the U.S. Congress and among the Joint Chiefs of Staff. The

winding-down of the Cold War in the latter portion of the 1980s dramatically

reduced the need for the aircraft, which was designed with the intention of

penetrating Soviet airspace and attacking high-value targets. During the late

1980s and 1990s, Congress slashed plans to purchase 132 bombers to 21. In

2008, a B-2 was destroyed in a crash shortly after takeoff, though the crew

ejected safely. A total of 20 B-2s remain in service with the United States Air

Force, which plans to operate the B-2 until 2058.

The B-2 is capable of all-altitude attack missions up to 50,000 feet (15,000 m),

with a range of more than 6,000 nautical miles (11,000 km) on internal fuel and

over 10,000 nautical miles (19,000 km) with one midair refueling. Though

originally designed primarily as a nuclear bomber, it was first used in combat

dropping conventional ordnance in the Kosovo War in 1999 and saw further

service in Iraq and Afghanistan.

Design

The B-2 Spirit was developed to take over the USAF's vital penetration missions,

able to travel deep into enemy territory to deploy their ordnance, which could

include nuclear weapons. The B-2 is a flying wing aircraft, meaning it has no

fuselage or tail. The blending of low-observable technologies with high

aerodynamic efficiency and large payload gives the B-2 significant advantages

over previous bombers. Low observability provides a greater freedom of action

at high altitudes, thus increasing both range and field of view for onboard

sensors. The U.S. Air Force reports its range as approximately 6,000 nautical

miles (6,900 mi; 11,000 km). At cruising altitude the B-2 refuels every six hours,

taking on up to 50 short tons (45 t) of fuel at a time.

Due to the aircraft's complex flight characteristics and design requirements to

maintain very-low visibility to multiple means of detection, both the development

and construction of the B-2 required pioneering use of computer-aided design

and manufacturing technologies. The B-2 bears a resemblance to earlier

Northrop aircraft: the YB-35 and YB-49 were both flying wing bombers that had

been canceled in development in the early 1950s, allegedly for political reasons.

The resemblance goes as far as B-2 and YB-49 having the same wingspan.

As of September 2013 about 80 pilots fly the B-2. Each aircraft has a crew of two,

a pilot in the left seat and mission commander in the right, and has provisions for

a third crew member if needed. For comparison, the B-1B has a crew of four and

the B-52 has a crew of five. The B-2 is highly automated and, unlike most

two-seat aircraft, one crew member can sleep in a camp bed, use a toilet, or

prepare a hot meal while the other monitors the aircraft; extensive sleep cycle

and fatigue research was conducted to improve crew performance on long

sorties.

Armaments and equipment

The B-2, in the envisaged Cold War scenario, was to perform deep-penetrating

nuclear strike missions, making use of its stealthy capabilities to avoid detection

and interception throughout missions. There are two internal bomb bays in

which munitions are stored either on a rotary launcher or two bomb-racks; the

carriage of the weapons loadouts internally results in less radar visibility than

external mounting of munitions. The B-2 is capable of carrying 40,000 pounds of

ordnance. Nuclear ordnance includes the B61 and B83 nuclear bombs; the

AGM-129 ACM cruise missile was also intended for use on the B-2 platform.

It was decided, in light of the dissolution of the Soviet Union, to equip the B-2 for

conventional precision attacks as well as for the strategic role of nuclear-strike.

The B-2 features a sophisticated GPS-Aided Targeting System (GATS) that uses

the aircraft's APQ-181 synthetic aperture radar to map out targets prior to

deployment of GPS-aided bombs (GAMs), later superseded by the Joint Direct

Attack Munition (JDAM). In the B-2's original configuration, up to 16 GAMs or

JDAMs could be deployed; an upgrade program in 2004 raised the maximum

carriable capacity to 80 JDAMs.

The B-2 has various conventional weapons in its arsenal, able to equip Mark 82

and Mark 84 bombs, CBU-87 Combined Effects Munitions, GATOR mines, and the

CBU-97 Sensor Fuzed Weapon. In July 2009, Northrop Grumman reported the B-2

was compatible with the equipment necessary to deploy the 30,000 lb (14,000 kg)

Massive Ordnance Penetrator (MOP), which is intended to attack reinforced

bunkers; up to two MOPs could be equipped in the B-2's bomb bays, the B-2 is

the only platform compatible with the MOP as of 2012. As of 2011, the AGM-158

JASSM cruise missile is an upcoming standoff munition to be deployed on the

B-2 and other platforms. This is to be followed by the Long Range Standoff

Weapon which may give the B-2 a standoff nuclear capability for the first time.

Avionics and systems

In order to make the B-2 more effective than previous bombers, many advanced

and modern avionics systems were integrated into its design, these have been

modified and improved following a switch to conventional warfare missions. One

system is the low probability of intercept AN/APQ-181 multi-mode radar, a fully

digital navigation system that is integrated with terrain-following radar and

Global Positioning System (GPS) guidance, NAS-26 astro-inertial navigation

system (first such system tested on the Northrop SM-62 Snark cruise missile)

and a Defensive Management System (DMS) to inform the flight crew of possible

threats. The onboard DMS is capable of automatically assessing the detection

capabilities of identified threats and indicated targets.

For safety and fault-detection purposes, an on-board test system is interlinked

with the majority of avionics on the B-2 to continuously monitor the performance

and status of thousands of components and consumables; it also provides

post-mission servicing instructions for ground crews. In 2008, many of the 136

standalone distributed computers on board the B-2, including the primary flight

management computer, were being replaced by a single integrated system. The

avionics are controlled by 13 EMP-resistant MIL-STD-1750A computers, which

are interconnected through 26 MIL-STD-1553B-busses; other system elements

are connected via optical fiber.

In addition to periodic software upgrades and the introduction of new

radar-absorbent materials across the fleet, the B-2 has had several major

upgrades to its avionics and combat systems. For battlefield communications,

both Link-16 and a high frequency satellite link have been installed, compatibility

with various new munitions has been undertaken, and the AN/APQ-181 radar's

operational frequency was shifted in order to avoid interference with other

operator's equipment. The arrays of the upgraded radar features were entirely

replaced to make the AN/APQ-181 into an active electronically scanned array

(AESA) radar.

To keep the B-2 relevant against increasingly sophisticated air defenses with

greater computer processing power that may be able to detect the aircraft

through its stealth measures, the Air Force is working to upgrade the Defensive

Management System (DMS), which uses sensors, passive receivers, and

computers to alert crew about the location of enemy air defense systems.

Though it will not counter air threats, it will be more capable of identifying and

locating them before the aircraft enters strike range. The upgraded DMS will help

detect radar emissions from air defenses to allow changes to the auto-router's

mission planning information while in-flight so it can receive new data quickly to

plan a route that minimizes exposure to dangers. The DMS upgrade is slated to

enter service by 2021.

Flight controls

In order to address the inherent flight instability of a flying wing aircraft, the B-2

uses a complex quadruplex computer-controlled fly-by-wire flight control system,

that can automatically manipulate flight surfaces and settings without direct pilot

inputs in order to maintain aircraft stability. The flight computer receives

information on external conditions such as the aircraft's current air speed and

angle of attack via pitot-static sensing plates, as opposed to traditional pitot

tubes which would negatively affect the aircraft's stealth capabilities. The flight

actuation system incorporates both hydraulic and electrical servoactuated

components, and it was designed with a high level of redundancy and

fault-diagnostic capabilities.

Northrop had investigated several means of applying directional control that

would least infringe on the aircraft's radar profile, eventually settling on a

combination of split brake-rudders and differential thrust. Engine thrust became

a key element of the B-2's aerodynamic design process early on; thrust not only

affects drag and lift but pitching and rolling motions as well. Four pairs of control

surfaces are located along the wing's trailing edge; while most surfaces are used

throughout the aircraft's flight envelope, the inner elevons are normally only in

use at slow speeds, such as landing. To avoid potential contact damage during

takeoff and to provide a nose-down pitching attitude, all of the elevons remain

drooped during takeoff until a high enough airspeed has been attained.

Stealth

The B-2's low-observable, or "stealth", characteristics enable the undetected

penetration of sophisticated anti-aircraft defenses and to attack even heavily

defended targets. This stealth comes from a combination of reduced acoustic,

infrared, visual and radar signatures (Multi-spectral camouflage) to evade the

various detection systems that could be used to detect and be used to direct

attacks against an aircraft. The B-2s stealth enables the reduction of supporting

aircraft that are required to provide air cover, Suppression of Enemy Air

Defenses and electronic countermeasures, making the bomber a "force

multiplier". as of September 2013, there have been no instances of a missile

being launched at a B-2.

To reduce optical visibility during daylight flights, the B-2 is painted in an

anti-reflective paint. The undersides are dark because it flies at high altitudes

(50,000 feet), and at that altitude a dark grey painting blends well into the sky. It is

speculated to have an upward-facing light sensor which alerts the pilot to

increase or reduce altitude to match the changing illuminance of the sky. The

original design had tanks for a contrail-inhibiting chemical, but this was replaced

in production aircraft by a contrail sensor that alerts the crew when they should

change altitude. The B-2 is vulnerable to visual interception at ranges of 20 nm/37

km or less.

Radar

Reportedly, the B-2 has a radar cross-section of about 0.1 m2. (for comparison, a

surface-to-air missile has roughly the same RCS, while a human has an RCS of

about 1 m2, see Radar cross-section) The bomber does not always fly stealthily;

when nearing air defenses pilots "stealth up" the B-2, the details of which are

secret. The aircraft is not seen by radar, except briefly when the bomb bay opens.

The B-2's clean, low-drag flying wing configuration not only provides exceptional

range but is also beneficial to reducing its radar profile. The flying wing design

most closely resembles a so-called infinite flat plate (as vertical control surfaces

dramatically increase RCS), the perfect stealth shape, as it would lack angles to

reflect back radar waves (initially, the shape of the Northrop ATB concept was

flatter; it gradually increased in volume according to specific military

requirements).

RCS reduction as a result of shape had already been observed on the Royal Air

Force's Avro Vulcan strategic bomber, and the USAF's F-117 Nighthawk. The

F-117 used flat surfaces (faceting technique) for controlling radar returns as

during its development (see Lockheed Have Blue) in the early 1970s, technology

only allowed for the simulation of radar reflections on simple, flat surfaces;

computing advances in the 1980s made it possible to simulate radar returns on

more complex curved surfaces. The B-2 is composed of many curved and

rounded surfaces across its exposed airframe to deflect radar beams. This

technique, known as continuous curvature, was made possible by advances in

computational fluid dynamics, and first tested on the Northrop Tacit Blue.

The leading edges of the wing converge at the nose of the aircraft, which

prevents radar reflections in the direction of flight; the W-shaped rear of the

aircraft is for similar reasons. The engine air intakes are placed on the superior

side of the fuselage to minimize reflection to ground-based radar. To avoid

detection by look-down radars, engines are buried within the B-2's wing (S-duct)

to conceal the engines' fans. The B-2 also carries all ordnance internally; the

aircraft's RCS becomes substantially larger while the bay doors are opens,

making it temporarily easier to detect.

Infrared

Some analysts claim Infra-red search and track systems (IRSTs) can be deployed

against stealth aircraft, because any aircraft surface heats up due to air friction

and with a two channel IRST is a CO2 (4.3 µm absorption maxima) detection

possible, through difference comparing between the low and high channel.

Burying engines deep inside the fuselage also minimizes thermal visibility of the

exhaust. At the engine intake, cold air from the boundary layer below the main

inlet enters the fuselage (Boundary layer suction, first tested on the Northrop

X-21) and is mixed with hot exhaust air just before the nozzles (similar to the

Ryan AQM-91 Firefly). According to the Stefan–Boltzmann law, this results in less

energy (thermal radiation in infrared spectrum) being released and thus a

reduced heat signature. The resulting cooler air is conducted over a surface

composed of heat resistant carbon-fiber-reinforced polymer and titanium alloy

elements, which disperse the air laterally, in order to accelerate its cooling. The

B-2 lacks afterburners as the hot exhaust would increase the infrared footprint;

breaking the sound barrier would produce an obvious sonic boom as well as

aerodynamic heating of the aircraft skin which also increased the infrared

footprint.

Materials

According to the Huygens–Fresnel principle, even a very flat plate would still

reflect radar waves, though much less than when a signal is bouncing at a right

angle. Additional reduction in its radar signature was achieved by the use of

various radar-absorbent materials (RAM) to absorb and neutralize radar beams.

The majority of the B-2 is made out of a carbon-graphite composite material that

is stronger than steel and lighter than aluminum. Perhaps most crucially, a

beneficial characteristic of this composite material is the absorption of

significant amounts of radar energy.

The B-2 is assembled with unusually tight engineering tolerances to avoid leaks

as they could increase its radar signature. Innovations such as alternate high

frequency material (AHFM) and automated material application methods were

also incorporated into the aircraft to enhance its radar-absorbent properties and

lower maintenance requirements. In early 2004, Northrop Grumman began

applying a newly developed AHFM to operational B-2s. In order to protect the

operational integrity of its sophisticated radar absorbent material and coatings,

each B-2 is kept inside a climate-controlled hangar (Extra Large Deployable

Aircraft Hangar System) large enough to accommodate its 172-foot (52 m)

wingspan.

Specifications

General characteristics

Crew: 2: pilot and commander (co-pilot)

Length: 69 ft (21.0 m)

Wingspan: 172 ft (52.4 m)

Height: 17 ft (5.18 m)

Wing area: 5,140 ft² (478 m²)

Empty weight: 158,000 lb (71,700 kg)

Loaded weight: 336,500 lb (152,200 kg)

Max. takeoff weight: 376,000 lb (170,600 kg)

Powerplant: 4 × General Electric F118-GE-100 non-afterburning turbofans, 17,300

lbf (77 kN) each

Fuel Capacity: 167,000 pounds (75,750 kg)

Performance

Maximum speed: Mach 0.95 (550 knots, 630 mph, 1,010 km/h) at 40,000 ft altitude

/ Mach 0.95 at sea level

Cruise speed: Mach 0.85 (487 knots, 560 mph, 900 km/h) at 40,000 ft altitude

Range: 6,000 nmi (11,100 km (6,900 mi))

Service ceiling: 50,000 ft (15,200 m)

Wing loading: 67.3 lb/ft² (329 kg/m²)

Thrust/weight: 0.205

Armament

2 internal bays for ordnance and payload with an official limit of 40,000 lb (18,000

kg); maximum estimated limit is 50,000 lb (23,000 kg).

80× 500 lb class bombs (Mk-82, GBU-38) mounted on Bomb Rack Assembly

(BRA)

36× 750 lb CBU class bombs on BRA

16× 2,000 lb class bombs (Mk-84, GBU-31) mounted on Rotary Launcher

Assembly (RLA)

16× B61 or B83 nuclear bombs on RLA (strategic mission)