F-15 Eagle

The McDonnell Douglas (now Boeing) F-15 Eagle is a twin-engine, all-weather tactical fighter designed to gain and maintain air superiority in aerial combat. It was developed for the United States Air Force, and first flew in July 1972. It is one of the most recognized fighters of the modern day. The F-15E Strike Eagle derivative is an all-weather strike fighter that entered service in 1989. The U.S Air Force plans to keep the F-15 in service until 2025.

Development
Origins
In 1967 U.S. intelligence was surprised to find that the Soviet Union was building a large fighter aircraft, known as the MiG-25 'Foxbat'. It was not known in the West at the time that the MiG-25 was designed as a high-speed interceptor, (not an air superiority fighter), so its primary asset was speed, not maneuverability. The MiG-25's huge tailplanes and vertical stabilizers (tail fins) hinted at a very maneuverable aircraft, which worried the Air Force that its performance might be higher than its American counterparts. In reality, the MiG's large fins and stabilators were necessary to prevent the aircraft from encountering inertia coupling in high-speed, high-altitude flight.
The F-4 Phantom II of the USAF and U.S. Navy was the only fighter with enough power, range, and maneuverability to be given the primary task of dealing with the threat of Soviet fighters while flying with visual engagement rules. As a matter of policy, the Phantoms could not engage targets without positive visual identification, so they could not engage targets at long ranges, as designed. Medium-range AIM-7 Sparrow missiles, and to a lesser degree even the AIM-9 Sidewinder, were often unreliable and ineffective at close ranges where it was found that guns were often the only effective weapon. The Phantom did not originally have a gun, but experience in Vietnam led to the addition of a gun. An external gun pod was tried and later the M61 Vulcan was integrated internally on the F-4E.

F-X program
There was a clear need for a new fighter that overcame the close-range limitation of the Phantom while retaining long-range air superiority. After rejecting the U.S. Navy VFX program (which led to the F-14 Tomcat) as being unsuited to its needs, the U.S. Air Force issued its own requirements for the Fighter Experimental (F-X), a specification for a relatively lightweight air superiority fighter. The requirements called for single-seat fighter having a maximum take-off weight of 40,000 lb (18,100 kg) for the air-air role with a maximum speed of Mach 2.5 and a thrust to weight ratio of nearly 1 at mission weight. Four companies submitted proposals, with the Air Force eliminating General Dynamics and awarded contracts to Fairchild Republic, North American Rockwell, and McDonnell Douglas for the definition phase in December 1968. The companies submitted technical proposals by June 1969. The Air Force announced the selection of McDonnell Douglas on 23 December 1969. The winning design resembled the twin-tailed F-14, but with fixed wings. It would not be significantly lighter or smaller than the F-4 that it would replace.

The Eagle's initial versions were designated F-15A for the single-seat configuration and F-15B (originally TF-15A, but this designation was quickly deprecated, as the F-15B is fully combat-capable) for the twin-seat. These versions would be powered by new Pratt & Whitney F100 engines to achieve a combat thrust-to-weight ratio in excess of 1 to 1. A proposed 25 mm Ford-Philco GAU-7 cannon with caseless ammunition was dropped in favor of the standard M61 Vulcan gun due to development problems. The F-15 retained conformal carriage of four Sparrow missiles like the Phantom. The fixed wing was put onto a flat, wide fuselage that also provided an effective lifting surface. Some questioned if the zoom performance of the F-15 with Sparrow missiles was enough to deal with the new threat of the high-flying MiG-25 "Foxbat", but its capability was eventually demonstrated in combat.

The first F-15A flight was made in July 1972 with the first flight of the two-seat F-15B (formerly TF-15A) following in July 1973.

The F-15 has a "look-down/shoot-down" radar that can distinguish low-flying moving targets from ground clutter. The F-15 would use computer technology with new controls and displays to lower pilot workload and require only one pilot to save weight. Unlike the F-14 or F-4, the F-15 has only a single canopy frame with clear vision forward. The USAF introduced the F-15 as "the first dedicated USAF air superiority fighter since the F-86 Sabre."

The F-15 would be favored by customers such as the Israel Air Force and Japan Air Self-Defense Force, and the development of the F-15E Strike Eagle would produce a strike fighter that would replace the F-111. However, criticism from the fighter mafia that the F-15 was too large to be a dedicated dogfighter, and too expensive to procure in large numbers to replace the F-4 and A-7, led to the Lightweight Fighter (LWF) program, which led to the USAF F-16 Fighting Falcon and the middle-weight Navy F/A-18 Hornet.

Further development
The single-seat F-15C and two-seat F-15D models entered production in 1978 with the models' first flights in February and June of that year. These new models have Production Eagle Package (PEP 2000) improvements, including 2,000 lb (900 kg) of additional internal fuel, provision for carrying exterior conformal fuel tanks and increased maximum takeoff weight of up to 68,000 lb (30,700 kg).

The F-15 Multistage Improvement Program (MSIP) was initiated in February 1983 with the first production MSIP F-15C produced in 1985. Improvements included an upgraded central computer; a Programmable Armament Control Set, allowing for advanced versions of the AIM-7, AIM-9, and AIM-120A missiles; and an expanded Tactical Electronic Warfare System that provides improvements to the ALR-56C radar warning receiver and ALQ-135 countermeasure set. The final 43 included the enhanced-capability Hughes APG-70 radar, which was carried forward into the F-15E. The earlier MSIP F-15Cs with the APG-63 were later upgraded to the APG-63(V)1, which significantly improves reliability and maintainability while providing performance similar to the APG-70. The improvements were retrofitted to existing F-15s.

Recent upgrades include retrofiting 178 F-15C fighters with the AN/APG-63(V)3 Active Electronically Scanned Array (AESA) radar with delivery beginning in early 2009. Additionally, the Air Force also plans to upgrade other F-15s with the Joint Helmet Mounted Cueing System (JHMCS).

Design
The F-15 has an all-metal semi-monocoque fuselage with a large cantilever shoulder-mounted wing. The empennage is all-metal twin fins and rudders with all-moving composite horizontal tail surfaces outboard of the fins. The F-15 has a spine-mounted air brake and retractable tricycle landing gear. It is powered by two Pratt & Whitney F100 axial-flow turbofan engines with afterburners mounted side-by-side in the fuselage. The cockpit is mounted high in the forward fuselage with a one-piece windscreen and large canopy to increase visibility.
The F-15's maneuverability is derived from low wing loading (weight to wing area ratio) with a high thrust-to-weight ratio enabling the aircraft to turn tightly without losing airspeed. The F-15 can climb to 30,000 ft (10,000 m) in around 60 seconds. The thrust output of the dual engines is greater than the aircraft's weight, thus giving it the ability to accelerate in a vertical climb. The weapons and flight control systems are designed so that one person can safely and effectively perform air-to-air combat. The "A" and "C" models are single-seat variants that make up the bulk of F-15 production. "B" and "D" models add a second seat behind the pilot for training. "E" models use the second seat for a bombardier/navigator.
A multi-mission avionics system includes a head-up display (HUD), advanced radar, inertial guidance system (INS), flight instruments, ultra high frequency (UHF) communications, and Tactical Air Navigation (TACAN) and Instrument Landing System (ILS) receivers. It also has an internally mounted, tactical electronic-warfare system, "identification friend or foe" system, electronic countermeasures suite and a central digital computer.

The heads-up display projects, through a combiner, all essential flight information gathered by the integrated avionics system. This display, visible in any light condition, provides the pilot information necessary to track and destroy an enemy aircraft without having to look down at cockpit instruments.

The F-15's versatile APG-63/70 Pulse-Doppler radar system can look up at high-flying targets and down at low-flying targets without being confused by ground clutter. It can detect and track aircraft and small high-speed targets at distances beyond visual range (the maximum being 120 nautical miles (220 km) away) down to close range, and at altitudes down to treetop level. The radar feeds target information into the central computer for effective weapons delivery. The capability of locking onto targets as far as 50 nautical miles (90 km) with an AIM-120 AMRAAM enables true beyond visual range (BVR) engagement of targets. For close-in dogfights, the radar automatically acquires enemy aircraft, and this information is projected on the head-up display. The F-15's electronic warfare system provides both threat warning and automatic countermeasures against selected threats.

A variety of air-to-air weaponry can be carried by the F-15. An automated weapon system enables the pilot to perform aerial combat safely and effectively, using the head-up display and the avionics and weapons controls located on the engine throttles or control stick. When the pilot changes from one weapon system to another, visual guidance for the required weapon automatically appears on the head-up display.

The Eagle can be armed with combinations of four different air-to-air weapons: AIM-7F/M Sparrow missiles or AIM-120 AMRAAM advanced medium range air-to-air missiles on its lower fuselage corners, AIM-9L/M Sidewinder or AIM-120 missiles on two pylons under the wings, and an internal M61A-1 20 mm Gatling gun in the right wing root.

Low-drag conformal fuel tanks (CFTs) were developed for the F-15C and D models. They can be attached to the sides of the engine air intake trunks under each wing and are designed to the same load factors and airspeed limits as the basic aircraft. However, they degrade performance by increasing drag and cannot be jettisoned in-flight (unlike conventional external tanks). Each conformal fuel tank can hold 750 U.S. gallons (2,840 L) of fuel. These tanks increase range thus reducing the need for in-flight refueling. All external stations for munitions remain available with the tanks in use. Moreover, Sparrow or AMRAAM missiles can be attached to the corners of the conformal fuel tanks. The 57 FIS based at Keflavik NAS, Iceland was the only C-model squadron to utilize CFT's on a regular basis due to its extended operations over the North Atlantic. With the closure of the 57 FIS the F-15E is the only U.S. variant to carry them on a routine basis. The American CFTs were also provided to Israel and Saudi Arabia but only Israel uses them (as needed) on their entire fleet.

The F-15E Strike Eagle is a two-seat, dual-role, totally integrated fighter for all-weather, air-to-air and deep interdiction missions. The rear cockpit is upgraded to include four multi-purpose CRT displays for aircraft systems and weapons management. The digital, triple-redundant Lear Siegler flight control system permits coupled automatic terrain following, enhanced by a ring-laser gyro inertial navigation system. For low-altitude, high-speed penetration and precision attack on tactical targets at night or in adverse weather, the F-15E carries a high-resolution APG-70 radar and LANTIRN pods to provide thermal imagery.

The APG-63(V)2 Active Electronically Scanned Array (AESA) radar has been retrofitted to 18 U.S. Air Force F-15C aircraft. This upgrade includes most of the new hardware from the APG-63(V)1, but adds an AESA to provide increased pilot situational awareness. The AESA radar has an exceptionally agile beam, providing nearly instantaneous track updates and enhanced multi-target tracking capability. The APG-63(V)2 is compatible with current F-15C weapon loads and enables pilots to take full advantage of AIM-120 AMRAAM capabilities, simultaneously guiding multiple missiles to several targets widely spaced in azimuth, elevation, or range.

Operational history
The largest operator of the F-15 is the United States Air Force. The first Eagle (F-15B) was delivered November 14, 1974. In January 1976, the first Eagle destined for a combat squadron, the 555th TFS, was delivered. These initial aircraft carried the Hughes Aircraft (now Raytheon) APG-63 radar.

The first kill in an F-15 was by IAF ace Moshe Melnik in 1979. In 1979–81 during Israeli-Lebanese border disputes, F-15As downed 13 Syrian MiG-21 "Fishbeds" and two Syrian MiG-25 "Foxbats", the latter being the aircraft the F-15 was designed to kill. F-15A and B models were used by Israel during the Bekaa Valley operation. During the 1982 Lebanon War, the Israeli F-15s shot down 40 Syrian jet fighters (23 MiG-21 "Fishbeds" and 17 MiG-23 "Floggers") and one Syrian SA.342L Gazelle helicopter. Later on, in 1985, IAF Eagles, in Operation Wooden Leg, bombed the PLO headquarters in Tunisia. This was one of the few times air superiority F-15s (A/B/C/D models) were used in tactical strike missions.

Royal Saudi Air Force F-15C pilots shot down two F-4E Phantom IIs flown by the Iranian Air Force in a skirmish in June 1984, and shot down two Iraqi Mirage F1s during the Gulf War.

The USAF deployed F-15C, D and E models to the Persian Gulf in 1991 in support of Operation Desert Storm where they accounted for 36 of the 39 Air Force air-to-air victories. F-15Es were operated mainly at night, hunting modified SCUD missile launchers and artillery sites using the LANTIRN system. According to the USAF, its F-15Cs had 34 confirmed kills of Iraqi aircraft during the 1991 Gulf War, mostly by missile fire: five MiG-29 "Fulcrums", two MiG-25 "Foxbats", eight MiG-23 "Floggers", two MiG-21 "Fishbeds", two Su-25 "Frogfoots", four Su-22 "Fitters", one Su-7, six Mirage F1s, one Il-76 cargo plane, one Pilatus PC-9 trainer, and two Mi-8 helicopters. After air superiority was achieved in the first three days of the conflict, many of the later kills were reportedly of Iraqi aircraft fleeing to Iran, rather than actively trying to engage U.S. aircraft. The single-seat F-15C was used for air superiority, and the F-15E was heavily used in air-to-ground attacks. An F-15E achieved an aerial kill of another Iraqi Mi-8 helicopter using a laser-guided bomb during the air war. The F-15E sustained two losses to ground fire in the Gulf War in 1991. Another one was damaged on the ground by a SCUD strike on Dhahran air base.
They have since been deployed to support Operation Southern Watch, the patrolling of the No-Fly Zone in Southern Iraq; Operation Provide Comfort in Turkey; in support of NATO operations in Bosnia, and recent air expeditionary force deployments. In 1994, two U.S. Army UH-60 Black Hawks were downed by USAF F-15Cs who thought they were Iraq Hinds in the Northern no-fly zone of Iraq in a friendly fire incident. USAF F-15Cs shot down four Yugoslav MiG-29s using AIM-120 missiles during NATO's 1999 intervention in Kosovo, Operation Allied Force.

The F-15 in all air forces had an air-to-air combined record of 104 kills to 0 losses in air combat as of February 2008. To date, no air superiority versions of the F-15 (A/B/C/D models) have ever been shot down by enemy forces. Over half of the F-15's kills were made by Israeli Air Force pilots.

Satellite killer
From January 1984 to September 1986, two F-15As were used as launch platforms for the ASM-135 anti-satellite (ASAT) missile. The F-15As (76-0086 and 77-0084) were modified to carry one ASM-135 on the centerline station with extra equipment within a special centerline pylon. The launch aircraft executed a Mach 1.22, 3.8 g climb at 65° to release the ASAT missile at an altitude of 38,100 ft (11,600 m). The flight computer was updated to control the zoom-climb and missile release. The third test flight involved a retired communications satellite in a 345-mile (555 km) orbit, which was successfully destroyed by kinetic energy. The pilot, USAF Major Wilbert D. "Doug" Pearson, became the only pilot to destroy a satellite.

The ASAT missile was designed to be a standoff anti-satellite weapon, with the F-15A acting as a first stage. The Soviet Union could correlate a U.S. rocket launch with a spy satellite loss, but an F-15 carrying an ASAT would blend in among hundreds of F-15 flights. The ASAT program involved five test launches. The program was officially terminated in 1988.

Structural defects
All F-15 aircraft were grounded by the U.S. Air Force after a Missouri Air National Guard F-15C came apart in flight and crashed on 2 November 2007. The newer F-15E fleet was later cleared for continued operations. The U.S. Air Force reported on 28 November 2007 that a critical location in the upper longerons on the F-15C model was suspected of causing the failure, causing the fuselage forward of the air intakes, including the cockpit and radome, to separate from the airframe.

F-15A through D-model aircraft were ordered grounded until the location received more detailed inspections and repairs as needed. The grounding of F-15s received media attention as it began to place strains on the nation's air defense efforts. The grounding forced some states to rely on their neighbors' fighter jets for air defense protection, and Alaska to depend on Canadian Forces' support.

On 8 January 2008, the USAF Air Combat Command (ACC) cleared a portion of its F-15A through D-model fleet for return to flying status. It also recommended a limited return to flight for units worldwide using the affected models. The accident review board report was released on January 10, 2008. The report stated that analysis of the F-15C wreckage determined that the longeron did not meet drawing specifications, which led to fatigue cracks and finally a catastrophic failure of the remaining support structures and breakup of the aircraft in flight. In a report released on 10 January 2008, nine other F-15s were identified to have similar problems in the longeron. As a result of these problems, General John D. W. Corley stated that "the long-term future of the F-15 is in question." On 15 February 2008, ACC cleared all its grounded F-15A-D fighters for flight pending inspections, engineering reviews and any needed repairs. ACC also recommended release of other U.S. F-15A-D aircraft.

Future
The F-15C/D model is being supplanted in U.S. service by the F-22 Raptor. The F-15E, however, will remain in service for years to come because of their different air-to-ground role and the lower number of hours on their airframes. The USAF will upgrade 178 F-15Cs with the AN/APG-63(V)3 AESA radar, and upgrade other F-15s with the Joint Helmet Mounted Cueing System. The Air Force will keep 178 F-15Cs as well as the 224 F-15Es in service beyond 2025.

Variants
Basic models
F-15A
Single-seat all-weather air-superiority fighter version, 384 built 1972-79.
F-15B
Two-seat training version, formerly designated TF-15A, 61 built 1972-79.
F-15C
Improved single-seat all-weather air-superiority fighter version, 483 built 1979-85.
F-15D
Two-seat training version, 92 built 1979-85.
F-15J
Single-seat all-weather air-superiority fighter version for the Japan Air Self-Defense Force 139 built under license in Japan by Mitsubishi 1981-97, 2 built in St. Louis.
F-15DJ
Two-seat training version for the Japan Air Self-Defence Force. 25 Built under license in Japan by Mitsubishi 1981-97, 12 built in St. Louis.
F-15N Sea Eagle
The F-15N was a carrier-capable variant proposed in the early 1970s to the U.S. Navy as an alternative to the heavier and, at the time, considered as "riskier" technology program: F-14 Tomcat. The F-15N-PHX was another proposed naval version capable of carrying the AIM-54 Phoenix missile. These featured folding wingtips, reinforced landing gear and a stronger tail hook for shipboard operation.
F-15E Strike Eagle
See F-15E Strike Eagle for F-15E, F-15I, F-15S, F-15K, F-15SG, F-15SE and other F-15E-based variants.

Research and test
F-15 Streak Eagle (72-0119)
One stripped and unpainted F-15A, demonstrated the fighter's acceleration – broke eight time-to-climb world records between 16 January and 1 February 1975. It was delivered to the National Museum of the United States Air Force in December 1980.

F-15 S/MTD (71-0290)
The first F-15B was converted into a short takeoff and landing, maneuver technology demonstrator aircraft. In the late 1980s it received canard flight surfaces in addition to its usual horizontal tail, along with square thrust-vectoring nozzles. It was used as a short-takeoff/maneuver-technology (SMTD) demonstrator.

F-15 ACTIVE (71-0290)
The F-15 S/MTD was later converted into an advanced flight control technology research aircraft with thrust vectoring nozzles.

F-15 IFCS (71-0290)
The F-15 ACTIVE was then converted into an intelligent flight control systems research aircraft. F-15B 71-0290 is the oldest F-15 still flying as of January 2009.

F-15 MANX
Concept name for a tailless variant of the F-15 ACTIVE, but the NASA ACTIVE experimental aircraft was never modified to be tailless.

F-15 Flight Research Facility (71-0281 and 71-0287)
Two F-15A aircraft were acquired in 1976 for use by NASA's Dryden Flight Research Center for numerous experiments such as: Highly Integrated Digital Electronic Control (HiDEC), Adaptive Engine Control System (ADECS), Self-Repairing and Self-Diagnostic Flight Control System (SRFCS) and Propulsion Controlled Aircraft System (PCA). 71-0281 was returned to the Air Force and became a static display at Langley AFB in 1983.
F-15B Research Testbed (74-0141)


Current operators of the F-15 in light blue, F-15E in red, both in dark blue
Acquired in 1993, it was an F-15B modified and used by NASA's Dryden Flight Research Center for flight tests.

B-52 Stratofortress

The Boeing B-52 Stratofortress is a long-range, subsonic, jet-powered, strategic bomber operated by the United States Air Force (USAF) since 1955.

Beginning with the successful contract bid on 5 June 1946, the B-52 went through several design steps; from a straight wing aircraft powered by six turboprop engines to the final prototype YB-52, with eight turbojet engines. The aircraft made its first flight on 15 April 1952 with "Tex" Johnston as pilot.

Built to carry nuclear weapons for Cold War-era deterrence missions, the B-52 Stratofortress replaced the Convair B-36. Although a veteran of a number of wars, the Stratofortress has dropped only conventional munitions in actual combat. The B-52 carries up to 70,000 pounds (32,000 kg) of weapons.

The USAF has had B-52s in active service since 1955, initially with the Strategic Air Command (SAC), with all aircraft later absorbed into the Air Combat Command (ACC) following SAC's disestablishment in 1992. Superior performance at high subsonic speeds and relatively low operating costs have kept the B-52 in service despite proposals to replace it with the Mach 3 XB-70 Valkyrie, supersonic B-1B Lancer and stealthy B-2 Spirit. In January 2005, the B-52 became the second aircraft, after the English Electric Canberra, to mark 50 years of continuous service with its original primary operator. There are six aircraft altogether that have made this list as of 2009; the other four being the Tupolev Tu-95, the C-130 Hercules, the KC-135 Stratotanker, and the Lockheed U-2.

Development
Background
On 23 November 1945, Air Materiel Command (AMC) issued desired performance characteristics for a new strategic bomber "capable of carrying out the strategic mission without dependence upon advanced and intermediate bases controlled by other countries". The aircraft was to have a crew of five plus turret gunners, and a six-man relief crew. It had to cruise at 300 mph (240 kn, 480 km/h) at 34,000 feet (10,400 m) with a combat radius of 5,000 statute miles (4,300 nmi, 8,000 km). The armament was to consist of an unspecified number of 20 mm cannon and 10,000 pounds (4,500 kg) of bombs. On 13 February 1946, the Air Force issued bid invitations for these specifications, with Boeing, Consolidated Aircraft, and Glenn L. Martin Company submitting proposals.

On 5 June 1946, Boeing's Model 462, a straight-wing aircraft powered by six Wright T35 turboprops with a gross weight of 360,000 pounds (160,000 kg) and combat radius of 3,110 statute miles (2,700 nmi, 5,010 km), was declared the winner. On 28 June 1946, Boeing was issued a letter of contract for US$1.7 million (1946 dollars) to build a full-scale mock-up of the new XB-52 and do preliminary engineering and testing. However, by October 1946, the Air Force began to express concern about the sheer size of the new aircraft and its inability to meet the specified design requirements. In response, Boeing produced Model 464, a smaller four-engine version with a 230,000 pound (105,000 kg) gross weight, which was briefly deemed acceptable.

Then, in November 1946, the Deputy Chief of Air Staff for Research and Development, General Curtis LeMay, expressed the desire for a cruise speed of 400 miles per hour (345 kn, 645 km/h), to which Boeing responded with a 300,000 pound (140,000 kg) aircraft. In December 1946, Boeing was asked to change their design to a four-engine bomber with a top speed of 400 miles per hour, range of 12,000 statute miles (10,000 nmi, 19,000 km), and the ability to carry a nuclear weapon. The aircraft could weigh up to 480,000 pounds (220,000 kg). Boeing responded with two models powered by the T-35 turboprops. The Model 464-16 was a "nuclear-only" bomber with a 10,000 pound payload, while the Model 464-17 was a general purpose bomber with a 90,000 pound (40,000 kg) payload. Due to the cost associated with purchasing two specialized aircraft, the Air Force selected Model 464-17 with the understanding that it could be adapted for nuclear strikes.

In June 1947, the military requirements were updated and the Model 464-17 met all of them except for the range. It was becoming obvious to the Air Force that, even with the updated performance, the XB-52 would be obsolete by the time it entered production and would offer little improvement over the Convair B-36. As a result, the entire project was put on hold for six months. During this time, Boeing continued to perfect the design which resulted in the Model 464-29 with a top speed of 455 miles per hour (395 kn, 730 km/h) and a 5,000-mile range. In September 1947, the Heavy Bombardment Committee was convened to ascertain performance requirements for a nuclear bomber. Formalized on 8 December 1947, these called for a top speed of 500 miles per hour (440 kn, 800 km/h) and an 8,000 statute mile (7,000 nmi, 13,000 km) range, far beyond the capabilities of 464-29.

The outright cancellation of the Boeing contract on 11 December 1947 was staved off by a plea from its president William McPherson Allen, and in January 1948 Boeing was instructed to thoroughly explore recent technological innovations, including aerial refueling and the flying wing. Noting stability and control problems Northrop was experiencing with their YB-35 and YB-49 flying wing bombers, Boeing insisted on a conventional aircraft, and in April 1948 presented a US$30 million (1948 dollars) proposal for design, construction, and testing of two Model 464-35 prototypes. Further revisions of specifications during 1948 resulted in an aircraft with a top speed of 513 miles per hour (445 kn, 825 km/h) at 35,000 feet (10,700 m), a range of 6,909 statute miles (6,005 nmi, 11,125 km), and a 280,000 pounds (125,000 kg) gross weight which included 10,000 pounds of bombs and 19,875 US gallons (75,225 L) of fuel.

Production

In May 1948 AMC asked Boeing to incorporate the previously discarded, but now more fuel-efficient, jet engine into the design. This resulted in Boeing developing yet another revision — in July 1948, Model 464-40 substituted Westinghouse J40 turbojets for the turboprops. Nevertheless, on 21 October 1948, Boeing was told to create an entirely new aircraft using Pratt & Whitney J57 turbojets.

On 25 October, Boeing engineers produced a proposal and a hand-carved model of 464-49. The new design built upon the basic layout of the B-47 Stratojet with 35° swept wings, eight engines paired in four underwing pods, and bicycle landing gear with wingtip outrigger wheels. A notable feature of the landing gear was the ability to pivot the main landing gear up to 20° from the aircraft centerline to increase safety during crosswind landings. The aircraft was projected to exceed all design specifications. Although the full-size mock-up inspection in April 1949 was generally favorable, range again became a concern since the J40s and the early model J57s had excessive fuel consumption. Despite talk of another revision of specifications or even a full design competition among aircraft manufacturers, General LeMay, now in charge of Strategic Air Command, insisted that performance should not be compromised due to delays in engine development. In a final attempt to increase the range, Boeing created the larger 464-67, stating that once in production, the range could be further increased in subsequent modifications. Following several direct interventions by LeMay, on 14 February 1951 Boeing was awarded a production contract for 13 B-52As and 17 detachable reconnaissance pods. The last major design change, also at the insistence of General LeMay, was a switch from the B-47 style tandem seating to a more conventional side-by-side cockpit which increased the effectiveness of the copilot and reduced crew fatigue. Both XB-52 prototypes featured the original tandem seating arrangement with a framed bubble-type canopy. The YB-52 (actually, the second XB-52 with more operational equipment) first flew on 15 April 1952, a 2 hour 21 minute flight from Renton Field in Renton, Washington to Larson AFB with Boeing test pilot Alvin M. Johnston and Air Force Lieutenant Colonel Guy M. Townsend. The XB-52 followed on 2 October 1952. The thorough development, including 670 days in the wind tunnel and 130 days of aerodynamic and aeroelastic testing, paid off with smooth flight testing. Encouraged, the Air Force increased its order to 282 B-52s.

Only three of the 13 B-52As ordered were built. All were returned to Boeing, and used in their test program. On 9 June 1952 the February 1951 contract was updated to order the aircraft under new specifications. The final ten—the first aircraft to enter active service—were completed as B-52Bs. At the roll out ceremony on 18 March 1954, Air Force Chief of Staff, General Twining said:

Design
Upgrades and modifications
In November 1959, SAC initiated the Big Four modification program (also known as Modification 1000) for all operational B-52s except early B models. The program was completed by 1963. The four modifications were:

Ability to perform all-weather, low-altitude (below 500 feet (150 m)) interdiction as a response to advancements in Soviet Union's missile defenses. The low-altitude flights were estimated to accelerate structural fatigue by at least a factor of eight, requiring costly repairs to extend service life.

Ability to launch AGM-28 Hound Dog standoff nuclear missiles
Ability to launch ADM-20 Quail decoys
An advanced electronic countermeasures (ECM) suite
The ability to carry up to 20 AGM-69 SRAM nuclear missiles was added to G and H models starting in 1971. Fuel leaks due to deteriorating Marman clamps continued to plague all variants of the B-52. To this end, the aircraft were subjected to Blue Band (1957), Hard Shell (1958), and finally QuickClip (1958) programs. The latter fitted safety straps which prevented catastrophic loss of fuel in case of clamp failure.

Ongoing problems with advanced avionics were addressed in the Jolly Well program, completed in 1964, which improved components of the AN/ASQ-38 bombing navigational computer and the terrain computer. The MADREC (Malfunction Detection and Recording) upgrade fitted to most aircraft by 1965 could detect failures in avionics and weapons computer systems, and was essential in monitoring the Hound Dog missiles. The electronic countermeasures capability of the B-52 was expanded with Rivet Rambler (1971) and Rivet Ace (1973).

In order to improve the ability to operate safely at low level during both day and night, the AN/ASQ-151 Electro-Optical Viewing System (EVS), consisting of a Low Light Level Television (LLLTV) and a Forward Looking Infra-Red (FLIR) system mounted in blisters under the noses of B-52Gs and Hs between 1972 and 1976. In order to further improve the B-52s offensive ability, it was decided to fit Air Launched Cruise Missiles (ALCMs). After testing of both the Air-Force backed Boeing AGM-86 and the Navy backed General Dynamics AGM-109 Tomahawk, the AGM-86B was selected for operation by the B-52 (and ultimately by the B-1 Lancer). A total of 194 B-52Gs and Hs were modified to carry AGM-86s, carrying 12 missiles on underwing pylons, with 82 B-52Hs further modified to carry another eight missiles on a rotary launcher fitted in the aircraft's bomb-bay. In order to conform with the requirements of the SALT II Treaty for cruise missile capable aircraft to be readily identified by reconnaissance satellites, the cruise missile armed B-52Gs were modified with a distinctive wing root fairing. As all B-52Hs were assumed to be modified, no visual modification of these aircraft was required. In 1990, the stealthy AGM-129 ACM cruise missile entered service. Although originally intended to replace the AGM-86 its high cost and the end of the Cold War stopped production after only 450 had been made. Unlike the AGM-86, no conventional (i.e. non-nuclear) armed version was built.

Structural fatigue, exacerbated by the change to low-altitude missions, was first dealt with in the early 1960s by the three-phase High Stress program which enrolled aircraft at 2,000 flying hours. This was followed by a 2,000-hour service life extension to select airframes in 1966-1968, and the extensive Pacer Plank reskinning completed in 1977. The wet wing introduced on G and H models was even more susceptible to fatigue due to experiencing 60% more stress during flight than the old wing. The wings were modified by 1964 under ECP 1050. This was followed by a fuselage skin and longeron replacement (ECP 1185) in 1966, and B-52 Stability Augmentation and Flight Control program (ECP 1195) in 1967.

Boeing has suggested re-engining the B-52H fleet with the Rolls-Royce RB211 534E-4. This would involve replacing the eight Pratt & Whitney TF33s (total thrust 8 × 17,000 lb) with four RB211s (total thrust 4 × 37,400lb). The RR engines will increase the range and payload of the fleet and reduce fuel consumption. However, the cost of the project would be significant. Procurement would cost approximately US$2.56 billion (US$36 million × 71 aircraft). A Government Accountability Office study of the proposal concluded that Boeing's estimated savings of US$4.7 billion would not be realized. They found that it would cost the Air Force US$1.3 billion over keeping the existing engines. This was subsequently disputed in a Defense Sciences Board report in 2003 and revised in 2004 that identified numerous errors in the prior evaluation of the Boeing proposal, and urged the Air Force to re-engine the aircraft without delay. Further, the DSB report stated the program would save substantial funds, reduce greenhouse gas emissions, and increase aircraft range and endurance, duplicating the results of a Congressionally funded US$3M program office study conducted in 2003. However, the re-engining has been approved as of 2009.

In 2007 the LITENING targeting pod was fitted and commissioned increasing the combat effectiveness of the aircraft during day, night and under-the-weather conditions in the attack of ground targets with a variety of standoff weapons under the guidance of LASERs and the help of high resolution forward-looking infrared sensor (FLIR) for visual display in the infrared portion of the electromagnetic spectrum and charged coupled device (CCD-TV) camera used to obtain target imagery in the visible portion, this technology could also be used in real-time transmission to ground communications networks and government agencies to gather battlefield intelligence, assess battlefield damage, assess terrorist activities and counter drug activity, further advancing the B-52H's capabilities and uses.

Fuel research platform
In September 2006, the B-52 became one of the first US military aircraft to fly using 'alternative' fuel. Syntroleum Corporation, a leader in Fischer-Tropsch process (FT) technology, announced that its Ultra-Clean jet fuel had been successfully tested in a B-52. It took off from Edwards Air Force Base with a 50/50 blend of FT and traditional JP-8 jet fuel which was burned in two of the eight engines on the aircraft. This marked the first time that FT jet fuel was tested in a military flight demo, and is the first of several planned test flights.

On 15 December 2006, tail number 61-0034, Wise Guy took off from Edwards with the synthetic fuel blend powering all eight engines, the first time an Air Force aircraft was completely powered by the mixture. The test flight was captained by Major General Curtis Bedke, commander of the Edwards Flight Test Center, the first time in 36 years that the installation's commander performed a first flight in a flight test program. The flight lasted seven hours, reached an altitude of 48,000 feet, and was considered a success.

On 8 August 2007, Air Force Secretary Michael Wynne certified the B-52H as fully approved to use the FT blend, marking the formal conclusion of the test program.

This program is part of the Department of Defense Assured Fuel Initiative, an effort to develop secure domestic sources for the military energy needs. The Pentagon hopes to reduce its use of crude oil from foreign producers and obtain about half of its aviation fuel from alternative sources by 2016. With the B-52 now approved to use the FT blend, the USAF will use the test protocols developed during the program to certify the C-17 Globemaster III and then the B-1B to use the fuel (the first B-1 test flight took place in March, 2008). The Air Force intends to test and certify every airframe in its inventory to use the fuel by 2011.

B-2 Spirit

The Northrop Grumman B-2 Spirit (also known as the "Stealth Bomber") is a multirole heavy bomber with "low observable" stealth technology capable of penetrating dense anti-aircraft defenses to deploy both conventional and nuclear weapons. Because of its considerable capital and operations costs, the project was controversial in Congress and among Pentagon brass during its development and placement into service. During the late 1980s and early 1990s, the United States scaled back initial plans to purchase 132 of the bombers. By the mid 1990s, Congress made appropriations to purchase a total fleet of just 21 of the bombers.

The cost of each air vehicle averaged US$737 million per plane in 1997 dollars. Total procurement costs averaged US$929 million per plane, which includes spare parts, equipment, retrofitting, and software support. The total program cost, which includes development, engineering and testing, averaged US$2.1 billion per aircraft in 1997 dollars.

Twenty B-2s are operated by the United States Air Force. Though originally designed in the 1980s for Cold War operations scenarios, B-2s have been used in combat to drop bombs on Kosovo in the late 1990s, and see continued use during the ongoing wars in Iraq and Afghanistan. One aircraft was lost when it crashed on takeoff in 2008.

The crew of two aboard the bomber can drop up to eighty 500 lb (230 kg) class JDAM "smart" bombs, or sixteen 2,400 lb (1,100 kg) B83 nuclear bombs in a single pass through extremely dense anti-aircraft defenses. It has been the subject of espionage and counter-espionage activity. The bomber has been a prominent public spectacle at air shows since the 1990s.

Development
ATB project
The B-2 Spirit originated from the Advanced Technology Bomber (ATB) black project that began in 1979. The Cold War was well underway, and on the campaign trail in 1979 and 1980, candidate Ronald Reagan promised a restoration of American military strength. On 22 August 1980, the incumbent Carter administration publicly disclosed that the Department of Defense was working to develop stealth aircraft including the ATB.

After the evaluations of the companies' proposals, the ATB competition was reduced to the Northrop/Boeing and Lockheed/Rockwell teams with each receiving a study contract for further work. The Northrop design was larger while the Lockheed design was smaller and included a small tail. The black project was funded under the code name "Aurora". The Northrop/Boeing team's ATB design was selected over the Lockheed/Rockwell design on October 20, 1981.

The Northrop design received the designation B-2 and the name "Spirit". The bomber's design was changed in the mid-1980s when the mission profile was changed from high-altitude to low-altitude, terrain-following. The redesign delayed the B-2's first flight by two years and added about US$1 billion to the program's cost. An estimated US$23 billion was secretly spent for research and development on the B-2 by 1989. At the program's peak, approximately 13,000 people were employed at a dedicated plant in Pico Rivera, California for the plane's engineering and portions of its manufacturing.

The B-2 was first publicly displayed on November 22, 1988, at Air Force Plant 42, Palmdale, California, where it was assembled. Its first public flight was on July 17, 1989 from Palmdale.

Procurement
A procurement of 132 aircraft was planned in the mid-1980s, but was later reduced to 75. By the early 1990s, the Soviet Union had disintegrated, which effectively rendered void the Spirit's primary Cold War mission. In light of budgetary pressures and congressional opposition, in his 1992 State of the Union Address, President George H.W. Bush announced B-2 production would be limited to a total of 20 aircraft. In 1996, however, the Clinton administration, though originally committed to ending production of the bombers once the 20th aircraft was completed, authorized the conversion of a 21st bomber, a prototype test model, to Block 30 full operational status at a cost of nearly $500 million.

The bomber's high costs reflected the innovation of a paperless computer aided design (CAD) system, and a computerized manufacturing control system. The costs also reflect the inefficiencies of separating design teams into different parts of the country for both design intelligence compartmentalization as a counter-espionage measure, and by parceling out the supply chain with the requisite lucrative contracts to congressional districts as a political reward.
Northrop made a proposal to the USAF in 1995 to build 20 additional aircraft with a flyaway cost of $566M each.

Espionage
In 1984 a Northrop employee, Thomas Cavanaugh, was arrested for trying to sell classified information to the Soviet Union, which apparently was smuggled out of the Pico Rivera, California factory. Cavanaugh was eventually sentenced to life in prison and released under parole in 2001.

Noshir Gowadia, a design engineer who worked on the B-2's propulsion system, was arrested in October 2005 for selling B-2 related classified information to foreign countries. His trial was initially scheduled for 12 February 2008, but he received a continuance.

Program costs
The program was the subject of public controversy for its costs to American taxpayers. In 1996 the General Accounting Office disclosed that the B-2 bomber "will be, by far, the most costly bombers to operate on a per aircraft basis" costing over three times as much as the B-1B (US$9.6 million annually) and over four times as much as the B-52H ($US6.8 million annually). In September 1997, each hour of B-2 flight necessitated 119 hours of maintenance in turn. Comparable maintenance needs for the B-52 and the B-1B are 53 and 60 hours respectively for each hour of flight. A key reason for this cost are the air-conditioned hangars large enough for the bomber's 172 ft (52.4 m) wingspan, which are needed to maintain the aircraft's stealthy properties, especially its "low-observable" stealthy skins. These maintenance requirements raise serious questions about the ability to deploy the B-2 overseas.

The total "military construction" cost related to the program was projected to be US$553.6 million in 1997 dollars. The cost to procure each B-2 "air vehicle" was US$737 million in 1997 dollars based only on air vehicle cost of US$15.48 billion. The procurement cost per plane as detailed in General Accounting Office (GAO) reports, which include spare parts and software support, was $929 million per plane in 1997 dollars.

The total program cost projected through 2004 was US$44.75 billion in 1997 dollars. This includes development, procurement, facilities, construction, and spare parts. The total program cost averaged US$2.13 billion per plane.

Opposition
In its consideration of the fiscal year 1990 defense budget, the House Armed Services Committee trimmed $800 million from the B-2 research and development budget, while at the same time staving off a motion to kill the bomber. The opposition was bipartisan, with Congressman Ron Dellums (D-CA), John Kasich (R-OH), and John G. Rowland (R-CT) authorizing the motion to kill the bomber; the growing cost of the B-2 appeared to be the factor driving the opposition. At the peak production period specified in 1989, the schedule called for spending US$7 billion to $8 billion per year in 1989 dollars, something Committee Chair Les Aspin (D-WI) said "won't fly financially."

In time, a number of prominent members of Congress began to oppose the program's expansion, to include former Democratic presidential nominee John Kerry who cast votes against the B-2 Stealth Bomber in 1989, 1991 and 1992 while a United States Senator representing Massachusetts. By 1992, Republican President George H.W. Bush called for the cancellation of the B-2 and promised to cut military spending by 30% in the wake of the collapse of the Soviet Union.

In May 1995, on the basis of its 1995 Heavy Bomber Force Study, the DOD determined that additional B-2 procurements would exacerbate efforts to develop and implement long term recapitalization plans for the USAF bomber force.

In October 1995, former Chief of Staff of the United States Air Force, General Mike Ryan, and Former Chairman of the Joint Chiefs of Staff, General John Shalikashvili, strongly recommended against Congressional action to fund the purchase any additional B-2s, arguing that to do so would require unacceptable cuts in existing conventional and nuclear-capable aircraft to pay for the new bombers, and because the military had much higher priorities on which to spend its limited procurement dollars.

Some B-2 advocates argued that procuring twenty additional B-2s would save money because B-2s would be able to deeply penetrate anti-aircraft defenses and use low-cost, short-range attack weapons rather than expensive standoff weapons. However, in 1995, the Congressional Budget Office (CBO), and its Director of National Security Analysis, found that additional B-2s would reduce the cost of weapons expended by the bomber force by less than US$2 billion in 1995 dollars during the first two weeks of a conflict, which is when the Air Force envisions bombers would make their greatest contribution. This is a small fraction of the US$26.8 billion (in 1995 dollars) life cycle cost that the CBO projected an additional twenty B-2s would cost.

In 1997, as Ranking Member of the House Armed Services Committee and National Security Committee, Congressman Ron Dellums, a long-time opponent of the bomber, cited five independent studies and offered an amendment to that year's defense authorization bill to cap production of the bombers with the existing 21 aircraft. The amendment was narrowly defeated. Nonetheless, Congress has never approved funding for the purchase of any additional B-2 bombers to date.

Radar modernization
On 29 December 2008, Air Force officials awarded a production contract to Northrop Grumman to modernize the B-2 fleet's radar. The contract provides advanced state-of-the-art radar components, with the aim of sustained operational viability of the B-2 fleet into the future. The contract has a target value of approximately US$468 million. The award follows successful flight testing with the upgraded equipment. A modification to the radar was needed since the U.S. Department of Commerce required the B-2 to use a different radar frequency.

Design
As with the B-52 Stratofortress and B-1 Lancer, the B-2 provides the versatility inherent in manned bombers. Like other bombers, its assigned targets can be canceled or changed while in flight, the particular weapon assigned to a target can be changed, and the timing of attack, or the route to the target can be changed while in flight. In addition, its low-observable, or "stealth", characteristics give it the ability to penetrate an enemy's most sophisticated anti-aircraft defenses to attack its most heavily defended targets.

The prime contractor, responsible for overall system design, integration and support, is Northrop Grumman. Boeing, Raytheon (formerly Hughes Aircraft), G.E. and Vought Aircraft Industries, are subcontractors.

The blending of low-observable technologies with high aerodynamic efficiency and large payload gives the B-2 significant advantages over previous bombers. The U.S. Air Force purports the aircraft has "high aerodynamic efficiency" and states its range is approximately 6,000 nautical miles (6,905 mi (11,113 km). Also, its low-observation ability provides the B-2 greater freedom of action at high altitudes, thus increasing its range and providing a better field of view for the aircraft's sensors. It combines GPS Aided Targeting System (GATS) with GPS-aided bombs such as Joint Direct Attack Munition (JDAM). This uses its passive electronically scanned array APQ-181 radar to correct GPS errors of targets and gain much better than laser-guided weapon accuracy when "dumb" gravity bombs are equipped with a GPS-aided "smart" guidance tail kit. It can bomb 16 targets in a single pass when equipped with 1,000 or 2,000-pound (450 kg or 900 kg) bombs, or as many as 80 when carrying 500 lb (230 kg) bombs.

The B-2's stealth comes from a combination of reduced acoustic, infrared, visual and radar signatures, making it difficult for opposition defenses to detect, track and engage the aircraft. Many specific aspects of the low-observability process remain classified.

The B-2's low observability originates from stealth technology exploited for the F-117. Russian-born physicist and mathematician Pyotr Ufimtsev, whose theoretical work made the F-117 and B-2 possible, was hired by Northrop at one time. Additionally, the B-2's composite materials, special coatings and flying wing design, which reduces the number of leading edges contribute to its stealth characteristics. Each B-2 requires a climate-controlled hangar large enough for its 172-foot (52 m) wingspan to protect the operational integrity of its sophisticated radar absorbent material and coatings. The engines are buried within the wing to conceal the induction fans and hide their exhaust.

The B-2 has a crew of two: a pilot in the left seat, and mission commander in the right. The B-2 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. B-2 crews have been used to pioneer sleep cycle research to improve crew performance on long sorties. The B-2 is highly automated, and, unlike two-seat fighters, one crew member can sleep, use a toilet or prepare a hot meal while the other monitors the aircraft.

In 1990, the Department of Defense accused Northrop of using faulty components in the flight control system. More recent issues with the bomber have included cracks in the tail. Efforts have also been made to reduce the probability of bird ingestion, which could damage engine fan blades.
In 2008, Congress funded upgrades to the B-2s weapon control systems.

Operational history
The first operational aircraft, christened Spirit of Missouri, was delivered to Whiteman A.F.B., Missouri, where the fleet is based, on 17 December 1993.[39] The B-2 reached initial operational capability on 1 January 1997.[40] Depot maintenance for the B-2 is accomplished by U.S. Air Force contractor support and managed at Oklahoma City Air Logistics Center at Tinker Air Force Base. Originally designed to deliver nuclear weapons, modern usage has shifted towards a flexible role with conventional and nuclear capability.

Into combat
The B-2 has seen service in three campaigns. Its combat debut was during the Kosovo War in 1999. It was responsible for destroying 33 percent of selected Serbian bombing targets in the first eight weeks of U.S. involvement in the War. During this war, B-2s flew non-stop to Kosovo from their home base in Missouri and back. The B-2 was the first aircraft to deploy GPS satellite guided JDAM "Smart Bombs" in combat use in Kosovo.

The B-2 has been used to drop bombs on Afghanistan in support of the ongoing War in Afghanistan. With the support of aerial refueling, the B-2 flew one of its longest missions to date from Whiteman Air Force Base, Missouri to Afghanistan and back.

During the ongoing War in Iraq, B-2s have operated from Diego Garcia and an undisclosed "forward operating location". Other sorties in Iraq have launched from Whiteman AFB. This resulted in missions lasting over 30 hours and one mission of over 50 hours. The designated "forward operating locations" have been previously designated as Guam and RAF Fairford, where new climate controlled hangers have been constructed. B-2s have been used during 22 sorties from Diego Garcia as well as 27 sorties from Whiteman AFB, and have released more than 1.5 million pounds of munitions, to include 583 JDAM "Smart Bombs" in 2003.

The B-2's combat use preceded a U.S. Air Force declaration of "full operational capability" in December 2003. The Pentagon's Operational Test and Evaluation 2003 Annual Report noted that the B-2's serviceability for Fiscal Year 2003 was still inadequate, mainly due to the maintainability of the B-2's low observable coatings. The evaluation also noted that the Defensive Avionics suite also had shortcomings with pop-up threats.

All B-2s, nuclear-capable B-52s, and nuclear intercontinental ballistic missiles, have shifted to the new nuclear-focused Global Strike Command scheduled to be set up by September 2009.

B-1 Lancer

The B-1 Lancer is a strategic bomber used by the United States Air Force. First envisioned in the 1960s as a supersonic bomber with sufficient range and payload to replace the B-52 Stratofortress, it developed primarily into a low-level penetrator with long-range and capable of supersonic speed. The design was canceled and reinstated multiple times over its lengthy development history, as the theory of strategic balance changed from flexible response to mutually assured destruction and back again. It eventually entered service more than 20 years after first being studied.

The B-1B production version has been in service with the United States Air Force (USAF) since 1986. The Lancer serves as the supersonic component of the USAF's long-range bomber force, along with the subsonic B-52 and B-2 Spirit. The bomber is commonly called the "Bone" (originally from "B-One"). With the retirement of the EF-111 Raven in 1998 and the F-14 Tomcat in 2006, the B-1B is the U.S. military's only variable-sweep wing aircraft.

Development
The B-1 was conceived as the Advanced Manned Strategic Aircraft (AMSA) program around 1965. AMSA was the last in a series of 1960s programs that looked at replacing the B-52 with a long-range multi-role supersonic aircraft that could drop bombs and launch nuclear missiles.

The Valkyrie and changing tactics
In December 1957, U.S. Air Force selected North American Aviation's proposal to replace the B-52 Stratofortress. This would lead to the B-70 Valkyrie. The Valkyrie was a six-engine bomber that could fly very high at Mach 3 to avoid interceptor aircraft, the only effective anti-bomber weapon in the 1950s. At the time, Soviet interceptors were unable to intercept the high-flying Lockheed U-2; the Valkyrie was to fly at similar altitudes and much higher speeds. But by the late 1950s, anti-aircraft surface-to-air missiles (SAMs) could threaten high-altitude aircraft, as demonstrated by the downing of Gary Powers' U-2 in 1960.

Recognizing this, the USAF Strategic Air Command had begun moving to low-level penetration before the U-2 downing. This greatly reduces radar detection distances while at that time SAMs were ineffective and interceptors less effective against low-flying aircraft. Also the flight path to a target could be routed around known anti-aircraft sites, and the landscape could be used to the bomber's advantage to stay out of the radar's line-of-sight operation. Aircraft speed became much less important. The targets themselves often had defenses located nearby to prevent this sort of approach all the way in, but stand-off weapons such as the AGM-69 SRAM provided an attack capability from outside the defensive missile's range. Low-altitude flight also made the bombers very difficult to detect from aircraft at higher altitudes, including interceptors, as radar systems of that generation could not "look down" due to the clutter that resulted from ground reflections.

Operations at low levels would limit the B-70 to subsonic speed, while dramatically decreasing its range due to much higher fuel requirements. The result would be an aircraft with similar speed but much less range than the B-52 it would have replaced. The Mach 2 B-58 was similarly limited to subsonic speeds at low altitudes. Unsuited for this new role, the viability of the B-70 as a bomber was questioned. Citing high cost, a growing ICBM force, and poor survivability against missiles, the operational bomber fleet was canceled in 1961 by President John F. Kennedy, and the program was changed to a supersonic research program with two XB-70 prototype aircraft.

B-1A program
President Richard Nixon re-established the program after taking office, in keeping with his administration's flexible response strategy that required a broad range of options short of general nuclear war. Secretary of Defense Melvin Laird reviewed the programs and decided to lower the numbers of FB-111s, claiming it lacked the required range, and recommended that the AMSA design studies be accelerated. In April 1969 the program officially became the B-1A. This was the first entry in the new bomber designation series, first created in 1962.

After the prolonged development period, the production contract was finally awarded in 1970. The original program called for two test airframes, five flyable aircraft, and 40 engines. This was cut in 1971 to one ground- and three flight test aircraft (74-0158 through 0160). First flight was set for April 1974. The company changed its name to Rockwell International and named its aircraft division North American Aircraft Operations in 1973. A fourth prototype (76-1074) was ordered in the FY 1976 budget. This fourth aircraft was to be built to production standards. At one time, some 240 B-1As planned to be built, with initial operational capability set for 1979.

Rockwell's design featured a number of features common to 1960s U.S. designs. These included the use of variable-sweep wings in order to provide both high lift during takeoff and landing, and low drag during a high-speed dash phase. With the wings set to their widest position the aircraft had considerably better lift and power than the B-52, allowing it to operate from a much wider variety of bases. Penetration of the USSR's defenses would take place in a dash, crossing them as quickly as possible before entering into the less defended "heartland" where speeds could be reduced again. The large size and fuel capacity of the design would allow this dash portion of the flight to be relatively long.

In order to achieve the required Mach 2 performance at high altitudes, the air intake inlets were variable. In addition, the exhaust nozzles were fully variable. Initially, it had been expected that a Mach 1.2 performance could be achieved at low altitude, which required that titanium be used in critical areas in the fuselage and wing structure. However, this low altitude performance requirement was lowered to only Mach 0.85, reducing the amount of titanium, and the overall cost.

Crew escape was provided for using an escape pod that ejected a portion of the entire cockpit with both pilots inside, as opposed to the more conventional ejection seats; it was felt that egress during a high-speed, high-altitude dash would be too dangerous without pressurization. A pair of small canards mounted near the nose are part of an active vibration damping system that smooths out the otherwise bumpy low-altitude ride, reducing crew fatigue and improving airframe life.

An extensive suite of electronics was planned, including a Litton LN-15 inertial navigation system, a Doppler radar altimeter, a Hughes forward-looking infrared, a General Electric APQ-114 forward-looking radar and a Texas Instruments APQ-146 terrain-following radar. The terrain-following radar, in particular, would allow the B-1 to fly at much lower altitudes during the "dash" phase of the mission than the B-52, which relied on older systems that demanded higher minimum altitudes during bad weather.

Overall it had a range similar to that of the B-52, although more of the flight could be low-level. A combination of flying lower due to better navigation systems and a greatly reduced radar cross section made it much safer from attack by missiles, and the latter also improved its odds against fighters as well. In situations where fighters were the expected competition (i.e. outside the USSR), its high-speed dash was a potentially useful technique the B-52 could not match. A convincing B-52 replacement had arrived.

M1 Abrams

The M1 Abrams is a main battle tank produced in the United States. The M1 is named after General Creighton Abrams, former Army Chief of Staff and Commander of US military forces in Vietnam from 1968 to 1972. It is a well armed, heavily armored, and highly mobile tank designed for modern armored ground warfare. Notable features of the M1 Abrams include the use of a powerful gas turbine engine, the adoption of sophisticated composite armor, and separate ammunition storage in a blow-out compartment for crew safety. It is one of the heaviest tanks in service, weighing in at close to 70 short tons.

The M1 Abrams entered U.S. service in 1980, replacing the 105 mm gun, full tracked M60 Patton main battle tank. It did, however, serve for over a decade alongside the improved M60A3, which had entered service in 1978. Three main versions of the M1 Abrams have been deployed, the M1, M1A1, and M1A2, incorporating improved armament, protection and electronics. These improvements, as well as periodic upgrades to older tanks have allowed this long-serving vehicle to remain in front-line service. It is the principal main battle tank of the United States Army and Marine Corps, and the armies of Egypt, Kuwait, Saudi Arabia, and since 2007, Australia.

Background
The first attempt to replace the aging M60 series of tanks was the abortive MBT-70, developed in partnership with West Germany. The M60 was itself a gradual evolution of a design starting with the World War II era M26 Pershing, with a very tall profile, and average armor and weaponry compared to the contemporary Soviet designs. The MBT-70 was very ambitious, like many American weapons programs of the 1960s. It had a gun launched missile system, kneeling suspension, a driver housed in the turret, and various other ideas that ultimately proved unsuccessful. Cancellation of this project paved the way for the much more successful M1 Abrams tank, which did not incorporate most of the troublesome innovations tried by the MBT-70.

Development
The XM1 Abrams was designed by Chrysler Defense (in 1979, General Dynamics Land Systems Division purchased Chrysler Defense Division) and is currently produced by General Dynamics Corporation in Lima, Ohio, and first entered US Army service in 1980. It was armed with the license-built version of the 105 mm Royal Ordnance L7 gun.

An improved version of the M1, the M1A1, was introduced in 1985. The M1A1 has the M256 120 mm smoothbore cannon developed by Rheinmetall AG of Germany for the Leopard 2, improved armor, and a CBRN protection system. The M1A2 is a further improvement of the M1A1 with a commander's independent thermal viewer and weapon station, position navigation equipment, digital data bus and a radio interface unit. The M1A2 SEP (System Enhancement Package) added digital maps, FBCB2 (Force XXI Battlefield Command Brigade and Below) capabilities, and an improved cooling system to maintain crew compartment temperature with the addition of multiple computer systems to the M1A2 tank.

Further upgrades include depleted uranium armor for all variants, a system overhaul that returns all A1s to like-new condition (M1A1 AIM), a digital enhancement package for the A1 (M1A1D), a commonality program to standardize parts between the U.S. Army and the Marine Corps (M1A1HC) and an electronic upgrade for the A2 (M1A2 SEP).

During Operations Desert Shield and Desert Storm and for Bosnia, some M1A1s were modified with armor upgrades. The M1 can be equipped with mine plow and mine roller attachments if needed. The M1 chassis also serves as a basis for the Grizzly combat engineering vehicle and the M104 Wolverine heavy assault bridge.

Over 8,800 M1 and M1A1 tanks have been produced at a cost of US$2.35–$4.30 million per unit, depending on the variant.

Design features
Armor
The Abrams is protected by the British designed 'Chobham armor', a further development of the British 'Burlington' armor. Chobham is a composite armor formed by spacing multiple layers of various alloys of steel, ceramics, plastic composites, and kevlar, giving an estimated maximum (frontal turret) 1320-1620 millimeters of RHAe versus HEAT (and other chemical energy rounds) and 940–960 mm versus kinetic energy penetrators. It may also be fitted with reactive armor over the track skirts if needed (as in the Urban Survival Kit) and Slat armor over the rear of the tank and rear fuel cells to protect against ATGMs. Fuel and ammunition are in armored compartments with blowout panels to protect the crew from the risk of the tank's own ammunition cooking off if the tank is damaged. Protection against spalling is provided by a kevlar liner. Beginning in 1987, M1A1 tanks received improved armor packages that incorporated depleted uranium (DU) mesh in their armor at the front of the turret and the front of the hull. Armor reinforced in this manner offers significantly increased resistance towards all types of anti-tank weaponry, but at the expense of adding considerable weight to the tank, as depleted uranium is 1.7 times denser than lead. The first M1A1 tanks to receive this upgrade were tanks stationed in Germany, since they were the first line of defense against the Soviet Union. US-based tank battalions participating in Operation Desert Storm received an emergency program to upgrade their tanks with depleted uranium armor immediately before the onset of the campaign. M1A2 tanks uniformly incorporate depleted uranium armor, and all M1A1 tanks in active service have been upgraded to this standard as well, the armor thickness is believed to be equivalent to 24 inches (610 mm) of RHA. The strength of the armor is estimated to be about the same as similar western, contemporary main battle tanks such as the Leopard 2. In the Persian Gulf War, Abrams tanks survived multiple hits at relatively close ranges from Iraqi Lion of Babylon tanks and ATGMs. M829A1 "Silver Bullet" APFSDS rounds from other M1A1 Abrams were unable to penetrate the front and side armor (even at close ranges) in friendly fire incidents as well as an incident in which another Abrams tried to destroy an Abrams that got stuck in mud and had to be abandoned.

In addition to the advanced armor, some Abrams, are equipped with a Missile Countermeasure Device that can impede the function of guidance systems of semiactive control line-of-sight (SACLOS) wire and radio guided anti-tank missiles (Russian AT-3, AT-4, AT-5, AT-6 and the like) and thermally and infrared guided missiles (ATGM). This device is mounted on the turret roof in front of the Loader's hatch, and can lead some people to mistake Abrams fitted with these devices for the M1A2 version, since the Commander's Independent Thermal Viewer on the latter is mounted in the same place, though the MCD is box-shaped and fixed in place as opposed to cylindrical and rotating like the CITV.

In the chance that the Abrams does suffer damage resulting in a fire in the crew compartment, the tank is equipped with a halon fire-suppression system that automatically engages and extinguishes fires in seconds.

Armament
Main armament
M68A1 rifled gun
The main armament of the original model M1 was the M68A1 105 mm rifled tank gun firing a variety of high explosive anti-tank (HEAT), high explosive, white phosphorus and an anti-personnel (multiple flechette) round. This gun is a license-built version of the British Royal Ordnance L7 gun. While being a reliable weapon and widely used by many NATO nations, a cannon with lethality beyond the 3 kilometer range was needed to combat newer armor technologies. To attain that lethality, projectile diameter needed to be increased. The M68A1's performance in terms of accuracy and armor-piercing penetration is on par with the M256A1 up to 3 kilometers out, but beyond that range the 105 mm projectile lacks the kinetic energy to defeat modern armor packages.

he main armament of the M1A1 and M1A2 is the M256A1 120 mm smoothbore gun, designed by Rheinmetall AG of Germany, manufactured under license in the United States by Watervliet Arsenal, New York. The M256A1 is a variant of the Rheinmetall 120 mm L/44 gun carried on the German Leopard 2 on all variants up to the Leopard 2A5. Leopard 2A6 replaced the L/44 barrel with a longer L/55.

The M256A1 fires a variety of rounds. The M829A2 was developed specifically to address the threats posed by a Soviet T-90 or T-80U tank equipped with kontakt-5 Explosive Reactive Armor. It also fires HEAT shaped charge rounds such as the M830, the latest version of which (M830A1) incorporates a sophisticated multi-mode electronic sensing fuse and more fragmentation which allows it to be used effectively against armored vehicles, personnel, and low-flying aircraft. The Abrams uses a manual loader, due to the belief that having a crewman reload the gun is faster and more reliable.[citation needed] and because autoloaders do not allow for separate ammunition storage in the turret.[citation needed]

The new M1028 120 mm anti-personnel canister cartridge was brought into service early for use in the aftermath of the 2003 invasion of Iraq. It contains 1,098 3/8 inch tungsten balls which spread from the muzzle to produce a shotgun effect lethal out to 600 m. The tungsten balls can be used to clear enemy dismounts, break up hasty ambush sites in urban areas, clear defiles, stop infantry attacks and counter-attacks and support friendly infantry assaults by providing covering fire. The canister round is also a highly effective breaching round and can level cinder block walls and knock man-sized holes in reinforced concrete walls for infantry raids at distances up to 75 meters.

In addition to this, the new XM1111 (Mid-Range-Munition Kinetic Energy) is also in development. Essentially a cannon-fired guided round, it has a range of roughly 12 km and uses a KE warhead which is rocket assisted in its final phase of flight. This is intended to be the best penetrator yet, an improvement over the US 3rd generation DU penetrator (estimated penetration 790 mm).

Secondary armament
The Abrams tank has three machine guns:
A .50 cal. (12.7 mm) M2 machine gun in front of the commander's hatch. On the M1, M1IP and M1A1, this gun is on a powered mount and can be fired using a 3× magnification sight, known as the Commander's Weapon Station (CWS for short), while the vehicle is "buttoned up" with all its hatches closed to protect the crew. On the M1A2 & M1A2SEP, this gun is on a flex mount (seen at right), the Commander having to expose himself to fire the weapon manually. With the forthcoming TUSK addon kit, an M2 or an Mk 19 grenade launcher can be mounted on the CROWS remote weapons platform (similar to the Protector M151 remote weapon station used on the Stryker family of vehicles). The upgrade variant called M1A1 Abrams Integrated Management (AIM) equips the .50 caliber gun with a thermal sight for accurate night and other low-visibility shooting.

A 7.62 mm M240 machine gun in front of the loader's hatch on a skate mount. Some of these have been fitted with gun shields during the ongoing conflict in Iraq as seen in the image at right, as well as night-vision scopes for low-visibility engagements.

A second 7.62 mm M240 machine gun in a coaxial mount to the right of the main gun. The coaxial MG is aimed and fired with the same computer fire control system used for the main gun.
(Optional) A second 12.7 mm M2 machine gun can be mounted directly above the main gun in a remote weapons platform as part of the TUSK upgrade kit mentioned below.

The turret is fitted with two six-barreled smoke grenade launchers (USMC M1A1s use an eight-barreled version). These can create a thick smoke that blocks both vision and thermal imaging, and can also be armed with chaff. The engine is also equipped with a smoke generator that is triggered by the driver. When activated, fuel is sprayed on the engine manifold, creating the thick smoke. However, due to change from diesel as a primary fuel to the use of JP-8, this system is disabled on most Abrams today, because JP-8 causes the tanks to catch fire when sprayed on the manifold. For the US Army in previous years, the Abrams usually maintained the provision for storing an M16 rifle or M4 carbine inside the turret in case the crew is required to leave the tank under potentially hostile conditions; while the crewmen were supplied with the M9 Beretta pistol as a personal sidearm. Considering the current (often dismounted) role of American armored crewmen and contemporary operating environments, though, current US Army crews maintain a rifle or carbine for each crewman. During Iraqi Freedom some crews were also issued M136 AT4 shoulder-fired anti-tank weapons under the assumption that they might have to engage heavy armor in tight urban areas where the main gun could not be brought to bear.

Aiming
The Abrams is equipped with a ballistic fire-control computer that uses data from a variety of sources, including the thermal or daylight Gunner's Primary Sight (GPS), all computing and displaying one of three components of the ballistic solution - lead angle, ammunition type, and range to the target. These three components are determined using a laser rangefinder, crosswind sensor, a pendulum static cant sensor, data on the ammunition type, tank-specific boresight alignment data, ammunition temperature, air temperature, barometric pressure, a muzzle reference sensor (MRS) that determines and compensates for barrel droop at the muzzle due to gravitational pull and barrel heating due to firing or sunlight, and target speed determined by tracking rate tachometers in the Gunner's or Commander's Controls Handles allowing for target speed input into the ballistic solution. The fire-control system uses these data to compute a firing solution for the gunner. The ballistic solution generated ensures a hit percentage greater than 95 percent at nominal ranges. Either the commander or gunner can fire the main gun. Additionally, the Commander's Independent Thermal Viewer (CITV) on the M1A2 can be used to locate targets and pass them on for the gunner to engage while the commander scans for new targets. In the event of a malfunction or damage to the primary sight system, the main and coaxial weapons can be manually aimed using a telescopic scope boresighted to the main gun known as the Gunner's Auxiliary Sight (GAS). The GAS has two interchangeable reticles; one for HEAT and MPAT (MultiPurpose AntiTank) rounds and one for APFSDS and STAFF (Smart Target-Activated Fire and Forget) ammunition. Turret traverse and main gun elevation can be accomplished with manual handles and cranks in the event of a Fire Control System or Hydraulic System failure. The commander's M2 .50 caliber machine gun on the M1 and M1A1 is aimed by a 3x magnification sight incorporated into the Commander's Weapon Station (CWS), while the M1A2 uses either the machine gun's own iron sights, or a remote aiming system such as the CROWS system when used as part of the TUSK (Tank Urban Survival Kit). The loader's M240 machine gun is aimed either with the built-in iron sights or with a thermal scope mounted on the machine gun.

Mobility
The M1 Abrams is powered by a 1500 hp (1119 kW) Honeywell AGT 1500 (originally made by Lycoming) gas turbine, and a six speed (four forward, two reverse) Allison X-1100-3B Hydro-Kinetic automatic transmission, giving it a governed top speed of 45 mph (72 km/h) on paved roads, and 30 mph (48 km/h) cross-country. With the engine governor removed, speeds of around 60 mph (97 km/h) are possible on an improved surface; however, damage to the drive train (especially to the tracks) and an increased risk of injuries to the crew can occur at speeds above 45 mph (72 km/h). The tank for all intents and purposes was built around this engine. The tank can be fueled with diesel fuel, kerosene, any grade of motor gasoline, JP-4 jet fuel, or JP-8 jet fuel; the US Army uses JP-8 jet fuel in order to simplify logistics. The Royal Australian Armoured Corps' M1A1 AIM SA uses diesel fuel; it is cheaper and makes practical sense for Australian military logistics. The gas turbine propulsion system has proven quite reliable in practice and combat, but its high fuel consumption is a serious logistic issue (starting up the turbine alone consumes nearly 10 gallons of fuel). The engine burns more than 1 gallon per mile and 12 gallons per hour when idle. The high speed, high temperature jet blast emitted from the rear of M1 Abrams tanks makes it difficult for the infantry to proceed shadowing the tank in urban combat. The turbine is very quiet when compared to diesel engines of similar power output and produces a significantly different sound from a contemporary diesel tank engine, reducing the audible distance of the sound, thus earning the Abrams the nickname, "whispering death" during its first REFORGER exercise. Honeywell was developing another gas turbine engine with General Electric for the XM2001 Crusader program that was also to be a replacement for the AGT-1500 engine already in the Abrams tank. The new LV100-5 engine is lighter and smaller (43% fewer parts) with rapid acceleration, quieter running and no visible exhaust. It also features a 33% reduction in fuel consumption (50% less when idle) and near drop-in replacement. The Abrams-Crusader Common Engine Program was shelved when the Crusader program was canceled, however Phase 2 of Army's PROSE (Partnership for Reduced O&S Costs, Engine) program calls for further development of the LV100-5 and replacement of the current AGT-1500 engine. Future US tanks may return to reciprocating engines for propulsion, as 4-stroke diesel engines have proven quite successful in other modern heavy tanks, e.g. the Leopard 2, Challenger 2 and Merkava.

The Abrams can be carried by a C-5 Galaxy or a C-17 Globemaster III. The limited capacity (two combat-ready in a C-5, one combat-ready tank in a C-17) caused serious logistical problems when deploying the tanks for the First Gulf War, though there was enough time for 1,848 tanks to be transported by ship.

Combat history
As the Abrams entered service in the 1980s, they would operate alongside M60A3 within the United States military, and with other NATO tanks in numerous Cold War exercises. These exercises usually took place in Western Europe, especially West Germany, but also in some other countries like South Korea. During such training, Abrams crews honed their skills for use against the men and equipment of the Soviet Union. However, by 1991 the USSR had collapsed and the Abrams would have its trial by fire in the Middle East.

Operation Desert Storm
The Abrams remained untested in combat until the Gulf War in 1991. A total of 1,848 M1A1s were deployed to Saudi Arabia. The M1A1 was superior to Iraq's Soviet-era T-55 and T-62 tanks, as well as Iraqi assembled Russian T-72s, and locally-produced copies (Asad Babil tank). The T-72s like most Soviet export designs lacked night vision systems and then-modern rangefinders, though they did have some night fighting tanks with older active infrared systems or floodlights—just not the latest starlight scopes and passive infrared scopes as on the Abrams. Only 23 M1A1s were taken out of service in the Gulf and one of these losses resulted in crew deaths from Iraqi fire. Some others took minor combat damage, with little effect on their operational readiness. Very few Abrams tanks were hit by enemy fire, and there was only one fatality, along with a handful of woundings as a result.

The M1A1 was capable of making kills at ranges in excess of 2,500 m. This range was crucial in combat against tanks of Soviet design in Desert Storm, as the effective range of the main gun in the Soviet/Iraqi tanks was less than 2,000 meters (Iraqi tanks could not fire anti-tank missiles like their Russian counterparts). This meant Abrams tanks could hit Iraqi tanks before the enemy got in range—a decisive advantage in this kind of combat. In friendly fire incidents, the front armor and fore side turret armor survived direct APFSDS hits from other M1A1s. This was not the case for the side armor of the hull and the rear armor of the turret, as both areas were penetrated at least in two occasions by friendly DU ammunition during the Battle of Norfolk.

M901 ITV

The M901 ITV (Improved TOW Vehicle) is a United States Army armored vehicle designed to carry a dual M220 TOW launcher. It is based on the ubiquitous M113 Armored Personnel Carrier chassis.

Equipment
The M901 ITV provides the crew and weapon system protection from small-arms fire and artillery fragments. The squad leader has a 270-degree range of view through the squad leader's periscope (SLP). The turret launcher has the capability for day and night acquisition and tracking of targets, and it provides firing coverages of 360 degrees in azimuth and +35 to −30 degrees in elevation. The ITV has stowage provisions for tripod-mounted TOW components configured so the ground system can be dismounted and set up in three to five minutes. In addition, the ITV can ford small bodies of water (40 inches or less) and is air transportable. It has the following characteristics:

A hydraulically and electrically powered "hammerhead" turret, attached to a modified M27 cupola, that can be operated manually.

A complete M220-series TOW weapon system stowed and strapped in fixed mounting brackets. The daysight tracker and nightsight (AN/TAS-4 or AN/TAS-4a) are mounted in an operational ready state at the head of the turret. The missile guidance system is also connected at the base of the turret.

A dual M220 TOW launcher.
M243 smoke grenade launchers.
A 3x acquisition sight with a 25-degree field of view.
Remote actuators that allow daysight tracker and nightsight adjustments.
A machine gun mounted on a traversing rail.

The system is capable of firing two missiles without reloading and carries ten TOW rounds in the missile rack. Reloading is performed under armor protection and is accomplished by tilting the launching apparatus back so that the crew can reach the turret through the carrier's rear roof hatch. The missile launcher targeting head is at the end of a pivoting arm which raises the launcher assembly for firing. When stowed, the turret is aimed down and to the rear of the vehicle. A major limitation of the M901 is that it is practically unable to move while the turret is in firing position, and unable to fire while it is in the stowed position. Moving from the firing to the stowed position is a procedure that takes several seconds and some skill on the part of the operator.