Quest for Performance: The Evolution of Modern Aircraft
Chapter 12: Jet Bomber and Attack Aircraft
Search for a New Strategic Bomber
[385] Even before the first flight of the Boeing B-52, studies of its successor were underway. As a means for reducing both weapons delivery time and vulnerability to enemy defenses, increasing speeds into the supersonic range seemed to offer the most favorable approach to the development of a more effective, large bomber aircraft. Although not too successful, the B-58 was a first result of this type of thinking. Next to appear was the impressive XB-70 Valkyrie supersonic bomber developed by North American Aviation - the same company that had already developed so many highly effective aircraft types for the armed services of the United States. A contract for this 521 000-pound Mach 3.0 strategic bomber was signed on December 23, 1957. Discovery of new methods for configuring a supersonic aircraft so as to achieve favorable aerodynamic interference effects made supersonic cruising efficiency much superior to that of the B-58 seem possible.
A growing conviction that a Mach 3.0 aircraft cruising at 75 000 feet had little chance of successfully penetrating enemy defenses, [386] however, resulted in cancellation of the program in 1961. Only two prototypes were constructed, and these were used for flight research studies of the problems of large supersonic cruising aircraft. Initial flight of the first XB-70 took place on September 21, 1964, and the second one that flew about a year later was destroyed in a mid-air collision in 1966. A joint NASA-USAF flight research program utilizing the remaining aircraft extended over a period of several years. Last flight of the unique XB-70 was on February 4, 1969, when it was delivered to the Air Force museum in Dayton, Ohio, where it is now on display. The first prototype XB-70 is shown in figure 12.18, and descriptive material on the aircraft may be found in references 127, 170, and 200.
Although cancellation of the XB-70 program was seen by some as signaling the end of the manned bomber as an important weapon, others continued to feel that a new and more capable bomber was essential for the defense of the United States. With due consideration of the state of existing and possible future technology in such areas as aeronautics, electronics, and weapons, detailed studies of the potential effectiveness of various types of aircraft and mission profiles were made. These studies extended over a period of several years and resulted in the issuance of new bomber requirements and a request for proposal. On June 5, 1970, Rockwell International, the conglomerate...

aerial view of a XB-70A with landing gear extended
Figure 12.18 - North American XB-70A supersonic bomber. [mfr] [Original photo was in color, Chris Gamble, html editor] 


[387]...that had absorbed North American Aviation, was awarded the contract for development of a new manned bomber to be known as the B-1A.

Requirements for the B-1A called for high-efficiency cruising flight at both subsonic speeds and at a supersonic Mach number of 2.2. In addition, a low-altitude penetration capability at near-sonic speeds was specified. To reduce vulnerability while on the ground, takeoff and landing field lengths short enough to permit operation from air fields other than large, established SAC bases were also called for. Not surprisingly, the configuration proposed by Rockwell incorporated variable-sweep wings; this feature really offered the only viable means for meeting the mission requirements of the aircraft. The manner in which variable sweep can be used to adjust the aircraft configuration to meet requirements of different flight regimes is illustrated in figure 11.29.
First flight of the prototype took place on December 23, 1974. By 1979, four prototypes had been constructed with the fourth vehicle representing a fully operational aircraft complete with electronic systems installed. In the meantime, the B-1A production program was cancelled in June 1977 during the first year of the Carter administration. High cost of the aircraft perhaps tempered with some lingering doubts regarding its ability to successfully penetrate enemy defenses were apparently reasons for the cancellation. During the first year of the Reagan administration, however, the B-1 program was revived, and production of a somewhat simplified and less costly version of the aircraft is now planned. In this version, known as the B-1B, the Mach 2.2 supersonic cruise requirement has been eliminated, and the maximum speed is limited to a Mach number of about 1.2 at high altitude. Because of a lack of definitive information on the B-1B, the following discussion is limited primarily to the B-1A version of the aircraft.
The B-1A is shown in figures 12.19 and 12.20 with the wings in the high and low sweep positions, respectively. Maximum and minimum sweepback angles are 67.5° and 15°, and the corresponding aspect ratios are 3.13 and 9.85. Maximum aspect ratio at the low sweep position is slightly higher than that of the B-52; the lower sweepback angle of 15° for the B-1A, as compared with 35° for the B-52, has a favorable effect on the drag due to lift. The outboard location of the wing pivots is in accordance with the NASA research findings discussed in chapter 10. The large-span single-slotted trailing-edge flaps as well as the full-span leading-edge slats are clearly visible in figure 12.20. The powerful leading- and trailing-edge high-lift devices together with the small minimum wing sweepback angle all contribute to the high-lift capability of the aircraft and the corresponding short landing and take-off...

aerial view of a B-1A
[388Figure 12.19 - Rockwell International B-1A bomber with wings in the maximum sweep position. [mfr]


...field lengths. Not visible in the photographs are the wing spoilers that, together with differential deflection of the horizontal tail, are used for roll control. Symmetrical deflection of the spoilers is used to aid in deceleration on the landing rollout.

Location of the two rectangular-shaped engine nacelles on the bottom of the fixed portion of the wing is shown in figure 12.20. Each nacelle contains two General Electric afterburning turbofan engines with bypass ratio of 2.0 and overall compression ratio of 27. The variable-geometry inlets on the B-1A will be replaced by more simple and less costly fixed-geometry inlets on the B-1B. Position of the exhaust nozzles behind the trailing edge of the wing may be seen in figure 12.20. The landing-gear arrangement is conventional and consists of four-wheel bogies on each of two main-gear struts and a two-wheel nose gear.
Other features of interest include the small size of the fuselage, which is carefully area ruled and blends smoothly into the wings. A small fuselage wetted area is an important factor in achieving a high...

photo of a B-1a comming in for a landing
[389Figure 12.20 - Rockwell International B-1A bomber with wings in the minimum sweep position. [mfr]

....value of the maximum lift-drag ratio. One of the small vanes located on either side of the nose of the aircraft can be seen in figures 12.19 and 12.20 These vanes are part of a special "structural mode control system." In response to signals received from sensors located in the aircraft, the vanes are actuated to aerodynamically attenuate gust-induced normal accelerations in the cockpit. In a similar manner, yawing motions are suppressed by automatic actuation of the lower part of the three-piece rudder. Crew fatigue encountered in the rough air of low-altitude high-speed penetration missions is reduced, and accuracy of weapons delivery is increased by the structural mode control system.
Normal crew of the B-IA consists of pilot, copilot, offensive-systems operator, and defensive-systems operator. Contrary to conventional practice in large aircraft, the pilots are provided with fighter-type control sticks instead of the traditional control wheels. The power-actuated control system is tailored to give light control forces and rapid response characteristics. For the first three prototypes, the entire crew compartment was ejected and lowered by parachute in the event that [390] crew escape was necessary. As a means of reducing aircraft costs, the fourth prototype had individual ejection seats; this type of escape equipment will be incorporated on all subsequent aircraft. No guns are carried on the B-1, but extensive gear for electronic warfare is provided. Various types of nuclear and conventional weapons can be carried in three internal bomb bays. Provision for mounting external weapons are also included. By careful attention to design, the radar signature of the B-1 is only about 5 percent of that of the B-52; hence the B-1 is much less visible to enemy radar than the earlier aircraft.
According to table VI, the gross weight of the B-1A is 389000 pounds and the cruising speed is listed as 648 miles per hour (Mach 0.85) at 50 000 feet. Gross weight of the B-1B is indicated by some sources to be about 476 000 pounds. No definitive range-payload data are available for either the B-1A or the B-1B.
According to present plans, 100 B-1B bombers will be built, with the last aircraft to be delivered in 1988. Thus with the reactivation of the B-1 program, the strategic bomber that traces its origins to the cumbersome Gotha and Handley Page biplanes of World War I seems assured of an important role in the front line of United States military power until well into the 21st century.