Calling the Boeing B-47 “the most significant multi-jet aircraft of all time” might be attributed to the bias of an old B-47 pilot, were it not for the incontrovertible facts of the situation.
The Boeing Company took a gigantic gamble to create the B-47. It combined an advanced planform, with wings and horizontal surfaces swept back 35 degree, new and relatively untried jet engines placed in pods beneath and forward of the wings, a bicycle landing gear, and new electronics. Its success was enormous, with more than 2,000 being built.
That success was conferred upon the aircraft which followed, including the B-52, the KC-135 and the 367-80. All of the subsequent Boeing transports, from the 707 to the 787, owe a massive debt to the B-47 for its revolutionary planform, its nurturing of the company’s financial basis and its outstanding subsonic performance. The multi-jet designs of many other manufacturers—including such sadly missed names as Douglas and Convair—owe a similar debt.
We are so accustomed to the swept-wing podded-engine layout that it is difficult to recall the utter wonder with which the B-47 was greeted when it made its first flight on December 17, 1947. It was so radical that even some of is own designers and builders, watching as it nosed toward its takeoff position at Boeing Field, wondered if in fact it would really fly. This is not a myth–it was told to me personally by one of its principal engineers.
To be fair, it must be noted that there are a large number of former B-47 crewmembers, pilots and radar observers alike, who hate the airplane. There were many reasons to do so. It was uncomfortable to fly on the very long missions, some of which exceeded twenty-four hours. It was in certain situations a dangerous aircraft, killing many a fine young crew in accidents, particularly on high gross weight takeoffs. In 1957, no less than 24 B-47s were lost in major accidents, killing 43 crew members. A loss rate like that today would not only cause a Congressional outcry–it would cut our bomber fleet by 1/5th.
The mission of the B-47 was demanding. It was to fly, alone, or in small cells of three or more aircraft, deep into the Soviet Union and destroy it with nuclear weapons. This hazardous undertaking was fortunately never required, and the B-47 in fact never dropped a bomb in anger. (It did serve briefly in the Vietnam War as an intelligence gathering aircraft.) Nevertheless, the B-47it served its primary purpose by deterring (a euphemism—the real term is “frightening”) the Soviet Union.
It did so by placing extraordinarily demands on the crews who flew and maintained it. The confining, restrictive nature of Alert duty, where crews were sequestered in monastic seclusion so that they could race to their airplanes and take off on a strike mission, took its toll of careers and marriages. Alert duty came often and for long periods, and was during the early years performed under primitive conditions.
In addition, B-47 units frequently deployed overseas for long periods of time, and this made life very difficult for families. There were good reasons not to like B-47 duty, particularly for hot young fighter pilots who graduated from flying school intent on becoming an ace and were instead placed in the back seat of a Stratojet.
But most people who flew the B-47 admire and respect it. It provided the United States with an unimaginable degree of power that was very obvious to the Soviet Union. By doing it quelled Soviet aggressive measures and helped maintain the uneasy peace between the two nations.
The B-47 came into being in a remarkably short time period, suffered delays as a result of the inevitable problems that ensued, and then swiftly equipped an expanding Strategic Air Command. Then just as it had expanded its numbers to become America’s premier strategic bomber, Robert McNamara ordered that the B-47s be withdrawn from service to save money for B-52s and missiles.
Just as they had suddenly appeared, forty-five to a wing, they disappeared, flown to Davis Monthan Air Force Base, and from there taxied to oblivion.
After a relatively short interval, during which salvageable parts were removed, they were towed to a huge guillotine that cut them to pieces and started their transformation back into aluminum ingots.
A total of 2,032 B-47s were built, 1,373 by Boeing, 274 by Douglas and 385 by Lockheed. For most of the twenty year interval between its first flight on December 17, 1947 and its last official Air Force flight on December 29, 1967, it reigned supreme as a bomber. For those of us that flew it, this is the way we will always remember it.
The Navy used an NB-47E as test bed for the General Electric TF-34 engine. The last NB-47E flight was in the summer of 1975.
For the record, the very last flight of a B-47 took place on June 17, 1986, when a restored B-47E made a precarious gear-down flight from the Naval Weapons Center at China Lake, California to Castle Air Force Base, California, to become a static display exhibit at the fine museum there.
During World War II, the Wright Field Bomber and Fighter Project Offices grew from one-man bands into large operations that routinely flew everything in the Army Air Forces’ inventory, tested foreign equipment and planned future developments. There was increasing interest in turbine engines, prompted by the success of both Great Britain and Germany. In 1943, the Army informally asked several manufacturers to put forward multi-jet bombers. The industry responded with a series of designs that displayed more than anything the lack of familiarity with the potential of the jet engine. This was followed in 1944 by a preliminary set of specifications, which called for a top speed of more than 500 miles per hour, a range of up to 3,000 miles and a service ceiling of 40,000 feet. By December, 1944, four firms responded—Boeing, North American, Martin and Convair. All of the paper designs would change over time, long before any metal was cut.
As might be expected, Boeing drew heavily on its past experience for its first offering, the Model 424, which was, in essence, a B-29 with four jet engines paired in nacelles under the wing, the design closely resembling a later competitor, the Convair XB-46.
A more advanced proposal followed, the Model 432, which retained the straight wings of the B-29, but placed the engines above the fuselage center section, with huge air intakes located beside the cockpit.
At this point fortune smiled upon Boeing. Its chief aerodynamicist, George Schairer, was a member of the Scientific Advisory Group (SAG) (later Scientific Advisory Board) that the famous Dr. Theodore von Kármán had established at the request of General of the Army Henry H. Arnold. Schairer and Kármán among others were part of Operation Lusty, which was tasked to gather up data on German advances in aeronautics, rocketry and nuclear science. In a heretofore secret German scientific institute at Volkenrode, (near Braunschweig) Schairer found an enormous amount of scientific data, drawings and papers dumped into a well. From these it was easy to confirm that there were great advantages in a swept wing for high speed flight, and Schairer immediately wrote Boeing, directing that the proposed jet bomber project be given swept wings. He specified twenty-nine degrees of sweep in his letter, but this was later changed to thirty-five degrees. (North American would similarly benefit from these findings, and the XP-86 would emerge with thirty-five degrees of sweepback.)
The next Boeing proposal was the Model 448, which retained much of the rotund’s Model 432’s layout, but featured thin swept wings of a high aspect ratio. The configuration of the wings meant that the undercarriage and all fuel had to be carried in the fuselage as was done in the next of the series, Model 450-1-1. To reduce interference, the engines were suspended in pods that placed them forward of the leading edge of the thin wings. This arrangement also permitted a lighter wing structure.
Although the successful conclusion of World War II seemed to lessen the urgency for a jet bomber, a mock-up inspection was held in Seattle in April, 1946. Boeing was awarded a $10,000,000 contract to build two prototypes. The sum seems laughably small today, but it was immensely important to Boeing at a time when literally hundreds of millions of dollars in contracts had been cancelled, and its work force was drawing down at an incredible rate.
George Martin was Boeing’s Project Engineer for the B-47 and focused from the start on two major challenges: the aerodynamics of the swept wings, and on the jet engines themselves. To husband funds for these two unknowns, he chose to use readily available elements wherever possible, including parts of the B-29 landing gear, a low pressure hydraulic system, and the old fashioned 28 volt D.C. electrical system. (Oddly enough, Martin was not certain initially that the risky B-47 represented Boeing’s future, believing that the B-54 (later the B-50) was the real “bread and butter” product of the firm.)
The configuration of the B-47 demanded new and more sophisticated structural methods, to deal with such phenomena as the incredibly flexible nature of the wings, which could deflect up to 17-1/2 feet in flight. Flying a B-47 in turbulence was daunting. If you looked to left or right you could see the wings and the podded engines doing a dance not unlike that of the Tacoma Narrows Bridge just before it disintegrated, an undulating, rolling flex that made you wonder how (or if) they would stay on.
The thin wing also created a problem, in that it was also flexible chord-wise, and at speeds above 425 knots indicated, the ailerons acted as a tab, twisting the wings rather than inducing a bank. At 456 knots, the ailerons were totally ineffective, and the control wheel could not be budged from side to side. The author flew the B-47 at the 4925th Test Group (Nuclear) at Kirtland Air Force Base in the early 1960s, and flying the B-47 at fifty feet or less above the desert at 456 knots would whiten the reddest knuckles. The wings would be locked solid, and if any sort of emergency occurred, the only recourse you had was to chop the throttles, pull back on the control column, and let the airspeed bleed off until you had aileron control again.
The bicycle landing gear, first tested on Martin’s B-26 “Middle River Stump Jumper” and used on the Martin XB-48, required that you land and take off in a fixed attitude, without the customary flare or rotation of conventional aircraft. This was a little difficult to adapt to, but after a few flights, you became accustomed to it.
The General Electric J47 engines used in production B-47s grew in power from an initial 5,200 lbs of thrust to 7,200 pounds in the later, water injected versions. They were reasonably reliable for the time, but were very slow to accelerate and had to be handled carefully both on takeoff and landing. On a hot day, especially at higher field elevations, the B-47’s slow acceleration often made it seem doubtful that it would lift off before you ran out runway. But, as long as the calculated speeds were reached at the calculated points—and as long as the calculations were correct—it would lift off right at the last moment.
The B-47 was very clean, even with the huge Fowler flaps extended, so an approach chute was installed to make the long, flat landing approaches easier. When deployed, the approach chute allowed you to carry more power on the engines. In the event of a go-around, a throttle application gave quick acceleration, and the approach chute could be jettisoned if necessary. The airplane also employed a brake chute, and had anti-skid brakes which were very advanced for the time. The fixed-attitude landing technique required that you land on the rear truck first. If a pilot landed on the front wheels, the airplane would bounce, and this could lead to a porpoise-like series of lunges that tended to increase rather than decrease. When you became proficient in the airplane you could save another pilot’s bad landing by deploying the brake parachute at exactly the right time; the bouncing aircraft would seem to catch its breath as the parachute took hold, then settle gracefully to the runway on all four wheels. (This was not something to do if you were not sure of yourself. Done at the wrong time, you could vastly compound your problems.)
After herding around a ten man crew in a B-50, the B-47’s small three-man crew was a relief. The Aircraft Commander sat in in the front seat of the tandem cockpit, with the pilot in the rear, both under a superb bubble canopy. The pilot’s seat rotated so that he could operate the only defensive armament, the two 20-mm cannons mounted in a turret at the end of the fuselage. Down in the black compartment of the nose, the Radar Observer acted as navigator and bombardier. Everyone kept busy on a mission, which in SAC ranged from as few as six hours to (rarely) longer than twenty-four.
Boeing tankers permitted the extended range. The piston engine powered Boeing KC-97 was slow, and refueling was difficult and not as productive as it should have been. Initially, the tanker could maintain a speed convenient for the bomber, which would be at a relatively low gross weight. As fuel transferred, the situation changed, for the increasingly heavy B-47 had to fly faster to stay above its stall speed. The KC-97 would begin a descent to keep its speed above the B-47’s stall speed. Further the B-47 used fuel in its descent, refueling and climb back to altitude, so its net gain was much less than would be the case using a jet tanker. The advent of the jet powered KC-135 made things much better, for its speed, altitude and wake were compatible, as it has proved to be with the B-47’s successor, the B-52 for forty years and more.
With the jet engine clearly the path to the future, the competition for the new United States Air Force’s primary bomber was extremely important, yet Boeing’s daring leap forward made it essentially no contest. The relatively state-of-the-art North American secured a production contract for its smaller, four-engine B-45 Tornado, which did its most important work in its reconnaissance versions. Convair’s entry, the slimly elegant XB-46, was handicapped by its straight wing and the fact that Convair was already under contract to produce large numbers of the huge B-36. Only one was built. Martin’s entry was the XB-48, also a straight wing aircraft with its six engines grouped in sets of three in huge, drag-inducing nacelles. Northrop’s XB-35 piston-engine flying wing nominally entered the competition later when modified to YB-49 status and fitted with eight jet engines. Lacking the computers that could sense the requirement for control inputs to dampen oscillations and maintain a stable bombing platform they were not available for another two decades), the YB-49 was relegated—for a time—to the reconnaissance role. None of the competition possessed either the performance or the potential of the B-47.
The Air Force eased into the B-47, recognizing that with all of the unknowns embodied in the radical design, there would be difficulties to overcome. Ten B-47As were purchased. These, built at Boeing’s mammoth Wichita plant, were essentially test aircraft, and did not have an in-flight refueling capability. A few were used by the 306th Bomb Wing at MacDill AFB to introduce SAC to its new weapon, beginning in May, 1951. This was forty-one months after the first flight, giving an indication of the development problems that had to be overcome.
Despite the ongoing troubles, in November, 1949, the Air Force made an initial order for eighty-seven B-47Bs. These had an in-flight refueling capability, but thanks to a terrible decision made by the Air Force to save weight, they lacked ejection seats. It is difficult to explain to a layman just how important an ejection seat is to the psychology of a crew member. No one ever wants to use it, but as you fly you are comforted by the fact that if all else fails, it is there as a last resort. A total of 399 B models were built, and many of these were later equippped with ejection seats and brought up to the B-47E configuration.
The Cold War was punctuated by many crises—the expansion of the Soviet system of satellite states, the Berlin Airlift and the Korean War among them—and these all accelerated rearmament in the United States. This was reflected in the massive growth in the Strategic Air Command, which went from a rag tag collection of 556 bombers in 1948 (35 B-36, 35 B-50, 486 B-29) to 1,895 in 1956 (247 B/RB 36, 97 B-52, 254 RB-47, 1,306 B-47). B-47 strength would peak in 1958, when there were 1,367 B-47s and 176 RB-47s on strength.
The most produced version was the B-47E, of which 1,341 were built. The increase in numbers was matched by an increase in proficiency as the standards imposed by Curt LeMay percolated down to every unit. There was an increase in readiness, as well, with half the force ultimately being on a fifteen minute alert. This meant that the aircraft was “cocked” (already fueled, loaded with bombs, pre-flighted, inspected and ready to start engines) and could launch within fifteen minutes. The alert was necessary to counter the growing intercontinental ballistic missile threat from the Soviet Union. It wore our air and ground crews to a frazzle.
The primary example of the type, the B-47E had a top speed of 527 knots and a cruise speed of 434 knots. The range, because of in-flight refueling was of course unlimited, but crew endurance was pretty well exhausted at the end of a twenty-four hour flight. I recall just finishing one twenty-four mission, debriefing and falling into bed into a deep slumber, when the Alert siren went off—back to Castle Air Force Base for another mission. Fortunately, they recognized the problem and split our crew up to fly with other rested crews, otherwise we might well have slept through the entire mission.
Even a standard mission of six to eight hours was tiring, because you stayed busy the entire time, and it was difficult to get out of the seat and stretch—many people just stayed in the seat from take-off to landing.
There were many variants of the aircraft, but none were more demanding of their crews than those RB and EB versions which carried a pressurized compartment in the bomb bay to accommodate three Electronic Warfar Officers—known as “Ravens” or, more familiarly, “Crows.” The EWOs—all heroes in my book– were jammed into a tiny compartment that was less than four feet high and crowded with electronic gear. They were equipped with ejection seats—but there were no ejection hatches. Steel blades on the seat’s platform were designed to punch a hole through the fuselage floor, through which the ejections would fire. Successful ejection was improbable.
Nonetheless, the Crows carried on, flying long missions, essentially trapped in their capsule, performing vitally important work in interpreting Soviet electronic intelligence. Their worked presaged the incredible assets found today in the AWACS, JointSTARS, Rivet Joint, U-2 and other intelligence gathering aircraft.
The radical design of the B-47 naturally endowed it with problems. This coupled with the inexorable demands to carry more weight resulted in structural difficulties. The aircraft’s original maximum 125,000 pound gross weight was ultimately increased to 230,000 pounds for taxi, with a maximum in-flight weight of 225,958 pounds—almost double. This placed a severe demand on the structure, which ultimately was limited to two positive “Gs” at maximum gross weight. No negative Gs were allowed. This meant that as maneuverable as the aircraft was, it had to be handled carefully even when new bomb delivery techniques were required.
The growth in Soviet anti-aircraft missile capability dictated a change from the B-47s high altitude mission. Tactics were evolved for two separate maneuvers. One, called LABS for Low Altitude Bombing System, involved the aircraft attacking on the deck, pulling up in a half loop and releasing the bomb at about the 45 degree point, arcing it out for as far as eight miles with considerable accuracy. The pilot continued the half-loop, rolling out at the top in an Immelmann turn—all without exceeding the G limits.
The second technique was called the “pop-up” and required the B-47 to fly low over the ground, then pull up rapidly to 18,000 feet, release its weapon, then turn away while dropping back down to ground level.
These maneuvers, and the intensive use of the aircraft in low-level flight (it was not unusual to fly eighty-hours per month as a crew member) resulted in a series of six crashes between March 13 and April 15, 1958 that rocked SAC and severely threatened B-47 air crew morale. A fleet-wide investigation revealed severe fatigue problems that ranged from fatigue in the lower wing skin to failure of the “milk bottle pin,” the main fitting that secured the wing to the fuselage. There followed a nightmare of fixes, new problems, new fixes and additional problems. Boeing, Lockheed and Douglas (the latter two companies also having built B-47s) eventually contained the problem, but at enormous expense. The alternative was to ground the fleet permanently, as would happen to the Royal Air Force’s Vickers Valiant in 1964.
Besides these very real problems, there were many rumors that circulated about the B-47 that were not so difficult to resolve. One of these was the infamous “coffin corner” where high speed stall and low speed stall were within a few knots of each other. It was possible to fly into the coffin corner if you took the aircraft above the best altitude for its current weight, but it was not something that occurred routinely, or that you could wander into casually. The requirement for precise speed control was another story that was often heard, and it was true. If you allowed your speed on approach to build up by an extra few knots, you added several hundred feet to your landing distance. However, it was easy to control the airspeed of the B-47 to exactly what you wished by minute adjustment to the throttle settings. If you wanted 140 knots on the gauge, and you were at 142, you simply throttled back slightly. And in a cross-wind, you could use asymmetric power on the one and six engines to keep you right on an ILS (Instrument Landing System) approach.
The entire fabric of the B-47 fleet was held together by the unremitting effort of the maintenance personnel who labored day and night to keep them flying. In the early 1950s, at Castle Air Force Base, I was struck by the fact that two of these beautiful $3 million dollar airplanes were supported by hard-working mechanics who had to share a $100 box of tools between them because hand tools were in short supply. As the aircraft aged, it became ever more difficult, but somehow, when takeoff time rolled around, the airplanes were ready.
Times have changed. Bombers are no longer procured by the thousands. Only twenty-one Northrop Grumman B-2A Spirits were purchased, and that number was reached only by equipping the static test article. No new bomber is expected to enter the fleet until sometime after 2037, and even then the number built will be very small. Never again will anyone experience the thrill and the pride that the B-47 crews did on a unit mission, when all of the aircraft in the wing, along with their tankers, would be started and serially wind their way around the taxiways to the run-up area and then onto the runway, doing minimum interval takeoffs (MITO). The B-47s, their wings drooping, their outrigger gear touching the ground, would moan and grown as they went into position and applied full power, waiting only fifteen seconds before following the preceding aircraft into the air. You could feel the vibration, hear the roar of engines, but see very little as water injection would darken the exhaust that drooped onto the runway obscuring visibility. The author recalls one of these takeoffs which had another hazard—off the end of the runway black smoke was rising from where an aircraft had crashed. Still the takeoffs went on, one after the other, through the exhaust and the through the smoke. It was another time—and it was a privilege to serve then, as it is now.
SIDEBAR 1: PERSONALITIES
As with every great airplane, the history of the B-47 was dominated by personalities at every level. At the very top, the nation owes a debt to General Curtis E. LeMay, the greatest combat air commander of all time, who whipped the Strategic Air Command into shape. He had the vision to look into the future and see that the tanker and the B-47 would provide an incomparable weapon system.
Boeing was fortunate to have its finely tuned engineering and management teams combine to create the B-47 at a rock-bottom price under the overall leadership of the famed William Allen. George Schairer was the chief aerodynamicist, and George Martin was the project manager. They were backed by Ed Wells and the rest of the company in an extremely demanding program that could have failed at many points. Schairer is the individual who had pushed Boeing to invest in its huge wind-tunnel, and this gave the company a tremendous advantage over its rivals.
Robert Robbins and Scott Osler made the first flight, and Robbins still speaks with affection and respect of the aircraft. Osler, unfortunately, was the first man to die in a B-47, losing his life to an unfortunate accident with a malfunctioning canopy.
Major (later Brigadier General) Guy Townsend, one of the most exacting test pilots in the business, as well as one of the most personable and far-seeing officers, stage managed the acceptance of the radical aircraft into the United States Air Force. It was Townsend who suggested the application of the ribbon-style approach chute to use on landing approach. At one point he induced a reluctant Major General K. B. Wolfe (the man who had introduced the B-29 to the USAAF) to fly in the airplane, and in that one flight convinced him to order the aircraft.
And mention should be made of the hardy Air Force instructor pilots at McConnell Air Force Base, Wichita Kansas, who taught thousands of young pilots how to fly this demanding aircraft.
SIDEBAR TWO: TAKEOFFS AND LANDINGS
For someone used to flying a Boeing B-50, the B-47 was a delight. The technique for takeoffs and landings were quite different, and reflected the jet engines of the era, with their comparatively low thrust and slow acceleration. The bicycle gear took some adjustment, but within a few hours, you could taxi the aircraft to the exact center of the runway, with the aft truck very near the beginning of the overrun area, for you wanted as much runway ahead of you as possible. Power was brought up on all six engines, and stabilized at 100 percent. Brakes were released, and water injection begun. The engine instruments were closely monitored, for if there was going to be trouble, you wanted to know about it as soon as possible. Prior to flight, the takeoff run had been calculated for the predicted temperatures and pressure altitude, and these were monitored prior to takeoff to be sure they were still applicable. There was no difficulty in controlling the aircraft during the takeoff, the rudder providing plenty of authority. Acceleration was pre-computed and monitored closely; if you were not accelerating properly, it was time to abort. There was no “rotation” you simply lifted off (if all was well) at the pre-computed unstick speed (around 155 knots at a typical takeoff weight), and as soon as a positive climb was established, the gear was retracted. Initial climb speed was unstick speed plus twenty. At 300 feet you could begin flap retraction, according to a previously calculated schedule. When the flaps were up you established a 310 knot climb airspeed.
What is not covered in the above is that often you rolled out to the runway in the blazing sun, with cockpit temperatures reaching well above 100, and you would be sweating so hard that the oxygen mask would slip on your face and the helmet slide down. One of the great boons of the B-47 was its air conditioner, and on climb out there would be a sudden refreshing blast of ice cold air that quickly chilled you down. Once established in flight, the aircraft’s temperature was quite comfortable for the pilots, but less so for the navigator.
The approach and landing were also refreshingly different from a piston-engine bomber. In the B-50, you might return to base to find yourself in a stack of ten or fifteen aircraft, and forced to orbit around a navigation aid descending in 1,000 foot increments as aircraft below you were cleared to land. The B-47’s thirst for fuel precluded this, so descents from altitude were made at high speed—6,000 feet per minute—and you were picked up by approach control and vectored in for an instrument approach, or cleared for a visual approach. . Landing the B-47 required a long flat approach, and careful management of speed. The “best flare” speed was based on aircraft weight, of course, and you entered the local pattern at “best flare” plus 30 knots. (For example, at a landing weight of 115,000 pounds, with flaps down, the best flare speed would be 137 mph with a touchdown speed of 128 mph.) The downwind leg was about two miles out (in a no-wind situation). Landing gear was down, the approach parachute was deployed and flaps were set for landing. You began the turn to base about forty-five seconds after passing the approach end of the runway, using about 30 degrees of bank, and of course varying this to compensate for any wind. The initial rate of descent was about 400 feet per minute. The turn was continued into the base, where you rolled wings level briefly to check for traffic, reduced your airspeed to best-flare plus twenty knots, and rolled in again for your final approach. When lined up on final, rate of descent maintained at 400 feet per minute, with a speed of best flare plus ten. As you crossed the numbers, you wanted to be at best flare speed, you reduced power further, slowed to touchdown speed, touched down, popped the brake chute, and then applied the anti-skid brakes.
Again, this rather clinical approach does not consider cross winds, turbulence, fatigue and the other normal factors that make landings interesting. But the B-47 was under normal circumstances not a difficult aircraft to land.
B-47 Stratojet: Boeing’s Brilliant Bomber, by Jan Tegler, the Walter J. Boyne Military Aircraft Series, McGraw Hill, New York, 2000
Boeing B-47 Stratojet by Lindsay Peacock, Osprey Air Combat series, Osprey Publishing Company, United Kingdom.
B-47 Stratojet by Alwyn T. Lloyd, Detail & Scale, Volume 18,
TAB Books, Inc. PA, 1986
The B-47 Stratojet Association has a great website at http://www.b-47.com/