The neutron initiator is a vacuum tube, having an electrically heated filament anode coated with uranium deuteride. Downstream from this ion source is a cathode consisting of a thin metal plate coated with uranium tritide. A potential of 500,000 volts is established between the anode and the cathode, and deuterium ions (deuterons) boil from the uranium deuteride and are attracted to and vigorously accelerated by the cathode. Splattering into the cathode, the deuterons fuse occasionally with tritium held in the uranium, and a burst of neutrons develops. The neutron flux is amplified by the uranium content of the cathode, which tends to fission and release 2.4 neutrons for each received.[195]
The safety measures used in the nuclear bombs and warheads were well designed and effective, but there were still problems with the delivery systems.[196] The end of World War II was the beginning of a very exciting quarter of a century for the United States Air Force, particularly with the steep rise in strategic bomber capability, radical changes in strategy, and one very ambitious enemy, the Union of Soviet Socialist Republics. The new ultimate weapon would not be carried in formations of a thousand bombers, but by one lone aircraft. One city, one bomb. Starting with the propeller-driven B-50 Superfortress, an upgrade of the old B-29 from the last war, the Air Force would lunge forward, developing an ever-advancing line of exotic airplanes to carry the nuclear bombs. These aircraft would have to fly higher, farther, and faster than anything had before, and new boundaries would be drawn describing the limitations of men and machines. Each new bomber was built way out on the edge of what was possible, using new materials and design techniques, and none were forgiving of a slight piloting error, a switch left in the wrong position, or a gas cap left off. Plane crashes, explosions, and disappearances occurred with appalling frequency.
The first notable strategic bomber of the Cold War was the Convair B-36 Peacemaker, the “aluminum overcast,” a 72,000-pound monster with a 230-foot wingspan. It was made airborne using six radial engines driving enormous propellers, swinging in circles 19 feet wide. Together, the engines developed 22,800 horsepower, but to top it off four General Electric J47-19 jet engines were added, two under each wing tip. Before takeoff, 336 spark plugs had to be changed, 600 gallons of oil were added to the engines, and a tanker truck had to top off the fuel after the B-36 had lumbered into takeoff position on the runway. For a while starting in 1949, it was the only airplane we had that could deliver atomic bombs to targets in the Soviet Union. It had a range of over 6,000 miles, and its bomb bay was the only one ever made that was big enough to carry the enormous MK-17 thermonuclear device.[197] It took a crew of 15 men to fly it, and amenities included a dining room, six bunk beds, and a wheeled trolley to ride from the tail to the nose. It was very prone to engine fires, due to the backward mounting of the engines, the fuel tanks in the wings would develop cracks and leak, and the vibration caused by firing the machine guns could disrupt the vacuum-tube electronics in the control system and send the big bird into an irrecoverable vertical dive. By the time it was retired from bomb-carrying service in 1955, B-36 airplanes had been involved in three crashes involving nuclear weapons.
In June 1951, the Air Force was introduced to its new, highly advanced jet-powered strategic bomber, the Boeing B-47 Stratojet. Everything about it was new, from its back-swept wings to its extensive electronic systems, and its many innovations helped Boeing to build its successful line of modern jet airliners. It was built like a fighter plane with six engines and a bomb bay, and it was flown by three very busy men: the pilot, the co-pilot, and the navigator/bombardier. It could almost break the sound barrier, and it remained on SAC alert, ready to fly with nuclear bombs against an enemy threat at a moment’s notice, until 1965. But it still had a few bugs.
It liked to take off slow and land fast. Its hesitancy to leave the ground was solved by attaching groups of solid-fuel rockets to either side of the airframe, pointed slightly down. The 18,000 pounds of thrust from the rockets were a big help. It wanted to land at an exciting 207 miles per hour, and its design was so aerodynamically clean, it would rather continue flying and not touch down. This situation was helped by popping out a 32-foot drogue parachute from the tail for landing. This would cut the speed and shorten the landing roll without burning up the brakes. At optimum fuel economy altitude, about 37,000 feet, the airspeed had to be maintained precisely, with an acceptable error of plus or minus three miles per hour. Four miles per hour too slow, and the plane would fall out of the sky. Four miles per hour too fast, and the wings came off. At lower altitudes it was easy to go faster than 489 miles per hour, but do so and the controls would reverse, going down when you say up.
The B-47 was a remarkable airplane and a forward leap in design, but it was involved in ten nuclear weapons incidents, the most interesting of which involved accidentally dropping bombs within the boundaries of the United States.
Learning a lot from having built the B-47, Boeing designed a new swept-wing bomber using eight jet engines mounted under the wings. The Boeing B-52 Stratofortress strategic bombers, introduced in February 1955, have been in service for an impressive 58 years, and they will probably be phased out around 2045. The grandchildren of people who flew the original batch of B-52s could be flying B-52s today. The last B-52H was built in 1962, and this last group of 85 planes still in service has been modified and improved several times. These bombers can go 650 miles per hour and climb to 50,000 feet with a range of 10,145 miles, and they have broken many flight records. They have flown around the world non-stop in 45 hours 19 minutes with in-flight refueling, and can fly from Japan to Spain with one load of fuel. A B-52 can land sideways in a heavy cross-wind, using its in-board landing gear with coupled steering.
The flight crew can vary between six and nine, sitting in a fuselage that has work stations on two levels. A B-52B first dropped an MK-15 mod 2 thermonuclear device over Bikini Atoll on May 21, 1956, in the test code-named Cherokee.[198] A B-52 has never dropped a nuclear weapon in warfare, although it has bombed targets in several wars since 1955. B-52s put out the lights in Baghdad in Operation Desert Storm, 1991, with gravity bombs and launched one hundred air-launched cruise missiles into Iraq in Operation Iraqi Freedom, 2003.
B-52s were involved in six nuclear weapon accidents. Common problems in the early versions of the plane were vertical stabilizers and wings breaking off in mid-flight. These structural deficiencies were identified, and all B-52s in service were eventually sent back to the Boeing factory for strengthening retrofits.
Next up was the B-58 Hustler, another wild experience in aircraft design from Convair of Fort Worth, Texas. Introduced to the Air Force on March 15, 1960, the B-58 could hasten along at Mach 2—twice the speed of sound. It looked like something out of science fiction, with its rakishly swept delta wings and its slender body, sharply pointed at both ends with a slight waist at the center, and four big General Electric J79 jet engines on pods under the wings. It was the first military aircraft in the inventory that was all transistorized, with computers monitoring and controlling everything. If the system detected a fault somewhere in the vast interplay of electronic, hydraulic, and human systems, a sexy recorded female voice, supplied by actress Joan Elms, would coo a warning into the communication system.
195
This is an educated guess. The vacuum-tube neutron initiator is among those aspects of nuclear weapon design which remain classified SECRET. However, vacuum-tube electrical neutron generators are fairly common industrial items, and all work using similar principles. The technique for making neutrons electrically was first patented in Germany in 1938. For practical reasons, electrical initiators are not used in missile warheads, where a solid-state, explosive thing that looks like a roll of quarters rounded off at one end is employed instead. It uses the old-school polonium-beryllium neutron source scheme, with the two metals explosively mixed together. Sandia Labs in New Mexico has recently developed a new, solid-state neutron generator called the neutristor, based on integrated circuit technology. If used in nuclear weapons, the neutristor would advance bomb technology, developed in the vacuum-tube days of the 1950s, by at least 30 years. Part number for the neutron generator on the MK-28 bomb was MC-890A.
196
There was another problem with nuclear bomb safety to deal with: a bomber crew could go rogue and decide to deploy a bomb or warhead without an order to do so. In early strategic bombers, all the bombardier had to do was push some buttons, and away she goes. To fix this flaw in the system, the Permissive Action Link (PAL) was devised by the Sandia National Laboratories and implemented across the board by September 1962. The system was basically a mechanical combination lock, with the unlocking code known only at the executive level. The USAF Strategic Air Command got around this restriction for the Minuteman ICBM force by having the lock codes set to all zeros, a terrifying fact discovered in a 1977 shakedown.
197
Although the B-36 could deliver the MK-17, there were doubts that it could make it back from the mission. The MK-17 was a high-yield weapon, 15 to 25 megatons, and the B-36 was a remarkably slow aircraft. With all the guns and nonessential equipment stripped out, it could probably reach 423 miles per hour, and at that speed it could not get out of the way of the fireball when the bomb went off, even if the bomb were dropped with a parachute. At the Operation Castle test in 1954, B-36s were flown at reasonable range to a 15-megaton explosion. They suffered extensive blast damage.
198
The mod 2 on the MK-15 bomb was interesting. This modification gave the bomb a piezoelectric contact detonator, so that it would explode when it hit the ground. Most nuclear weapons were made to explode in the air, at least 1,000 feet up, so as to cover as much ground as possible with the shock wave while avoiding making dust. The contact explosion made the mod 2 useful against deeply buried targets. The experimental TX-15 version, code-named