Silverplate

Silverplate B-29 Straight Flush. Note lack of gun turrets and tail code of 444th BG painted on for security reasons.

Silverplate was the code reference for the United States Army Air Forces participation in the Manhattan Project during World War II. Originally the name for the aircraft modification project for the B-29 Superfortress to enable it to drop an atomic weapon, Silverplate eventually came to identify the training and operational aspects of the program as well. The airplane modification project fell under the purview of Project Alberta after March 1945. The original directive for the project had as its subject line "Silver Plated Project" but continued usage of the term shortened it to the one word "Silverplate".

Between February 1944 and December 1947 a total of 65 B-29s were modified to Silverplate specifications in five increments. Ultimately 53 of them served with the first nuclear weapons unit, the 509th Composite Group.

Initial phase

The project was initiated in October 1943 when Dr. Norman F. Ramsey, a member of the Los Alamos National Laboratory Group E-7, identified the B-29 Superfortress as the only airplane in the U.S. inventory capable of carrying either type of the proposed weapons shapes: the tubular "gun-type fission weapon" shape and the oval plutonium implosion weapon shape. Furthermore, because the attachment box for the main wing spars was located between the bomb bays on the B-29, the gun-type weapon could only be a maximum of 2 ft (0.6 m) in diameter. Prior to the decision to use the B-29 serious consideration had been given to using the British Avro Lancaster to deliver the weapon, which would have required much less modification, but the idea was vetoed by General Groves who thought it "beyond comprehension to use a British plane to deliver an American A-bomb".

USAAF sent instructions to its Army Air Forces Materiel Command at Wright Field, Ohio, on November 30, 1943, for a highly-classified B-29 modification project. The Manhattan Project would deliver full-sized mockups of the weapons shapes to Wright Field by mid-December, where AAFMC would modify an aircraft and deliver it for use in bomb flight testing at Muroc Army Air Field, California.

B-29-5-BW 42-6259 (referred to as the "Pullman airplane" from an internal code name assigned it by the Engineering Division of AAF Materiel Command) was delivered to the 468th Bomb Group at Smoky Hill AAB, Kansas on November 30, 1943, and flown to Wright Field, Ohio, on December 2. Modifications to the bomb bays were extensive and time-consuming. Its four 12 ft (3.7 m) bomb bay doors and the fuselage section between the bays were removed and a single 33 ft (10 m) bomb bay configured (the length of the gun-type shape was approximately 17 ft (5.2 m)). New bomb suspensions and bracing were attached for both shape types, with the gun-type suspension anchored in the aft bomb bay (although its length protruded into the forward bay) and the implosion type mounted in the forward bay. Separate twin-release mechanisms were mounted in each bay, using modified glider tow-cable attach-and-release mechanisms. To document the tests, motion picture camera mounts were installed in the rear bay.

The modifications were made using the dummy bomb shapes as models, and the gun-type shape (code-named Thin Man) proved to be a very close fit. All modifications were made by hand and the process required more than 6000 man-hours of labour which could not be completed until February. Engine problems systemic to the B-29 delayed delivery of the Pullman B-29 for flight testing until February 20, 1944.

Testing of bomb shapes

The first test drop at Muroc on March 6 involved a Thin Man, followed on March 14 by two drops of an implosion device shape (codenamed Fat Man) fitted with a circular tail fin stabilizer designed by engineers at the National Bureau of Standards. The Thin Man performed without major problems but the Fat Man shapes exhibited significant wobble characteristics, apparently due to poor workmanship and misalignment of the tail fins.

All three bombs had also failed to release immediately, frustrating calibration tests. A fourth testing flight resulted in the premature release of a Thin Man shape while the B-29 was still en route to the test range and severely damaged the aircraft. The modified glider mechanisms had apparently caused all four malfunctions, because of the weight of the bombs, and were replaced with British Type G single-point attachments and Type F releases.

After repair of the Pullman B-29 at Wright Field, testing resumed with three Thin Man and nine Fat Man shapes dropped in the last two weeks of June 1944. Various combinations of stabilizer boxes and fins were tested on the Fat Man shape to eliminate its persistent wobble until an arrangement dubbed a "California Parachute", a cubical tail box with fins angled at 45° to the line of fall, was approved.

The Thin Man gun-type design was based on the fissibility of the very pure isotope produced in microgram quantities by the Berkeley cyclotron. When the Hanford production reactors came on-line in the spring of 1944, the mix of ²³⁹
Pu
and obtained was found to be much more fissile. To avoid pre-detonation, the muzzle velocity would need to be greatly raised, making the bomb impractically long. The weapon was re-designed to use . The muzzle velocity required for a ²³⁵
U
fission reaction is much lower, reducing the barrel length of the resulting bomb (now code-named Little Boy) to less than 10 ft (3.0 m). This allowed the device to fit into a standard B-29 bomb bay.

The Pullman was modified to its original configuration with the rear bomb bay a standard B-29 design. All subsequent Silverplates were also configured in this manner. The Pullman B-29 was flown to Wendover and assigned to further drop testing in September 1944 with the 216th Base Unit until it was damaged in a landing accident in December.

Wartime production versions

File:Enola Gay.gif

B-29 Enola Gay, 31st Silverplate Superfortress

On August 22, 1944, to meet the requirements of the USAAF group about to be formed to train in the atomic mission, a production phase of Silverplate B-29s was ordered under the designation Project 98146-S from the Glenn L. Martin Company's modification center at Omaha, Nebraska. In mid-October the first three of these second increment Silverplate B-29s were delivered to the USAAF and flown to Wendover Army Airfield, Utah. They were fitted with British single-point bomb releases mounted on a re-designed H-frame suspension rack fitted in the forward bomb bay, so that additional fuel tanks could be carried in the aft bay. A new crew position, called the "weaponeer station", was created in the cockpit with a panel to monitor the release and detonation of the bomb during the actual combat drops. Fourteen production aircraft were assigned to the 393rd Bomb Squadron and three to the 216th Base Unit for bomb drop testing.

By February 1945 the seventeen aircraft of the second increment were themselves in need of upgrades, particularly those of the 216th Base Unit. Four of the planes assigned to the 393rd Bomb Squadron (now a squadron of the 509th Composite Group) were immediately transferred to the 216th to meet an increase in its testing tempo. Rather than attempt to modify the existing aircraft a few at a time, a decision was made to start a new production series. The first five of this third increment, known as Project 98228-S, also went to the test unit. The order totalled an additional 28 aircraft, with delivery of 15 designated combat models for the 393rd Bomb Squadron beginning in April; the final 8 were not delivered until after the atomic bomb missions in August.

The final wartime Silverplates incorporated all technical improvements to B-29 aircraft, as well as the final series of Silverplate modifications that included fuel-injected Wright R-3350-41 engines, Curtiss Electric reversible-pitch propellers, and pneumatic actuators for rapid opening and closing of bomb bay doors. Weight reduction was also accomplished by removal of all gun turrets and armor plating. These B-29s represented a significant increase in performance over the standard variants.

Silverplate operational units

Including the Pullman B-29, a total of 65 Silverplate B-29s were produced both during and after World War II. Twenty-nine of these were assigned to the 509th Composite Group during World War II, with 15 used to carry out the atomic bombings of Hiroshima and Nagasaki. An additional twenty-four were assigned to the group for post-war operations as the 509th Bomb Group. Fifty-seven Silverplates were produced by Martin-Omaha and 8 by Boeing-Wichita. Thirty-two were eventually converted to other configurations, 16 were placed in storage and later scrapped, and 12 were lost in accidents (including four of the Tinian bombers). Two, the Enola Gay and Bockscar, are displayed in museums.

The only other USAF combat unit to use the Silverplate B-29 was the 97th Bomb Wing at Biggs Air Force Base, El Paso, Texas. In the summer of 1949 it received 27 of the aircraft from the 509th Bomb Wing when the latter transitioned to B-50D bombers, but within a year all were converted to TB-29 trainers. One other Silverplate B-29, on temporary assignment in the United Kingdom was converted into a weather reconnaissance aircraft (WB-29) and transferred to the 9th Bomb Wing at Travis Air Force Base, California.

Followup program

The U.S. Joint Chiefs of Staff issued a directive in January 1948 for the modification of 225 B-29, B-50, and B-36 bombers to carry nuclear weapons. The project was intended to be completed by December 1948 and was known by the code name "Gem". The modification of 80 B-29s was authorized by Air Materiel Command project DOM-595C, known as Saddletree.

The project included "winterization" of 36 B-29s for operating from Arctic bases, and the modification of 36 others to have an air refueling capability under project DOM-599C ("Ruralist"). With the addition of the 80 Saddletree bombers, a total of 145 B-29s were modified to carry nuclear weapons, and 117 of these were assigned to operational units.