08 December 2006

Flightdeck Friday - Nuclear Fleas

Last week we went big – big planes, big ships, big nukes, big hair (no wait, that’s another story...). This week we go to the other end of the spectrum.

Arms Race - 1950

1950 and the nuclear arms race is well and truly underway. The Soviets had surprised the world with their first nuclear detonation the previous year at Semipalatinsk – a good 5 or more years earlier than the intel community had predicted. In a remote part of Kazakhstan, Sergei Korolev and a group of engineers were embarked on another endeavor which would prove equal, if not greater shock to the Free World seven years later. In the US, the press was on for both the “Super” (a thermonuclear device) and miniaturization of weapons for carriage on a wider variety of aircraft.

At Los Alamos, scientists and engineers of Sandia labs were working on producing a weapon smaller than the Mk 5, which was a more efficient implosion device, similar to “Fat Man” but suitable only for internal carriage on bomber aircraft. The core of the Mk7 was scaled down in size from that of the Mk5, retaining the same 92-point HE (high explosive) geometry of the Mk5. Similar economies of scale were realized with the common components with the Mk5 (e.g., arming, fuzing, and firing equipment of the HE core). Reduced in size (and weight) the now “lighter” Mk7, weighing in at some 1700 lbs, could be placed in a shape streamlined for supersonic, external carriage. Douglas Aircraft Company was contracted to develop the shell and interface for the weapon. Significant changes were required in the approach to arming and safing nuclear weapons now as well with external carriage. Previously, arming was accomplished by in-flight insertion of the physics package, thereby safing the weapon for launch/recoveries. Obviously, that would not be the case with tactical aircraft.

Code-named Thor, the Mk7 would be comprised (initially) of the Mk7 Mod 0 weapon and Mk 5 Mod 0 barometric fuse. The use of the latter ensured the earliest possible entry to operational use, but would require specialized delivery tactics and use of dive brakes on the weapon to ensure subsonic deliveries. Proof testing of the Thor concept was conducted during Operation Buster-Jangle test series, conducted at the (then) new Nevada proving grounds in late 1951 with yields of 3 – 31 Kt. With continued testing and refinement, an “emergency capability” was available by May of 1952. Later in 1952, the USAF embarked on a series of tests with the Low Altitude Bombing System (LABS) that showed the Mk7 could be safely and accurately released at angles of up to 75 degrees. Further work on and incorporation of a radar fuzing system to the barometric and contact/timer fuses formed the basis of the production model Mk7. Full production began in November 1952 and ran until February 1963 with the weapon remaining in the inventory until final retirement in 1967. A total of 17-1800 were produced.

The 'Competition'

While the Air Force was busy adapting some of its tactical aircraft for carriage of the Mk7 (notably the F-84F, -84G and F-100), the Navy was slower to adapt tactical aircraft. In part, this was due to resistance form the heavy attack community (which had the A3D in the pipeline), but a concerted effort on the par to of a core of Naval aviators, chief among them being CAPT Ramage (assigned to the Armed Forces Special Weapons Project at the time) and VADM T. L. Sprague, COMNAVAIRPAC started to turn this thinking around. Envisioning wide deployment among the many Essex class carriers still in the fleet (Note: at the time, only the larger Midway class were “nuclear capable” with their complements of AJ Savages – and these chiefly on the Atlantic coast) they also saw this as a means of forestalling monopoly of nuclear weapons by SAC. A successful demonstration of a Mk7 training shape with an AD on USS Princeton led to a series of briefings by a three man team, led by Ramage and consisting of a couple of academics from Sandia labs (CDR Jack Sloatman and Ted Youngs) set off to convince others of the efficacy of tactical jets carrying nukes off the smaller decks. Eventually they presented their arguments to SECNAV, Dan Kimball who was so impressed he set up an appointment for the team to present their proposal to CNO, who was duly impressed.

In the meantime, Douglas Aircraft was also noticeably busy, and not just from developing the A3D and nuclear weapons aero-shells. Work on a successor to the venerable AD was underway in the form of the A2D Skyshark. Basically starting off as an AD body mated to a turboprop, the XA2D evolved into a different aircraft in its own right. Twin counter-rotating props were mounted to an Allison T-40 turboprop engine. The program, however, was encountering severe setbacks, including one instance where the aircraft shed its props during a test flight. Clearly a replacement was needed and once again, Ed Heinemann led the way with another inspired design.

More Inspired Design from Ed H.

Examining the growing weight and complexity of modern combat aircraft, Heinemann’s team worked to reduce both and provide the Navy a proposal that was half of the 30,000 lb official specification. The Navy placed an order for a prototype and some pre-production model and thus the AD4 (later A-4) Skyhawk was born. With a small wingspan based on a modified delta planform, the AD4 shed the requirement for folding wings (wing span was a mere 27 ft – compare to the A3D at 72 ft). Without the compromise required of folded wings, the AD4 sported a much stronger, 3-spar wing with integral fuel tanks. The weight saving paid off in the form of a weight-carrying capability of around 5,000 lbs of mixed conventional and nuclear ordnance and fuel tanks. Working on the design of the Mk7, Douglas Aircraft had an idea of the compromises required by the size of the tactical nuke, especially when examining older aircraft designed before the Mk7 and the carriage challenges it provided (for example, the struts on the F2H-2 had to be over-inflated to accommodate the Mk7 and also carried a significant recovery-weight constraint). The characteristic “stilt” landing gear of the AD4 was but one direct result of addressing these limitations.

The XA4D-1 prototype first flew on 22 Jun 1954 and was followed by the first of the pre-production models in August of that year. Production AD4-1’s followed in September 1956 with initial deliveries to VA-72. The AD4-2 (later A-4B) differed form the -1 models in many details, chief of which were a slightly more powerful J65 engine (7800 lb static thrust vs. 7700 lb s.t.) and most importantly, in-flight refueling capability. The A-4B provided a significant boost to the Navy’s nuclear capabilities when it deployed. By the time of the Cuban Missile Crisis, the Navy had the capability to deploy nuclear strikes across a broad range of mission areas – from long-range penetration as provided by the A3D (and soon, the new A-6 Intruder) from the big deck super carriers to short/medium range from Essex class CVs as provided by the Skyhawk and Skyraider. Of course these were also available from the big decks as well.


(From Nuclear Weapons and Aircraft Carriers):

Practicing the delivery tactics was fun with the LABS. A pilot approached the target at an altitude as low as he wanted, at a speed of 500 knots for jets. As he crossed over the IP or the target itself, depending on if he was going to loft/toss or do the OTS (Over The Shoulder) delivery, he pushed a bomb-release pickle on the control stick and started a 4-G pull up, now concentrating on the LABS instrument in the cockpit. The instrument gave an indication of wings level and vertical directional stability, plus the amount of deviation from the required 4-G force level. If the plane were steady in the pull-up, the crosshairs on the instrument remained in perfect horizontal/vertical presentation. When the bomb release pickle was pressed, a whistling tone was broadcast in the pilot's earphones and remained on until the bomb released, at approximately 45 degrees for the loft/toss maneuver and 110 degrees for the OTS. At that time the pilot could start his escape maneuver, rolling out or performing whatever routine he elected for escape.

Passing the Torch

However, change was afoot and soon the primary nuclear attack mission would be assumed by Polaris-equipped SSBNs which could provide a secure deterrent from their submerged patrols. Nevertheless, the conventional capabilities of these same aircraft would soon serve requirements in the skies over Southeast Asia very well. Nuclear capabilities would still be maintained by carriers through the remainder of the Cold War, finally being removed at the end of that period with the dissolution of the Soviet Union.


Hansen, Chuck. U.S. Nuclear Weapons: The Secret History, 1988.

Miller, Jerry. Nuclear Weapons and Aircraft Carriers: How The Bomb Saved Naval Aviation. Smithsonian Institute Press, 2001.

Swanborough, Gordon and Bowers, Peter M, United States Navy Aircraft since 1911. Naval Institute Press, 1990.

Cochran, Thomas; Arkin, William and Hoenig, Milton M. Nuclear Weapons Databook: Volume 1, U.S. Nuclear Forces and Capabilities. Ballinger Publishing, 1984.