The Douglas (later McDonnell-Douglas) A-4 Skyhawk -- Part I:
The original Douglas A4D (A = attack, 4 = 4th model, D = Douglas) was designed to deliver a single nuclear bomb when launched from an aircraft carrier or Marine Corps airfield. Douglas had learned their lesson during the latter part of World War II: Simplicity was the key. The A4D (later A-4) was simplicity taken to the extreme. A turbojet engine (the Wright J65), a semi-delta wing of such compact dimensions that the normal Navy wing-folding was not required, simple avionics, etc. The price per aircraft was much less than contemporary jets. There were three store stations that could be used for external fuel or weapons; the initial concept was for a nuclear bomb on the centerline and fuel tanks under each wing. The fuel capacity for this diminutive attack plane was limited and you will note that photos of the A-4 almost always include one or two (and sometimes three) external fuel tanks.
Navy/Marine attack aviation had essentially been the AD (later A-1) Skyraider since just after WW2; the A-4 brought attack aviation into the jet age.The program started in 1952 and the prototype first flew in June of 1954.
The Navy bought 166 production A4D-1s (later A-4As) and named the aircraft the Skyhawk. The -1 was soon superseded on the production line by the -2 (A-4B) and the Navy and Marines went big on this model, taking delivery of 542 aircraft. The A4D-2 introduced a couple of changes, the most notable of these being an aerial refueling probe to allow extended range and endurance. The A4D-1s were soon relegated to secondary roles.
Simple was good, but the services wanted more capability, and they got it with the next model, the A4D-2N (later A-4C). The -2N was the most-produced Navy/Marine variant with 638 built. The A4D-2N featured a slightly lengthened nose with a lightweight radar and had an autopilot. By the 1960s, Douglas ruled the light attack skies with the A-4B and A-4C.
The A-4 was not perfect, though -- there were engine problems with the Wright J65. (My Dad had to eject from an A4D in 1958 during an air show, no less, when the engine quit on him.) Like defense contractors always do, Douglas offered more capability -- for more money, of course. There was even a proposal to abandon the simple wing for swept wings with folding as the A4D-4, but the Navy didn't bite.
Pratt & Whitney, meanwhile, had come up with a new scaled-down version of the outstanding J57 jet engine. This smaller engine -- the J52 would be a perfect fit in the A4D and would offer greater thrust. The Navy finally agreed to a new A4D model, the A4D-5 (later A-4E). The -5 was a major step forward and I'll discuss it and other more advanced models in the next part.
The designations -- old versus new -- are confusing and from now on I will stick to the new designations; the A4D-1 became the A-4A, the A4D-2 became the A-4B and the prolific A4D-2N became the A-4C, all in late 1962. Stay tuned.
A note on the photos: The second photo shows a variety of stores that could be carried by the A4D-1 including two different nuclear bombs, as well as fuel tanks and conventional bombs and rockets. The third photo shows one A4D-2 with 3 tanks refueling another carrying a Mark 12 practice bomb. The last photo shows a Marine Corps A-4C using JATO rocket assist for short takeoff; until adequate airfields could be built, Marine A-4s used this technique when first deployed to Vietnam in 1965-66.
https://en.wikipedia.org/wiki/Douglas_A-4_Skyhawk