The history of the Delta rocket begins with the PGM-17 Thor, which was an intermediate range ballistic missile (IRBM) with a range of 2,400km. It was designed to be able to hit Moscow from a launch site in the UK, and first flew in 25 January 1957, seven months after Douglas Aircraft was contracted to build it.
The missile used an LR-79 engine for its first (and only) stage, along with two LR-101 Vernier engines, for roll control. Its propellants were LOX and RP-1.
However, the missile was modified many times between this and the Delta, including changing the engine, and I won't go over them all.
The engine of missile 101 failed immediately after lifting off the pad, and the rocket fell back to the pad and exploded. It was determined that the failure was due to debris in the engine, from a LOX filling line that crews dragged over a patch of sand.
Missile 102 was erroneously destroyed by range safety (footage later in the above video), missile 103 exploded four minutes before the planned launch, and 104 broke up due to an electronics failure.
105 finally succeeded, but the Thor missile remained unreliable for many flights, mostly due to its turbopump design, which was eventually fixed.
The only other notable event in the Thor's history was the launch Bluegill Prime, on July 26th 1962, as part of Operation Fishbowl, which was a series of upper atmosphere nuclear weapons tests.
The rocket exploded on the pad, destroying the nuclear warhead and contaminating the pad with plutonium.
Thor was modified with several upper stages for use as a orbital launch vehicle. This first was Thor-Able (Able is the name of the upper stage, but was named Able because it is the first modification of Thor, and the first in the allied military phonetic alphabet).
The next variant was not Thor-Bravo, but Thor-Agena. Agena was an already existing upper stage/satellite bus, with stabilization, communications, and power built in. When those were removed, it was known as an Ascent Agena. It used Bell 8048, 8081, and 8096 engines burning JP-4 and IRFNA, on Agena -A, -B, and -C respectively. Attitude control was provided by nitrogen-freon cold gas thrusters.
Then the Thor-Ablestar was designed, which was the same as Thor-Able, but with a larger Able upper stage.
After this, the Thor-Delta was built. This is the first Delta rocket, which will be henceforth referred to as the Delta-A. The first stage used a Rocketdyne MB-3 engine, and the second stage (derived from the able upper stage) used an AJ-10-118 engine burning hydrazine and nitric acid. It first flew on May 13 1960, with a solid third stage. The first flight was a failure, but the second flight, in August, successfully launched NASA's first communications satellite, Echo 1A, ("A" due to the previous launch being a failure) into orbit.
