Delta Missiles

In 1959, NASA's Goddard Space Flight Center commissioned Douglas to create a civilian launch vehicle based on Thor-Able and to produce and integrate 12 launch vehicles. As this would be the fourth modification of the Thor vehicle (after Able, Able-Star, and Agena), it would be called Delta (radio code word for the fourth letter of the alphabet).

The original Thor-Delta used a Thor booster powered by a Rocketdyne MB-3 engine burning liquid oxygen and kerosene; and upper stages derived from Vanguard: the Aerojet AJ-10-118 (nitric acid/hydrazine) second stage previously used in Thor-Able, and the Allegheny Ballistics Laboratory X-248 solid motor as a third stage. This continuing use of available parts allowed Delta to be ready just 18 months after receiving the go-ahead. It could place a 100 pound payload in geostationary transfer orbit, though it was used for lower orbits only, and was initially regarded as an interim vehicle until more powerful rockets could be developed.

In response perhaps to the successes of the first batch, in 1962 Douglas Aircraft began a series of upgrades and modifications which would increase Delta's capacity tenfold. The Thor name was dropped from subsequent models, and eventually 34 different versions were developed.

As it was never used in the manned space flight program, Delta never received the name recognition enjoyed by the Atlas, Titan and Jupiter; it has been the unsung workhorse of the US space program. It success rate has been exceptional and includes the following launch "firsts": Echo I in 1960, the first communication satellite in Earth orbit to relay voice and TV signals from one ground station to another; Syncom I in 1963, the first geostationary satellite; and Early Bird in 1965, the first Intelsat (International Telecommunications Satellite Organization) satellite. Delta also launched may Explorer satellites, Pioneer interplanetary probes and most of the satellites in the TIROS and Landstat series.

The Delta A used an improved MB-3 engine (Block II), and the B model was an A with a lengthened second stage using higher energy oxidizer. The C model added a bulbous fairing for greater payload space and used as its third stage the X-258 motor developed for the Scout rocket. The Delta D, also known as the Thrust Augmented Delta, took the C and added three Castor I strap-on solid boosters from the Thrust Augmented Thor-Agena D. These boosters gave the Delta the extra thrust necessary to propel the Syncom satellites into geosynchronous transfer orbits.