Titan rockets where initially developed as intercontinental ballistic missiles but were re purposed as expendable space launch vehicles
Titan I was manufactured for the U.S Air force in the late 1950's and fuelled by a mixture of kerosene and liquid oxygen. it was a 2 stage Intercontinental ballistic missile (ICBM), that could deliver a four megaton nuclear war head to targets in the Soviet union, about 8000 km away. The first stage was powered by two LR-87 rocket engines and the second stage used a single LR-91 rocket. The missiles were stored in underground silos and had to be raised to ground level for launch. The Titan I provided greatly needed experience in handling large, multi-stage, ballistic missiles. Experience that would prove useful in later missile programs
Titan II replaced the Titan I in 1965, It was much larger than its predecessor (approximately 30 metres [100 feet] long) and could be launched directly from its silo. It could deliver a nine-megaton warhead—the most powerful nuclear explosive ever mounted on a U.S. delivery vehicle and was the principal weapons in the land-based U.S. nuclear arsenal. Titan II was fueled by internally stored hypergolic fuels (self-igniting liquids such as hydrazine and nitrogen tetroxide). The last Titan II's were decommissioned between 1982 and 1987, and converted by NASA as launch vehicles for Gemini manned spacecraft during the 1960's. Titan II was modified by Lockheed Martin to launch satellites for U.S. government use.
To obtain greater thrust, Titan III was fitted with two additional strap-on boosters burning solid propellants, one on either side of the liquid-fuel first stage.
Several different upper stages could be mounted atop the second stage like the Agena or Centaur.The most successful vehicle was a the Titan IIIE, a combination of the Centaur atop of a Titan III which had a height of 160 feet. This vehicle was used in the 1970's to launch the Viking, Voyager and Helios space probes to Mars, the giant outer planets and the sun respectively. Some other configurations for the titan includes,
Titan IIIA mounted with a Tran-stage upper stage, Titan IIIB mounted with an Agena-D, Titan IIIB Ascent Agena mounted with an Ascent Agena, Titan IIIC mounted with a restart-able trans-stage and fitted with Solid rocket boosters, Titan IIIC MOL mounted with a trans upper stage, Titan III4D with optional upper stage which includes a Tran-stage, an inertial upper stage or no upper stage.
Titan IV was designed for heavy payloads such as the U.S space shuttle. It was fitted with much larger and powerful engines compared to its predecessors.The Titan IV was fitted with two solid rocket boosters and combined with an upper stage a Centaur upper stage. For its time, it was the largest expendable launch vehicle in U.S with a height of 200 feet. Several civilian and military payloads have been delivered into space aboard the Titan IV, most famously is the Cassini-Huygens probe to Saturn in 1997. The last Titan rocket blasted off in 2005