It is the Titan-a name synonymous with space exploration and national defence.

First developed as an intercontinental ballistic missile (ICBM), the Titan family of launch vehicles was born in October 1955 when the U.S. Air Force awarded a contract to the Martin Company (which evolved into Martin Marietta and later Lockheed Martin) to build a second ICBM to supplement the still-untested Atlas missile. The result was the Titan I-the United States' first two-stage ICBM and first missile to be based in a hardened silo buried deep underground. 

 
Titan IIs served as the U.S. Air Force's most powerful intercontinental ballistic missile (ICBM) until they were withdrawn from service in 1987.

The Air Force accepted delivery of the first Titan I in June 1958, followed by a test program that culminated in the successful inaugural launch of the missile from Cape Canaveral, Florida, on February 6, 1959. Soon after, in May 1959, the U.S. Army Corps of Engineers Ballistic Missile Construction Office started construction of the first Titan I underground silos at Lowry AFB (Air Force Base), Colorado-each 160 feet (49 meters) deep and 44 feet (13 meters) in diameter, topped off with a pair of horizontal doors on the surface that weighed 125 tons apiece. The first Titan I squadron at Lowry AFB and its complement of nine Titan I's was placed on operational alert in 1962.

Production of an upgraded version of the Titan I with greater range and lifting capacity was initiated in 1958, with the Martin Company again selected as the prime contractor. The new missile, named Titan II, used fuels that could withstand long-duration storage in the missile's fuel tanks, effectively creating an ICBM that could be launched almost instantaneously by eliminating the laborious pre-launch fuelling process. By 1965, all of the original Titan Is deployed at five different bases in the western United States were phased out in favour of the more capable Titan II.

The first Titan II was launched on March 16, 1962 from Cape Canaveral. The missile system promptly achieved all of its test program objectives and was placed on operational status in 1963. NASA, too, was quick to recognize the vehicle's capabilities and ordered a modified version of the Titan II to launch its two-person Gemini spacecraft. Ten crewed Gemini missions were launched during 1965 and 1966 from Cape Canaveral, developing the techniques required for the upcoming Apollo Moon landing program.

 
Titan II launch of Lockheed Martin Missiles Space DMSP satellite from Vandenberg Air Force Base, Ca.

Eventually, more than 140 Titan II ICBMs were manufactured, deployed at two missile squadrons located at Davis-Monthan Air Force Base, Arizona; two at Little Rock Air Force Base, Arkansas; and two at McConnell Air Force Base, Kansas. Titan missiles armed with nuclear warheads remained on strategic alert for 25 years until the last one was decommissioned in 1987. Fourteen of the retired Titan II ICBMs were refurbished for use as space launch vehicles, boosting a variety of military and civilian spacecraft into polar orbits from Vandenberg AFB, California.

In August 1962, the U.S. Department of Defence announced a further enhancement of the Titan. This new vehicle, designated the Titan III, was specially designed as a space launch vehicle (instead of as an ICBM) and could be flown in several different configurations, some of which included a pair of powerful solid rockets strapped-on to the basic Titan core missile.

 
Launch of the Gemini-Titan 3 mission, 1965.

The Titan IIIC variation (with the twin solid rocket motors) was intended to launch the Air Force's planned X-20 Dyna Soar piloted spacecraft as well as a variety of heavy unpiloted military satellites. A massive complex of launch facilities and assembly buildings was constructed at Cape Canaveral to support the Titan III program, which was projected for up to 40 launches per year.

The first Titan III vehicle (designated Titan IIIA), consisting of the core vehicle without the strap-on solid rockets, was launched from Cape Canaveral on September 1, 1964, followed by three additional test launches in 1964 and 1965. A more efficient and cost effective version, the Titan IIIB, was created by the introduction of an Agena upper stage-its first launch occurred on June 29, 1966, from Vandenberg AFB, California.

A total of 59 Titan IIIBs was launched between 1966 and 1987 exclusively from Vandenberg, all carrying classified reconnaissance spacecraft. An improved version with a stretched core, the Titan 34B (also equipped with an Agena upper stage), was flown 11 times between 1975 and 1987, all from Vandenberg and again exclusively launching classified intelligence spacecraft.

The first Titan III vehicle with the twin solid rocket motor configuration (designated the Titan IIIC) was launched on June 18, 1965, from Cape Canaveral, but its original payload, the X-20 Dyna Soar, had been cancelled about 18 months earlier in December 1963. In its place, a more powerful Titan III variation was proposed to propel the Air Force's newest human spaceflight project, the Manned Orbiting Laboratory, formally approved by President Lyndon B. Johnson on August 25, 1965.

Designated the Titan IIIM (“M” for manned), the new launch vehicle would be “man-rated” (that is, incorporating numerous safety features to permit its use by humans) and powered by a pair of improved seven-segment solid rocket motors. The Titan IIIM was never flown-the Manned Orbiting Laboratory program was cancelled in June 1969-but the technologies developed would be reused in later versions of the Titan.

The Air Force continued to refine the Titan III configuration by stretching its core vehicle, adding a variety of different upper stages and upgrading its twin solid rocket motors. These vehicles, launched from both Cape Canaveral and Vandenberg under the designations of Titan IIID and Titan 34D, carried a wide range of military and intelligence spacecraft into orbit for almost a quarter-century.

 
Titan IV launch of second Milstar satellite from Cape Canaveral, Fl.

NASA also used a Titan III variation equipped with a powerful Centaur upper stage, labelled the Titan IIIE, in the mid-1970s to launch two Helios missions to the Sun, two Viking Mars landing expeditions as well as the Voyager I and II spacecraft to explore the outer planets. A commercial variation of the Titan 34D, known as Commercial Titan, was launched four times in the early 1990s, but proved too costly for practical purposes. In all, a total of 84 Titan III variants with strap-on solid rocket motors were launched from 1965 to 1992.

Built to boost the heaviest national security spacecraft into orbit-satellites so large that they were effectively grounded after the Shuttle Challenger exploded-the Titan IV (later designated the Titan IVA) was first launched in 1989, incorporating the solid rocket motor designed for the Titan IIIM of the 1960s. Flown with payload shrouds of varying dimensions and three different upper stage configurations (Centaur, Inertial Upper Stage, or No Upper Stage), Titan IVs are launched from both Cape Canaveral and Vandenberg.

Equipped with more powerful and reliable solid rocket motors, the Titan IVB was first flown in 1997, and is used exclusively to launch military and intelligence spacecraft, although NASA did use one Titan IVB (equipped with a Centaur upper stage) to launch the Cassini and Huygens spacecraft to Venus in 1997. Capable of lifting more than 47,000 pounds (21,319 kilograms) into orbit, the Titan IVB is also the most costly rocket ever constructed-some custom-built models cost more than $400 million each.

Lockheed Martin delivered the last Titan IV heavy-lift launch vehicle to the U.S. Air Force in early 2002, ending a string of missions that began in February 1959 and a heritage dating back to the earliest days of the space age. The final Titan IVB launches are scheduled to occur in 2003. Lockheed Martin is phasing out the Titan IV in favour of its new Evolved Expendable Launch Vehicle (EELV) designated as the Atlas V.