The earliest launch vehicles used by the Air Force were Thor and Atlas missiles modified by the Air Force Ballistic Missile Division and Space Systems Division to serve as space boosters. Indeed, the Air Force achieved its first success in space with Project Score, an Atlas booster containing a communications repeater that transmitted President Eisenhower's Christmas message to the world in December 1958. Thor and Atlas missiles with only minor modifications continued to be used as space boosters for a long time, especially for military and civilian weather satellites. The last Thor launch occurred in July 1980, and the last launch of a modified Atlas missile occurred in March 1995, with both boosters carrying military weather satellites. As time went by, Thor and Atlas vehicles were improved and standardized, and families of Standard Launch Vehicles were created. The Thor gave rise to the series known as Standard Launch Vehicle 2, and the Atlas gave birth to the several varieties of Standard Launch Vehicle 3. Upper stages such as the Agena, the Burner II, and the Stage Vehicle System were developed for use with these vehicles. Together with their associated upper stages, Thor and Atlas launch vehicles once constituted the backbone of the US space program.

A Thrust-Augmented Thor (Standard Launch Vehicle 2A) lifting off from its launch pad at Vandenberg AFB. This version of the Thor employed three strap-on solid rocket motors that almost doubled the thrust of the core vehicle.

An Atlas Agena booster sitting on the launch pad at Cape Canaveral. This particular booster launched NASA's Mariner II spacecraft in August 1962.

The launch vehicles developed by the Ballistic Missile Division and its successors were used not only by the Air Force but also by the National Aeronautics and Space Administration, created in 1958. Civilian programs began using boosters based on the Thor missile immediately, and in 1959, NASA began developing the Delta upper stage for it--the first step in developing the highly successful Delta launch vehicle. NASA started using the Atlas vehicle in 1959, and its first manned space program, Project Mercury, relied on the Atlas for its orbital flights. Project Gemini, the agency's next manned program, employed Titan II boosters developed and procured by Space Systems Division. The Gemini Target Vehicle, an Agena upper stage, was also developed by Space Systems Division. The Agena was later modified by NASA and employed extensively by both agencies. The Centaur upper stage, the most powerful upper stage in the national inventory, was born as an Air Force program before being transferred to NASA in 1960. It is noteworthy that much of this cooperation in developing and using launch vehicles was the result of a carefully considered series of written agreements--initiated in 1959 and expanded during the early 1960s--which made up a National Launch Vehicle Program.

An Atlas booster lifting off the pad at Cape Canaveral with a Gemini Agena Target Vehicle as its payload.

A Titan II booster being launched from the Cape with a Gemini capsule as its payload.

A Gemini Agena Target Vehicle in space, photographed by astronauts aboard a Gemini capsule.

An artist's conception of a Gemini capsule and an Agena Target Vehicle joined together in space. In the Gemini missions of 1966, manned Gemini capsules accomplished rendezvous and docking with unmanned Agena Target Vehicles, perfecting techniques that would be used later in the Apollo program.

Thor and Atlas boosters were complemented by the Titan III--a powerful booster capable of launching large, heavy payloads. Development of the Titan III was initiated in late 1961, and the first research and development vehicle was flown in September 1964. This vehicle, a Titan IIIA, consisted of a modified Titan II core topped by an upper stage called the Transtage. A new configuration, the Titan IIIC, was successfully launched from Cape Canaveral in June 1965. The IIIC used two strap-on solid rocket motors that generated around one million pounds of thrust each. From 1965 through 1989, Titan III vehicles performed well in a wide variety of missions and configurations. The family expanded to include the Titan IIIB/Agena D, the Titan IIID, and the Titan IIIE/Centaur, which was used by NASA for space projects such as the Viking missions to Mars. The final variety of Titan III, the Titan III (34)D, was used during the 1980s as a backup and alternative to the manned Space Shuttle. The last 34D was launched in September 1989.

Four versions of the Titan III: the IIIB (far left), the IIIC (center left), the IIID (center right), and the IIIE (far right). The IIIB consisted of an unaugmented Titan core and was used with an Agena D upper stage. The remaining versions employed a Titan core augmented by two strap-on solid rocket motors. The IIIC was sometimes used with a Transtage upper stage and the IIIE used a Centaur upper stage. The IIID did not employ an upper stage.

During the 1970s, NASA developed a Space Transportation System employing a manned, reusable Space Shuttle to replace most expendable launch vehicles. In addition to monitoring the development of the Shuttle to ensure that it would satisfy DOD's requirements, SAMSO contributed several important elements to allow DOD to make full use of the system. It developed and almost completed a launch and landing site at Vandenberg AFB to allow the Shuttle to be launched into polar orbits. It also developed the Inertial Upper Stage (IUS), an upper stage for large Shuttle payloads requiring higher orbits. The IUS was adapted for use with the Titan III and, later, the Titan IV expendable system as well. Although it had a troubled and costly developmental period, the IUS came to be considered one of the most accurate and reliable launch systems ever built.

Launch of the Space Shuttle Columbia from NASA's Kennedy Space Center.

DSP satellite and IUS being deployed from the cargo bay of the Space Shuttle Discovery on 25 November 1991. The launch, which had occurred the day before, was the first unclassified launch of a DSP satellite and the first and only launch of a DSP satellite by the Shuttle.

In January 1986, a Space Shuttle exploded during launch, killing the crew of the orbiter Challenger. NASA was forced to suspend all Shuttle launches while it investigated the cause of the explosion and assessed its implications. Military payloads as well as civilian payloads scheduled for the Shuttle had to obtain launches on expendable boosters or wait. Shuttle flights did not resume until September 1988. The disaster had further implications for SSD. Development of the Shuttle facilities at Vandenberg ended after the disaster because design changes in the Space Shuttle diminished its capability for polar launches.

Although eventually the Air Force was able to shift some of its most critical payloads to Titan vehicles, the Titan program happened to be suffering from launch failures of its own when the Challenger disaster occurred. After consecutive launches of Titan 34Ds failed in August 1985 and April 1986, further launches were suspended while the causes were investigated. They resumed in October 1987, restoring the only available alternative to the Space Shuttle for large payloads.

The Challenger disaster gave added weight to the argument for having a variety of expendable launchers available, so that failures in one type would not again affect so many payloads. Space Division had already begun the development of a larger, more capable Titan booster known as the Titan IV in 1985. Launched for the first time in June 1989, the Titan IV could be used with either an IUS or a newly-developed version of the Centaur upper stage. It was capable of placing 10,000 pounds into geosynchronous orbit using the Centaur. The Titan IV's performance would be considerably enhanced by upgraded solid rocket motors. Their development was delayed when the first qualification motor exploded during a test firing, but they successfully completed the final test firing in September 1993. For some smaller payloads, Space Division began converting the 55 obsolete Titan II ballistic missiles removed from their silos in 1982-1987. They could place about 4,200 pounds into low-earth, polar orbit, and the first was launched in September 1988.

A Titan IVA booster on the pad awaiting launch.

A Titan II booster on the pad at Vandenberg AFB prior to launch.

During the suspension of Shuttle flights, Space Division began procuring two new medium launch vehicles--the Delta II and the Atlas II. Development and production of the Delta II, an improved version of the Delta launch vehicle, began in January 1987. It was procured primarily to launch the constellation of 24 operational Global Positioning System (GPS) satellites, and it did so without a single failure. The Delta II was developed in two consecutive configurations. The first of these launched the first nine GPS satellites from February 1989 to October 1990, while the second, more powerful version launched the later, heavier GPS satellites from November 1990 to March 1994, completing the constellation. During this entire period, a Delta II successfully launched a GPS satellite about every two months, an accomplishment without equal. Delta IIs also launched other payloads, both military and commercial. In April 1993, SMC awarded a contract for additional Delta II launch vehicles to replenish the GPS constellation. Development and production of the Atlas II, an improved version of the Atlas/Centaur launch vehicle, began in June 1988. The Atlas II would be able to launch somewhat heavier payloads in the medium-weight class, and DOD intended it for Defense Satellite Communications System (DSCS) satellites as well as some experimental satellites. It was also used in many commercial launches. The Atlas II launched its first commercial payload in December 1991, and its first DSCS III satellite in February 1992. Although launches of the Atlas II and the Centaur upper stage suffered some delays because of failures that occurred during commercial launches, no failures had occurred during Air Force launches as of September 1995. At that time, the Atlas II was just beginning a period of intense launch activity.

A Delta II booster carrying a Block IIA Global Positioning System satellite being launched from Cape Canaveral AFS in October 1990.

An Atlas II booster carrying a Defense Satellite Communications System Phase III (DSCS III) satellite, on the pad at Cape Canaveral in July 1993.

Programs to develop a new generation of launch vehicles had more trouble getting off the ground. In 1987, the Air Force and NASA had begun a cooperative program to develop a more efficient family of boosters to replace the Space Transportation System and expendable launch vehicles. The program was known at first as the Advanced Launch System and later as the National Launch System before Congress ceased to fund it. In 1993, the Air Force and SMC tried a new, more frugal approach known as the Spacelifter program, which intended to develop a new launcher for medium and heavy payloads using existing technology. Nevertheless, the Secretary of Defense canceled it for reasons of cost later that year. Efforts to develop a new, more efficient launcher received a badly needed endorsement when President Clinton signed a National Space Transportation Policy in August 1994. Among other things, it assigned responsibility for expendable launch vehicles to DOD and directed DOD to prepare for the evolution of an expendable launch vehicle. The response was SMC's Evolved Expendable Launch Vehicle (EELV) program, which aimed to develop a family of launch vehicles for medium to heavy payloads, based on existing vehicles or their components using existing technology. SMC awarded contracts for the initial phase of the EELV program in August 1995. The program was one of the Air Force's standard bearers in streamlined acquisition reform.