I.

Introduction

Delta (rocket), family of rockets that began service as satellite-launching vehicles for the United States government in 1960. By the year 2000, Delta rockets had successfully launched more than 250 missions. These missions included the launch of Telstar 1, the first telephone and television communications satellite, in 1962, and the launch of 24 Global Positioning System (GPS) satellites for navigation, beginning in 1989.

II.

Delta I

Several generations of Delta rockets have been developed to meet the changing needs of government and private industry. Development of the first generation of Delta rockets, Delta I, was sparked by the launch of the first artificial satellite, Sputnik 1, by the Soviet Union in 1957. In 1959 the National Aeronautics and Space Administration (NASA) contracted with Douglas Aircraft Company to build Delta rockets. The first Delta I rocket launched in 1960, but the rocket failed to deliver its payload, the Echo 1 communications satellite, into Earth orbit. The Delta program recovered and successfully launched a rocket later that year. Early Delta rockets had two separate stages, or engines. After the first several minutes of flight, the lower stage burned out and fell away, leaving the upper stage to lift the craft the rest of the way to its destination in space. Delta I was initially known as the Thor-Delta because its first stage was based on the technology of its predecessor, the Thor rocket, developed in 1957 by the United States Air Force.

The future of Delta rockets became uncertain in 1972, when NASA announced plans to develop the space shuttle. Unlike Delta rockets, which could be used only once, the space shuttle was reusable. NASA planned to use the space shuttle for missions previously performed by Deltas and other expendable rockets. By 1979, however, development of the space shuttle had fallen behind schedule. The Air Force contracted with McDonnell Douglas Corporation to upgrade Delta to launch payloads previously intended for the shuttle.

As satellites became larger and heavier, Delta underwent numerous upgrades to boost its payload capacity. Between 1960 and 1982, 34 versions of the Delta I were developed. The first Delta I rocket carried a maximum cargo of 45 kg (99 lb); the final version of the series, the Delta 3920/PAM, could carry 1,312 kg (2,892 lb). Upgrades to the design of Delta I improved several aspects of the rocket. The addition of a third stage and booster rockets improved lift and range. The propellant tanks were expanded to increase fuel capacity, and payload areas were made larger to hold bigger cargoes.

III.

Later Delta Models

In the 1980s Delta rockets declined in importance as the government and industry relied increasingly on space shuttles for launching cargo. After the space shuttle Challenger exploded in 1986, however, NASA eliminated most of the shuttle program’s satellite-launching operations. The following year, the Air Force contracted with McDonnell Douglas to create a second generation of Delta rockets. The first Delta II rocket launched in 1989.

The Delta II 7925, first launched in 1990, became the most widely used rocket of the Delta II series. It stands approximately 38 m (125 ft) tall and measures about 5 m (15 ft) across its base. The rocket’s weight at liftoff is about 230,000 kg (506,000 lb). The first stage burns liquid kerosene and liquid oxygen and features as many as nine strap-on booster rockets to provide extra power. The first six booster rockets burn out and fall away one minute after liftoff, then the remaining three booster rockets ignite and carry the rocket farther into space. The second stage burns a specialized liquid fuel, called Aerozine 50, and liquid nitrogen tetroxide.

Delta II 7925 can deliver a payload weighing 5,140 kg (11,330 lb) into an orbit with an altitude of about 160 km (100 mi). When modified with a third stage, it can send a payload of up to 1,870 kg (4,120 lb) into geostationary orbit. Satellites in geostationary orbit circle the Earth at an altitude of 35,700 km (22,200 mi), appearing to stay in one place because their orbit speed matches the rate of Earth’s spin. Modified versions of the Delta II 7925 can also launch spacecraft weighing as much as 1,300 kg (2,900 lb) beyond Earth’s orbit. In 1989 a Delta II launched the Cosmic Background Explorer (COBE), an astronomical satellite that gathered evidence about the beginnings of the universe. In 1996 Delta II rockets launched NASA’s Near-Earth Asteroid Rendezvous (NEAR) spacecraft, Mars Global Surveyor 96 spacecraft, and Mars Pathfinder spacecraft.

In the late 1990s the Boeing Company developed the Delta III to carry satellites weighing as much as 3,800 kg (8,400 lb), twice the capacity of Delta II, into geostationary orbit. The first Delta III exploded shortly after takeoff in August 1998, but a later Delta III lifted off successfully in October 1999. At the end of the decade, Boeing began work on a fourth generation, the Delta IV, scheduled for launch in 2002. The largest of the series, the Delta IV Heavy, was expected to carry payloads weighing as much as 13,200 kg (29,100 lb) into geostationary orbit.