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.