The series was phased out in favor of the Vulcan Centaur, with the Delta IV Heavy rocket's last launch occurring on April 9, 2024.

[1] The original Delta rockets used a modified version of the PGM-17 Thor, the first ballistic missile deployed by the United States Air Force (USAF), as their first stage.

[2][3] NASA intended Delta as "an interim general-purpose vehicle" to be "used for communication, meteorological, and scientific satellites and lunar probes during 1960 and 1961".

From this point onward, the launch vehicle family was split into civilian variants flown from Cape Canaveral, which bore the Delta name, and military variants flown from Vandenberg Air Force Base (VAFB), which used the more warlike Thor name.

The Delta design emphasized reliability rather than performance by replacing components that had caused problems on earlier Thor flights; in particular, the trouble-prone inertial guidance package made by AC Spark Plug was replaced by a radio ground guidance system, which was mounted to the second stage instead of the first.

NASA made the original Delta contract to the Douglas Aircraft Company in April 1959 for 12 vehicles of this design:[citation needed] These vehicles would be able to place 290 kg (640 lb) into a 240 to 370 km (150 to 230 mi) LEO or 45 kg (99 lb) into GTO.

Eleven of the twelve initial Delta flights were successful, and until 1968, no failures occurred in the first two minutes of launch.

The high degree of success achieved by Delta stood in contrast to the endless parade of failures that dogged West Coast Thor launches.

The Delta B introduced the upgraded AJ10-118D upper stage, a three-foot propellant tank extension, higher-energy oxidizer, and solid-state guidance system.

The Altair 2 had been developed as the ABL X-258 for the Scout vehicle and was 3 in (76 mm) longer, 10% heavier, and with 65% more total thrust.

[citation needed] The Delta M first stage consisted of a Long Tank Thor with MB-3-3 engine augmented with three Castor 2 boosters.

[11] A license-built version of the Long Tank Thor stage with the MB-3 engine was also used for the Japanese N-I launch vehicle.

[11] The Extended Long Tank Thor stage was also used in the Japanese N-II and H-I launch vehicles.

The Delta 2000 introduced the new Rocketdyne RS-27 main engine on an Extended Long Tank first stage with the same constant 8-foot diameter.

The first stage had the MB-3 main engine and Extended Long Tank of the 3000-series and mounted upgraded Castor 4A motors.

[citation needed] The Delta 5000 series featured upgraded Castor 4A motors on an Extended Long Tank first stage with the new RS-27 main engine and only launched one mission.

The Delta 6000-series introduced the Extra Extended Long Tank first stage, which was 12 ft (3.7 m) longer, and the Castor 4A boosters.

[citation needed] The Delta 7000-series introduced the RS-27A main engine, which was modified for efficiency at high altitude at some cost to low-altitude performance, and the lighter and more powerful GEM-40 solid boosters from Hercules.

As part of the Air Force's Evolved Expendable Launch Vehicle (EELV) program, McDonnell Douglas / Boeing proposed Delta IV.

[23] The Delta was a launch success, but it has also been a significant contributor to orbital debris, as a variant used in the 1970s was prone to in-orbit explosions.

The nature of the propellant and the thermal environment occupied by the derelict rockets made explosions inevitable.

The new system could better accommodate the various changes and improvements to Delta rockets and avoided the problem of a rapidly depleting alphabet.

The digits specified (1) the tank and main engine type, (2) number of solid rocket boosters, (3) second stage (letters in the following table refer to the engine), and (4) third stage:[25] This numbering system was to have been phased out in favor of a new system that was introduced in 2005.