Delta (rocket family)

Delta origins

The original Delta rockets used a modified version of the PGM-17 Thor, the first ballistic missile deployed by the United States Air Force, as their first stage. The Thor had been designed in the mid-1950s to reach Moscow from bases in Britain or similar allied nations, and the first wholly successful Thor launch had occurred in September 1957. Subsequent satellite and space probe flights soon followed, using a Thor first stage with several different upper stages. The fourth upper stage used on the Thor was the Thor "Delta," delta being the fourth letter of the Greek alphabet. Eventually the entire Thor-Delta launch vehicle came to be called simply, "Delta."

The Delta name stems from its position as the fourth, or D version, of the Thor based rocket combination. The vehicle has been known both as Thor-Delta and simply Delta.

NASA intended Delta as "an interim general purpose vehicle" to be "used for communication, meteorological, and scientific satellites and lunar probes during '60 and '61". The plan was to replace Delta with other rocket designs when they came on-line. 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 which used the more warlike Thor name. The Delta design emphasized reliability rather than performance by replacing components which had caused problems on earlier Thor flights, in particular the trouble-prone AC Spark Plug inertial guidance package which was replaced by a radio ground guidance system mounted to the second stage rather than the Thor itself. NASA let the original Delta contract to the Douglas Aircraft Company in April 1959 for 12 vehicles of this design:

These vehicles would be able to place 650 pounds (290 kg) into a 150 to 230 miles (240 to 370 km) LEO or 100 pounds (45 kg) 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 total project development and launch cost came to $43 million, $3 million over budget. An order for 14 more vehicles was let before 1962.

Delta A

13. EPE2
14. EPE3

Delta B

15. 13 December 1962. Relay 1, second NASA communications satellite, NASA's first active one
16. 13 February 1963. pad 17b. Syncom 1; Thiokol Star 13B solid rocket as apogee kick motor
20. July 26, 1963. Syncom 2; geosynchronous orbit, but inclined 33° due to the limited performance of the Delta

Delta C

Delta D

25. 19 August 1964. Syncom 3, the first geostationary communications satellite
30. 6 April 1965. Intelsat I

Delta E

First Delta E. 6 November 1965; Launched GEOS 1

Delta F

Delta G

Delta J

Delta K

Delta L

Delta M

Delta N

'Super Six'

Launch reliability

From 1969 through 1978 (inclusive), Thor-Delta was NASA's most used launcher, with 84 launch attempts. (Scout was the second-most used vehicle with 32 launches.)
NASA used it to launch its own satellites, and also to launch satellites for other government agencies and foreign governments on a cost-reimbursable basis.
Sixty-three of the satellites NASA attempted to launch were provided by other parties. Out of the 84 attempts there were seven failures or partial failures (91.6% successful).

Delta numbering system

In 1972, McDonnell Douglas introduced a four-digit numbering system to replace the letter-naming system.
The new system could better accommodate the various changes and improvements to Delta rockets (and avoided the problem of a rapidly depleting alphabet).
It specified (1) the tank and main engine type, (2) number of solid rocket boosters, (3) second stage (letters refer to engine, not earlier letter system), and (4) third stage.

This numbering system was to have been phased out in favor of a new system that was introduced in 2005. In practice, this system has never been used.

Delta 904

Delta 1000-Series

Delta 2000-Series

Delta 3000-Series

Delta 4000-Series

Delta 5000-Series

Delta II series

The Delta II series consists of the retired Delta 6000, the active Delta 7000, and two variants (Lite and Heavy) of the latter. It was developed by Rockwell International Rocketdyne Div. at Chatsworth and Canoga Park, California, and its engines were tested at Rockwell International Rocketdyne Div's Santa Susana Field Laboratory, (SSFL), located northwest of Los Angeles in the Simi hills.

Delta 6000-Series

When in 1986 the Challenger accident demonstrated that Delta launches would continue, the Delta II was developed.

Delta 7000-Series

Delta II Med-Lite

A 7000-series with no third stage and fewer strap-ons (often three, sometimes four). Usually used for small NASA missions.

Delta II Heavy

A Delta II 792X with the enlarged GEM-46 boosters from Delta III.

Delta III (8000-Series)

A McDonnell Douglas/Boeing-developed program to keep pace with growing satellite masses:

Of the three Delta III flights, the first two were failures and the third carried only a dummy (inert) payload.

Delta IV (9000-series)

As part of the Air Force's EELV (Evolved Expendable Launch Vehicle) program, McDonnell Douglas/Boeing proposed Delta IV. As the program implies, many components and technologies were borrowed from existing launchers. Both Boeing and Lockheed Martin were contracted to produce their EELV designs. Delta IVs are produced in a new facility in Decatur, Alabama.

The first stage is referred to as a common booster core (CBC); a Delta IV Heavy attaches two extra CBCs as boosters.

Delta IV Heavy

The Delta IV Heavy (Delta 9250H) uses two additional CBCs as boosters. These are strap-on boosters which are separated earlier in the flight than the center CBC.
The initial demonstration flight on December 21, 2004 was a partial failure, due to the premature cutoff of CBCs. The DemoSat reached incorrect orbit and the 3CS satellites entered orbit at a height of only 105 km, which led to a rapid decay.
The cause of the problem was a faulty first-stage LOX depletion sensor signal that resulted when LOX cavitation occurred in the LOX feedline. The LOX feedline/sensor design was modified and the problem did not recur on subsequent Delta IV Heavy missions.