Delta (rocket family)

The Delta rocket family was a versatile range of American rocket-powered expendable launch systems that provided space launch capability in the United States from 1960 to 2024. Japan also launched license-built derivatives from 1975 to 1992. More than 300 Delta rockets were launched with a 95% success rate. The series was phased out in favor of the Vulcan Centaur, with the [Delta IV Heavy|Delta IV Heavy rocket]'s last launch occurring on April 9, 2024.

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 combination of the Thor was named the Thor "Delta", reflecting the fourth letter of the Greek alphabet. Eventually the entire Thor–Delta launch vehicle came to be called simply "Delta".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". The plan was to replace Delta with other rocket designs when they came on-line. 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:

Stage 1: Modified Thor IRBM with a Block I MB-3 engine group consisting of one Rocketdyne LR-79 main engine and two Rocketdyne LR-101 vernier thrusters for roll control, producing a total of thrust, including LOX/RP1 turbopump exhaust.
Stage 2: Modified Able. Pressure-fed UDMH/nitric acid-powered Aerojet AJ-10-118 engine producing. This reliable engine cost US$4 million to build and is still flying in modified form today. Gas-jet attitude control system.
Stage 3: Altair. A spin-stabilized at 100 rpm by two solid rocket motors before separation. One ABL X-248 solid rocket motor provided of thrust for 28 seconds. The stage weighed and was largely constructed of wound fiberglass.

These vehicles would be able to place into a LEO or 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 numerous West Coast Thor failures. The total project development and launch cost came to US$43 million, US$3 million over budget. An order for 14 more vehicles was made before 1962.

Evolution

Image:Delta-M with Skynet-1A.jpg|thumb|Launch of the first Skynet satellite by Delta M
launch vehicle in 1969 from Cape Canaveral.

Delta J

Delta 2000-series

Delta II (6000-series and 7000-series)

The Delta II series was developed after the 1986 Challenger accident and consisted of the Delta 6000-series and 7000-series, with two variants of the latter.
The Delta 6000-series introduced the Extra Extended Long Tank first stage, which was longer, and the Castor 4A boosters. Six SRBs ignited at takeoff, and three ignited in the air.
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. The Delta II Med-Lite was a 7000-series with no third stage and fewer strap-ons that was usually used for small NASA missions. The Delta II Heavy was a Delta II 792X with the enlarged GEM-46 boosters from Delta III.

Delta III (8000-Series)

The Delta III 8000-series was a McDonnell Douglas / Boeing-developed program to keep pace with growing satellite masses:

The two upper stages, with low-performance fuels, were replaced with a single cryogenic stage, improving performance and reducing recurring costs and pad labor. The engine was a single Pratt & Whitney RL10, from the Centaur upper stage. The hydrogen fuel tank, 4 metres in diameter is exposed, showing its orange insulation; the narrower oxygen tank and engine are covered until stage ignition. Fuel tank contracted to Mitsubishi and produced using technologies from Japanese H-II launcher.
To keep the stack short and resistant to crosswinds, the first-stage kerosene tank was widened and shortened, matching the upper-stage and fairing diameters.
Nine enlarged GEM-46 solid boosters were attached. Three of them have thrust-vectoring nozzles.

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 Evolved Expendable Launch Vehicle program, McDonnell Douglas / Boeing proposed Delta IV. As the program name implied, many components and technologies were borrowed from existing launchers. Both Boeing and Lockheed Martin were contracted to produce their EELV designs. Delta IVs were produced in a new facility in Decatur, Alabama.

The first stage changed to liquid hydrogen fuel. Tank technologies derived from Delta III upper stage, but widened to 5 metres.
The kerosene engine replaced with Rocketdyne RS-68, the first new, large liquid-fueled rocket engine designed in the United States since the Space Shuttle Main Engine in the 1970s. Designed for low cost, it had lower chamber pressure and efficiency than the SSME, and a much simpler nozzle. Thrust chamber and upper nozzle was a channel-wall design, pioneered by Soviet engines. Lower nozzle was ablatively cooled.
The second stage and fairing were taken from the Delta III in smaller models; widened to 5 metres in Medium+ and Heavy models.
Medium+ models had two or four GEM 60, solid boosters.
The plumbing was revised and electrical circuits eliminated need for a launch tower.

The first stage was referred to as a Common Booster Core ; a Delta IV Heavy attached two extra CBCs as boosters.

Launch reliability

From 1969 through 1978, Thor-Delta was NASA's most used launcher, with 84 launch attempts. Satellites for other government agencies and foreign governments were also launched on a cost-reimbursable basis, totaling 63 satellites. Out of the 84 launch attempts there were 7 failures or partial failures, a 91.6% success rate.

Orbital debris

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. Eight Delta second stages launched between 1973 and 1981 were involved in fragmentation events between 1973 and 1991 usually within the first 3 years after launch, but others have broken apart 10 or more years later. Studies determined that explosions were caused by propellant left after shutdown. The nature of the propellant and the thermal environment occupied by the derelict rockets made explosions inevitable. Depletion burns were started in 1981, and no fragmentation events for rockets launched after that have been identified. Deltas launched before the 1970s variant have had fragmentation events as long as 50 years after launch.

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. The digits specified the tank and main engine type, number of solid rocket boosters, second stage, and third stage:

Number	First digit	Second digit	Third digit	Fourth digit	Letter
0	Long Tank Thor MB-3 engine Castor 2 SRBs	No SRBs	Delta F, with Aerojet AJ-10-118F engines. References uprated Aerojet AJ-10-118 engine*	No third stage	N/A
1	Extended Long Tank ThorMB-3 engine Castor 2 SRBs	N/A	Delta P, Douglas built with TRW TR-201 engines. Exception: AJ-10-118F engine for Anik-A1 launch*.	N/A	N/A
2	Extended Long Tank ThorRS-27 engine Castor 2 SRBs	2 SRBs	Delta K, with AJ-10-118K engines. References uprated Aerojet AJ-10-118 engine*	FW-4D	N/A
3	Extended Long Tank ThorRS-27 engine Castor 4 SRBs	3 SRBs	Delta III cryogenic upper stage, RL-10B-2 engine	Star 37D	N/A
4	Extended Long Tank Thor MB-3 engine Castor 4A SRBs	4 SRBs	Delta IV 4m diameter cryogenic upper stage, RL-10B-2 engine	Star 37E	N/A
5	Extended Long Tank Thor RS-27 engine Castor 4A SRBs	N/A	Delta IV 5 metre diameter cryogenic upper stage, RL-10B-2 engine	Star 48B / PAM-D	N/A
6	Extra-Extended Long Tank Thor RS-27 engine Castor 4A SRBs	6 SRBs	N/A	Star 37FM	N/A
7	Extra-Extended Long Tank Thor RS-27A engine GEM 40 SRBs	N/A	N/A	N/A	GEM 46 SRBs
8	Strengthened Extra-Extended Long Tank Thor RS-27A engine GEM 46 SRBs	N/A	N/A	N/A	N/A
9	Delta IV Common Booster Core RS-68 engine	9 SRBs	N/A	N/A	2 additional CBC parallel first stages

This numbering system was to have been phased out in favor of a new system that was introduced in 2005. In practice, the new system was never used, as all but the Delta II have been retired:

Number	First digit	Second digit	Third digit	Fourth digit	Letter
0	N/A	No SRBs	N/A	No third stage	N/A
1	N/A	N/A	N/A	N/A	N/A
2	Extra-Extended Long Tank Thor RS-27A engine GEM 40 SRBs	2 SRBs	Delta K, with AJ-10-118K engines	N/A	GEM 46 SRBs
3	Strengthened Extra-Extended Long Tank Thor RS-27A engine GEM 46 SRBs	3 SRBs	N/A	N/A	-
4	Delta IV CBC RS-68 engine	4 SRBs	Delta IV 4 metre diameter cryogenic upper stage, RL-10B-2 engine	N/A	2 additional CBC parallel first stages
5	N/A	N/A	Delta IV 5 metre diameter cryogenic upper stage, RL-10B-2 engine	Star 48B / PAM-D	N/A
6	N/A	N/A	N/A	Star 37FM	N/A
7	N/A	N/A	N/A	N/A	N/A
8	N/A	N/A	N/A	N/A	N/A
9	N/A	9 SRBs	N/A	N/A	N/A