The Delta family of rockets is used in an expendable launch system that has provided space launch capability for the United States since 1960. Delta has a history of over 300 launches, with a 95% success rate. Of the many types of Delta rockets that have been used, two -- the Delta II and the Delta IV -- are currently scheduled for future space launches. Manufacture and launch of Delta rockets are currently provided by the United Launch Alliance.

Delta origins

Early Delta flights

Delta Evolution
Block II MB-3 engine, 170,000 lbf (756 kN) vs. 152,000 lbf (676 kN)
13. EPE2 14. EPE3

Delta A
15. 13 Dec 1962. Relay 1, second NASA communications satellite, NASA's first active one. 16. 13 Feb 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.

Upgraded AJ10-118D upper stage--3 foot tank stretch, higher energy oxidizer, solid-state guidance system.
Delta program goes from 'interim' to 'operational' status.
200 lb (90 kg) to GTO. Delta B

Third stage Altair replaced with Altair 2—its engine having been developed as the ABL X-258 for the Scout vehicle; 3 in (76 mm) longer, 10% heavier, but 65% more total thrust. Delta C
25. 19 Aug 1964. Syncom 3, the first geostationary communications satellite. 26. Intelsat 1

Also known as Thrust Augmented Delta.
A Delta C with the Thrst Augmented Thor core plus three Castor 1 boosters. Delta D
First Delta E. 6 Nov 1965. Launched GEOS 1.

Also known as Thrust Augmented Improved Delta.
1965.
100 lb (45 kg) more to GTO than Delta D.
Castor II vs. Castor boosters. Same thrust, longer duration.
MB-3 Block III core engine, 2000 lbf (9 kN) more thrust.
AJ10-118E second stage widened from 33 to 55 inches (0.8 to 1.4 m) diameter. Double burn time.
Additional helium tanks allow for almost unlimited restarts.
Two available third stages: Altair 2 or FW-4D. The latter caused the Delta to be known as a Delta E1.
New payload fairing from Agena. Delta E
1. 14 Dec 1966. Biosatellite 1. 2. 7 Sep 1967. Biosatellite 2

Two stage Delta Es.
used for Biosatellite 1 and 2 flights. Delta G
4 Jul 1968. Explorer 38.

Used larger Thiokol Star 37D motor as third stage. Delta J

Introduced Extended Long Tank first stage- 8-foot diameter throughout.
FW-4d motor for third stage. Delta L

Star 37D for stage 3. Delta M

Two stage version of Delta N. Delta N

Delta M or Delta N with three extra strap ons.
1000 lb (450 kg) to GTO. 'Super Six'
From 1969 through 1978 (inclusive), Thor-Delta was NASA's most popular launcher, with 84 launch attempts. ( Scout was the second most used vehicle with 32 launches.)

Launch Reliability
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 boosters, (3) second stage, and (4) third stage. [1]
This numbering system was phased out in favor of a new system that was introduced in 2005. [2] In practice, vehicles designated by the new system will not launch before 2007 at the earliest.

Delta Numbering System
On July 23, 1972, the launch of Landsat 1 marked the first use of nine strap-on boosters, and the new uprated second-stage engine (AJ 10-118F). This Thor-Delta model was designated the 904.

Delta 904

Extended Long Tank with eight-foot diameter payload fairing; nicknamed "Straight-Eight".
Nine Castor II strap-on solid boosters.
The first successful 1000 series Thor-Delta launched Explorer 47 on September 22, 1972. Delta 1000-Series

Features new Rocketdyne RS-27 main engine on Extended Long Tank. Same constant eight-foot diameter. Delta 2000-Series

Introduced upgraded Castor IV solid motors. Same first stage as 1000- and 2000-series.
Also introduced PAM (Payload Assist Module)/Star 48B solid-fueled kick motor. Later used as Delta II third stage. Delta 3000-Series

Used old MB-3 main engine on Extended Long Tank with Castor IV motors.
Only launched two missions. Delta 4000-Series

Featured upgraded Castor IVA motors on Extended Long Tank first stage with RS-27 main engine.
Only launched one mission. Delta 5000-Series

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

Introduced Extra Extended Long Tank first stage. 12 additional feet provide more propellant.
Introduced Castor IVA boosters. Six ignite at takeoff, three ignite in flight. Delta 6000-Series

Introduces RS-27A main engine, modified for efficiency at high altitude, at some cost to low-altitude performance.
Introduces GEM-40 (Graphite-Epoxy Motor) solid boosters from Hercules (now Alliant). Besides being longer, their lighter casings allow higher payload capability. Delta 7000-Series
A 7000-series with no third stage and fewer strap-ons (often three, sometimes four). Usually used for small NASA missions.

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

Delta II Heavy

Main article: Delta III rocket