Delta II is a space launch system originally designed and built by McDonnell Douglas. Delta II is part of the Delta rocket family and has been in service since 1989. Delta II vehicles include the retired Delta 6000, the active Delta 7000, and two 7000 variants (light and heavy).

Delta II rockets were later built by Boeing Integrated Defense Systems until Delta rocket production became the responsibility of United Launch Alliance (ULA) on December 1, 2006.^[1][2] ULA now markets Delta II to U.S. government customers, and Boeing Launch Services (BLS) markets Delta II to commercial companies.^[3]

History

All United States expendable launch vehicles were to be phased out for the Space Shuttle, but in 1986 the Challenger accident restarted Delta development. The Delta II, specifically, was designed to accommodate the GPS Block II series of satellites. Delta IIs have successfully launched 125 projects (through August 2007), including several NASA missions to Mars:

• Mars Global Surveyor in 1996
• Mars Pathfinder in 1996
• Mars Climate Orbiter in 1998
• Mars Polar Lander in 1999
• Mars Odyssey in 2001
• Mars Exploration Rovers (MER-A, Spirit and MER-B, Opportunity) in 2003
• Mars Phoenix lander in 2007

Delta II manufacturing, assembly and integration currently take place in Decatur, Alabama, Harlingen, Texas, San Diego, California, and Denver, Colorado.^[3]

Vehicle description

Deltas are expendable launch vehicles (ELVs), which means they are used only once. Each launch vehicle consists of:

• Stage I: Kerosene and liquid oxygen tanks that feed the Rocketdyne RS-27 main engine for the ascent.
• Solid rocket booster motors: Used to increase thrust during the initial two minutes of flight. The medium-capacity Delta II has nine motors total (six fire on the ground, three in flight); the other models use only three or four.
• Stage II: Fuel and oxidizer tanks feeding a restartable, hypergolic Aerojet AJ10-118K engine that fires one or more times to insert the vehicle-spacecraft stack into low Earth orbit. This stage also contains the vehicle's "brains", a combined inertial platform and guidance system that controls all flight events.
• Stage III: Optional ATK-Thiokol solid rocket motor (some Delta II vehicles are two-stage only, and generally used for Earth-orbit missions) provides the majority of the velocity change needed to leave Earth orbit and inject the spacecraft on a trajectory to Mars or other target beyond Earth orbit. It is connected to the spacecraft until it is done firing, and then separates. This stage is spin-stabilized and has no active guidance control; it depends on the second stage for proper orientation prior to Stage II/III separation. It also includes a yo-yo de-spin mechanism to slow the spin before spacecraft release, as many spacecraft cannot handle the high spin rates needed for stability of this stage.
• Payload fairing: Thin metal or composite payload fairing (aka "nose cone") to protect the spacecraft during the ascent through Earth's atmosphere.

Naming system

The Delta II family is more technically named by a four-digit system:^[4]

• The first digit is either 6 or 7, denoting the 6000- or 7000-series Deltas. The 6000-series, last flown in 1992, had an Extra Extended Long Tank first stage with RS-27 main engine, plus Castor IVA solid rocket boosters. The current model 7000-series have an RS-27A engine, with a longer nozzle for higher expansion ratio and better high-altitude performance, and GEM (Graphite-Epoxy Motor) boosters. GEMs are larger, and have a composite casing to reduce mass versus the steel-case Castors. In addition, two LR101-NA-11 vernier engines provide guidance for the first stage.

• The second digit indicates the number of boosters, usually 9. In such cases, six are lit at liftoff and three are lit one minute into flight. On vehicles with 3 or 4 boosters, all are ignited at liftoff.

• The third digit is 2, denoting a second stage with an Aerojet AJ10 engine. This engine is restartable, for complex missions. Only Deltas prior to the 6000-series used a different engine, the TR-201.

• The last digit denotes the third stage. 0 denotes no third stage, 5 indicates a Payload Assist Module (PAM) stage with Star 48B solid motor, 6 indicates a Star 37FM motor.

For example, a Delta 7925 has the later first stage, nine GEM boosters, and a PAM third stage. A Delta 7320 is a two-stage vehicle with three boosters.

• A Delta II-Heavy has the larger GEM-46 boosters, originally designed for the Delta III. These are designated 79xxH.

Three payload fairings are available. The original aluminum fairing, seen above, is 9.5 feet in diameter. A 10-foot fairing is made of composite, and can be distinguished by its tapering front and rear. A lengthened 10-foot fairing is used for the largest payloads.

Comparison of standard vs. heavy Delta II

Launch description

Launch vehicle build-up

A Delta II launch vehicle is assembled vertically on the launch pad. Assembly starts by hoisting the first stage into position. The solid rocket boosters are then hoisted into position and mated with the first stage. Launch vehicle build-up then continues with the second stage being hoisted atop the first stage.^[5]

Fueling

It takes approximately 20 minutes to load the first stage with of fuel.^[6]

Delta II launches

The Delta II system has been used for 135 launches. On September 18, 2007, Delta II completed its 75th consecutive successful launch.^[7] This is a record for modern launch vehicles.^[8] It is the second most reliable launch vehicle currently in service, behind the Tsyklon 2.^[9] Eight launches took place in 2007.

However, the Delta II system does not have a perfect success record. One mission, the launch of Koreasat-1, was a partial failure in which the satellite payload was able to compensate when the launch system placed the vehicle in an incorrect orbit.^[10]

Another failure, this time complete, occurred on January 17, 1997, when a Delta II 7925 carrying the first GPS Block IIR satellite exploded only 13 seconds after liftoff, raining flaming debris all over Launch Complex 17 at Cape Canaveral Air Force Station. No one was injured, and the launchpad itself was not seriously damaged, though several cars were destroyed and a few buildings were damaged.^[11]

Notable payloads

Between May 1997 and November 1998 Delta II vehicles placed 55 Iridium satellites into orbit.^[12]

Retirement of system

An article published by the Wall Street Journal speculates about the fate of the Delta II launch system after U.S. Air Force discontinues its use of the Delta II.^[13] Thomas Young, who was director of Goddard Space Flight Center from 1980 to 1982, is quoted as saying, "It's definitely an item people are quite worried about."

The final payload currently scheduled for Delta II is a NASA moon mission in 2011.^[14] ULA has indicated it has "around half a dozen" unsold Delta II rockets on hand.^[15] A spokesperson indicated ULA will change some aspects of the Delta II system once the current Medium Launch Vehicle 3 contract with the Air Force ends and requirements imposed by the contract are lifted. The Air Force contract required that Delta II be kept ready to launch within 40 days of call up, which led ULA to maintain two launch pads at Cape Canaveral. ULA indicated it would not continue to operate two launch pads.^[15]

Future applications

The Aerojet-built second-stage engine has been chosen by NASA to be used as the main propulsion engine for the Orion spacecraft that will replace the Space Shuttle after 2010. The engine was chosen due to its restart capabilities along with a switch from the original liquid oxygen/liquid methane (LOX/LCH₄) application to hypergolic fuel and oxidizer similar to that in use on the Shuttle's OMS and RCS systems.

References

External links

• Boeing Delta II Page
• Delta I, II und III Site from Gunter Krebs
• History of the Delta Launch Vehicle
• Article about 1997 failed launch and self-destruction (with video)

Delta rocket evolution (U.S. Govt.)

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