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Delta II was an expendable launch system designed and built by McDonnell Douglas, sometimes known as the Thorad Delta 1. Delta II was part of the Delta rocket family, derived from the Delta 3000, and entered service in 1989. There were two main variants, the Delta 6000 and Delta 7000, with the latter also having "Light" and "Heavy" subvariants. During its career, Delta II flew several payloads, including 24 Global Positioning System (GPS) Block II satellites, several dozen NASA payloads, and 60 Iridium communication satellites. The rocket flew its final mission, ICESat-2, on 15 September 2018, earning a streak of 100 consecutive successful missions, with the last failure being GPS IIR-1 in 1997. In the late 1990s, Delta II was developed into the unsuccessful Delta III, which was in turn developed into the more capable and successful Delta IV, though the latter shares little heritage with the original Thor and Delta rockets.
McDonnell Douglas, at that time the manufacturer of the Delta family, signed a contract with the U.S. Air Force in 1987 to provide seven Delta II. These were intended to launch a series of Global Positioning System (GPS) Block II satellites, which had previously been manifested for the Space Shuttle. The Air Force exercised additional contract options in 1988, expanding this order to 20 vehicles, and NASA purchased its first Delta II in 1990 for the launch of three Earth-observing satellites. The first Delta II launch occurred on 14 February 1989, with a Delta 6925 boosting the first GPS Block II satellite (USA-35) from Launch Complex 17A (SLC-17A) at Cape Canaveral into a high medium Earth orbit.
The first Delta II 7000-series flew on 26 November 1990, replacing the RS-27 engine of the 6000-series with the more powerful RS-27A engine. Additionally, the steel-cased Castor 4A solid boosters of the 6000 series were replaced with the composite-cased GEM 40. All further Delta II launches except three were of this upgraded configuration, and the 6000-series was retired in 1992 with the last launch being on July 24.
McDonnell Douglas began Delta III development in the mid-1990s as increasing satellite mass required more powerful launch vehicles. Delta III, with its liquid hydrogen second stage and more powerful GEM 46 boosters, could bring twice as much mass as Delta II to geostationary transfer orbit, but a string of two failures and one partial failure, along with the development of the much more powerful Delta IV, led to the cancellation of the Delta III program. The upgraded boosters would still find use on the Delta II, leading to the Delta II Heavy.
On 28 March 2003, the Air Force Space Command began the process of deactivating the Delta II launch facilities and infrastructure at Cape Canaveral once the last of the second-generation GPS satellites were launched. However, in 2008, it instead announced that it would transfer all the Delta II facilities and infrastructure to NASA to support the launch of the GRAIL (GRAIL) in 2011.
On 14 December 2006, with the launch of USA-193, was the first launch of the Delta II operated by United Launch Alliance.
The last GPS launch aboard a Delta II and the final launch from SLC-17A at Cape Canaveral occurred in 2009. The GRAIL Launch in 2011 marked the last Delta II Heavy launch and the last from Florida. The final five launches would all be from Vandenberg Air Force Base in California.
On 16 July 2012, NASA selected the Delta II to support the Orbiting Carbon Observatory (OCO-2), Soil Moisture Active Passive (SMAP), and Joint Polar Satellite System (JPSS-1 – NOAA-20) missions. This marked the final purchase of Delta II. OCO-2 was launched on 2 July 2014, Soil Moisture Active Passive (SMAP) was launched on 31 January 2015, and JPSS-1 was launched on 18 November 2017. All three of these launches were placed into orbit from SLC-2 at Vandenberg.
The Delta II family launched 155 times. Its only unsuccessful launches were Koreasat 1 in 1995, and GPS IIR-1 in 1997. The Koreasat 1 launch was a partial failure caused by one booster not separating from the first stage, which resulted in the satellite being placed in a lower-than-intended orbit. By using reserve fuel, it was able to achieve its proper geosynchronous orbit and operated for 10 years. The GPS IIR-1 was a total loss as the Delta II exploded 13 seconds after launch. The explosion occurred when a damaged solid rocket booster casing ruptured and triggered the vehicle's flight termination system. No one was injured, and the launch pad itself was not seriously impacted, though several cars were destroyed and a few buildings were damaged.
In 2007, Delta II completed its 75th consecutive successful launch, surpassing the 74 consecutive successful launches of the Ariane 4. With the launch of ICESat-2 in 2018, Delta II reached 100 consecutive successful launches.
During its career, Delta II achieved a peak launch rate of 12 launches in a single year, although its infrastructure was capable of supporting up to 15 launches per year.
While all completed Delta II rockets were launched, many flight-qualified spare parts remained in inventory. These spare parts were assembled, alongside a few structural simulators, to create a nearly complete Delta II for exhibition in its 7320-10C configuration. The rocket is displayed vertically at the Kennedy Space Center Visitor Complex, and bears its popular "shark teeth" livery on its fairing, which was painted on past Delta II rockets for the GPS launches.
The two first-stage tanks were constructed from aluminum isogrid panels, providing high strength at a lower mass. Nitrogen gas was used to pressurize the tanks. These tanks were stretched a total of from those of the Extended Long Tank Thor that flew on older Delta rockets, providing more propellant. Between the two tanks was the "center body", where first-stage avionics and communications equipment were housed. The solid rocket booster attachment points were located on the outside of the liquid oxygen tank and the aft skirt, the latter of which also contained a gyroscope for vehicle stability.
The Castor 4A boosters were an improvement over the Castor 4 motors flown on the earlier Delta 3000 rocket, replacing the propellant with more modern HTPB-based propellant and providing an 11% increase in performance. The GEM 40 boosters on the 7000 series further improved Delta II's performance by each featuring of extra propellant than the Castor 4A thanks to a lengthening of . In addition, the GEM boosters also boasted a lower dry mass than the Castors due to the former's carbon composite construction.
In 2003, the Delta II Heavy debuted, featuring larger GEM 46 motors from the abandoned Delta III program. These new motors allowed the vehicle to carry over of extra payload into low-Earth orbit. Only Cape Canaveral Space Launch Complex 17B was capable of flying the Heavy configuration, as it was previously reinforced to handle the Delta III.
The Castor motor nozzles were angled 11° from vertical to aim their thrust into the vehicle's center of gravity, while the GEM motors had a slightly lower 10° tilt. On the nine-engine configuration, the three air-lit motors featured longer nozzles to allow the boosters to perform better in the upper atmosphere. All solid motors that flew on the Delta II featured fixed nozzles, meaning the first stage was solely responsible for vehicle control during the initial portions of the flight.
The Delta-K consisted of stainless steel tanks and a lightweight aluminum structure. The tanks were pressurized with helium gas, and the stage featured nitrogen thrusters for roll control during burns and for complete attitude control during coasts. The stage had a mass of when empty, and when fully fueled.
Two third-stage options were available, both consisting of a single solid rocket motor. The most common by far was Star 48, flying on over 70 missions. The Star 48, also referred to as the Payload Assist Module-Delta (PAM-D, PAM-Delta), was the more, powerful of the two options, producing an average thrust of about during its 87.1 seconds of burn time. The stage would end up flying primarily on the more powerful Delta variants and never flew on the three-booster configuration.
The other third-stage option was Star 37FM. This stage flew four times, and only on three- and four-booster configurations of Delta. Star 37FM produced about of thrust during its 66.4-second burn.
For example, a Delta 7925H-10L used the RS-27A, nine GEM 46 boosters, a PAM third stage, and a lengthened diameter fairing. A Delta 6320–9.5 is a two-stage vehicle with an RS-27 first-stage engine, three Castor 4A boosters, a diameter fairing, and no third stage.
Interplanetary
The last Delta II launch was the ICESat-2 satellite in September 2018.
In 2008, ULA indicated that it had "around half a dozen" unsold Delta II rockets on hand, and by October 2017, ULA CEO Tory Bruno stated that there were no complete Delta II rockets left in ULA inventory.
There were some unused Delta II components, but not enough to build another vehicle. A Delta II, made of these leftover parts alongside some Simulation parts, is located at the Rocket garden.
Delta rockets have been involved in multiple fragmentation events as they were routinely left in orbit with enough fuel to explode. A large amount of current "Space debris" is Delta rocket debris.
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