Delta II Primed for Return to Flight OCO-2 Mission on Tuesday

The Orbiting Carbon Observatory (OCO)-2 will form part of the "Afternoon" Constellation (or "A-Train") of U.S. and French-provided global monitoring satellites. Image Credit: NASA
The Orbiting Carbon Observatory (OCO)-2 will form part of the “Afternoon” Constellation (or “A-Train”) of U.S.- and French-provided global monitoring satellites. Image Credit: NASA

For the first time in almost three years, the thunderous roar of a Delta II booster will rattle the mountainous landscape of Vandenberg Air Force Base, Calif., on Tuesday, 1 July, carrying NASA’s Orbiting Carbon Observatory (OCO)-2 into a near-polar, near-circular orbit of 98.2 degrees at an altitude of about 370 nautical miles (686 km). Liftoff of the Delta II—which is flying under the auspices of United Launch Alliance (ULA)—is scheduled to take place from Space Launch Complex (SLC)-2 during a 30-second “window,” which opens at 2:56:44 a.m. PDT (5:56:44 a.m. EDT). When it attains operational status, OCO-2 will become NASA’s first spacecraft wholly devoted to the global study and characterization of atmospheric carbon dioxide levels. In doing so, it will replace OCO-1, which was lost following a launch failure in February 2009.

Tuesday’s mission—for which weather conditions at the mountain-ringed California launch site are anticipated to be 100 percent favorable, according to ULA’s Jessica Rye—marks the 152nd flight of a Delta II vehicle since its maiden voyage in February 1989, as well as the rocket’s 51st NASA payload and its 42nd overall launch from Vandenberg’s SLC-2. In its earliest incarnation, the Delta II was fabricated by McDonnell Douglas and, following the company’s merger with Boeing in 1997, by Boeing Integrated Defense Systems and, most recently, by ULA since December 2006. Today, U.S. government missions which require the lifting capability of the Delta II are supported by ULA, whilst Boeing Launch Services oversees missions within the commercial sector.

In the immediate aftermath of the January 1986 loss of Challenger, plans to gradually phase out U.S. expendable launch vehicles in favor of the shuttle were abandoned and the reliable Delta, with an ancestry stretching back to the dawn of the Space Age, was brought back into production with the Delta II. First flown on St. Valentine’s Day in 1989, its maiden payload was a Global Positioning System (GPS) Block II Navstar satellite, and in the ensuing two decades it has also delivered seven orbiters and landers toward the Red Planet, including the two Mars Exploration Rovers (MER), Spirit and Opportunity, in the summer of 2003. Moreover, Delta IIs have lofted dozens of other pivotal voyages of robotic exploration, including the Near-Earth Asteroid Rendezvous (NEAR) in February 1996, the Genesis solar wind sample-return spacecraft in August 2001, the ill-fated Comet Nucleus Tour (CONTOUR) in July 2002, the Spitzer Space Telescope in August 2003, the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) mission to Mercury in August 2004, Deep Impact in January 2005, Dawn in September 2007, and the Kepler observatory in March 2009. In addition to these scientific payloads, the Delta II has also transported several dozen Iridium communications satellites into orbit.

In spite of a partial failure to deploy KoreaSat-1 into geostationary orbit in August 1995, due to the failure of one of its Solid Rocket Motors (SRMs) to separate properly, and a catastrophic explosion, just 13 seconds after liftoff in January 1997, which destroyed the first GPS Block IIR satellite, the Delta II has a virtually unblemished record and remains one of the world’s most reliable launchers currently in active service, with a 99.3 percent success rate.

The Orbiting Carbon Observatory (OCO)-2 is readied for launch. Photo Credit: NASA
The Orbiting Carbon Observatory (OCO)-2 is readied for launch. Photo Credit: NASA

Yet Tuesday’s mission will be the first flight of a Delta II in almost three years. Not since 28 October 2011, when it lofted the Suomi National Polar-Orbiting Partnership (NPP) weather satellite into orbit from Vandenberg’s SLC-2 on behalf of the National Oceanic and Atmospheric Administration (NOAA), has the combined roar of its RS-27A engine and strap-on SRMs been heard at the outset of a space mission. Original plans called for it to be phased out of service, following U.S. Air Force plans to discontinue their use of the vehicle. However, in September 2011 it was announced that NASA had added the Delta II to its NASA Launch Services (NLS)-II contract, which provided for the delivery of payloads weighing about 550 pounds (250 kg) into minimum circular orbits of 124 miles (200 km). Additionally, four scientific missions—OCO-2, together with NASA’s Soil Moisture Active Passive (SMAP) environmental research satellite, targeted for launch in November 2014, the Ice, Cloud and Land Elevation Satellite (ICESat)-2, scheduled for July 2016, and the Joint Polar Satellite System (JPSS), currently planned for November 2016—are firmly booked for launches aboard the Delta II.

In traditional style, the vehicle for the OCO-2 mission has received a four-digit Delta designation of “7320.” This describes its membership of the 7000-series of the rocket (“7”), its three-strong complement of strap-on SRMs (“3”), the presence of a second stage (“2”), and the absence (“0”) of a third stage. Fabrication of the booster took place at ULA’s Decatur, Ala., facility, after which it was transported to Vandenberg in March 2014. Assembly of the Delta II got underway with the erection of the first stage, followed by the mating of the three SRMs. The last of the SRMs was in place by 14 April. By the end of the month, the Delta II’s second stage had been positioned atop the first, and its hypergolic propellants of nitrogen tetroxide and Aerozine-50 were loaded on 25-26 June. Due to their highly corrosive nature, after loading it is mandatory that the Delta II must launch within 37 days; failure to do so will require destacking and refurbishment or replacement of the entire 19.4-foot-tall (5.9-meter) second stage. In the meantime, a Flight Readiness Review (FRR) was concluded Tuesday, 24 June, and over the weekend of 28/29 June the two-piece (or “bisector”) Payload Fairing (PLF) was installed around the OCO-2 satellite and mated to the Delta II. The 27.9-foot-tall (8.5-meter) fairing serves to protect the payload whilst on the launch pad and from aerodynamic, thermal, and acoustic stresses encountered during its violent ascent to orbit. Late Sunday, 29 June, a formal Launch Readiness Review (LRR) issued a definitive “Go” to proceed with an opening launch attempt on Tuesday.

“Delta II countdown operations will get underway on Monday at SLC-2, when the Mobile Service Tower is rolled back to its launch position about nine hours before launch to uncover the Delta II rocket for its comeback mission after standing down for 2.5 years,” explained Spaceflight101. “Terminal Countdown Operations begin four hours before liftoff, heading into liquid oxygen loading on the first stage, one hour and 45 minutes before launch.” The process of loading the first stage with its propellants—liquid oxygen and a highly refined form of rocket-grade kerosene, known as “RP-1″—is anticipated to require about 20 minutes to complete in the early hours of Tuesday morning. Four minutes before the opening of the exceptionally brief, 30-second “launch window” for OCO-2, the automated countdown sequence will commence.

A Delta II 7320, akin to the one earmarked for Tuesday's OCO-2 mission, launches Argentina's SAC-D Earth sciences satellite in June 2011. Photo Credit: NASA
A Delta II 7320, akin to the one earmarked for Tuesday’s OCO-2 mission, launches Argentina’s SAC-D Earth sciences satellite in June 2011. Photo Credit: NASA

Three seconds before liftoff, the single RS-27A main engine of the Delta II’s first stage will roar to life, producing 200,000 pounds (90,700 kg) of thrust at sea level. This Rocketdyne-built engine is fueled by liquid oxygen and RP-1, and will support the Delta II for the first 4.5 minutes of its ascent. Strapped around the base of the first stage, the trio of 42-foot-long (12.8-meter) SRMs will ignite at T-0. Together with the RS-27A main engine, and a pair of verniers for roll controllability, these will power the Delta II stack away from SLC-2 and establish it onto the proper flight azimuth for the precise injection of OCO-2 into its 98.2-degree-inclination orbit.

By 35 seconds into Tuesday’s flight, the vehicle will pass Mach 1, and at 49.7 seconds it will encounter a period of maximum aerodynamic stress on its airframe, a phenomenon known as “Max Q.” The three SRMs will exhaust their powder-based solid fuel and burn out at T+64 seconds, but will remain attached to the rapidly ascending Delta II for more than half a minute, before finally being jettisoned at T+99 seconds. After this point, the RS-27A engine will continue to burn hot and hard, shutting down at T+264.2 seconds. Its role in the mission now complete, the first stage—which measures 87 feet (26.5 meters) in length—will be jettisoned shortly afterward, making way for the ignition of the Aerojet-built AJ-10-118K second stage engine at T+277.7 seconds.

Unlike the RS-27A, the Aerojet engine is capable of being restarted in flight and will support two discrete “burns” to deliver OCO-2 into orbit. Capable of producing 9,850 pounds (4,470 kg) of propulsive yield, the AJ-10-118K is fed by a hypergolic mix of nitrogen tetroxide and a 50/50 combination of hydrazine and unsymmetrical dimethyl hydrazine, known as “Aerozine-50.” Its propellant tanks are insulated by “blankets” of Dacron and Mylar. The second stage also houses an inertial platform and guidance system to control all critical phases of the flight. Less than half a minute into the first burn, at T+301 seconds, the Payload Fairing (PLF)—responsible for protecting OCO-2 from aerodynamic, thermal, and acoustic stresses in the lower atmosphere—will be jettisoned, exposing the satellite to the space environment for the first time.

The second stage engine will shut down for its First Cutoff (known as “SECO-1”) at T+620.5 seconds, a little over 10 minutes after departing Vandenberg. It will then coast for the next 40.5 minutes, ahead of the First Restart at T+3050 seconds. This second burn will last 12.4 seconds, ending with SECO-2, and will be followed by another period of coasting—lasting some 313 seconds, or just over five minutes—preparatory to the separation of the OCO-2 satellite at T+3375 seconds. Assuming that all planned milestones are met, NASA’s global carbon dioxide watcher will be in orbit a little over 56 minutes after launch.

Successful insertion into its correct operational “slot” will come a little more than five years after the ignominious failure of its predecessor, OCO-1. Launched atop an Orbital Sciences Corp. Taurus-XL booster from Vandenberg in February 2009, the mission suffered a failure in its payload fairing, which apparently did not detach properly. The result was that the excess mass of the fairing impaired the thrust of the Taurus-XL’s third stage and the entire vehicle plunged to a watery Pacific grave, just off the coast of Antarctica, a mere 17 minutes after liftoff. The importance of the mission was such that a replacement satellite was funded and in June 2010 NASA announced that another Taurus-XL would deliver OCO-2 from Vandenberg in February 2013. However, in the spring of 2012, Orbital Sciences and NASA jointly decided not to pursue the launch on a Taurus-XL, which imposed further delay upon the already-snakebitten program. Several months later, in July 2012, NASA contracted with ULA to launch OCO-2.

OCO-2 and Delta II mission artwork. Image Credit: ULA/NASA
OCO-2 and Delta II mission artwork. Image Credit: ULA/NASA

At length, in April 2014, the 970-pound (440-kg) satellite arrived at Vandenberg for final checks ahead of its long-awaited mission. Built by Orbital Sciences, it carries a single scientific instrument to undertake the most precise measurements of atmospheric carbon dioxide levels ever acquired from space. Three parallel, high-resolution spectrometers will perform simultaneous observations of the carbon dioxide and molecular oxygen absorption of sunlight reflected from Earth’s surface at near-infrared wavelengths. This will provide OCO-2 investigators with the “spectral fingerprints” of these absorption profiles as part of efforts to determine the number of molecules between the upper atmosphere and the surface. The instrument has been developed by Hamilton Sundstrand Sensor Systems of Pomona, Calif., together with NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

“Carbon dioxide in the atmosphere plays a critical role in our planet’s energy balance and is a key factor in understanding how our climate is changing,” said Michael Freilich, NASA’s Earth Science director, as quoted by Spaceflight101. “With the OCO-2 mission, NASA will be contributing an important new source of global observations to the scientific challenge of better understanding our Earth and its future.”

The measurements performed by OCO-2 are expected to be of sufficient accuracy to show the distribution of carbon dioxide “sources” and “sinks” on a regional scale and, by extension, should offer greater insights into our understanding of the global carbon cycle and our own influence upon it. Planned for a baseline mission of two years in near-polar orbit, OCO-2 will fly in conjunction with five other U.S.- and French-provided satellites of the Earth Observing System Afternoon Constellation (or “A-Train”).



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