Barely a month since the last of its kind—the OA-4 mission—returned to a fiery demise in Earth’s atmosphere, Orbital ATK will deliver another Cygnus cargo ship towards the International Space Station (ISS) on Tuesday, 22 March. Coming five weeks after the 19 February departure of its predecessor, the OA-6 launch represents the shortest interval between two visiting vehicles by either of NASA’s Commercial Resupply Services (CRS) providers. Like OA-4, Tuesday’s mission will fly atop a United Launch Alliance (ULA) Atlas V 401 booster from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station, Fla., and will benefit from the lower mass, greater power and larger payload capabilities of the Enhanced Cygnus spacecraft. The OA-6 flight will deliver 7,228.9 pounds (3,279 kg) of payloads and supplies to the station’s incumbent Expedition 47 crew.
Together with SpaceX, Orbital ATK has received contracts from NASA to provide resupply services to the ISS. Under the original $1.9 billion contract, signed in December 2008, Orbital Sciences Corp. was tasked with delivering 44,000 pounds (20,000 kg) of equipment and supplies on eight dedicated Cygnus missions, to be flown by 2016. Following an initial demonstration mission to the station in the fall of 2013, Orbital flew two of those dedicated missions in January and July 2014, before calamity struck the following October and ORB-3 was lost, seconds after liftoff.
In keeping with tradition, OA-6 is named in honor of a former astronaut; in this case, Rick Husband, who commanded the final voyage of shuttle Columbia in early 2003. Its formal title is “Spaceship (S.S.) Rick Husband”. Although it has not revealed the specific criteria for selecting certain astronauts’ names for its resupply craft, Orbital ATK has noted that Husband’s “spirit for exploration” was a key factor. “We will further demonstrate the flexibility of the Cygnus spacecraft during the OA-6 mission,” said former astronaut Frank Culbertson, president of Orbital ATK’s Space Systems Group. “The S.S. Rick Husband will launch aboard an Atlas V rocket, once again exhibiting Cygnus’ compatibility with various launch vehicles.”
In readiness for OA-6, the 41.5-foot-long (12.6-meter) Centaur—whose single RL-10C engine, powered by liquid oxygen and hydrogen, will burn for about 14 minutes, late in Tuesday’s ascent, to inject Cygnus into low-Earth orbit—was delivered to Cape Canaveral on 23 January. Two weeks later, the Atlas V’s 106.5-foot-long (32.5-meter) first stage, known as the Common Core Booster (CCB), arrived and the pair were mated in the Vertical Integration Facility (VIF) at SLC-41 on 26 February. In the meantime, Cygnus’ Pressurized Cargo Module (PCM) had reached the Cape in mid-January, followed by the Service Module (SM) a few days later. The pair were integrated in the Space Station Processing Facility (SSPF) and transferred to the Payload Hazardous Servicing Facility (PHSF) on 29 February for fueling. At length, the Cygnus, encapsulated within its bulbous Extra-Extended Payload Fairing (XEPF), was attached to the top of the Atlas V on 14 March. A successful Integrated Systems Test of the functionality of the vehicle and payload allowed teams to press ahead with final preparations for the opening launch attempt.
In a similar fashion to OA-4, tomorrow’s launch benefits from a 30-minute “window”, which extends from 11:05 to 11:35 p.m. EDT. This is substantially longer than the “instantaneous” or approximately 5-10 minutes usually allowed for ISS-bound missions and has been made possible by the performance and flexibility of the Atlas V. According to Jim Sponnick, ULA’s vice president for Atlas and Delta Programs, instantaneous windows “can significantly limit the probability of an on-time launch” and in remarks provided to AmericaSpace he stressed that ULA “strives to implement longer launch windows in our mission designs to maximize the probability of a first-day launch for our customers”. Ironically, the OA-4 launch campaign in December was thwarted on no fewer than three occasions by foul weather at the Cape.
Weather conditions for Tuesday night offer a more promising outlook, with an 90-percent probability of acceptable conditions at T-0. According to the 45th Weather Squadron at Patrick Air Force Base, the key meteorological concern centers on the risk of violating the Cumulus Cloud Rule; a risk which is expected to deteriorate to 80-percent-favorable in the event of a 24-hour scrub to Wednesday.
“Breezy today,” tweeted ULA President and CEO Tory Bruno, “but weather is still looking good for launch.” In fact, winds are anticipated to remain breezy, following a cold front over the weekend, with high pressure expected to build into Central Florida with steadily improving conditions by Tuesday. “On launch day, high pressure pushes east of the peninsula with on-shore winds during the day and becoming from the southeast during the window with gusts in the low to mid-teens,” the 45th Weather Squadron reported in its Monday morning summary. “With the on-shore flow, there is a small coastal shower threat. No thunderstorms are expected. The primary concern for launch, although slight, is Cumulus Clouds.” In the event of a 24-hour scrub, high pressures are expected to persist off the coast, with a continuation of on-shore winds and a coastal shower threat.
When the Atlas V arrived at the pad, it was carefully centered and propellant umbilicals and electrical and data connections were established. The track mobiles from the Mobile Launch Platform (MLP) were removed and ULA engineers prepared to begin formal countdown operations. This process includes the initiation of flight control systems, the bringing online of ground command, control and communications instrumentation and the verification of interfaces between the Atlas V and the Launch Control Center (LCC).
The CCB first stage, equipped with a Russian-built RD-180 engine, is powered by liquid oxygen and a highly refined form of rocket-grade kerosene, known as “RP-1”. Although the RP-1 is storable, the liquid oxygen loading occurs relatively late in the countdown. In tandem, liquid oxygen and hydrogen will be tanked aboard the Centaur, whose RL-10C engine will be responsible for delivering the OA-6 Cygnus spacecraft into low-Earth orbit. When all loading is completed, the vehicle will enter “Replenishment Mode”, with boiled-off cryogens being rapidly replaced until shortly before T-0.
A final “Go/No-Go” poll of all stations, led by the ULA Launch Conductor, will allow teams to press ahead into the Terminal Countdown and the clock will be released from its final pre-planned hold at T-4 minutes at 11:01 p.m. EDT Tuesday. The RD-180 engine will ignite at T-2.7 seconds, punching out 860,000 pounds (390,000 kg) of thrust, and the climb-out from SLC-41 will occur at T+1.1 seconds. After clearing the launch complex, the Centaur avionics will command a pitch, roll and yaw program maneuver to set the Atlas V onto its proper flight azimuth of 44.4 degrees. Four minutes into the flight, the RD-180 will shut down and the Centaur’s engine—capable of 22,900 pounds (10,390 kg) of thrust—will execute a 14-minute “burn”. During this period, the XEPF will be jettisoned. By the time the Centaur shuts down at T+21 minutes, the Cygnus will be circling the Home Planet at an altitude of 144 miles (232 km), inclined 51.6 degrees to the equator.
This will set the pieces in motion for a rendezvous and capture of the spacecraft by the station’s 57.7-foot-long (17.6-meter) Canadarm2 on Saturday, 26 March. Leading the effort from inside the multi-windowed cupola will be Expedition 47 Commander Tim Kopra and Britain’s Tim Peake, assisted by newly-arrived NASA astronaut Jeff Williams. Early Monday, Peake tweeted a photograph of himself performing Cygnus rendezvous training in the U.S. Destiny lab. “Practicing a #Cygnus capture,” he told his followers, “ahead of our @OrbitalATK launch and rendezvous this week.”
Grapple is scheduled to occur at 6:40 a.m. EDT, after which the Robotics Officer (ROBO) in the Mission Control Center (MCC) at the Johnson Space Center (JSC) in Houston, Texas, will command Canadarm2 to maneuver Cygnus to its berthing location at the Earth-facing (or “nadir”) port of the Unity node. According to the present schedule, the entire process is expected to be completed by 9:45 a.m. EDT.
The three U.S. Orbital Segment (USOS) crew members will then work together to outfit Unity nadir’s Common Berthing Mechanism (CBM), prior to accessing the Cygnus. When they open the hatch, they will be greeted by a sign welcoming them into “Spaceship Rick Husband”, together with an image of the late astronaut himself. For Kopra and Williams, who have been members of NASA’s astronaut corps for almost two decades, Husband was a friend and a colleague. And for their Russian Expedition 47 crewmate, Yuri Malenchenko, there is also a link. At one stage, Husband and Malenchenko might have flown STS-96 together. However, schedules changed and the pair ended up flying on two separate shuttle missions.
In a sense, therefore, the pair will end up flying together after all. And Rick Husband—who lost his life when Columbia disintegrated during re-entry—will posthumously be granted a second chance to visit the space station which, long ago, he helped to build.
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