A shroud of secrecy hangs over United Launch Alliance’s (ULA) next scheduled Atlas V mission, which is slated to transport the classified NROL-67 payload into orbit on behalf of the National Reconnaissance Office. Liftoff of the venerable booster—which will be flying, for only the second time in its career, in its “541” configuration, with a 17.7-foot (5.4-meter) payload fairing, four strap-on solid-fueled rockets, and a single-engine Centaur upper stage—will take place from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station, Fla., at 2:48 p.m. EDT (6:48 p.m. GMT) Tuesday, 25 March. This will be the 44th launch of an Atlas V since the rocket’s inaugural mission back in August 2002. With 42 outright successes and only one partial failure, it has one of the most reliable track records of any operational launch vehicle in the world today.
With the potential to deliver payloads weighing up to 38,455 pounds (17,443 kg) into low-Earth orbit and up to 18,276 pounds (8,290 kg) into geostationary transfer orbit, the Atlas V 541 was previously utilized to loft NASA’s Mars Science Laboratory (MSL) in November 2011, carrying the Curiosity rover on its journey to the Red Planet. This choice of configuration—the second most powerful variant of the Atlas V—offers a clue to the size and weight of the NROL-67 primary payload for Tuesday’s mission. Stacking operations associated with the Atlas V began on 7 February in the Vertical Integration Facility (VIF) and were completed last week with the installation of NROL-67 atop its Centaur upper stage.
Assuming that all final processing milestones run according to plan, the Atlas V will be transferred to the SLC-41 launch complex atop the Mobile Launch Platform (MLP) early Tuesday and should be “hard down” on the pad surface about 7.5 hours ahead of liftoff. Its arrival will be followed by pad and MLP connections, flight control systems checks, and weather briefings. The Flight Termination System (FTS), which is tasked with destroying the Atlas V in the event of a major, off-nominal event during ascent, will be armed and tested. At T-2 hours, a 30-minute built-in hold in the countdown will begin, ahead of the loading of cryogenic propellants aboard the Common Core Booster (CCB) and the Centaur. Liquid oxygen will be transferred into the tanks of the Centaur and should reach flight levels and enter a topping-off mode to replenish the effects of cryogenic boil-off by about T-64 minutes.
Meanwhile, the three-stage operation of fueling the CCB with liquid oxygen and a highly refined form of rocket-grade kerosene (known as “RP-1”) will get underway, proceeding through Slow Fill, Fast Fill, and Topping modes. With all tanks confirmed at flight levels, the final checkout of the FTS will be performed and the ascent software, based upon the real-time weather situation on the Florida coast, will be updated. Four minutes before launch, following a “Go-No Go” poll of all stations, the terminal countdown will commence. The vehicle will be transitioned to internal power, and, 60 seconds ahead of liftoff, the Launch Control System will be enabled and the Atlas’ computers will assume primary command of all critical functions. Two and a half seconds before liftoff, the Russian-built RD-180 engine—fueled by liquid oxygen and RP-1—will roar to life, spooling up to its full 860,000 pounds (390,000 kg) of thrust by T-0. Climb-out of the stack from SLC-41 will get underway at T+1.1 seconds.
Shortly after clearing the tower, the vehicle will execute a combined pitch, roll, and yaw program maneuver to position it onto the proper flight azimuth for the injection of NROL-67 into orbit. A little over a minute into the flight, with the RD-180 still burning hot and hard, the rocket will burst through the sound barrier, at which point maximum aerodynamic stresses (known as “Max Q”) will be experienced through the Atlas’ airframe. In response to this aerodynamic situation, the RD-180 will be temporarily throttled back to 95 percent of its rated performance.”Guidance steering is enabled approximately 120 seconds into flight,” noted ULA in the mission brochure for its most recent Atlas V launch in January. “At 212 seconds, the vehicle throttles up to a constant 5.0 G-level. Approximately ten seconds prior to Booster Engine Cutoff (BECO), the Atlas V throttles down to a constant 4.6 Gs.” This final throttling-down of the RD-180 should occur at T+4 minutes and 22 seconds, and, after separation, the turn will come for the Centaur upper stage, which carries the key responsibility for delivering NROL-67 into orbit. The Centaur’s RL-10A engine, built by Pratt & Whitney, is capable of restarting in flight, although the number of “burns” it will perform to deliver its classified payload remains unknown.
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