With weather conditions at Cape Canaveral Air Force Station expected to be 80 percent favorable, United Launch Alliance (ULA) stands ready to light up the Florida sky tomorrow night, lofting NASA’s 12th Tracking and Data Relay Satellite (TDRS-L) into geostationary transfer orbit. Liftoff of the venerable Atlas V rocket—flying in its “401” configuration, with a 13-foot (4-meter) payload fairing, no strap-on boosters and a single-engine Centaur upper stage—is scheduled to occur from Space Launch Complex (SLC)-41 during a 40-minute “window”, which opens at 9:05 p.m. EST Thursday, 23 January. The new satellite will serve to bolster a critical network of communications and data-relay assets which currently support dozens of low-Earth-orbiting spacecraft, including the International Space Station and the Hubble Space Telescope.
Tomorrow’s launch marks the second member of the current third generation of TDRS satellites, the inaugural contracts for which were signed between NASA and Boeing in December 2007. Under the provisions of that agreement, the aerospace giant built TDRS-K—which was launched in January 2013—and TDRS-L, at a cost of $695 million, in order to “ensure vital operational continuity” of an orbital network which saw its first member (TDRS-A) placed into orbit more than 30 years ago and whose heritage stretches back even further. The NASA-Boeing agreement is expandable to $1.2 billion if all options are exercised, and this indeed seems to be the case, for the space agency ordered a third satellite, TDRS-M, in November 2011. Current plans anticipate its launch in December 2015. All three satellites are designed to support 15-year operational lives, which enables continuity until the middle or even the end of the 2020s.
The orbital emplacement of these new satellites coincides broadly with the retirement of the TRW-built first generation of TDRS, designated “A” through “G”, which were launched by the shuttle between April 1983 and July 1995. Of this first generation, one (TDRS-B) was lost in the Challenger disaster, whilst two others (TDRS-A and D) have already been shut down. The others are expected to follow in the near future. A second generation of three satellites—TDRS-H, I and J, all built by Boeing—were launched aboard expendable rockets between June 2000 and December 2002 and remain fully functional, despite having endured a handful of technical troubles.
With last January’s successful launch of TDRS-K, a hiatus of more than a decade since the flight of the last satellite was closed. The new third generation is visually quite distinct from its cousins of the first generation. It is based upon Boeing’s 601 spacecraft bus, first introduced more than two decades ago, but heavily upgraded over the years, and can support multiple payloads and objectives, including direct TV broadcasts and the needs of private businesses and mobile communications users. The size and output of its communications payload has also expanded and it is capable of housing up to 60 transponders and producing 10,000 watts of power.
As well as enabling all navigation, power, propulsion and command capabilities, the bus has twin solar arrays—each measuring 15 feet (4.5 meters) in diameter—for use whilst in direct sunlight and battery packs for use whilst in Earth’s shadow. Its “spring-back” antennas are designed with flexible membrane reflectors, which fold up for launch and spring back into their original, “cupped” circular shape after orbital insertion. The communications hardware consists of microwave equipment, a pair of gimbaled antennas and a phased-array antenna for forward, return, and tracking services. In addition to operating at S-band and Ku-band frequencies, the second- and third-generation TDRS provide improved overall service and substantially higher bandwidth through the Ka-band.
In March 2009, Boeing selected ULA’s 19-story Atlas V as its vehicle of choice to deliver the third-generation satellites into orbit. The core of the rocket arrived at Port Canaveral last 1 November, aboard the Delta Mariner barge, and was transported to the Atlas Spacecraft Operations Center at Cape Canaveral Air Force Station for initial checkout. Six weeks later, on 16 December, the Centaur upper stage was mated to the Atlas in the Vertical Integration Facility (VIF) at SLC-41.
Concurrently, processing efforts to ready TDRS-L for launch were moving into high gear. The 7,600 pound (3,450 kg) satellite arrived at the Kennedy Space Center (KSC) aboard an Air Force C-17 Globemaster III aircraft on 6 December, having traveled across the continental United States from Boeing’s Space and Intelligence Systems factory in El Segundo, Calif. Following five days of electrical tests at the Astrotech payload processing facility at KSC, the satellite’s attitude-control system was readied for the loading of monomethyl hydrazine oxidizer on 20 December, ahead of the Christmas break.
Earlier this month, NASA’s TDRS-L test team conducted a countdown simulation, after which the satellite was installed onto its payload adapter and encapsulated within the two-piece protective shroud. In the pre-dawn darkness of 13 January, it emerged from Astrotech’s high bay at 1:50 a.m. EST and had arrived at the VIF at SLC-41 by 5:20 a.m. A little over four hours later, by 10:00 a.m., the payload had been mated atop the Atlas V.
Over the following days, electrical and other connections were established and the Integrated Systems Test on 15 January verified that the booster and payload were fully compatible in a launch configuration. Two days later, on 17 January, NASA and ULA managers completed their Flight Readiness Review (FRR) and issued a unanimous “Go” for launch at 9:05 p.m. EST on 23 January. Yesterday, a Launch Readiness Review was concluded and managers approved the rollout of the Atlas V/TDRS-L stack from the VIF to SLC-41. First motion occurred today (Wednesday) at 10:22 a.m. EST and the vehicle was hard-down on the pad surface less than 30 minutes later.
Tomorrow’s opening launch attempt presents an 80 percent likelihood of favorable weather conditions, with a minimal chance of a thick ground fog layer forming the primary concern for violation. Countdown operations are scheduled to commence at 2:05 p.m. EST and will include the final loading of propellants aboard the Atlas V. Two-and-a-half seconds before liftoff, the Russian-built RD-180 engine—fueled by liquid oxygen and a rocket-grade form of kerosene, known as “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 Atlas V from SLC-41 will commence 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 TDRS-L 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. Just ahead of engine cutoff, at T+4 minutes and 22 seconds, it will throttle back again to limit acceleration loads and, after separation, the turn will come for the Centaur upper stage. This carries a Pratt & Whitney Rocketdyne-built RL-10A engine and will carry the key responsibility for delivering TDRS-L into geostationary transfer orbit.
The RL-10A—which remains subject to an extended investigation, following an incident of abnormally low thrust during its otherwise successful effort to loft a Global Positioning System (GPS) satellite into orbit on 4 October 2012—is capable of restarting in flight and will support two “burns” to inject TDRS-L. The first burn will last for a little over 14 minutes and serves to place the TDRS-L/Centaur into a “parking orbit,” after which the combo will coast for about 80 minutes, ahead of a second burn, lasting just 60 seconds. Shortly after the completion of this second burn, the Centaur will begin a “Spacecraft Separation Attitude Alignment” and will spin itself up to five revolutions per minute. Finally, 106 minutes after launch, TDRS-L will be released into space to begin the complex process of deploying its solar arrays and communications payload. As the 12th satellite in NASA’s fleet, TDRS-L will continue a proud heritage of providing near-continuous tracking, voice and data communications relay services between ground stations and more than 20 users, including the Hubble Space Telescope and the International Space Station.
Yet TDRS arose in the post-Apollo era at one of the most uncertain times for the U.S. human space program and even its long-awaited maiden launch in April 1983 hung for a time under the shadow of abject failure. The concept was born in the early 1970s as one of the recommendations of a study group led by Don Hearth, then-deputy director of NASA’s Langley Research Center in Hampton, Va., which charted possible post-Apollo roadmaps for America’s future in space. The group, which included astronaut Joe Allen, felt that a system of tracking and relay satellites placed into 22,000 mile (35,000 km) geosynchronous orbits by the shuttle and operated from a single ground terminal at White Sands, N.M., would provide near-continuous voice and data traffic and eliminate the older generation of ships and costly ground stations. In fact, as well as supporting low-orbiting missions, it could relay data from satellites up to 3,000 miles (4,800 km) above Earth’s surface. Since the dawn of human space flight, astronauts had been out of contact with Mission Control for up to 80 percent of every orbit; furthermore, satellites had to tape record data and transmit it when they came within range of a tracking ship or ground station. As the shuttle effort gained momentum in the mid-1970s, it was envisaged that two TDRS relays would provide astronauts with space-to-ground voice and data links for between 85-98 percent of each orbit.
TDRS was no miracle worker. In its original incarnation, it could not process or adjust communications traffic in either direction. Rather, it operated as a “bent pipe” repeater, relaying signals and data between its Earth-circling users and the highly automated ground terminal. Signals processing, therefore, occurred on the ground, and the satellite’s sophistication was devoted to its very high throughput. Located in the inhospitable New Mexico desert, White Sands provided a clear line of sight with the satellites and its limited amount of annual rainfall meant that weather conditions would not interfere with uplink or downlink capabilities.
It was envisaged that a pair of TDRSes—one stationed over the equator, just off the north-eastern corner of Brazil, known as “TDRS-East,” and a second over the central Pacific Ocean, near the Phoenix Islands, known as “TDRS-West”—would fill this urgent communications and tracking need. TDRS-A was launched in April 1983, but was almost lost when its Boeing-built Inertial Upper Stage (IUS) booster failed to insert it into its proper orbit. Only by using the satellite’s own hydrazine thrusters were controllers able to gradually maneuver it into its final location, although the result was that its operational lifetime was shortened. Ongoing problems with the IUS meant that it was almost three years before the second satellite, TDRS-B, could be launched … and that was the primary payload aboard the ill-fated Challenger on 28 January 1986.
Two more TDRS satellites (C and D) were launched in September 1988 and March 1989, the former replacing the doddery TDRS-A in the west (slightly south of Hawaii) and the latter taking up position in the east, near Brazil. Unfortunately, TDRS-C also succumbed to anomalies which affected its Ku-band relay capability. A fourth satellite, TDRS-E, was launched in August 1991 and positioned at 175 degrees West longitude to become the primary provider of communications services over the Pacific from October 1991. TDRS-A and TDRS-C, meanwhile, were relegated to the status of on-orbit “spares.”
This left only TDRS-D and TDRS-E in fully-operational status … which meant that no spare existed to support them in the event of problems. The successful arrivals of TDRS-F in January 1993 and TDRS-G in July 1995 filled this backup capability. This enabled the network to be rearranged to include two fully-operational satellites in the East and West spots, plus the fully-functional TDRS-A as a spare and the partially-functional TDRS-C designated to support NASA’s Compton Gamma Ray Observatory.
A few months before the launch of the final first-generation TDRS, in February 1995, NASA’s Goddard Space Flight Center of Greenbelt, Md., chose Boeing to build three second-generation satellites under a contract valued at $481.6 million. Based upon the 601 “bus,” the new satellites were intended to augment the Ku-band and S-band capabilities of the first generation with the higher-bandwidth Ka-band. The ground stations at White Sands were modified to accept the new satellites. TDRS-H was launched atop an Atlas booster in June 2000, followed by TDRS-I in March 2002 and TDRS-J the following December. Although TDRS-H suffered problems with its multi-access antenna and TDRS-I lost pressure in one of its four fuel tanks shortly after launch, the second generation has supported International Space Station and other operational assets for more than a decade.
On 4 April 2013, NASA marked the 30th anniversary of the TDRS-A launch. It has been a long and rocky road for a network which was born with such promise, but very soon fell on hard times, yet matured to shine throughout the heyday of the shuttle era and today’s International Space Station. It has provided the data-relay capability for the astonishing scientific returns of the Hubble Space Telescope … and even the doddery first satellite has contributed to more down-to-earth achievements. In 1998, NASA allowed scientists at the Amundsen-Scott base in Antarctica to employ TDRS-A as a relay for research data and it supported a medical emergency at McMurdo Station, allowing scientists to conduct a telemedicine conference with doctors in the U.S. Several of the first-generation satellites are now out of service.
TDRS-A—“the queen of the fleet,” according to NASA-Goddard’s Space Network Project Manager Roger Flaherty—was deactivated in October 2009, followed by TDRS-D in November 2011. The remaining first-generation satellites (C, E, F and G) are expected to be retired by 2015. And by the end of that year, the entire third generation will be complete, with TDRS-M inserted into orbit. These satellites are immeasurably more powerful than their predecessors, but, like them, they will enable TDRS to evolve through the middle of the next decade as the United States’ primary tracking and data-relay service provider for its key human-exploration programs and scientific endeavors.
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