Twenty years ago today, power—in the form of two immense, electricity-generating solar arrays, together with associated batteries and radiators—began the long journey to the International Space Station (ISS).
Launched on 30 November 2000, shuttle Endeavour and her STS-97 crew of Commander Brent Jett, Pilot Mike Bloomfield and Mission Specialists Joe Tanner, Carlos Noriega and Canadian Space Agency (CSA) astronaut Marc Garneau became the first humans to visit the infant station’s incumbent Expedition 1 crew and, during the course of three sessions of Extravehicular Activity (EVA), delivered, installed and activated one of the largest, longest and most massive structures ever put into space.
The $600 million P-6 Integrated Truss Structure (ITS) would go on to play a key role in powering the station during its early evolution and has taken center-stage in a number of recent operations, including last year’s multi-spacewalk campaign to swap outdated nickel-hydrogen batteries for more modern lithium-ion ones.
Its story extends back to the dawn of the ISS era, when Boeing was selected in January 1995 to design and develop the new station, under the language of an initial $5.63 billion contract from NASA. Over the following years, its various components took shape, with agreements reached in August 1995 for the United States to purchase the $190 million Russian-built Functional Cargo Block (FGB)—later renamed “Zarya” (“Dawn”)—which would become the first ISS element to be launched into orbit.
By the end of 1996, the first of (originally) two U.S.-provided nodes had sailed through its final pressure test and the FGB had attained structural completion. Plans called for the FGB to launch atop a Russian Proton-K rocket in November 1997, after which the STS-88 shuttle crew would deliver Node-1 in early December. This would followed by the Russian-built Service Module (SM), later named “Zvezda” (“Star”), thereby establishing essential life-support and crew quarters, and the STS-92 shuttle assembly mission, ahead of Expedition 1.
Following the arrival of the Expedition 1 crew—Commander Bill Shepherd of NASA and Russian cosmonauts Yuri Gidzenko and Sergei Krikalev—the stage would be set for a stream of shuttle missions to deliver and activate major station hardware. The first piloted flight to visit an occupied ISS was STS-97, originally targeted for a March 1999 liftoff.
In readiness for what was expected to be an eight-day mission, astronauts Tanner and Noriega were assigned in June 1997 to support the development and execution of a pair of EVAs to install and activate the P-6 truss segment. “The EVA crews…were named quite early,” Tanner told a NASA interviewer, “so that we could work the hardware issues that came up and there were numerous issues to work.” As the mission evolved over the next few years, these two EVAs developed into three, each baselined at 6.5 hours.
However, as circumstances transpired, the launch of Zvezda was repeatedly delayed, first from April to December 1998, then to mid-1999 and eventually into the summer of 2000, which created a domino-like effect on the shuttle manifest. By August 1998, when Jett, Bloomfield and Garneau were named to the STS-97 crew, their launch had slipped to August 1999. And as Zvezda’s launch date slipped further and further to the right, eventually reaching orbit in July 2000, STS-97 also slipped and eventually settled on a target of No Earlier Than (NET) 30 November.
“One of the jobs as a Commander is to keep your crew focused,” explained Jett in a pre-flight NASA interview, “make sure you don’t train too early, peak too early. Preparing for a shuttle mission is a little bit tricky, because you know towards the end of your training flow there are going to be a lot of things that are going to be required to be done late and there’s going to be a lot of changes, so you need to—just like a football team or a basketball team, as you’re going through the season—you need to make sure you’re really on your game at the end.”
Bloomfield added that Jett made sure the crew had time off when they needed it, “and that we haven’t gone without vacations for two years”, whilst Tanner stressed that he and Noriega had certainly not been focused solely on STS-97 from their June 1997 assignment to their November 2000 launch.
“But it has been a little bit difficult as the launch date kept changing and we knew when we were assigned that the date would probably change,” said Tanner. “And we have a record of it on our board: I think we’re up to our 11th launch date for the same mission!” For Noriega, there were tough times, “but we kind of just joked about it, because it wasn’t just us!”
Garneau had a slightly different perspective, having trained for 60 days as a Payload Specialist for his first shuttle flight, then for nine months for his second…and more than two years for his third. “NASA has developed a training system and training rate and rhythm that optimized towards peaking at the moment of launch,” he said, “and if you find that the goalposts are continuously moved to the right, you have to recalibrate yourself, because you don’t want to peak too early, because then you run the risk of burnout or fatigue or being less than at your optimum capability.”
In the months before launch, Jett realized that none of the STS-97 crew—with a total of eight prior shuttle missions between them—had ever scrubbed a launch and had always flown on their first journey out to the pad. This left him with one of two thoughts as night fell on 30 November 2000: either that the five astronauts were on a good streak and would launch on-time or that they had used up most of their good luck.
As circumstances transpired, the former was the case, and Endeavour lit up the night sky with an on-time liftoff from Pad 39B at the Kennedy Space Center (KSC) in Florida at 10:06 p.m. EST. According to Bloomfield, the weather conditions were so clear that the launch could be seen from as far north as New York City. (But for Jett, Bloomfield and Tanner, they had indeed used up their good luck; for on their next shuttle missions, all three would succumb to repeated launch delays.)
With the P-6 ITS component tipping the scales at around 35,000 pounds (15,875 kg), STS-97 marked the second-heaviest shuttle payload at that time, sitting just behind STS-37 in April 1991, which delivered the 37,000-pound (17,000 kg) Compton Gamma Ray Observatory (CGRO) into orbit.
Within minutes of achieving orbit, the crew opened Endeavour’s payload bay doors and received a “Go” for orbital operations from Ascent Flight Director Wayne Hale. After a sleep period, their second day aloft focused on a pair of rendezvous “burns” to bring the shuttle into the proper alignment for its docking with the ISS on 2 December, as well as checking out Tanner and Noriega’s U.S.-built Extravehicular Mobility Unit (EMU) space suits and the Canadian-built Remote Manipulator System (RMS) mechanical arm by Garneau.
By the early hours of Flight Day 3, Endeavour had closed to a distance of about 700 miles (1,100 km), approaching the space station from “below”, along the so-called “R-Bar” (or “Earth Radius Vector”). The Termination Initiation (TI) maneuver was executed about 2.5 hours prior to docking, after which Jett—situated at the aft flight deck controls—assumed manual control and oversaw the final phases of the rendezvous. At 500 feet (150 meters), he executed a 180-degree yaw maneuver to position Endeavour into a “tail-forward” attitude, pausing at 30 feet (10 meters) for approval from U.S.- and Russian-based flight control teams, before accomplishing a smooth docking at Pressurized Mating Adapter (PMA)-3 on the Earth-facing (or “nadir”) port of the Unity node at 3 p.m. EST.
Docking in this fashion was quite dissimilar to previous shuttle-ISS missions, which had typically maneuvered around the “top” of the station, before moving in to the forward-facing port of the Unity node. Before launch, the Center of Gravity (CG) constraints of having the two vehicles linked, almost tip-to-tip, with the station’s center of mass nowhere near Endeavour’s center of mass, had posed a measure of concern for Jett.
“What I think is very unique about where we’re docking is the fact that, as we close in and actually make contact to get capture with the station, the CG of the shuttle is not going through anywhere close to the CG of the station,” he told a NASA interviewer. “If you think about momentum and mass and velocity and, in all the other dockings we’ve had as the shuttle’s approached and actually made contact with the station, the CG of the shuttle was pretty close to being in a line through the CG of the station; not exactly, but there was a significant amount of mass along that line.”
With Endeavour having docked in a tail-forward attitude, its CG mass was way back in the payload bay and the station’s CG was way out over the shuttle’s nose. “So you can imagine, if you took two objects and had them close together and then they just made contact at the tip, the post-contact dynamics could be substantially different,” Jett explained. “That has gotten the most attention in my mind in terms of what does that do to our probability of capture; we don’t want to be the first shuttle ever to bounce off the station and not capture. That sometimes keeps me up at night!” In order to avoid any unwanted “tip-off” motions, Jett had to align the docking hardware to within just 3 inches (7.6 cm) of tolerance.
By this time, the Expedition 1 crew had been in space for a month, but were restricted to just the two Russian-built modules, Zvezda and Zarya, with Unity temporarily sealed-off until STS-97’s arrival, as part of a power-conservation plan to maximize solar heating on the recently-installed Z-1 ITS element and keep its dormant Control Moment Gyroscopes (CMGs) warm. This situation was expected to change after Jett’s crew had installed the P-6 ITS, whose twin Solar Array Wings (SAWs) would effectively end the reliance of the U.S. Orbital Segment (USOS) upon the smaller Zvezda and Zarya solar arrays, ahead of the arrival of the U.S. Destiny lab in early 2001.
The sheer size of the P-6 ITS cannot be underestimated, for its installation transformed the ISS from an exceptionally large, inhabited spacecraft into one of the brightest and most easily identifiable objects in the night sky. When folded into its launch configuration, it virtually filled Endeavour’s 60-foot-long (15-meter) payload bay, and consisted of three discrete components.
Of these, the Photovoltaic Array Assembly (PVAA) was equipped with a pair of dual-channel SAWs—each of which extended, in opposite directions, to a length of 115 feet (35 meters) when fully deployed—and a Beta Gimbal Assembly (BGA) to support the mast canister, Electronic Control Units (ECUs) and Sequential Shunt Units (SSUs) to regulate voltages and a Bearing, Motor and Roll Ring Module (BMRRM) to deploy and rotate the structure and perform power-transfer functionality. With over 65,000 solar cells in the two SAWs on the P-6 element, an electrical potential of close to 64 kilowatts would be generated by the PVAA, then routed to the 17,000-pound (7,700 kg) Integrated Equipment Assembly (IEA) for control, storage and distribution.
Primary and secondary distribution, together with protection and fault-isolation, was then provided by Direct Current Switching Units (DCSUs), DC-to-DC Control Units (DDCUs), Battery Charge/Discharge Units (BCDUs) and storage batteries. In addition to power, the Photovoltaic Thermal Control System (PVTCS) housed an ammonia coolant supply, twin Pump Flow Control Systems (PFCSs) and Photovoltaic Radiators (PVRs) were designed to cool the hardware and reject waste heat into deep space.
Yet the intricate docking of Endeavour with the space station on 2 December 2000 was merely the prelude for one of the most challenging shuttle missions of all time. Over the next several days, the STS-97 and Expedition 1 crews were intentionally separated from each other, as the visitors brought power to the residents and labored outside the windows of their orbital home. Aside from a few “goodies” left on either side of the hatches, it was not until close to the very end of the docked phase that the respective skippers of both vehicles would clasp hands and exploit their shared military heritage by ringing the ceremonial ship’s bell for the first time.
One Comment
One Ping
Pingback:Expedition 64 Spacewalkers Prepare for ISS Power Upgrades « AmericaSpace