With the critical Zvezda module now in place, NASA began a rapid series of Shuttle launches to further outfit the young International Space Station.
NASA paced itself. Many ISS components or additional modules that could fit in the remaining room of an Orbiter’s payload bay along with the supplies were not yet ready for deployment. Even if an Orbiter could pack more components for assembly, the amount of orbital time needed to assemble (as well as the ground training required) would exceed an Orbiter’s normal consumables span. NASA and the Russian Space Agency took care to make sure that each mission completed their work without complication.
The STS-92 mission, the last before mission before Expedition 1’s arrival, carried two new ISS elements for Assembly 3A. In the payload bay of Discovery was the Z1 Truss and Pressurized Mating Adapter (PMU) #3.
A truss is a load-bearing structure. In this case, Z-1 formed the first part of the Integrated Truss Structure, the station’s backbone. Over additional missions, the truss would expand port and starboard to hold additional, larger ISS elements such as the solar arrays and cooling panels.
The Z1 Truss, while a small element, included four control moment gyroscopes, devices that would later aid in keeping the station at a stable position to the earth and sun, minimizing the use of limited propulsion fuel.
Also included in the Z1 Truss were communications gear and plasma contactors, devices that would shunt away any electrical charges built up on the ISS exterior to avoid dangerous electrical shock to components or crew. The Z-1 Truss also provided the first elements needed for cooling the station.
The third and final PMU would be installed for now to a spare Common Berthing Mechanism (CBM) port on Unity as a backup docking port for future Orbiters. PMU #3 would be moved about the station during later assembly missions, typically used when the Orbiter needed to install components along the truss while keeping the Orbiter’s body out of the way, docked to a PMU on the nadir of a node.
STS-92 launched on October 10, 2000, only twenty-three days before the Expedition 1 crew were scheduled to man the station as the first permanent residents.
Also aboard the Z1 Truss were the first of many DC-to-DC Converter Units, or DDCUs, for use throughout trusses and modules.
This devices were critical first elements of the upcoming installation of the first of the four solar arrays. Like transformers in an area of a city that steps-down the higher transmission AC voltages from an electrical power substation to a voltage suitable for home electrical appliances, the DDCU steps down the power from the solar array’s 31 kilowatt primary power subsystem to the 160-volt secondary power subsystem, a distribution system which would eventually charge batteries on the secondary power system for use when the ISS orbited in earth’s shadow. The secondary power also (with an additional step-down) provided direct power for all module systems.
After four spacewalks and with the use of the Shuttle’s robotic manipulator arm, the two components were installed. During the last spacewalk, still connected to tethers, the spacewalkers made the first of a series of tests by Shuttle crews of the SAFER emergency jetpack.
On the next trip to outfit the ISS, the STS-97 crew on Endeavour would be greeted by the station’s first expedition crew as they added more to the truss and installed the station’s first large solar array.
Next: Assembly 4A.