Maintaining a space station
International Space Station partners will face many challenges once assembly is complete - one of which is the maintenance and upkeep of this new satellite.
by Frank Bokulich, Editor
In addition to the numerous challenges throughout assembly of the International Space Station (ISS), station partners have been preparing themselves for the daunting task of maintaining the station throughout its lifetime. Since construction of such a structure has never been undertaken, it presents unique challenges for station crew maintaining the massive satellite. It also requires the use of many unique diagnostic and repair tools.
 IVAs are maintenance activities crew members perform inside the station. |
Unlike the Space Shuttle, the ISS never returns to the ground. Therefore, all repairs must be made in orbit. Since the station is designed to operate for many years, all repairs must be designed to be permanent. Temporary fixes may prove to be too risky and take more of the crew's time than permanent repairs (the crew operate and maintain station systems).
Both ISS crew and flight controllers will be required to have some on-orbit maintenance experience. Crew members will be required to troubleshoot, service, and/or replace any defective orbital replacement unit (ORU). Flight controllers will have to perform procedures to determine the status of a particular ORU or isolate it from the rest of the system so that a crew member can remove and replace it.
NASA and the Russian Space Agency differ in their approaches to on-orbit maintenance. NASA's approach has been to remove and replace defective ORUs in their entirety. In limited cases where time considerations and the lack of spare ORUs do not permit replacement, repairs are made. This approach is based on the idea that replacing ORUs requires less crew training and reduces the amount of crew time required to make repairs, thus increasing the amount of time to conduct research.
The RSA approach for Mir was to repair ORUs in-situ on orbit. If the particular ORU was a critical one, the temporarily repaired ORU was replaced by a spare when delivered by the Progress resupply vehicle. In the past, the RSA has had limited down-mass capabilities, requiring all ORUs to be maintained on orbit. Generally, ground servicing has not been an option.
Currently, three distinct methods for performing on-orbit maintenance exist: intravehicular activity (IVA), extravehicular activity (EVA), and extravehicular robotics (EVR). IVAs are maintenance activities crew members perform inside the station, while EVAs are performed outside the habitat using special tools, restraints, and aids. EVRs are conducted using the Space Station Remote Manipulator System (SSRMS) alone or in conjunction with EVA to perform external maintenance. The SSRMS may be used to move the crew member to the work site or move the work to the crew member.
 EVRs are maintenance or assembly activities performed using special robotic equipment such as the Space Shuttle robotic arm or the ISS robotic arm. |
One challenge faced by crew members conducting maintenance activities is the zero-g environment. Tools and loose ORUs have to be tethered and small parts restrained so they do not float away. Crew members have to also determine the best method of anchoring themselves while they attempt to remove and replace defective ORUs and parts. Also, when conducting maintenance in an IVA environment, particles of debris caused by drilling and filing operations have to be collected and disposed of so they do not contaminate the crew's environment.
To meet this challenge as well as others, crew members are equipped with tools designed for the upkeep of the station. One such piece of equipment is the maintenance work area (MWA), which is basically a portable work table with a tabletop measuring 36 x 25 in. The MWA can be folded and stowed inside a storage drawer. In its folded configuration, the MWA measures about 9.25 x 15 x 26 in. It clamps to a slotted mechanism - similar to aircraft seat tracks - on either side of a rack and can be rotated either up or down. The MWA is sometimes used to restrain ORUs while maintenance is being performed. ORUs may be attached to the table via seat track mechanisms, clamps, and bungee cords.
To contain debris created by maintenance operations such as cutting, drilling, filing, or soldering, the MWA features a containment system, which is a clear plastic enclosure surrounding the work area. Enclosure rigidity is provided by seven structural members, two arched members located at each end of the enclosure, and five straight horizontal members that connect the two end pieces. When used, the enclosure and its structural members are removed from a storage drawer; the structural members are inserted into the plastic envelope then clamped to the surface of the MWA. Fully assembled, the containment system measures 34 x 24 x 26 in. The containment system also features four glove ports, one at each end and two on one side; a 6 x 12-in rigid, clear plastic viewing window; a 14 x 26-in access flap for ORU access; a 6 x 12-in filtered air intake; and two utility ports that can be connected to a vacuum hose or an electrical cable. The MWA and its containment system are scheduled for delivery on assembly flight 6A in April.
Other equipment used by crew members consists of both hand and diagnostic tools. Hand tools can be further subdivided into IVA and EVA types. IVA tools are common tools found in most automobile repair shops: ratchets, adapters, sockets, screw drivers, wrenches, pliers, hacksaws, chisels, files, and hammers. A hose and cable kit, a tap and die set, and a sewing kit have been developed along with special tool kits for repair of electronics, fiber optics, and fluid lines. EVA tools have special provisions for tethering and can be used when wearing EVA gloves.
Diagnostic tools will be primarily used to perform fault isolation. Crew members will remove a defective ORU, carry it to the MWA, open it up, and use the diagnostic equipment to pinpoint the faulty component. Once identified, the component will be replaced, and the diagnostic equipment is again used to determine if the repair was successful. For situations in which the ORU cannot be removed, the diagnostic tools can be carried to the unit for maintenance.
 EVAs are performed outside the station using special tools, tethers, and aids. |
One such diagnostic tool is a scopemeter, which is a combination of a multimeter and oscilloscope manufactured by the Fluke Co. It can be used to measure voltage, current, and resistance to detect, digitize, store, and display waveforms with frequencies up to 100 MHz. Using special probes, it can also measure temperature and pressure. The scopemeter features a liquid crystal display and is powered by a rechargeable power pack. This tool was shipped to ISS during flight 2R in October 2000. Another one will be sent up on assembly flight 6A in April.
Another piece of equipment to be hauled on flight 6A is a pin kit, which is housed in a Nomex pouch and has some of the same components as the one used by the Space Shuttle crew. Items unique to the ISS pin kit include prefabricated jumper/test cables, materials for manufacturing custom jumper cables, alligator clips, and assorted fuses.
ISS crews also use a logic analyzer, which consists of a portable computer, a portable computer memory card international adapter (PCMCIA), and LabVIEW software. The PCMCIA card has numerous probes to allow the computer to monitor several different points in a circuit or several circuits simultaneously. The logic analyzer application software is designed to monitor the logical state ("1" or "0"; high or low, etc.) of particular points within a circuit or electronic component.
A function/sweep generator produces standard waveforms - sine, saw-tooth, and square - to diagnose electronic circuits and perform fault isolation. Generally, this tool is used to inject a known reference signal (wave) into a circuit; the output of the circuit is monitored with the scopemeter or the logic analyzer. The function/sweep generator used onboard the ISS is an off-the-shelf unit that has been repackaged in a new cabinet and adapted to operate from 120 V dc power.
A power strip is also employed. It plugs into a standard UOP and provides four UOP type sockets, each of which can be switched on and off independently. The four sockets can be used to provide data connections. The unit also features a removable fuse.
Improving the view
L-3 Communications Corp.'s Interstate Electronics Corporation division was selected by MacDonald Dettwiler to supply its Warrior Vision flat panel displays for the robotic work station aboard the International Space Station. Scheduled for launch on flight 5A.1 in 2001, the displays are used as a visual reference to control the station's sophisticated robot arm, also known as the Canadarm used aboard space shuttles. The displays enable astronauts to perform formidable tasks such as manipulating large payloads and satellites and assembling projects that are too big to be launched completely assembled from Earth. Two robotic workstations will be located aboard the space station in the U.S. Lab Module and one in the Cupola, from which astronauts can build and perform maintenance on the station.
 The Warrior vision system of flat panel displays from Interstate Electronics Corp. will be integrated into the robotic workstations aboard ISS. |
The flat panel displays will be integrated into the robotic work station with special software to provide visual feedback from the Space Station Remote Manipulator System, Special Purpose Dexterous Manipulator, Mobile Base System, and Artificial Vision Unit. The units are lightweight active matrix liquid crystal displays with high quality 640 x 480 resolution. They are designed to provide the required viewing angle while meeting critical space and weight limitations.
Ruggedized to accommodate the stresses of launch and space travel, the displays are designed to meet severe operational and environmental requirements encountered by ISS. Because the system is critical to the overall mission, there is no tolerance for breakdowns. According to the company, the system has a mean time between failure rating of 13,000 h.
