Have Mobile Base System, Will Travel

A year ago Canadarm2 was the star of the show when Canadian Astronaut Chris Hadfield and his crewmates, in a series of spacewalks, installed it on the International Space Station (ISS) during shuttle flight STS-100.

Now the robot arm's supporting cast is about to be launched. The Mobile Base System (MBS), a 1450-kilogram aluminum work platform that will move Canadarm2 and other equipment around the Station, will be installed during STS-111, scheduled to lift off from the Kennedy Space Center on May 30, 2002.

During this mission Canadarm2 will place the Mobile Base System atop another platform—the U.S.-built Mobile Transporter (MT)—that can run along tracks on the trusses that span the Station. The arm will then step off the U.S. laboratory module, for the first time since its installation, and onto the Mobile Base System, a move that will greatly extend its reach.

"The intent is to cover the great length of the Space Station," said Alan Robins, the Canadian Space Agency's (CSA) chief systems engineer for the Mobile Servicing System (MSS). The Mobile Servicing System is the collection of technologies that Canada has contributed to the Station. He compared the Mobile Base System/MT system to a utility truck cruising down the highway, carrying construction materials and equipment and a robot arm for lifting things on and off the truck. "It drives from the depot to your house and it's got everything it needs."

The MSS was built by MD Robotics Inc., the Brampton, Ontario, company that also built Canadarm2 and the shuttle's Canadarm. Along with other MSS components, it is essential to the construction of the Space Station, which is being built by an international partnership involving Canada, the United States, Europe, Russia and Japan. The final MSS element—a smaller advanced robot called the Special Purpose Dexterous Manipulator (SPDM)—is scheduled for launch later in the Station assembly, to perform maintenance activities that would require astronaut spacewalks. The mobility provided by the Mobile Base System is critical, despite the fact that Canadarm2 can "walk" around on its own. Canadarm2's jointed design gives it the unique ability to move around the Station end-over-end by grasping connection devices known as Power Data Grapple Fixtures (PDGFs) located on exterior surfaces. These connection devices, the PDGFs, provide electricity to the arm and transfer computer commands and video signals between the arm and two robotic workstations inside the Station.

"If the Mobile Base System was not available, you'd have to be extremely careful about placing the PDGFs so Canadarm2 could independently walk along," said John Dunlop, assistant program manager for MSS Systems with MD Robotics. "The MBS gives a lot more freedom of range to Canadarm2—it will allow the arm to access considerably more of the Station than it's currently able to when it's anchored on the lab." The Mobile Base System is important for another reason: walking end-over-end occupies both ends of the arm, leaving none to carry other objects needed for construction or repair jobs. There is no point in walking Canadarm2 to a work site "if you can't do the job at the other end without the other bits and pieces," Robins said. "By putting it on the MBS, you've got one end free to manipulate and carry elements."

The arm can attach itself to any one of four Power Data Grapple Fixtures (PDGF) located at each corner of the Mobile Base System; in fact, when not in use, it might be stowed with both arms grasping PDGFs to protect its "hands"—known as Latching End Effectors or LEEs. In the year it has been attached to the U.S. lab, one of its hands has often been left free, exposed to the space environment. This creates the risk of damage due to space debris or to movements caused when the Station's orbit is periodically reboosted.

"If it's double-grappled, it's in a safer configuration," said Adam Mizera, the CSA's operations flight lead for the mission on which the Mobile Base System will be installed. He said that during the mission, Canadarm2 will be placed in several double-grappled positions to test the PDGFs on the Mobile Base System.

Eventually the Special Purpose Dexterous Manipulator (SPDM) will also ride on the Mobile Base System attached to a PDGF. Designed to perform delicate repair and maintenance tasks, it will work directly from the Mobile Base System or be plucked up by Canadarm2 and delivered to a work site.

"We anticipate the Special Purpose Dexterous Manipulator will spend the majority of its life held by Canadarm2 to give it the reach to get to works sites," said Dunlop. "But when Canadarm2 has its own assembly operations or the SPDM has to do a maintenance function on Canadarm2 itself, it will sit on one of the MBS Power Data Grapple Fixtures."

The Mobile Base System has another device known as a Payload and Orbit Replaceable Unit Accommodation (POA). It functions just like the Canadarm's Latching End Effector but does not have the "sense of touch" of Canadarm2. The POA is able to provide electrical power and exchange data while grappling very large and heavy payloads or Orbit Replaceable Units (ORUs). ORUs are self-contained packages that can be swapped for new units when they wear out or fail on the Station.

Payloads with grapple fixtures can be parked on the Mobile Base System when attached to the POA by either the Shuttle's Canadarm or the Station's Canadarm2 and then carried to wherever they're needed on the Station.

"A payload may require power to keep it alive for a period of time," said Dunlop. Instead of occupying the manipulator as a source of power, "you can sit the payload out of the way and keep it happy with power and data for hours or days until you're ready to put it where it belongs."

The POA will be used to carry large structural elements such as trusses, said Mizera. In an upcoming mission, Canadarm2, riding on the Mobile Base System, will lift a 15.4-tonne truss from a temporary location on the Station and stow it on the POA; the Mobile Base System will then carry this truss about 36 metres to the end of another truss, allowing Canadarm2 to assemble the two components together.

"The Mobile Base System will be involved in building the whole truss structure of the Station," said Mizera. "During the next few missions, there will be a lot of truss assembly, so it has a pretty crucial role."

The Mobile Base System also has another device allowing it to carry payloads that do not have a PDGF. It is known as MBS Common Attach System (MCAS). The MCAS consists of three payload alignment V-guides with ready-to-latch indicators, a powered claw to grip a special capture bar on payloads, a camera target for payload berthing and an Umbilical Mating Assembly (UMA), which provides power and data exchange to payloads such as scientific experiments. A number of experiments will be positioned on the external structure of the Station, each payload held in devices similar to the MCAS.

In total, the Mobile Base System can transport 20,900 kilograms of payloads, the 1800 kilogram Canadarm2, and the 1700 kilogram SPDM – all on an aluminum structure that itself only weighs 1450 kilograms. It's not a speed demon, however; it will trundle along the tracks at a maximum rate of about 90 metres per hour.

Finally, the Mobile Base System will serve as a work platform not only for Canadarm2 maintenance, but also for astronauts and cosmonauts during spacewalks or Extravehicular Activities (EVAs). It's equipped with foot restraints and hand-rails, as well as room for any special tools and equipment needed during an EVA.

Canadian Astronaut Steve MacLean, who was recently assigned to fly on STS-115 in April 2003, will participate in two spacewalks to expand the Station's truss structure.

"Being the first Canadian to operate Canadarm2 and its Mobile Base will certainly be an emotional moment for me," he said.

Canadarm2 will install the Mobile Base System

The Mobile Base System is scheduled to be installed five days after STS-111 is launched and the second day after the shuttle docks with the Station. Reaching into the shuttle's cargo bay, Canadarm2 will grapple the Mobile Base System, providing power needed to operate a Mobile Base System camera, as well as a capture claw that will be used to mate it to the MT (installed on a previous mission, STS-110 in April 2002.) Once assured that the Mobile Base System is receiving power and is warmed up, the astronauts will then remove the thermal blankets used to protect it from the extreme temperatures of space. The blankets are no longer required once the Mobile Base System is powered. This will be the first time Canadarm2 handles a powered payload.

Canadarm2 will then lift the Mobile Base System out of the cargo bay and, after a checkout to make sure everything is working properly, will position it close to the MT where it will be left overnight to allow the temperatures of the two elements to equalize.

The next day, flight day 6, astronauts inside the Station will command Canadarm2 to mate the Mobile Base System to the MT, using visual images from a Mobile Base System camera to guide them.

"That's going to be a critical part of the mission," said Robins. "The Mobile Base System is a major piece of structure that weighs well over a tonne. Manoeuvring it into position to within five centimetres and a degree is going to be a key element in the deployment."

On flight day 7, EVA astronauts will connect cables between the Mobile Base System and the MT that will allow power and data to flow to the Mobile Base System through the MT rather than through Canadarm2. This reversal of the power distribution system will become the status quo; under normal operating conditions, Canadarm2 will receive its power through the Mobile Base System when riding on it, not the other way around.

During STS-111, "we'll be going through a series of tests to ensure the system is operational in that configuration," said Robins.

The MT's own cable provides enough power to move the Mobile Base System, but not enough to operate it, so the MT will have to "plug in" at one of ten work sites around the Station to draw enough power for the Mobile Base System, Canadarm2 or the SPDM to perform their tasks.

"As the Mobile Transporter is travelling, no power is being provided to the Mobile Base System, so if it ever gets stuck between worksites, it's a problem because a lot of MBS components are sensitive to cold," said Mizera.

There are two ways of dealing with this. One is to position the Station to keep the Mobile Base System in direct sunlight and the other is to send astronauts out to plug in the Mobile Base System with what is essentially an extension cord.

During the EVA on flight day 7, the astronauts will adjust the position of the POA, which had to be launched folded in on itself, so the Mobile Base System could fit in the shuttle's cargo bay. They will also move the camera that was used for the mating task to its final position on a mast rising up from the Mobile Base System. This camera will be very useful to help position payloads on either the MCAS or POA as well as view specific work sites and Canadarm2 or SPDM. Throughout the installation procedures, Canadarm2 will remain attached to both the Mobile Base System and the lab, still able to provide power in case there are any problems with the power link between the MT and the Mobile Base System. Later, the arm will be commanded to "step off" the lab for the first time. This will allow engineers to run tests on the end effector that has been attached to the lab for the past year, Mizera said.

The Mobile Base System/MT system will not be moved during the STS-111 mission, but tests are planned sometime before the next assembly flight, STS-112, scheduled for August. It's anticipated the Mobile Base System will be used for a truss assembly task, but it can do the work without being moved, Mizera said. For the following flight, STS-113 in September, the work site is on the other side of the truss and the Mobile Base System will have to be moved.

"Other pieces of the truss will be added over the next two years until the Station is complete," said Dunlop. "When it's finally complete, it will be the length of a Canadian football field. The big thing is extending the reach of Canadarm2 along that entire length."

Unlike Canadarm2, the Mobile Base System won't have a Canadian astronaut in space supervising its installation, but the two MSS operators, cosmonaut Valeri Korzun and astronaut Peggy Whitson, part of the fifth ISS crew, have been trained and certified at CSA headquarters in Saint-Hubert, Quebec.

Canada also provides a group of qualified Flight Controllers and other support crew on the ground. The CSA, which is responsible for Canadarm2, has a Space Operations Support Centre and an Engineering Support Centre in Saint-Hubert, Quebec, that have direct data and voice links with NASA's Mission Control during flights in which the arm is used.

Computer data sent down from the Station will enable them to closely monitor the entire installation process "to ensure the Mobile Base System, as well as the rest of the system, is doing exactly what the operation calls for," Robins said.

There will also be Canadians in Mission Control. Astronaut Steven MacLean will be lead Capcom on STS-111, a job that involves acting as the primary conduit of information between the shuttle crew and Mission Control. Astronaut Bob Thirsk will be the lead Capcom talking to the Space Station crew during the mission.