Spaceflight Now STS-110

Station's mobile transporter to take test-drive today
Posted: April 15, 2002

The Atlantis astronauts will operate the first space railroad today, running a $190 million flatcar about 32 feet along tracks that currently lead nowhere. In fact, there's a 240-mile drop off on either end.

But as NASA adds sections to the international space station's solar power truss, the flatcar eventually will creep the length of a football field, carrying the station's $600 million Canadarm2 space crane to work sites as required to continue the lab's assembly.

"The mobile transporter is nothing more than simply a railroad car," said Atlantis skipper Michael Bloomfield. "There's actually a set of railroad tracks that will span the length of this truss once it's up there. And you have the robotic arm that's up on the international space station, and we want to be able to use this robotic arm, all the way out to both ends of the truss.

"And so what the robotic arm can do is, it can actually grab on to a grapple that's on top of this mobile transporter and it can move from one end of the truss to the other. And so we can use it to transport, in the assembly phase, other truss sections to the end of the truss that's already built. Kind of like you're building a railroad, you run the locomotive out to the end of the track and then you lay more track and you keep going. That's exactly how we're going to build the international space station."

The first section of the station's solar power truss was bolted to the top of the Destiny laboratory module during spacewalks Thursday and Saturday. The mobile transporter and the tracks it will creep along are mounted on the forward face of the $600 million S0 truss. During a spacewalk Sunday, astronauts Steven Smith and Rex Walheim removed launch locks that held the transporter in place during the shuttle's climb to space, clearing the way for "first motion" today.

The 44-foot-long S0 truss is the central element in a nine-section beam that eventually will stretch 356 feet. Huge solar arrays will be mounted on each end and radiators will be deployed halfway out each side to keep the station's electronics cool.

The truss was designed to be built using the station's robot arm, riding atop a mobile transporter that will run the length of the beam as it's being assembled. The completed truss will have 10 work sites where the arm can lock itself down and plug into power sockets and video distribution leads.

The S0 truss, the 27,000-pound central element mounted atop the Destiny module, has two such work sites. During tests today, the astronauts will send commands from a laptop computer to move the transporter from its launch position to work site 4 on the starboard side of S0. From there it will move the other way to work site 5 on the port side and then back to work site 4.

"A lot of the work is going to be behind the scenes," pilot Stephen Frick said in a pre-flight NASA interview. "While we're sleeping, the station flight controllers are going to be going through some very long checkout procedures of the mobile transporter to make sure everything's up and operating and ready when we're ready to move it. So they're going to have worked for hours before we even get ready to go.

"And then Carl (Walz) and I are going to get on the PCS, the control software on the station, and we're going to command the mobile transporter to move from its launch site to two work sites. We have one called work site 4 and one called work site 5 on the S0 truss.

"It's going to be exciting for us because it'll be the first time we'll get it to see it move," Frick said. "But I've got to say it moves at an inch a second, so I don't want people to get their hopes up too much about a NASCAR race up there on the truss."

As it turns out, an inch per second is the design specification of the transporter when it is unloaded. Ben Sellari, the launch package manager for the S0 truss, said the speed will be even slower for the tests planned today. The transporter measures 102 inches by 107 inches and weighs 1,923 pounds.

The transporter moves using a linear drive unit that "is basically like the motor, transmission and brakes in your car," Sellari said. "This is what makes the mobile transporter go. It has two drive devices on it that engage the rail and through friction, allow the mobile transporter to go up and down the rails."

The friction is provided by five spring-loaded wheels. Power and telemetry is provided through two redundant ribbon cables that play out from reels on either side of the transporter.

For the tests today, "our most important role up there is as a safety observer," Frick said. "The commands we're going to send could be sent from the station (but) we're actually going to send them from the shuttle on the station laptop. The reason we're going to be over there is because we can look directly out the overhead window and watch the mobile transporter as it moves.

"The software has been checked out on the ground and it's worked very well. But it's never been done in space and if anything should happen that's unexpected, we want to be there, ready to just stop it and give us a chance to look at what happened. So really we're safety observers. We're going to command it to leave (the) launch site, translate across, stop at the next one, plug itself back in, and then we're just going to be watching it very closely to make sure it does exactly what we expect it to do."

If all goes well, Canadarm2 will mount itself on the mobile transporter this summer, after delivery of a required mounting assembly in early June.

Along with testing the mobile transporter, the astronauts plan to hold a traditional in-flight news conference today at 12:44 p.m. After that, the combined crews will enjoy a bit of off-duty time.