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STS-112 S1 Truss
Question and Answer Board

Katie from Tampa
It sounds like the S1 Truss and P1 Truss (which will be launched later) are identical. Are there any differences between them?
Well Katie, they are very similar. As a matter of fact, they are mirror images of each other. If you think of S1 and P1 as kind of coming out from opposite directions from each other off the center segment, the S0 segment, so they have all the same components, they're all just kind of reversed. Now, that created some challenges for us when we were working on them because we'd kind of get mixed up on our orientations between the two since we we'd be working on them parallel. There are some differences. The S-band communication system flies on S1, versus on P1, a UHF communication system, and that's just different forms of antenna and transmission devices used to communicate down with JSC's ground communications. But they are very similar in function. They both have thermal control systems that were talked about earlier, and the segment-to-segment attach systems and the other items. But they are very similar.
Sean from Orlando
When the truss was designed, did the length have to be adapted to fit into the STS for transport to the ISS?
Actually, when the truss was originally designed, we knew what the launch vehicle was going to be, we knew it would be a shuttle, so we had all the envelopes to design to. So basically, flying in the shuttle was designed into the original concept of the space station elements. In fact that's what kind of drove how many launches there would be in getting those segments into space by what we could get into the shuttle.
Sean from Orlando
What additional work had to be done to ensure the truss would remain straight and not be bent by the uneven temperatures in light and shade while exposed to space?
Yes, very good question, Sean. The thermal extremes that we're going to go through in orbit with all the truss elements is extreme. What we do to try to counter that is thermal testing. There's a lot of thermal chambers, thermal vacuum chambers that we put hardware into, so we can check how much bending and the Moment arm changes we get as a result of those environmental extremes that we're going through. So it's kind of built into the design stage of this, knowing what the environments are going to be, how we go about building them, what alloys we choose for the different structural components. So there's a lot of up-front thermal dynamic type testing that goes on before the first piece of metal is actually cut. Then after we get the segments assembled, there's also some thermal vacuum testing that goes on there, to validate what our choices of materials were, and to ensure that they'll be good on orbit.
Oswaldo Medina from River Grove, Illinois
How do they get the truss ready for liftoff to ensure it doesn't get damaged?
Very similar to Sean's question. We do vibration testing. Our concern during the launch environment is the shaking that we're going to see as we go up the hill into orbit. The shuttle creates quite a bit of vibration -- you've probably seen pictures of astronauts as they getting shaken around - and our concern is for the hardware in the payload bay. What's that going to do to the hardware as we go up? There's a lot of deployable components on the space station, the radiators and other things that you have to worry about working once you get on orbit. So we've got shaker tables that we put components on, we shake them and do testing. And we have found failures in the past doing that shaker table, and that'll drive us back to do some design work to come up with a better design approach to preclude that from happening. So we try to test it on the ground and our history for space station tells us that we've done an extremely good job, and we've seen no problems on orbit from the environment we see during the launch ascent phase.
Brianna from Chicago
What is the S1 Truss? What does it do, how does it work, and why is it needed on the ISS?
Okay, Brianna. S1's major purpose is structural integrity. As we grow out that 100 yards of truss, we're looking for a structural integrity system because when you get out to the end, you've got your solar arrays. The solar arrays, they're going to be spinning, so we're looking for something that is structurally sound enough to hold -- you can imagine that thing out there vibrating back and forth -- we needed something structurally sound enough to handle that. As I mentioned earlier, there's also the thermal rejection capability that we bring up, and also the ability to take the Mobile Transporter and the CETA cart back and forth to the outer reaches of the truss. So those are pretty much the major purposes of it.
Howard from Fresno, California
How are structural elements such as S1, which are fabricated in Earth's gravity but intended for service in microgravity, designed and tested? How big a job is it to re-write the engineering math for this change in gravitational environment?
Well actually, if we could build on orbit, it would be a little different, because you wouldn't have to endure the launch loads you're going to see if you launch through the shuttle vibration phase I spoke of earlier. So that would make a difference if we could do that, but obviously we can't, so we have to build it to meet the standards of launch loads. Once you get up on orbit, it's a whole different dynamic as far as bending and moment arms and that kind of thing. What we do is we've got the neutral buoyancy tank in Houston in which we take structural test articles very similar to the ones that we're putting on orbit, we put those in the swimming pool and then the astronauts can go out there and manipulate the different mechanisms and make sure that everything fits together in place well, and that they are actually able to do what they need to do once they get up on orbit. And that's how we try to account for what's going to happen on orbit. We do a lot of testing in the Space Station Processing Facility but the neutral buoyancy tank is in Houston.
Brendon from Lafayette, Indiana
The truss portion of the ISS has been referred to as the station's backbone. Is there a system of bolts or latches that holds the S1 and other truss segments together, that allows it to be so structurally sound?
Yes, Brendon, there is. As a matter of fact, to me that's one of the most interesting parts of the space station, is how those all attach together. It's a system called the segment-to-segment attach system. It's a mechanism that what we do is we come out of the payload bay with the robotic arm and put S1 in very close proximity to, in this case, S0. S0 has an active segment-to-segment attach system with a capture latch. And they're close enough, there's a capture bar on the S1. And it'll grab hold of that bar, and pull it up close. And when it gets real close, there's ready-to-latch plungers, if you will, that actually compress. This gives the crew the indication that they're close enough to fire a command off that will drive the motorized bolts. There's four motorized bolts that will engage into the S1 truss and pull it very tightly and securely together. And that's how they get attached on orbit.
Rick from Sarasota
Why is the S1 (and other station parts) so massive (i.e., heavy)? It's designed to work in 0-G; Couldn't these pieces have been made of a lighter material, which does not need to hold itself up in 1-G? Thus, allowing larger parts for the same weight.
That's a good question too. Lighter is more expensive. That would be the real reason. When you go through the original design concept of how you're going to build a truss or any space element, you have to go look at what you're really intending to do as far as a structure's on orbit mechanism. You have to endure the launch loads as well. So we could have built it with a lighter material but we didn't need to. It would have cost more money, and that's why it's made of aluminum versus, say, titanium. Titanium would have cost us more, and it wasn't necessary.
Zain from Atlanta
How will the module that STS-112 is taking up be connected to the space station?
Zain, right now we have S0 on orbit. That's the center point of all the truss segments, and it's connected to the laboratory module. If you think of a hundred yards connected to one point, you can think about all the bending and gyrating going on on orbit, and think that might not be enough. And there's a system of tripods and bipods that attach S0 to the laboratory module. That gives us the structural integrity to be able to build out from the lab -- S0, S1, P1, as we build on out to the end -- and be structurally sound enough to endure any kind of environment we see up there with the truss segments.
Cindy from Richmond
If we understand it correctly, the individual parts of the space station have been constructed in pieces over the years. How do you guarantee that the S1 Truss will fit with the parts that are already installed? What is the margin of error?
Very good question, Cindy. That's been a concern. The way we've dealt with that concern is by using master tooling. Every element that launches has a master tool that it has to fit up to. That ensures that everything is identical and fits up to whatever it's going to be like on orbit. So we've gotten a high confidence from using these special tooling jigs to make each one of the elements that when we get up on orbit that they will fit up correctly. That question is also pertinent to electrical test and software compatibility so earlier this year and in previous years actually, for the laboratory and airlock modules, we ran what's known as the Multi-Element Integration Test, in which we took every piece of hardware we could get our hands on, and hooked them up together to run electrical testing and software compatibility. And we did find a few problems, and we got those fixed. So we're very happy that we ran that test on the ground and we're highly confident and history has proven so far that everything once we've gotten up on orbit operates just perfectly and flawlessly.
Bruce from Dayton, Maryland
What is the metal composition of the truss segment? What is the Earth weight of the segment?
OK, I mentioned aluminum, and that's what makes up the major structural components. There are a lot of other alloys and metals that make up the different components, there's a lot of (orbital replacement units) and things that fly on S1 so but aluminum is the major structural component to make up the S1 truss. And we weighed it right before we went into the payload bay and it came out at 27,880 pounds.
Po Kee from Silver Springs, Maryland
Are the precise locations and velocities of the ISS completely controlled by the ground-tracking stations and/or can they also be controlled by the jets on the structures of the ISS? Are there instrument panels for the crew members to read and determine, and to know the ISS locations and velocities at any time?
I think that question can be answered yes to both. It can be controlled from Johnson Space Center and it can also be controlled from on orbit. There are panels and telemetry and guidance available to the astronauts on station, so they've always got a knowledge of where they're at, what orbit they're in, if their orbit is degrading or if there's anything like that. By and large, the space station gets reboosted not that often. When the shuttle's up, they generally take advantage of the shuttle, with its extra fuel, to be boosted a little bit, and then of course the space station itself has the ability to reboost itself. So from that standpoint, there's not a lot of astronaut involvement required to keep the thing flying but they do have the capability.
NASA's John F. Kennedy Space Center