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Jennifer from Green Bay, WI: How much does the payload weigh? What will be the combined weight of the payload and the orbiter?
Ask The Mission Team - Question and Answer Session
Ashley: Well, that's a very specific question, which tells me that the person asking has been following the space program rather closely for some time. And so I'll give a very specific answer. The combined weight of the payload and the orbiter at liftoff will be 269,840 pounds.
Jennifer from Green Bay, WI: What kind of power will the new truss give to the station and what kind of power capabilities does the station have now compared to when the truss will be installed? Will the station be able to do more things that it never had before with the new truss?
Ashley: Well, this is actually a very good question. The Port 3/Port 4 truss, designated P3/P4 for short, includes the second of four power modules to be brought up to the International Space Station. The P4 power module is essentially identical to the one currently on orbit, called P6, which was launched in November 2002 on shuttle mission STS-97 and it's been the main source of power for the space station ever since. P4 is capable of generating roughly 30 kilowatts of power, basically doubling the power-generating capability onboard the station. During the next shuttle mission, STS-116, currently scheduled to launch in December, P4 will take over the role from P6 of primary provider of power for the station, while P6 begins the process of having its solar arrays stowed in preparation for its eventual relocation out to the end of the port-side truss structure. In the meantime, P4 will provide the power necessary to continue the construction of the space station, including the addition of the international partner laboratory modules.
Matthew from Big Lake: How will the P3/P4 truss connect to P1?
Nitin from Hyderabad: How are the P3/P4 truss segments joined to the P1 truss?
Ashley: Well, the P3/P4 truss will be attached to the P1 truss using four large structural bolts which are part of what is called the Segment-to-Segment Attach System, or SSAS, for short. When P3/P4 is aligned and positioned to within 3 to 4 inches of the P1 truss using the robot arm, three ready-to-latch sensors on P1 will make contact with three corresponding striker plates on P3, sending a signal to the station command and control computers indicating to the crew that the segments are ready to be mated. The crew will command a capture latch on P1, which resembles a large metal claw, to close, and as it does, it will close around a capture bar on P3, drawing the two truss segments together. Once the two segments are physically together, the crew will command the four motorized bolts on P1 to engage in the corresponding nuts on P3, thus completing a hard mate of the two truss components.
Pawan from Sharjah: How important will the role of the robotic arm be during the construction? What will the astronaut's roles be and how many spacewalk hours will it take for the construction?
Ashley: Well, both the shuttle and station robotic arms will be used during this shuttle mission and both are absolutely critical to connecting the P3/P4 truss segment to the International Space Station. Astronauts Chris Ferguson and Dan Burbank will use the shuttle robot arm to lift the P3/P4 truss out of orbiter Atlantis' payload bay and maneuver it to a position where it will be handed off to the station's robot arm. Astronauts Steve McLean from the Canadian Space Agency, along with station crew member Jeff Williams, working from the station's robotic work station in the U.S. Laboratory module, will position the station robot arm to receive the P3/P4 truss from the shuttle robot arm. Once the handoff is complete, the truss will remain grappled to the station arm overnight in a parked position while the crew sleeps. The next day, MacLean and Williams will slowly and carefully position the P3/P4 truss at the edge of the P1 truss and use television cameras to properly orient and position the two truss components for mating. Once in position, the Segment-to-Segment Attach System will be used to complete the mating of the two segments.
In terms of how many hours of spacewalk are planned for the construction during this flight, there are three planned spacewalks and each are scheduled for the maximum time limit of 6-1/2 hours, so there will be a total of almost 20 hours of crew time outdoors during the assembly that's going to occur on this flight.
Leon from Bordentown: What voltage are the new solar panels that are going to be installed on the ISS and why do the Russians use a different voltage? Thanks, Lee
Ashley: Well, the station solar array wings, designed and built by Lockheed Martin in Sunnyvale, California, generate power at 160 volts, but DC-to-DC Converter Units are used throughout the station to convert the power down to 120 volts which is the operating voltage for the U.S. segments and subsystems. Transmitting the power at the higher 160 volts and then stepping it down to 120 volts at the point of use allowed designers to utilize smaller wires for electrical power transmission, thus saving on the overall weight of the space station.
The operating voltage on the U.S. side of the space station differs from the Russians, basically because we use a different system of measurements than the rest of the world, essentially, for the same reason that we have different weights and measures than the Europeans and the Russians. We use 120 volts as our standard and the Russians, like the rest of the world, use 110 volts.
Mark from Phoenix: I read somewhere on the NASA website that the P3/P4 payload is one of the heaviest payloads ever at 35,000 pounds. What kind of adjustments have to be made to the external tank engines and the solid rocket boosters to get that increased mass into space?
Ashley: As far as the adjustments made to accommodate the weight of the P3/P4 payload, several steps have been taken to ensure Atlantis will have sufficient up-mass capability to get the beefy P3/P4 truss all the way up to the orbiting space station. As an example, the orbiting altitude of the space station has been lowered to 185 nautical miles, or will be for the rendezvous with the space shuttle, thus shortening the trip and making it easier on the space shuttle vehicle.
In addition, the shuttle program has been looking at the complement of hardware on the shuttle for a couple of years now, and all non-essential hardware has been removed from the shuttle. So if it's not being used for this flight, it's been taken off to allow more up-mass capability for the payload.
Mark from Charleston, SC: When is the payload loaded onto the shuttle?
Ashley: The P3/P4 payload was installed into the orbiter's payload bay at the launch pad. When the payload is delivered to the launch pad, it is installed in what is called the payload changeout room, which is located on the rotating part of the launch pad known as the rotating service structure. P3/P4 was delivered to the launch pad on July 27, last month.
After a couple of days' delay due to bad weather, the shuttle arrived at the launch pad on Aug. 2. Once the shuttle was secured at the pad, the rotating service structure was rotated around to the shuttle and the P3/P4 payload was loaded into the shuttle's payload bay on Aug. 5.
Stan from Cincinnati: I have a question about the payload. The story line about Atlantis includes that the payload will be loaded at the launch pad. When I visited the Kennedy Space Center, I thought that I remember hearing that the payload was loaded in the Vehicle Assembly Building before moving to the launch pad. Can you clarify?
Ashley: Yes. All space station modules and trusses are loaded into the orbiter payload bay at the launch pad. Payloads can be installed in one of the three orbiter processing hangars known as the Orbiter Processing Facility while the payload bay doors are open. However, the payload bay doors are closed for the trip over to the Vehicle Assembly Building, and over in the Vehicle Assembly Building, we do not have the capability to open the payload bay doors. Therefore, we cannot install payloads in the Vehicle Assembly Building.
Brandon from Orlando: How many pounds is the STS-115 shuttle carrying to the International Space Station?
Ashley: Well, the P3/P4 truss weighs 34,885 pounds and, in addition to the truss, roughly 750 pounds of International Space Station hardware will be flown up in the crew compartment middeck.
Dave from Sugarloaf, PA: When the P4/P5 truss is added to the ISS, does the asymmetric configuration of the station cause any problems for the CMG system?
Ashley: Well, I don't know if I would characterize it as a problem for the control moment gyro system, or CMGs. However, the asymmetric configuration will make it more complicated to maintain attitude control of the International Space Station once P3/P4 is installed. But the flight directors out in Houston, along with the engineers, have been working on this for several years now and they have their plans in place. They've simulated the attitude control in this configuration many times out at the Johnson Space Center and they're ready to go and are prepared for this asymmetric configuration.
Dave from Pennsylvania: From the illustrations of the installed new solar array and P3/P4 truss structure, it looks like the current P6 Truss and Solar array will remain in its current place. When will it be moved to the opposite end of the truss structure?
Ashley: Well, the P6 truss element will be relocated to its permanent location at the end of the portside truss structure during STS-120, the International Space Station flight 10A, which is currently fourth in line for launch after this current mission, P3/P4, is launched.
Nitin from Hyderabad: What is fixed at the end of P1 now? Will there be way to move from the P1 truss to P3/P4? How is the joint made leak proof?
Ashley: Well, the P1 truss is currently open-ended. It's been configured and is awaiting the addition of the P3/P4 truss element which will be launched on this upcoming flight, STS-115. Once P3/P4 is installed to the end of P1, there will be a way to translate from P1 across the interface to the P3/P4 truss segment. P3/P4 includes a work site for the mobile transporter, and during the third spacewalk on this shuttle mission, the astronauts will clear the translation path for the mobile transporter, and the mobile transporter will actually be translated to the work site on the P3/P4 truss segment.
As far as how the joint will be made leak-proof, well, the joint is actually strictly a mechanical interface. The two elements are hard-mated together mechanically, so there's really nothing that needs to be leak-proofed at that interface.
Nitin from Hyderabad: Why are the S2 and P2 trusses missing on the ISS?
Ashley: S2 and P2 were attach locations for rocket thrusters which would be used on the space station Freedom for reboost and attitude control. When the Russians were brought into the partnership of the International Space Station, the S2 and P2 reboost capability was no longer necessary.
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