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Spacewalk Spotlights on Canada

Dave Williams lends a hand in expanding the International Space Station

On flight STS-118 space shuttle Endeavour, astronauts suited up and stepped outside into the vacuum of space in the ongoing quest to assemble and construct the most ambitious space project in history – the International Space Station (ISS). Canadian crewmember Dave Williams played a key role in three of the four strenuous spacewalks, helping to install a new truss segment on the space station, replace a faulty gyroscope, pack-up a set of solar panels heat radiator, and install a new communication antenna. He also took centre stage during his second excursion, where he moved by hand a giant gyroscope unit from the shuttle cargo bay onto the station while riding at the tip of the station's Canadarm2. With such a jam-packed mission, Williams ventured outside a total of three times, logging no less than 19 hours of spacewalk, making him the Canadian record holder.

Note that during this mission, the crew worked to assemble the Station-Shuttle Power Transfer System (SSPTS), an upgraded power distribution module that allowed the shuttle to draw power from the ISS power supply. Mission managers for STS-118 decided to add three more days and an additional spacewalk after the SSPTS is activated and inspected. Only if the SSPTS worked as planned would Astronaut Williams perform a fourth spacewalk. William therefore performed spacewalks EVA1, EVA2 and EVA4.

Shuttle Rendezvous and Docking

• Once the shuttle had gone through on-orbit inspections and had docked with the outpost on Flight Day 3, Endeavour's Canadarm hoisted the newly delivered station component from its cargo bay and positioned it overnight in front of the station's own robotic crane, Canadarm2, in preparation for the next day's installation.
• Meanwhile shuttle crewmates Dave Williams and Rick Mastracchio camped out overnight in the Quest Airlock, preparing for the first gruelling 6.5-hour spacewalk the next morning on Flight Day 4. The pressure in the airlock was lowered from 14.5 psi (that onboard the station) to the spacesuit pressure of 4.3 psi. A strict protocol was followed, similar to those used by deep-sea divers, in order to prevent decompression sickness and reduce the amount of nitrogen in the blood.
• The shuttle arm, controlled by Tracy Caldwell, handed off the newly delivered S5 truss segment, a 3.2-by-4.5-m aluminum structure that weighs approximately 5000 pounds (2267.96 kg), to the Canadarm2, which carried it close to its final attachment point on the ISS.

EVA1 – Making Additions and Upgrades

• On Flight Day 4, Pilot Charles Hobaugh manoeuvred the S5-laden Canadarm2 in a 3-centimetre corridor, just narrow enough to avoid a high-voltage electrical box, into Ready-to-Latch Install position.

• Next the spacewalkers secured the new starboard truss segment (S5) onto the existing (S4) backbone of the station using bolts and electrical connectors. Williams and Mastracchio got spectacular views of the ISS and Earth below, since they were working out on the farthermost sections of the space station.
• The S5 truss was a critical connecting joint for new truss platforms and the power-generating solar arrays that would be delivered by future missions.
• Once the S5 truss was installed, the astronauts took a stroll along it using handrails to guide them to the centre of ISS. From there they climbed to the highest point on the station, folded up a solar panel heat radiator and stowed it away for future relocation.
• This day also saw the first tryout of a new Power Distribution Module, the SSPTS (Station-Shuttle Power Transfer System), which enabled the Shuttle to convert about 8 kW from the ISS electrical power system (EPS). This would eventually allow the orbiter to remain docked at the station for an additional 3 or 4 days.

EVA2 – Going for a Ride

• On Flight Day 6, Williams and Mastracchio once again headed outside, this time to replace a faulty gyroscope, which the orbiting outpost uses for attitude control.
• Located just above the central part of the station, the Control Moment Gyros (CMG) was removed and stowed away temporarily on a storage platform on the side of the ISS, while a new replacement CMG was taken from the shuttle cargo bay and installed.
• Mastracchio walked out to the CMG worksite, and Williams joined him by climbing onto the end of the Canadarm2 and, bootstraps secured, was flown into working position by the station's robotic crane.

An astronaut at the tip of the Canadarm2 (Photo: NASA)

• While crewmate Charles Hobaugh piloted Canadarm2 from the robotic workstation aboard the Destiny module, Williams rode at the tip of the station's 17-metre construction crane to position himself above the old CMG, and manually grabbed and moved it to a secured storage area.
• Riding atop Canadarm2, Williams was then swung over to Endeavour's cargo bay to pick up the new CMG.
• Using only his arms, Williams picked up the nearly 1,322.77-lb (600-kg) CMG and its mounting hardware from Endeavour's payload bay, and was slowly flown back to the ISS.
• The CMG's intertia in microgravity was of great concern to the astronauts, so Williams and his payload were moved very slowly. The trip back to the ISS took about 15 minutes.
• Williams likened the slow trip to driving a zamboni on a skating rink: drive it too fast, and the zamboni's mass will keep you gliding off in a straight line.
• Holding the washing machine-sized CMG in front of his face, Williams' forward vision was totally blocked; so Mastracchio gave voice commands to Hobaugh to move Canadarm2 and Williams into position for the final installation.
• Williams rode Canadarm2 for most of the 6-hour spacewalk.

EVA3 – Improving Station Signals – 'Can you hear me now?'

• Williams took a rest from working outside of the ISS in preparation for his third mission spacewalk, on Flight Day 8.
• Working like orchestra conductors from the shuttle's flight deck, Tracy Caldwell provided support and went through spacewalk to-do lists, making sure everything on the EVAs ran smoothly.
• ISS crew member (Expedition 15 astronaut) Clay Anderson and Rick Mastracchio replaced parts of the high-flying station's communication system, installing a new signal processor and relocating a communication antenna. Mastracchio used the Canadarm2 to help relocate an S-band antenna.
• The two spacewalkers moved the CETA (Crew and Equipment Translation Aid) cart located on the starboard ISS truss to allow for the installation of a new transponder. The handrail cart was used to carry spacewalkers and equipment to different points along the main trusses.

EVA4 – Battening down the hatches

• On the mission's fourth and final spacewalk, Williams and Anderson wrapped up the outside work on the station, installing a new wireless transmission assembly: a new antenna system used with helmet cameras during EVAs.
  
  

• They also outfitted the station's hull with new foot restraints, video cameras and toolboxes, to be used by future construction EVAs.
  
  
  
  
• William mounted a special stand onto the side of station to which the shuttle's extension boom (OBSS) can be attached in a future shuttle mission.
  
  

• Most of Williams' and his crewmates' spacewalks on STS-118 involved many months of rigorous on-the-ground training using virtual-reality simulators and oversized swimming pools back at the Johnson Space Centre, in Houston. Some of the tasks, however, required a new adaptable training routine. The astronauts alternated between doing carefully choreographed tasks and tasks that were simply objective- or skills-based. It was a bit like knowing how to pilot a plane, having a map to a new destination, and flying to that location without practicing the details of the flight path.
• The new skills-based spacewalks now being conducted on ISS assembly missions are helping to establish standards for the methods astronauts may have to use to construct future lunar bases.

Spacewalkers