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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 are suiting up and stepping 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 is set to play 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 attach new communication antenna. He will also be taking centre stage during his second excursion where he will be moving 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 will be donning his construction helmet and venturing 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 will work to assemble the Station-Shuttle Power Transfer System (SSPTS), an upgraded power distribution module that will allow the shuttle to draw power from the ISS power supply. Mission managers for STS-118 could add three more days and an additional spacewalk after the SSPTS is activated and checked out. Only if the SSPTS works as planned, will Astronaut Williams perform three spacewalks: EVA1, EVA2 and EVA4.

Shuttle Rendezvous and Docking

• Once the shuttle has gone through on-orbit inspections and has docked with the outpost on flight day 3, Endeavour's Canadarm will hoist the newly delivered station component from its cargo bay and position it overnight in front of the station's own robotic crane, Canadarm2, in preparation for the next day installation.
• Meanwhile shuttle crewmates Dave Williams and Rick Mastracchio will camp-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 is lowered from 14.5 psi, which is found onboard the station, to the spacesuit pressure of 4.3 psi. A strict protocol is 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, will hand-off the newly delivered S5 truss segment, a 3.2m by 4.5m aluminium structure that weighs approximately 5000 pounds (2267.96 kg), to the Canadarm2 which will manoeuvre it close to its final attachment point on the ISS.

EVA1 – Making Additions and Upgrades

• First on tap for the fourth flight day will be for the Pilot, Charles Hobaugh, to manoeuvre S5-laden Canadarm2 in a 3-centimetre corridor, just narrow enough to avoid a high-voltage electrical box, into a Ready to Latch Install position.

• Next, for the spacewalkers will be to firmly fix the new starboard truss segment (S5) onto the existing (S4) backbone of the station using bolts and electrical connectors. Williams and Mastracchio will get spectacular views of the ISS and Earth below since they will be working out on the farthest most sections of the space station.
• The S5 truss is a critical connecting joint for new truss platforms and power-generating solar arrays that will be delivered by future missions.
• Once installed, the astronauts will take a stroll along the main ISS truss using handrails to guide them to the centre of ISS. From there they will climb to the highest point on the station and fold-up a solar panel heat radiator and stow it for future relocation.
• This day will also see the first tryout of a new Power Distribution Module, the SSPTS (Station-Shuttle Power Transfer System) which enable the Shuttle to convert about 8 kW from the ISS Electrical Power System. This will eventually allow the orbiter to remain docked at the station for an additional 3-4 days.

EVA2 – Going for a Ride

• Flight Day 6 will see Williams and Mastracchio once again head 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) will be removed and stowed temporarily on a storage platform on the side of the ISS while a new replacement CMG will be taken from the shuttle cargo bay and installed.
• While Mastracchio walks out to the CMG worksite, Williams will join him by climbing onto the end of the Canadarm2, and with bootstraps secured, will be flown into working position by the station's robotic crane.

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

• With crewmate Charles Hobaugh piloting the Canadarm2 from the robotic workstation aboard the Destiny module, Williams riding at the tip of the stations 17-metre construction crane, will position himself above the old CMG and manually grab it and move it to a secured storage area.
• Williams on top of Canadarm2 will then be swung over to Endeavour's cargo bay to pick up the new CMG.
• Despite weighing nearly 1322.77 lbs (600 kg), Williams will pick up the CMG and its mounting hardware from Endeavour's payload bay using only his arms and will be slowly flown back to the ISS.
• In microgravity, the inertia that the CMG possesses is of great concern to astronauts, so Williams along with his payload in hand will be moved very slowly, taking about 15 minutes to complete his flight over to the ISS.
• Williams likens it to driving a zamboni on a skating rink; if you drive it too fast, the mass of the zamboni will keep you gliding off in a straight line.
• Because Williams will be holding the washing machine-sized CMG in front of his face, his forward vision will be totally blocked. So Mastracchio will give voice commands to Hobaugh on how to move the Canadarm2 and Williams into the proper position for the final installation.
• Williams will be riding the Canadarm2 for most of the 6-hour spacewalk.

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

• Williams will take a rest from working outside of the ISS as the third mission spacewalk will take place on flight day 8.
• Working like orchestra conductors from the shuttle's flight deck, Tracy Caldwell will be giving support and going through spacewalk to-do lists, making sure everything on the EVAs is running smoothly.
• ISS crew member Clay Anderson (Expedition 15 astronaut) and Rick Mastracchio will be replacing parts of the high-flying station's communication system, installing a new signal processor and relocating a communication antenna. Mastracchio will also be using the Canadarm2 again to help relocate an S-band antenna.
• The two spacewalkers will move the CETA (Crew and Equipment Translation Aid) cart located on the starboard ISS truss to allow for installation of a new transponder. This handrail cart allows both spacewalker and equipment to ride to different points along the main trusses.

EVA4 – Battening down the hatches

• On the mission's fourth and final spacewalk, Williams and Anderson will wrap up the outside work on the station with the installation of a new wireless transmission assembly, which is a new antenna system used with helmet cameras during EVAs.
  
  

• They will also be outfitting the station's hull with new foot restraints, video cameras, and toolboxes to be used by future construction EVAs.
  
  
  
  
• William will also mount 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 involve many months of rigorous on-the-ground training using virtual-reality simulators and oversized swimming pools back at the Johnson Space Centre in Houston. But some of these spacewalk tasks on STS-118 are being carried out using a new adaptable routine to training. There is an ongoing shift from doing carefully choreographed spacewalks to ones that are simply objective or skills based. This is analogous to knowing how to pilot a plane, and having a map to a new destination, and then flying to that location without practicing the details of the flight path.
• These new skills-base spacewalks now being conducted on these ISS assembly missions are setting the standards for how astronauts may have to carry out construction of future lunar bases.

Spacewalkers