Why space repairs aren't easy
Posted: Friday, May 15, 2009 6:00 PM by Alan Boyle
NASA
Click for video: Perched on the end of the space shuttle Atlantis' robotic arm,
spacewalker Andrew Feustel works on the Hubble Space Telescope with his Pistol
Grip Tool tucked at his side like a sword. Click on the image for a larger
version, or click here for a video about the tools developed for the mission.
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Stray rivets? Stuck bolts? Parts that don't quite fit? Sometimes it sounds as if fixing the Hubble Space Telescope is like trying to put a new headlight in my VW Beetle. OK, there are a couple of differences - like the fact that all this is happening in zero-G, where a stray rivet or a broken bolt could ruin a $10 billion investment.
The differences between space repairs and earthly repairs go a long way toward explaining why spacewalkers' tools have to be built from scratch rather than bought off the shelf, and why it takes seven hours or more to install parts that would take much less time on Earth.
The big reason doesn't have as much to do with the technology as it does with the fact that the help desk is a few hundred miles below. During this mission, the small cameras attached to the spacewalkers' helmets ("helmet-cams") have been instrumental for showing Mission Control how the job is going so that experts on the ground can offer timely advice. Nevertheless, nice and easy does it, particularly when you're wearing thick gloves that have been compared to oven mitts.
We've already seen several examples of typical fix-it troubles, writ large because of the space environment:
- When veteran Hubble spacewalker John Grunsfeld opened his tool bag, a stray rivet started rising up in zero-G. Fortunately, Grunsfeld made a goalie grab and rescued the rivet. Just imagine the worries that bit of metal might have caused if it floated up and around Hubble's delicate instruments. (It could have been worse.)
- Grunsfeld's partner, Drew Feustel, had lots of trouble freeing up Hubble's Wide Field and Planetary Camera 2 for removal. The bolt holding WFPC2 in place wouldn't budge: When Feustel pushed too hard, his power tool would engage a mechanism designed to avoid putting too much torque on the bolt. He got the go-ahead to override that mechanism, but he had to be careful not to push so hard that he broke the bolt. If that happened, WFPC2 would have to stay in the telescope, and its $130 million replacement camera would have to be sent back down to Earth unused. Fortunately, the bolt loosened up with a little extra elbow grease.
- During today's spacewalk, one of the three replacement gyroscope units just wouldn't fit in its slot, no matter how hard the astronauts tried. (Maybe they tried too hard. At least that was the problem with my VW headlight.) Fortunately, Atlantis' crew packed along a spare refurbished unit that did the trick.
- Throughout the spacewalks so far, astronauts had to deal with doors that were tricky to open and close (what do you expect when the doors haven't been used for seven years?), and tight spots that made it hard to use some of the custom-made tools (such as the "PikStik" gyro grabber).
More than 60 new tools were created for the Hubble mission, in part because the tools have to fit those bulky gloved hands. As I mentioned last fall, tool designers at NASA's Goddard Space Flight Center brought an astronaut glove with them to the local hardware store so they could see what kinds of grips worked best for the Mini Power Tool they were making.
The Mini Power Tool is due to get its prime workout this weekend, when Atlantis' spacewalkers try to replace balky circuitry for the Hubble's Advanced Camera for Surveys and the Space Telescope Imaging Spectrograph (STIS for short). That's an unprecedented repair job in space, and if it works, it would be a testament to the cleverness of the gearheads at Goddard and ATK.
Your typical power tool would never work in space - and not just because it's hard to hold the darn thing with your glove. The materials and mechanisms have to stand up to the rough environment in space, where temperatures can fluctuate by dozens of degrees in the course of just one orbit. Even the lubricants have to be different: Dry-film lubricants are used in place of the wet lubricants and oils used in earthly tools.
The outer-space power tool of choice, for Hubble as well as the international space station, is the Pistol Grip Tool - a hefty version of a cordless drill/screwdriver. That tool came into play quite a bit during today's replacement of the space telescope's gyroscopes and batteries. But it's too big for unscrewing the more than 100 screws that need to be taken out in order to fix STIS and the Advanced Camera for Surveys.
The Mini Power Tool is smaller, with a screwdriver bit that's sized just right to poke through the holes in specially designed plastic capture plates. The plates will be fitted over the access panels for the two instruments to be repaired, and catch all the screws before they float away. The tool is designed for speed rather than torque: With that many screws, you can't wait around and count the turns one by one, as spacewalkers have done during previous fix-up jobs. For the STIS repair job, one screw should be removed every 15 seconds on average (which adds up to almost a half-hour of solid unscrewing).
So what happens when the work is done? Will all those screws have to be screwed back in? Fortunately, no. STIS' original electronics access panel wasn't meant to be taken off easily, but spacewalkers should be able to clamp down the redesigned panel in a snap, just by throwing two levers. Chalk up another one for the gearheads.
