Thanks,

GAry 7

Oh, sorry. When you used “replace” you meant substitute something different for the 111 screws, i.e. the easy new cover with the latches. I thought you meant “put the 111 screws back in place” and I suspect others may have read it that way, too.

Well, you might say that. And you might be right. Let’s just say that once we get the ACS Repair hardware down to the cape next week, I’m going to take a short vacation before the launch.

And no problem on the specific names of the tools. It’s hard enough keeping the names straight even working with them on a regular basis.

The photo in your gallery is actually the STIS “Fastener Capture Plate”. There is a separate screw capture tool (I think that’s what it’s called, but the name may be slightly different), used to take out four larger (#10-32) screws in the ACS repair. It looks like a screwdriver with a little claw that reaches out to grab the screw as you remove it. Not nearly as photogenic as the FCP, for sure. Good article.

We briefly considered using ants to sort the screws, but Dan Castellaneta wasn’t available, and Hubble only hires the best!

As for the post, all of this talk of screws makes me inevitably think of Deep Space Homer: “You fool! Now we may never know if ants can be trained to sort tiny screws in space.”

DISCOVER mag (and Web site) recently covered the save-the-Hubble mission, focusing on Massimino. The last picture in the story’s photo gallery shows a “screw capture tool,” similar to what BA and 01101001 are talking about.

A box. It’s definitely more of a box. The FCP I mean, not Morey Amsterdam.

Umm…why not use lock washers or lock nuts? I understand the need for redundancy, but wow!

Ah, good question! Hey, the answer is obvious to me, but I didn’t really explain it, did I? Sorry about that.

It’s not a question of each fastener failing; even #4-40 screws (yes, English units, what can I say?) are pretty strong as far as holding the cover on. And we do lock them down so they don’t back out during the vibrations of launch. Usually with a locking insert or a dab of glue, rather than with lock washers; lock washers tend to create little metal shards, which can float around and short things out later.

The reason we need lots of fasteners is that we need to tie down the cover at lots of points along the edge. Box covers are (generally) relatively thin aluminum, so they’re fairly flexible. If we just put in screws at the corners, they would tend to flex in the middle of each edge, both during launch and due to temperature changes in orbit. And we generally need the covers to be light-tight, to prevent electromagnetic interference. Also in many cases the heat has to flow through the box covers, which also means they need to be tightly held down. When you do the analysis, in the end you want to put a fastener every couple inches or less. They can be small fasteners, but you need a lot of them. Which makes repair difficult. Especially when you’re in a pressurized space suit!

I hope that’s a little clearer.

ACS, failing much closer to the planned repair mission, was a harder decision yet. There was a long period of “Well go ahead and see how far you get, and if it looks like you’ll get there in time we’ll make it official.” But hey, no problem, just open up a can of can-do attitude (and some really smart engineers), and again, good to go.

I hadn’t heard the complaint about the cameras being lower quality than the $50 Wal-Mart ones. Hmm, this may call for some outreach work. I guess the Wal-Mart cameras are better quality, if by “quality” you mean “number of advertised pixels”. If on the other hand, you define quality based on video noise, sensitivity, actual angular resolution, quantum efficiency, crosstalk, etc., that’s a different story. After the trouble I’ve had getting decent shots at rock concerts (with a camera I chose based on its low light capability), I cry inside a little every time I take a test image with the ACS spare detector. In a sealed box, taped everywhere, with two layers of black cloth over the top, it still picks up light leaking in. Now that would be an awesome camera to take to the next Carbon Leaf show!

Course the whole keeping it at -80 degrees C. is a bit of a difficulty. As is the 10cm x 10cm CCD, and the 100 seconds it takes to read it out. But man, talk about image quality! Oh yeah, plus, it all works in space, with radiation and the vibration of launch, and no air to help cool the parts, and…

And madge, the screwdriver and fastener capture plate (2 really, one for STIS and one for ACS) are just the beginning. There are more new tools designed for this mission than for any before.

Umm…why not use lock washers or lock nuts? I understand the need for redundancy, but wow!

http://www.youtube.com/watch?v=YQVyQOZ1Ozw

So, why so many screws? The basic answer is this:

Thanks for the not-always-obvious explanation. (Do you speak from experience?) It’s like the other oft-heard complaint about why the really expensive cameras on the spacecraft are so “low quality”, and they can “buy a better camera for $50 at Wal-Mart”.

“The challenge was the 111 screws that are holding it on, and the screws are not captivated. So they have to go in there and take all these screws out. Well, you can imagine what went through a lot of people’s minds when we first started thinking about this, you know, the 111 screws floating all around inside Hubble. That was unacceptable.

“So we came up with a very clever device called the fastener capture plate which is basically made out [inaudible]-type material, and this plate goes over top of the MEB cover that is aligned and fastened on there. Then this fastener capture plate has a series of little holes in it that line up little screws. The holes are small enough to allow the tool bit to go in, so you can turn the screw, but they are small enough to keep the screw from falling out. So, once you get all 111 screws taken care of, the cover stays attached to the fastener capture plate and moves the whole thing out. So all the debris and all the screws is captured in there.

“And then we have come up with an astronaut-friendly replacement cover. Once we are done servicing, they take the new cover, put it on, and there are two latches. They just throw the latches, and Bingo, it is on there, and then there is a third latch that they throw that has some fingers that grab the electronics board and mates them to the cover because this cover also acts like [inaudible]. Okay. So that was challenge number one.”

This particular cover was never designed to be removed in space. The normal cycle has been to build a new instrument when we want to replace something. The instruments themselves are designed to be replaced: Turn one handle a few times (with a fancy space-capable screwgun), demate four circular connectors (which have nice wing-tabs so they’re easy to rotate), and slide out the old instrument. Reverse the process to put in the new one. That’s how they’re going to remove COSTAR and replace it with COS (Cosmic Origins Spectrograph).

With STIS, the failure happened late enough in the game that it was too late to build a whole new instrument. Then the question is, “Can we repair it?” It was never meant to be repaired like that, so it took a lot of work to figure out what needed to be done.

So, why so many screws? The basic answer is this: Rockets are really loud. Obvious, huh? Well, that is, they shake the hell out of things during launch. Which means you need to build stuff strong. And light, of course, because lighter is easier to get to orbit. (Servicing mission 4 will be using about all of the lift capability of Atlantis, which is the most powerful Shuttle.) And for a large cover like that, to make it strong you need lots of fasteners. Which means lots of screws.

ACS (Advanced Camera for Surveys) is also being repaired. That only needs to have 32 screws removed, but then we take out 4 circuit boards and replace them, and strap on a new power supply. There’s also a metal grid that has to be cut, on that one. ACS only failed a year and a half ago, so it turns out to have been very helpful that the STIS repair had already been in progress. A lot of technology developed for STIS has been adapted for ACS repair.

Now if the hurricanes will cooperate, this is going to be a pretty amazing mission!

But I’m just guessing here!

Presumably the FSM forgot that astronauts don’t have as many noodly appendages.

There is too much good information here to waste behind an intentionaly crippled system.

111 tiny screws??? If those guys had to repair their design under water, wearing big, bulky gloves, I bet they would have come to the conclusion six BIG screws were adequate,,,

GAry 7

This is, of course, a simplified revenue model, but there’s some truth to it.
The other thing it does is prevent those scummy spam blogs from stealing content and auto-posting it to steal traffic.

That’s why I (and many others) have a feed reader, so I don’t have to click all over the Tubes to read what I want. And if the feed is useless… off the list it goes, and rarely do we remember to visit. Phil, please kindly tell your editors to let you be nice to your readers, and Discover will come out ahead.