I'm building a vacuum chamber to make sure that my satellite doesn't go kablooey when it hits space. Vacuum is a nasty environment. We have no pressure, we have outgassing, and (least we not forget) we have the simple removal of air.
|If only the alien had used my vacuum testing first! |
Materials 'outgas', which is to say, liquids and volatile solids boil off. This outgassing can then coat itself onto nearby parts. Ever buy something wrapped in plastic, and the item smells like plastic for the entire day after you unwrapped it? That's outgassing. Now imagine plasticy vapour depositing itself on, oh, your eyes, and bonding there. Because in space, the outgassing stuff just may hit parts of the satellite as it evaporates away.
That is bad. It can coat detectors or solar cells. If anything outgassing conducts (unlikely but possible), you can mess up circuits. Plus, it's not tidy, letting stuff swirl away like that. However, in a vacuum chamber, you can have the bulk of the outgassing occur safely on the ground, sparing your components once in orbit.
Vacuum testing also ensures your soldered links are stable, and aren't going to break due to air pockets, air conducting where there should be solder, and similar mishaps. That's part of the 'no pressure' and 'removal of air' issues. Maybe there's a part or two that is fragile and can't take the drop to zero pressure. Maybe there are air bubbles in a component due to poor manufacturing, that will break when it hits vacuum.
So that's why you want to test your satellite in conditions as near to where it will be to make sure it can handle space. It's pretty straightforward, actually. Imagine if you were building an underwater detector-- you'd want to send it underwater. You don't have to enumerate everything that might go wrong, you just immerse and see what happens.
So it is with vacuum. Instead of imagining every possible mishap, just test the sucker. And one usually does thermal testing along with vacuum testing. Heat and cool your satellite while it's in a vacuum to mimic the space conditions you expect. Hope it survives. Repeat if necessary.
And if you're curious what happens to humans in space, here's the scoop from Imagine the Universe. Summary: a human in vacuum doesn't explode, but does black out in about 15 seconds.
As it happens, I ran into another 'human vacuum' issue when buying the pump for my DIY chamber. For preliminary testing, I picked up a hand vacuum pump, the type often used to check automobile break lines. It's not the hardest vacuum. But it's enough to do the primary outgassing and basic sanity checks I'll need early on.
The vacuum hand pump presented an unexpected challenge, particulary when hunting on eBay. It turns out there is an, err, type of vacuum hand pump that is used for, umm... enhancing the size of the male sexual organ. I did not want (nor need) that.
Besides, those pumps look to be flimsy plastic, unable to hold a hard vacuum. And I want to test the satellite at levels that would make a typical sex organ simply explode (and not in a good way). As a matter of fact, even for automotive-grade parts, the plastic pumps are to be avoided; I went with metal.
In the end, I bought it at Harbor Freight. On sale for $20, bargain! And best of all, I can safely certify that my satellite is 100% sex gadget free. Still keeping our PG rating, woo!
I hunted up DIY vacuum chamber construction ideas from others, and the best set of informational links was at links was at DVForum They use vacuum to make bubble free wax. Okay.
Among the other materials I need, is a primary retort, and material I can use to make a lid. The lid will be plastic-- thick acrylic. As it happens, I learned how to work acrylic when making custom pickguards for the electric guitars I like to build.
So a technique from building electric guitars come in handy when making a musical satellite. That shows that DIY hobby work is just like being in orbit. What goes around comes around.
Until next week,
Satellite details on ProjectCalliope ('Music from Space', Launching Spring 2011)
Or read it every Tuesday here at The Satellite Diaries, plus science every Friday at the Daytime Astronomer
p.s. Any good plan needs a fallback. If my DIY isn't sufficient to past InterOrbital spec, I can always hit up a nearby university to see if I can borrow their chamber-- it's a lot easier to borrow something if you've already tested the payload in your chamber so it's not likely to explore in theirs.