Beryllium is a good conductor of both heat and electricity, so I don’t know what he meant.
Heat conductivity but electrical insulation is an oddball combination, but it does exist – for instance, in the pastes used with heat sinks. But Be doesn’t do that.
He’s talking about an oxide though, not the metal.
Yes, I just realized that. He’s right about the oxide.
Correct.
It’s still toxic, though which is why I’d wash up with the grease and happy I’d never broken a ceramic.
Well, the mirrors have to be large, which implies the need for a considerable volume of material to make them. And low mass is, for a rocket payload, highly desirable. It thus follows that any suitable real-world mirror material will be of low density.
I don’t see how it can be the website’s fault … wasn’t it always supposed to get to L2
today (Sunday) - day 29 ?
(Sounds very suspicious to me !)
Precisely. But this morning they abruptly changed the “distance remaining” and added a “Day 30” to the timeline.
Yes, but that’s 'cos nasa told them it was going to be a day late for some reason.
They got out there, and found this sign.
Well … i suppose that would explain it !
As mentioned already, the total distance changed. When I calculated earlier that it was 95% to L2 the total distance was about
898,705 (from my calculator’s register)
Now it is
911,131
For a difference of 12,426.
That’s approximately consistent with the amount that “distance remaining” changed this morning. Really, I think the whole thing was just a clerical error on the website.
Nope. They need a large area, but thickness is not a virtue. The material they are made from needs to be strong (stiff) enough.
The woman in the video said what’s great about Beryllium is it’s light weight. That was not wrong - it just left unstated the assumption that she meant light weight to achieve a given strength.
You claimed that she should have said what’s great about Beryillium is it’s large volume for a given weight. This is worse than an unstated assumption - it is wrong. They do not use Beryllium because (ceteris paribus) they want a large volume of material.
For essentially any material that could be used to make these mirrors (any structural material at all?), thickness and stiffness are correlated.
No, what’s great is its low mass for a given volume. (Along with its other desirable qualities.)
So what if they are correlated. That does not imply that large volume is a design criterion. For a given strength and weight, if anything less thickness is probably desirable.
No, what the designers cared about is its low mass for a given strength.
You nitpicked the video for being imprecise in just saying that low weight is a virtue, yet your claim that low density is the true design criterion is worse that being imprecise, it is wrong.
This is puzzling. If stiffness is (as you note) desirable, and if it correlates with thickness (which you seem to accept), then how can thickness be irrelevant?
No, I objected to the narrator saying that beryllium is “super light weight”. A pound of Be weighs almost exactly what a pound of aluminum, steel, lead, or gold does. Its virtue lies in how much each pound can deliver in the way of mirror-base performance, as compared to other metals.
Why is it puzzling? If larger X is a design objective, the fact that (for any given material) Y is positively correlated with X does not imply anything about Y. Y may be irrelevant, or it may often be a design objective to minimize Y.
For any given fuel, the total mass of fuel is positively correlated to the total maneuvering delta-V it can provide. Does this imply that having a large mass is a design objective? Of course not. It is desirable to find a fuel with minimal mass for a given delta-V.
That’s just ridiculous. The narrator never claimed that Beryllium was lighter than something with equal weight. She simply left unstated the fact that it is lighter than other materials for a given strength.
You’re so desperate not to concede that high strength (not large volume) is the design criterion that you are now circumlocuting with “mirror-base performance”?
This is turning into a sidetrack for this thread. I’m done here. Feel free to have the last word if you want to be so stubborn.
Thank you. I was thinking of starting a competitive derailing involving the obviously unwise decision to have the mirrors gold-coated, because it’s bound to attract alien space pirates! 
But it would be nice to get back to a discussion of the JWST for which I started this thread.
I apologize, I should have stopped flogging this dead horse 3 posts ago.
Thank you for ending that sidetrack.