NASA used pure O2 in space until the end of the Apollo program, but the Shuttle and later used the same air we breathe today, 1atm 80% N2/20% O2. Note that in space, the pure O2 was at 0.4atm, so roughly twice the oxygen partial pressure, but only slightly more dangerous than the air we are breathing now. (You need about 0.4atm to keep your lungs from collapsing, so that's the lower limit.)
Why the difference? It's a question of what risks you were most afraid of. Even today, every single spacewalk is done at 0.4atm pure O2- trying to do a spacewalk at 100kPa even the strongest man in the world would have trouble bending his arms- so before a spacewalk the astronauts need to spend several hours pre-breathing pure O2 to get all the nitrogen out of their bloodstream before they can do a spacewalk. The Apollo program thought it was safer if the astronauts could do a spacewalk at literally any point in the mission, so that's what the spacecraft was designed around.
On the other hand, for long duration spaceflight, introducing a different pressure and atmosphere is just another potential source of health problems. Even today, the largest source of information on how human bodies last under 0.4atm pure O2 is the three Skylab missions from 1973-1974. And so the Soviets- who were always more interested in space stations than the moon- and NASA during the Shuttle era went with the atmosphere that seemed like it offered less health risks for people staying on a space station.
Okay, so what about the Apollo 1 fire? To speed up testing, Apollo 1 did two tests at the same time: the Plugs-Out Test, where the astronauts were in the spacecraft with everything running and practicing their countdown, and the Overpressure test where they pressurized the spacecraft to 1.4 atm (to mimic the pressure differential in outer space). And they did it with pure O2. So you had all of these electronics running in an environment at 1.4atm pure O2. And that was incredibly dangerous, in a way that actual spaceflight, a mere 0.4atm O2, was not. But it was just a test, another in a long string of them, and no one involved ever really analyzed it as a potential hazard.
After Apollo 1 a few things were changed: one was that they did the Plugs Out test and the Overpressure test at different times, and a lot of stuff was turned off for the Overpressure test. Another was that the Apollo capsule at takeoff was 1atm 80/20 until a couple of minutes into flight, when it dumped the cabin atmosphere overboard and replaced it with pure O2 at 0.4atm. That's why the astronauts carried little packs in their arms in all the pictures of them getting into the spacecraft, that's the pure O2 tank that they were breathing off of until they could switch to the atmosphere in the cabin after it was replaced.
They were definitely used on Mir- in 1997 one caught fire, blocking the crew's access to their escape Soyuz, though they put it out.
It looks like NASA helped redesign it to be safer, creating the modern Solid Fuel Oxygen Generator (SFOG) system still in use on the ISS as the backup.
There are indexes which explicitly try to capture the entire market- the Russell 3000 is most prominent, but the Wiltshire 5000 is another one, and Vanguard's Total Market Funds and ETF follow the CRSP US Total Market Index. I believe all of them plan to include SpaceX/OpenAI etc. within a few weeks of its listing, which is what I'd expect from their goal of tracking the total market. Other indexes follow just a few stocks- most famously, the Dow Jones Industrial Average (built during an era of when it had to be calculated by hand every night) looks at just 30 stocks in a weird way(1).
The S&P 500 isn't either of those. It has a list of criteria for inclusion, one of which is profitability. They are sticking with that criteria. If you don't like it, sell your VOO and buy VTI instead.
1: It is essentially impossible to build an index that tracks the DJIA because, since it was done on pencil and paper, it isn't actually market-cap weighted, but is share price weighted, with a correction factor for each stock to account for splits, one stock replacing another, etc. Because of that nature, the weights of the DJIA change minute by minute, so someone attempting to track it would be subject to enormous error.
In September 2016 almost exactly the same thing happened to a Falcon 9 at the Cape, also on a static fire. New Glenn is bigger, so bigger bang, but pretty much exactly the same thing.
Off the top of my head, I recall in SpaceX's case it was a helium tank failure- a helium tank weld failed and the helium tank itself shot through the cryogenic oxygen, hit the far wall, and gave off a spark. But that sort of failure is only apparent when everything is pressurized correctly, which means tanks have to be full. The goal of the test is that you detect that sort of failure before it goes boom and then can fix it.
Wasn't a bad weld; it was a bad interaction between liquid or solid oxygen and what were previously thought to be inconsequential defects in the composite-overwrapped pressure vessel the helium was loaded into.
Quoting from one of the press releases:
"The recovered COPVs showed buckles in their liners. Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition.
"The investigation team identified several credible causes for the COPV failure, all of which involve accumulation of super chilled LOX or SOX in buckles under the overwrap."
Was that when a SpaceX engineer demanded immediate "roof" access to ULA's pad because they suspected someone at ULA had used a sniper rifle to shoot at the Falcon? Crazy times.
>Externally, they sent the site director for their Florida operations, Ricky Lim, to inquire whether he might visit the roof of the United Launch Alliance building... ULA told SpaceX’s Ricky Lim to get lost when he wanted to see the roof of their building in Florida.
> This theory appealed to SpaceX founder Elon Musk, who was asleep at his home in California when the rocket exploded. Within hours of hearing about the failure, Musk gravitated toward the simple answer of a projectile being shot through the rocket.
Man, the signs were always there, right? I think I only fully realized it in 2018 during the cave "incident".
I think this makes sense, but then what’s the learning - dont make bad welds? I imagine they were already trying to do as best they could. Or perhaps “however stringent you think your checks are, they need to be more stringent”. And then learning that repeatedly is somewhat spectacular.
There's another comment that it wasn't the weld but even if it was the welders would build to spec and "better" (if it's known what better is) only if it's straightforward. There are certainly scenarios where a fabricator could design a better jig or use a more precise process but if the spec doesn't call for it then it's probably not going to happen because there are also the dimensions of time and money that matter as well.
LL Bean for me. Original at first, then the deluxe later as I got more homework. And I used an accordion folder, one slot for each class, for holding handouts etc. in addition to the notebooks and binders.
A report by some judges tried to carefully conceal the name of a judge who did a lot of terrible things (having a multi-year affair with a senior police officer, including sexual encounters in their chambers), even trying to hide their gender to make it more anonymous. But within about 10 minutes two different LLM's identified who did it, reasoning through the various possibilities and ruling most of them out.
Couples (in prime reproducing age) where both members WFH at least 1 day a week have 0.32 more live births per woman per lifetime than couples where neither does.
The reason that duplicates were treated as dangerous was that SO viewed their most important user not as anyone you have mentioned but instead they prized the lurker most- the person who typed their problem into Google and got brought to SO, and never asked or answered a question because they got what they wanted from that one page load. The entire structure of SO was built around this user.
So why does that mean that duplicates are dangerous? Because of updates. When someone answered a question about how to do something in Python (but it was 2008 so it was written in Python2) SO had ways to get a more correct, up-to-date answer to that question written in 2015 (and then again in 2019) and get that upvoted, and moderators could reward that new answer by editing the original etc.
That is why duplicates were a major threat: if the same question is asked and answered thousands of times, no one is going to go do the work to update all of those answers all across the site. Those lurkers are now dependent on the whims of Google as to which of the many answers you get taken to, and whether it has the latest answer or some answer that stopped working years ago.
And that is why they were so hostile to duplicate questions.
To extend this, the eventual problem is that you eventually lose all of your new users, which means you lose the extremely valuable intermediate users (ones that know enough to ask complex questions, but not so much that they can figure out the answer).
But in trying to solve that problem, they threw the baby out with the bathwater!
They were so fixated on solving that that they failed to realize: training all their power users to grief anyone who didn't behave like a power user off the site was detrimental (to having a site with non-power users). Everyone came for a question and answer site, but when it transformed it into a "question and get downvoted and modded into oblivion" site, everyone left.
AI put the final nail in the coffin, but SO was dead before AI arrived ... from this self-inflicted wound.
Why the difference? It's a question of what risks you were most afraid of. Even today, every single spacewalk is done at 0.4atm pure O2- trying to do a spacewalk at 100kPa even the strongest man in the world would have trouble bending his arms- so before a spacewalk the astronauts need to spend several hours pre-breathing pure O2 to get all the nitrogen out of their bloodstream before they can do a spacewalk. The Apollo program thought it was safer if the astronauts could do a spacewalk at literally any point in the mission, so that's what the spacecraft was designed around.
On the other hand, for long duration spaceflight, introducing a different pressure and atmosphere is just another potential source of health problems. Even today, the largest source of information on how human bodies last under 0.4atm pure O2 is the three Skylab missions from 1973-1974. And so the Soviets- who were always more interested in space stations than the moon- and NASA during the Shuttle era went with the atmosphere that seemed like it offered less health risks for people staying on a space station.
Okay, so what about the Apollo 1 fire? To speed up testing, Apollo 1 did two tests at the same time: the Plugs-Out Test, where the astronauts were in the spacecraft with everything running and practicing their countdown, and the Overpressure test where they pressurized the spacecraft to 1.4 atm (to mimic the pressure differential in outer space). And they did it with pure O2. So you had all of these electronics running in an environment at 1.4atm pure O2. And that was incredibly dangerous, in a way that actual spaceflight, a mere 0.4atm O2, was not. But it was just a test, another in a long string of them, and no one involved ever really analyzed it as a potential hazard.
After Apollo 1 a few things were changed: one was that they did the Plugs Out test and the Overpressure test at different times, and a lot of stuff was turned off for the Overpressure test. Another was that the Apollo capsule at takeoff was 1atm 80/20 until a couple of minutes into flight, when it dumped the cabin atmosphere overboard and replaced it with pure O2 at 0.4atm. That's why the astronauts carried little packs in their arms in all the pictures of them getting into the spacecraft, that's the pure O2 tank that they were breathing off of until they could switch to the atmosphere in the cabin after it was replaced.
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