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#51 2026-01-25 17:45:41
- SpaceNut
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- From: New Hampshire
- Registered: 2004-07-22
- Posts: 30,326
Re: Why Artemis is “better” than Apollo.
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#52 Yesterday 10:15:28
- GW Johnson
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- From: McGregor, Texas USA
- Registered: 2011-12-04
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Re: Why Artemis is “better” than Apollo.
Well, NASA managers are "playing the odds". Yes, the Artemis-2 heat shield is going to spall out chunks, maybe more of them since it was built less permeable, or so the story says. The Artemis-1 heat shield was mostly identical, and spalled out chunks. But those ugly and alarming craters left behind were not enough to cause a burn through.
Less likely would be two craters close enough together to be one big crater, at the bottom of which another chunk happens to spall out! That would penetrate most of the way through the heat shield, leading to a burn-through, in turn very likely fatal for the crew.
You have to understand what Avcoat really is. It is a cycolac polymer loaded with little microballoons to lower its density and increase its ablation rate. Denser is lower ablation rate. Too dense, and gas has more difficulty getting out of the less permeable char. But the more filled with microballoons, the more viscous it is, and hard to mold into where you want it to go. Artemis 2 has fewer microballoons in more of the tiles they made, according to the story. That's denser, with a slower ablation rate, but a less permeable char, increasing the risk of the gas not getting out, and so causing chunks to spall off.
Their theory is that gas unable to get out fast enough blows off chinks of char, exposing virgin material beneath too soon. That’s probably right. Probably.
Have you ever handled real charcoal? Not the pressed crap they sell for your grill, but an actual charred piece of wood? Depending upon the species, it can be quite weak. The more fibrous and stronger the virgin wood, if the charred fibers hold together, the physically tougher a charred piece of it is. Think of oak as strong char (good coals in the fireplace), versus mesquite as weak (doesn't even form coals, just burns immediately to soft ash).
The fundamental mistake NASA made with the Orion heat shield has NEVER BEEN ADDRESSED! That was deleting the fiberglass hex from the Avcoat! They continue to think of that as an either/or decision. Either we build it like Apollo, or we make tiles. That is just plain BS !!!!
You can put the hex back into every tile you make, where as the hex chars and melts, but at a rate slower than the cycolac, its fibrous structure ties the cycolac char together, as a sort of composite material. THAT is why the heat shields built Apollo-style did not spall chunks! THAT is the real difference! Not piddling around with more or less microballoon content in the cycolac polymer!
Here is where you have to see beyond the either/or thinking, in order to put the hex back into the Avcoat. Apollo (and EFT-1 Orion) had the hex bonded to the capsule structure. Into each and every hex cell, you had to manually gun viscously-stiff cycolac with a high microballoon content, to make sure its ablation rate was faster than the fiberglass hex (likely epoxy or vinyl ester resin on glass fiber, I don't know such details). There were 300,000-ish cells on Apollo, and nearly 400,000 such cells on Orion EFT-1. That's a LOT of manual labor to be paid!
Managers do not like to spend money. The good ones will spend what it takes to protect lives. The bad ones will not. There are more bad ones than good ones, we’ve already seen that during 2 shuttle-loss inquests. In fact, the good ones might well now be extinct, near as I can tell.
Here is how you do it OUTSIDE their either/or thinking:
Put a hex core into a tile mold with no bottom. Instead of pouring your cycolac /microballoon mix into an empty mold with a bottom, put that hex-containing bottomless mold on the outlet of an extrusion press (such are very common in the molded plastics industry). Load your cycolac / microballoon mix into the press, and extrude it through the cells in the hex inside that bottomless mold. Use a trowel to scrape most but not all of the extruded strings of mix off the bottom of the hex, and install the bottom of the mold. Then remove the mold from the press, add a bit more mix on top with the trowel, and install the top of the mold. Go cure the thing.
Result: hex-reinforced tiles that will not spall, because they are fiber-reinforced AND you used the correct high-microballoon mix ratio to get the correct ablation-rate ratio! AND you did it with low labor, saving scads of money! The labor relates to a few dozen tiles, not a third of a million cells. But you filled a third of a million cells doing it this way!
I gave that notion free of charge to NASA long ago. I was able to confirm that their heat shield group in Houston actually got it, and that some folks in that group thought very well of it. Then I never heard another word from anybody at NASA about it.
Meanwhile, NASA has had the 2 years necessary to make tiles that way and confirm that the process actually works. But they DID NOT DO THAT! They instead spent all their time and money trying to make unreinforced tiles look OK by analysis, while ONLY considering the hand-gunned Apollo process as the alternative!
Either/or thinking among top managers. The very same as what killed 2 shuttle crews! They so very clearly do NOT listen to their own engineers, much less engineers from outside the agency!
And THAT, sadly, is my assessment of NASA, regardless of who leads it!
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#53 Yesterday 12:24:51
- SpaceNut
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- From: New Hampshire
- Registered: 2004-07-22
- Posts: 30,326
Re: Why Artemis is “better” than Apollo.
Photos: NASA's Orion Spacecraft Heat Shield on Display
For the manual filling.
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