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It seems to me that there could be two concerns about frost damaging heat shield tiles.
Frost accumulation, water expands as it turns to ice. But I suspect that the way the frost would accumulate may not produce much of that effect, as the accumulation would be from air to a developing ice mass.
As for the thawing process in a greater partial vacuum, evaporation at 0 degrees C would produce only 6 mbar of pressure per solid to gas phase.
Liquid pockets in the ice would actually contract in volume.
So, it would be a matter of time and temperature to dry out the heat shield prior to reentry attempts.
So, it is not hopeless, perhaps. The "Pins" if perfected may allow some wiggle on the tiles without breakage. That would be the hope. Perhaps some spring action in them.
The cargo portion of the heat shield may not have these problems as much as cold air will flow down during tanking of propellants prior to launch.
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Last edited by Void (2024-03-22 11:14:10)
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Some tiles are coming loose during the first stage ascent. But what I saw in the entry video was a whole bunch of them coming off during entry. Recovery of intact tiles would suggest the problem is not so much breakage of fragile low-density ceramic, but the attachment method. That's where the failure is. The question is: why are forces trying to dislodge the tiles so much higher than they anticipated?
I so far have seen two notions: (1) ice accumulating in the void space under the tile suddenly vaporizing in a steam explosion during entry, blowing the tiles straight off the attach pins, and (2) air getting underneath the edge of the tiles along an edge of the heat shield inadvertently exposed to strong surface-scrubbing winds (when that was not supposed to happen), pressuring under the tile to blow it off the attach pins. This would unzip from the edge into the interior of the heat shield, as long as that direction of fluid scrubbing is maintained.
There should have been no steam explosions of heated ice during the ascent, because there was no aeroheating at only subsonic transonic conditions, with the tanks so full of cold liquid. Yet some tiles came loose, since intact tiles were found on the beach.
That favors the wind getting under the edge of the heat shield for tiles lost during ascent. There's not many locations where that can occur unless the vehicle tumbles. So there's not that many tiles lost on ascent.
Both causes could be active during entry, especially since the videos suggest the vehicle was tumbling as entry began. The Earth in those images is moving all the way around the vehicle, which is probably both pitch/yaw and roll motions that should not be there. You can also see plasma glowing in places and directions that should not be occurring, such as radial streamers coming from the periphery of the engine bay, indicating tail-first attitude into the oncoming windstream.
If the vehicle is tumbling, edges of the heat shield are repeatedly exposed to surface-scrubbing flow directed toward the local edge of the heat shield, allowing air to get underneath the tile and pressure it off from the attach pins. This would strip row by row like a zipper from the edge into the interior of the heat shield. It stops only when the flow direction changes as the vehicle tumbles. Which would show up as "pulses" of multiple simultaneously-lost tiles in the video, which is EXACTLY what we saw!
Now, if the vehicle did not tumble, that loss mechanism does not happen, and flow never scrubs along the surface into an edge of the heat shield. But ice deposits under the tiles can still suffer steam explosions, since the heating is hundreds of watts per square centimeter at peak. Tiles could still be lost to that mechanism, unless they make the attachments stronger.
I think it is pretty clear the vehicle was tumbling randomly, and not under control a all, as entry began. That enabled the wind-under-the-edge-of-the-heat shield mechanism to strip off a lot of tiles in quick pulses (which we saw in SpaceX's own video), leading to vehicle breakup before it even reached peak heating, much less peak deceleration gees.
These tiles are some sort of reduced density ceramic, but are not as low a density as the old Shuttle tiles, or those hexagonal shapes would weight much less than 3/4 pound. They ride on that white felt layer, because their thermal conductivity is not low enough to effectively cut off inward conduction. The felt does that. In that sense, the tile and felt are doing exactly the same thing as the two-part Tufroc tiles used on the X-37B's nose and leading edges.
I don't know what ceramic or ceramic blend they are using to make these low-density tiles, but it is not the alumino-silicates used in NASA's shuttle tiles, because reportedly the maximum temperature is higher at 2500 F. It is clear that re-radiative cooling is intended, or they would not be black. They must re-radiate sufficiently to equal stagnation heating rates, at temperatures a tad below that 2500 F service rating. (With shuttle, that was 2000 F max nominal rating, limited by the phase-change temperature of 2350 F.) Maybe mag oxide? I dunno. If it is, it's the first porous mag oxide I ever heard of.
SpaceX's problem is actually two-fold: (1) they clearly need larger and more reliable attach forces, and (2) they absolutely cannot allow the vehicle to tumble or assume off-design attitudes during entry, since their heat shield has edges that are vulnerable to getting stripped by surface scrubbing forces if they ever get directed toward those edges.
GW
Last edited by GW Johnson (2024-03-22 11:53:59)
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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I think I feel comfortable with your diagnosis, so far.
But an interesting idea has formed in my mind about this. For a very future type of ship, not the Starship, could active cooling be achieved by pushing a fluid under the tiles, and outside of the metal skin of the ship? As I say, not for this ship, as the tiles are fragile, and the attachment mechanism is not perfected.
Just an interesting variation on active cooling perhaps. A ship with lizard scales, where a cooling fluid is flowed under the tiles.
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I think something like that could be developed more for a Stokes Space type ship. Perhaps something that could survive a return from the Moon at that sort of speed.
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Vapors then flowing from under the tiles, and then through the gaps between tiles then might also cool the outside of the tiles. The rate of flow of vapors might be controlled as a consumable, and only be at a needed rate to protect the tiles.
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For Starship:
I think that if they could flow cold dry air down the sides of the heat shield when it is on the launch pad and being filled with propellants, they might be able to keep moisture from condensing under the tiles. I suppose it may depend on the wind conditions.
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Last edited by Void (2024-03-22 12:51:01)
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Void:
One of the things you brought up is transpiration cooling. Until now, nobody has actually tested this concept during entry, although it was supposed to have been flown experimentally on the X-20 Dyna-Soar that was cancelled in 1963. It would seem that one of the new space outfits intends to use transpiration cooling in its design. I just don't recall which outfit or the vehicle. You said something about Stokes Space, maybe that's it. It will be interesting to see how well transpiration cooling really works. Finally. After all these decades.
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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Stokes space is in the family, but the "Heat Shield" is sort of a boiler that boils Liquid Hydrogen, and then the output is vented from thrusters.
I am also interested in the pinhole sweaty type.
What I speculated on might be a sort of hybrid, where the pinholes would vent to a place under tiles, and then the gas flow from under the tiles, to exit between the tiles, and then that perhaps to even cool the outside of the tiles.
But of course, it would be really easy to blow the tiles off that way, so I think such a method could be considered for trial in some way after both the Stokes Space method and the sweaty methods have been tried and confirmed to work.
Thanks for your tolerance. I know I am not at all measured as a professional and am happy to confess to that fact.
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Last edited by Void (2024-03-23 10:55:06)
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