Heat Shield Design Manufacture Application Maintenance

Void · 2024-03-22 11:09:48

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.

Done

Last edited by Void (2024-03-22 11:14:10)

GW Johnson · 2024-03-22 11:33:15

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)

Void · 2024-03-22 12:01:27

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.

Done

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.

Done

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.

Done

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.

Done

Last edited by Void (2024-03-22 12:51:01)

GW Johnson · 2024-03-23 10:10:59

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

Void · 2024-03-23 10:51:57

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.

Done

Last edited by Void (2024-03-23 10:55:06)

tahanson43206 · 2024-08-06 13:23:50

Here is an item from Quora, about heat shields...

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Do spacecraft heat shields play a role in the deceleration of the spacecraft on re-entry or are they merely to protect the astronauts from heat?
Originally Answered: My friend and I were having a friendly debate on the heat shields of spacecraft, do heat shields play a role in the deceleration of the spacecraft on re-entry or are they merely to protect the astronauts from heat?
Yes. Heat shields are intentionally designed to have high drag and thus more rapid deceleration.
Julian Allen and A.J. Eggers of NACA made the counter-intuitive discovery, in 1951, that a blunt shape (high drag) made the most effective heat shield. They showed that the heat load experienced by an entry vehicle was inversely proportional to the drag coefficient - the greater the drag, the less the heat load. Through making the reentry vehicle blunt, air can’t ‘get out of the way’ quickly enough, and acts as an air cushion to push the shock wave and heated shock layer forward. Since most of the hot gases are no longer in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere.

It’s usually assumed that the mechanism of heating in reentry is by friction (i.e. viscous drag in the atmosphere). In fact, this is the predominant mechanism only at lower altitudes, as air density increases. During the fastest and hottest part of the descent, less familiar physics is in play. A reentering vehicle develops a very energetic pressure wave at its leading surfaces. The energy density is sufficient to cause atmospheric molecules to dissociate, and their component atoms to become ionized. The vehicle thus descends in a superheated shroud of incandescent plasma.

The history cited goes back to Mercury.

An item of particular interest is the idea that a blunt shield does not experience the worst of the heating that might be expected, because a buffer zone of atmosphere takes the beating that would otherwise happen to the shield.

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tahanson43206 · 2024-08-07 05:14:32

In another topic, Void recently described an interesting concept for a heat shield that might use CO2 as a coolant.

https://newmars.com/forums/viewtopic.ph … 00#p225600

I am hoping NewMars members will provide feedback for Void on the design of such as system.

This is another area where innovation is needed, and once again, Void has shown a possible way forward.

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tahanson43206 · 2024-08-23 06:46:15

In another topic, it was proposed that Silicon Carbide might be used as a heat shield for Earth return from the Moon.

This post is intended to show a snippet or two about Silicon Carbide...l.

AI Overview
Learn more
…
Pure silicon carbide (SiC) is made of 29.95% carbon and 70.05% silicon. Technical silicon carbide also contains impurities, such as:
Free carbon and SiO2
Silicon and iron
Aluminum and calcium
Oxygen
Titanium
Nickel
Vanadium
Impurities are often introduced during smelting, milling, or storage.
Silicon carbide is a hard, synthetic, crystalline compound and semiconductor. It's also known as carborundum. It's been mass-produced since 1893 as a powder and crystal for use as an abrasive. It's also used in steel additives, structural ceramics, refractory linings, heating elements, wear-resistant parts, and semiconducting substrates.
Occurrence forms of major impurity elements in silicon carbide
Jan 1, 2022 — The experimental results showed that in SiC powders prepared using the Acheson process, the contents of O, free-Si, fre...
ScienceDirect.com
Silicon Carbide | AMERICAN ELEMENTS ®
Ceramics Refractory CeramicsTransparent CeramicsAluminates Arsenides Borides Carbides Ferrites Niobates Nitrides Phosphides Silici...
American Elements
Silicon Carbide - Top Seiko Co,. Ltd.
Ceramics List. Alumina（Al₂O₃） Excellent electrical insulation properties , High temperature resistance , Plasma resistance , High ...
Silicon carbide - Wikipedia
Silicon carbide (SiC), also known as carborundum (/ˌkɑːrbəˈrʌndəm/), is a hard chemical compound containing silicon and carbon.
Silicon carbide - Wikipedia
Wikipedia
https://en.wikipedia.org › wiki › Silicon_carbide

The Carbon for this application would have to be imported from elsewhere in the Solar System.

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tahanson43206 · 2024-08-23 06:56:54

Gemini found support for the idea of using Silicon Carbide as a heat shield material for lunar return.

Hello Again! It has been proposed to use Silicon Carbide as a heat shield for flights returning from the Moon. Please see if any research has been done using Silicon Carbide for a heat shield.
Silicon Carbide as a Heat Shield for Lunar Returns: A Promising Candidate
Yes, there has been significant research and development into using Silicon Carbide (SiC) as a heat shield material for lunar return missions. SiC's exceptional properties make it a strong contender for this demanding application.
Key Properties of Silicon Carbide:
High Temperature Resistance: SiC can withstand extremely high temperatures, making it ideal for the intense heat experienced during atmospheric reentry.
Low Density: SiC is relatively lightweight, which is crucial for minimizing the mass of the heat shield.
Excellent Thermal Conductivity: SiC can efficiently dissipate heat, helping to protect the spacecraft from excessive temperatures.
Chemical Stability: SiC is resistant to oxidation and other chemical reactions, ensuring its durability in harsh environments.
Research and Development:
NASA's LOFTID Mission: NASA's Low-Frequency Asteroid and Planetary Science (LOFTID) mission is testing an inflatable heat shield made of a woven silicon carbide fabric. This innovative approach aims to reduce the mass and complexity of heat shields while providing effective thermal protection.
Other Projects: Various research institutions and aerospace companies have explored the use of SiC in different heat shield designs, including tiles, blankets, and ablative materials.
Challenges and Considerations:
Manufacturing: While SiC is a promising material, its production can be complex and expensive. Researchers are working on developing more efficient manufacturing processes to make SiC more accessible.
Integration with Spacecraft: Integrating a SiC heat shield into a spacecraft's design requires careful consideration of factors like weight distribution, structural integrity, and compatibility with other systems.

Overall, Silicon Carbide shows great promise as a heat shield material for lunar return missions. With ongoing research and development, it could become a viable and effective solution for protecting spacecraft from the extreme temperatures encountered during atmospheric reentry.
Would you like to learn more about specific research projects or the manufacturing processes involved in producing SiC heat shields?
Sources and related content

Unless a source of Carbon is found elsewhere in the Solar System, the most logical source for Carbon would be the Earth.

A carbonaceous asteroid that passes near the Earth/Moon system would/could provide plenty of Carbon for a Lunar heat shield industry.

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tahanson43206 · 2024-08-23 07:32:15

Because the Apollo heat shield was a success, and because GW Johnson has mentioned it often in favorable comparison to other designs that have been considered, I decided to ask Gemini to collect information about the Apollo design:

Hello Again! The heat shield developed for the Apollo Moon flights is a notable success. Please see if you can find details of how the heat shield was made, and why that method was chosen.
The Apollo Heat Shield: A Technological Marvel
The Apollo heat shield was a groundbreaking piece of engineering that played a crucial role in the success of the lunar missions. It was designed to protect the Command Module from the extreme temperatures and pressures experienced during atmospheric reentry.
Construction and Materials:
Ablative Material: The primary heat shield material was a phenolic-impregnated fiberglass honeycomb. This ablative material was designed to vaporize and erode as it encountered the intense heat of reentry, absorbing and carrying away the heat from the spacecraft.
Honeycomb Structure: The honeycomb structure provided a strong and lightweight foundation for the ablative material. It helped to distribute the heat load evenly and prevent the heat shield from collapsing.
Why This Design?
Ablation: The ablative design was chosen because it could effectively absorb and dissipate the immense heat generated during reentry. As the material vaporized, it created a protective layer that shielded the spacecraft from the worst of the heat.
Lightweight: The honeycomb structure and ablative material combined to create a relatively lightweight heat shield, which was essential for reducing the overall mass of the spacecraft and improving its performance.
Reliability: The Apollo heat shield design was thoroughly tested and proven to be reliable in multiple missions. It provided the necessary protection for the astronauts and the spacecraft to return safely to Earth.
The Apollo heat shield was a testament to the ingenuity and engineering prowess of its designers. Its success paved the way for future space exploration missions and continues to inspire innovations in heat shield technology.

Sources and related content
Heat Shield, Fragment, Ablated, Apollo CM 009 | National Air and Space Museum
airandspace.si.edu
Heat Shield, Fragment, Ablated, Apollo | National Air and Space Museum
airandspace.si.edu
What are the chemical elements that were used to make the Apollo heat shield?
The Apollo heat shield was primarily made of a phenolic-impregnated fiberglass honeycomb structure. Here's a breakdown of the chemical elements involved:
Phenolic Resin: This is a synthetic polymer derived from phenol and formaldehyde. It's used as a binder to hold the fiberglass fibers together.
Fiberglass: Fiberglass is a composite material made from glass fibers. These fibers are typically composed of silica (SiO2), alumina (Al2O3), and other oxides.
Honeycomb Structure: The honeycomb structure itself is typically made from a lightweight material like aluminum or a composite.
So, while the primary heat-resistant material was the phenolic resin, the overall structure also included elements like silicon, oxygen, aluminum, and possibly others depending on the specific composition of the fiberglass and honeycomb.
What are the elements in the phenolic resin

Phenolic resins are a type of synthetic polymer formed from the condensation of phenol and formaldehyde. The primary elements present in phenolic resins are:
Carbon (C): Found in both phenol and formaldehyde.
Hydrogen (H): Found in both phenol and formaldehyde.
Oxygen (O): Found in both phenol and formaldehyde.
In addition to these elements, phenolic resins may also contain small amounts of other elements, such as sulfur (S) or nitrogen (N), depending on the specific formulation and additives used.
There is no Carbon on the Moon. Can you imagine a heat shield that would function as well as the Apollo heat shields did, made from materials that are abundant on the Moon?
You're absolutely right! Carbon is a vital element in the phenolic-impregnated fiberglass used for the Apollo heat shield, and it's not abundant on the Moon. However, there are other materials found on the Moon that could potentially be used for a heat shield.
Here are some possibilities:
Regolith: The lunar regolith, which is a fine-grained material covering the Moon's surface, is primarily composed of silicate minerals. While not as effective as a traditional heat shield, it could be used in combination with other materials to provide some level of thermal protection.
Lunar Ice: If significant amounts of lunar ice are found, it could be used as a coolant or ablative material. However, its effectiveness would depend on factors like the amount of ice available and the efficiency of its application.
Lunar Rocks and Minerals: Certain types of lunar rocks and minerals, such as basalt or anorthosite, might have properties that could be beneficial for heat shielding. Research is ongoing to explore these possibilities.
Challenges and Considerations:
Material Processing: Extracting and processing materials from the lunar surface would be a significant challenge, requiring advanced technology and infrastructure.
Thermal Properties: The thermal properties of lunar materials would need to be carefully studied and tested to ensure they can withstand the intense heat of reentry.
Structural Integrity: Any heat shield made from lunar materials would need to be designed to maintain its structural integrity during the harsh conditions of reentry.
While it's challenging to predict the exact composition of a future lunar heat shield, these materials offer promising possibilities for in-situ resource utilization. Researchers are actively exploring these options to develop sustainable and efficient heat shields for future lunar missions.

Sources and related content

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tahanson43206 · 2024-08-23 08:02:44

In the session below, Gemini came up with the possibility that the Apollo heat shield may have been classified during the Cold War. I would like to think that a Freedom of Information Act petition could release the information, if it is indeed still classified.

Thank you for gathering information about the heat shield developed by NASA and contractors for the Apollo Moon flights. NASA has failed to reproduce the Apollo design, and it has instead attempted to build a design that turned out to be deficient. What would it take to recreate the original successful Apollo Moon flight heat shields? Was there a prime contractor for that work?
The Apollo heat shield was a technological marvel, and its exact design and manufacturing processes remain proprietary. While NASA has not been able to fully replicate it, the agency has made significant strides in understanding and developing new heat shield technologies.
Key Factors for Recreating the Apollo Heat Shield:
Material Composition: The specific composition of the phenolic-impregnated fiberglass honeycomb used for the Apollo heat shield is a closely guarded secret. Replicating this exact material would require extensive research and development.
Manufacturing Techniques: The manufacturing processes used to create the honeycomb structure and impregnate it with the phenolic resin were highly specialized and may have involved techniques that are no longer in widespread use.
Testing and Validation: Thorough testing and validation would be essential to ensure that any new heat shield design meets the stringent requirements for lunar return missions. This would involve subjecting the heat shield to extreme temperatures and pressures in a simulated reentry environment.
While NASA has not been able to fully replicate the Apollo heat shield, the knowledge gained from that program has been invaluable in developing new heat shield technologies. It's possible that future advancements in materials science and manufacturing techniques could enable the creation of a new heat shield that is comparable or even superior to the Apollo design.
As for a prime contractor, it's likely that multiple companies and organizations were involved in the development and production of the Apollo heat shield. NASA often collaborates with industry partners to develop and test new technologies for space missions. It's possible that the exact details of the prime contractor or contractors for the Apollo heat shield are not publicly available.
Thank you for this very interesting information. Did the American public pay for the Apollo heat shield?
Yes, the American public did pay for the Apollo heat shield. The Apollo program was a massive undertaking funded by the U.S. government, and the costs were borne by taxpayers. The development and production of the heat shields, along with other components of the Apollo spacecraft, were significant expenditures within the overall program budget.
It's important to note that the Apollo program was a significant investment in scientific exploration and technological advancement. While the costs were high, the benefits in terms of national pride, scientific knowledge, and technological innovation were also substantial.
Please explain how the manufacture of the heat shield could be kept from the public? What agency of government is responsible?
The secrecy surrounding the Apollo heat shield was likely due to a combination of factors:
National Security: During the Cold War, many aspects of the Apollo program were classified for national security reasons. The heat shield, as a critical component of the spacecraft, may have been considered sensitive technology that could have potential military applications.
Competitive Advantage: Protecting the details of the heat shield's design and manufacturing could have been seen as a way to maintain a technological advantage over other nations.
Proprietary Information: Some aspects of the heat shield's development may have been the result of proprietary research conducted by private companies or universities. These entities might have retained ownership of certain information and kept it confidential.
The agency primarily responsible for the Apollo program, and thus the heat shield, was NASA (National Aeronautics and Space Administration). However, NASA often collaborated with private companies and research institutions, and some aspects of the heat shield's development may have been handled by these entities.
While the exact level of secrecy surrounding the Apollo heat shield may vary depending on the specific details, it's likely that a combination of national security, competitive advantage, and proprietary information played a role in keeping certain aspects of its development confidential.
Does it require an act of Congress to release any and all information about the Apollo heat shield?
Yes, it is likely that an act of Congress would be required to release any and all information about the Apollo heat shield.
Given the potential sensitivity of the information, particularly due to its historical context and potential implications for national security and technology, it's highly probable that the classification or declassification of such documents would fall under the purview of the U.S. government.
Congress has the authority to enact laws that govern the classification and declassification of government documents. While some documents may be automatically declassified after a certain period, others might require specific legislation to release.
If you are interested in obtaining information about the Apollo heat shield, you might consider contacting your Congressional representatives to express your interest and advocate for the declassification of relevant documents.

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GW Johnson · 2024-08-23 15:00:37

I addressed this in the other thread. If that was ever classified at all, it was declassified long ago.

It might have been some sort of company trade secret, but I think NASA had control of it, because both McDonnell and North American Rockwell used the very same technology on Gemini (McDonnell) and Apollo (North American Rockwell), and they were competing companies back then.

The original versions of the technology actually flew on Mercury (also built by McDonnell).

There is a rather complete Wikipedia article on Avcoat. It pretty much tells you what it is, what it is made with, and how it was made. There's absolutely NO excuse for NASA not to have detailed specifications and process instructions in its Apollo records!

GW

update 8-23-2024 4:44 PM CDT --here is the link: https://en.wikipedia.org/wiki/AVCOAT

Last edited by GW Johnson (2024-08-23 15:45:07)

tahanson43206 · 2024-08-23 19:42:56

For GW Johnson and all members....

The link provided by GW in Post #37 contains a reference to NASA documentation that appears to be from 1968. I am offering a quote here because it seems to show why the shield was made by "gunning" material into the individual cells of the honeycomb. The answer appears to be that the designers decided to fabricate the metal understructure in one piece, and they laid the honeycomb on that base also as one continuous piece. That explains why the ablative material had to be injected into the individual cells. There was no mass production of tiles.There was no putty knife smoothing of the material over the cells. My guess is that this meticulous procedure was followed (at great cost) due to the need to be absolutely certain that EACH CELL was properly loaded with material.

My argument is that modern robotic manufacturing allows the same meticulous preparation of the individual cells to be done far more rapidly and at far less cost to the same level of accuracy.

References
Wilson, Jim. "NASA - NASA's Exploration Systems Architecture Study -- Final Report". www.nasa.gov.
"Fire-Resistant Reinforcement Makes Steel Structures Sturdier". January 12, 2007. Archived from the original on 2007-01-12.
Textron Systems History Archived November 30, 2010, at the Wayback Machine, 1984 History, "Textron acquires Avco, including Lycoming, to become Avco Systems Textron", 2010, accessed 2010-11-27.
Flight-Test Analysis Of Apollo Heat-Shield Material Using The Pacemaker Vehicle System NASA Technical Note D-4713, pp. 8, 1968-08, accessed 2010-12-26. "Avcoat 5026-39/HC-G is an epoxy novolac resin with special additives in a fiberglass honeycomb matrix. In fabrication, the empty honeycomb is bonded to the primary structure and the resin is gunned into each cell individually. ... The overall density of the material is 32 lb/ft3 (512 kg/m3). The char of the material is composed mainly of silica and carbon. It is necessary to know the amounts of each in the char because in the ablation analysis the silica is considered to be inert, but the carbon is considered to enter into exothermic reactions with oxygen. ... At 2160° R (1200° K), 54 percent by weight of the virgin material has volatilized and 46 percent has remained as char. ... In the virgin material, 25 percent by weight is silica, and since the silica is considered to be inert the char-layer composition becomes 6.7 lb/ft3 (107.4 kg/m3) of carbon and 8 lb/ft3 (128.1 kg/m3) of silica."
Apollo Experience Report - Thermal Protection Subsystem (Jan. 1974)

It seems to me there is a business opportunity for a company that is willing to recover the Apollo heat shield technique, and offer heat shields suitable for missions returning from the Moon or elsewhere in the solar system.

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SpaceNut · 2024-08-24 10:55:33

Gw's post

Sounds to me that they need to go back to honeycomb design to keep it from poor performance as it is being used.

tahanson43206 · 2024-08-24 12:29:28

For SpaceNut ... thanks for your post #39, and for noting the proven Apollo design. As you may have noted, manufacturing techniques have improved greatly since 1968, While the original Apollo heat shields had to be made with enormous manual effort, it is now possible to build the entire shield using 3D Printing and robots. I'm hoping that NewMars members may be able to find records of the original design.

My understanding is that the base of the shield was a matrix of thin metal plates that provided the rounded/curved surface upon which the Fiberglas hexagonal cells was laid, and the cells were then filled by hand by experts.

I believe that the entire operation can now be automated.

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tahanson43206 · 2024-11-29 10:40:13

NASA published a detailed report on Orion, and I've selected the portion of the text that covers the heat shield that is installed in the Orion for Artemis II.

The article describes the Apollo system, and specifically cites the 320,000 individual honeycomb cells that had to be filled by hand.

The heat shield currently installed has NO honeycomb cells at all, and is vulnerable to failure during the Earth return.

Heat Shield
The heat shield is one of the most critical elements of the Orion spacecraft, protecting Orion and the astronauts as they enter Earth’s atmosphere travelling about 25,000 mph and reaching temperatures of nearly 5,000 degrees Fahrenheit — about as half as hot as the Sun.
Measuring 16.5 feet in diameter, it’s the largest ablative heat shield in the world. The outer surface is made of blocks of an ablative material called Avcoat, a reformulated version of the material used on the Apollo capsules. During descent, the Avcoat ablates, or burns off in a controlled fashion, transporting heat away from Orion.
The Avcoat is first made into large blocks at NASA’s Michoud Assembly Facility, then shipped to NASA’s Kennedy Space Center in Florida and machined into 186 unique shapes before being applied onto the heat shield’s underlying titanium skeleton and carbon fiber skin. Engineers conduct non-destructive evaluations to look for voids in the bond lines, as well as measure the steps and gaps between the blocks. The gaps are filled with an adhesive material and then reassessed.
After the thermal protection system has been applied and inspected, engineers and technicians put the heat shield through a thermal cycle test in the high bay of the Operations and Checkout (O&C) Building at Kennedy. The thermal cycle test ensures the thermal protection blocks are properly bonded and will perform as expected when they are exposed to the extreme temperatures during the mission.
Inside the Neil Armstrong Operations and Checkout Building high bay at NASA’s Kennedy Space Center in Florida, from left, technicians Kenny Leidner, Diamond ScharSenstine, Russ Novak and Darlene Beville with ASRC Federal, inspect AVOCAT block bonding on the Artemis II heat shield on July 2, 2020.
NASA
The heat shield is then given a coat of white epoxy paint. Aluminized tape is applied after the painted surface dries to dissipate electrical surface charges and maintain acceptable temperatures. Once all testing has been completed, the heat shield is bolted to the crew module.
The Orion heat shield was redesigned from a single piece system to individual blocks of material following Exploration Flight Test-1 in 2014. Before the time-saving block system was used, a fiberglass-phenolic honeycomb structure was bonded to the structure’s skin. Then each of the 320,000 tiny honeycomb cells were individually filled with Avcoat by hand, inspected by X-ray, cured in a large oven, and robotically machined to meet precise thickness requirements.
This new design introduced several considerations that prompted further testing for risk reduction. Engineers performed more than 30 tests across the United States on the new design to investigate the effects of the block structure that could disrupt the smooth airflow and cause localized heating spots. Understanding both effects is critical to confirm the heat shield will thermally protect the astronauts during entry into Earth’s atmosphere. The Avcoat material was tested at the NASA Ames Research Center Arc Jet Complex and at the NASA Johnson Space Center’s Atmospheric Reentry Materials and Structures Evaluation Facility through its closing in 2014. The Arc Jet facilities were used to simulate the heating and airflow conditions that occur during entry. Thermal testing also has been performed at Johnson Space Center’s Radiant Heat Test Facility (RHTF) periodically from the onset of the Orion Program up until March 2021. During these tests, the Avcoat surface reached temperatures of over 3,000 degrees Fahrenheit. Heat shield testing concluded in March 2019 at NASA’s Langley Research Center with a six-inch Orion heat shield model in the 20-inch Mach 6 wind tunnel. The six-inch Orion heat shield model was machined to represent small-scale features, including the patterns expected as the heat shield ablates during return to Earth.

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GW Johnson · 2024-12-08 11:29:09

Avcoat is an epoxy novolac resin filled with microballoons to reduce its density and "tailor" its ablation rate. I keep thinking silicone resin, but that was Gemini, and also the best ablative that I used in ramjet combustors. Sorry.

Avcoat is supposed to be gunned into the hex cells of a fiberglass hex core. The glass melting produces silica, and the epoxy pyrolizing produces carbon. The char this stuff produces atop the virgin is supposed to be composed of both silica and carbon, and it is inherently porous. That's the way it was during Apollo, and on the first flight test of the Orion capsule atop a booster other than SLS. Those performances were quite adequate for simple entries into Earth's atmosphere at its escape speed or thereabouts.

Plus, the glass fibers soften at a slightly higher temperature than the epoxy resin pyrolizes (real silica fiber even higher). What that means is that the glass fibers penetrate into the char a little ways, as the "pyrolysis front" moves into the material during heating. That effect helps to "tie" the weak porous char layer more strongly to the virgin beneath. Which in turn makes the char harder to rip off of the virgin beneath, by whatever forces get applied (for any reason).

If you leave out the fiberglass hex, the char has only carbon, and is less dense, and less strong. Anybody who was ever handled real charcoal that wasn’t made in a press, knows how crumbly this stuff really is. It is NOT strong at all! Plus, it usually embrittles upon cooldown, becoming even more vulnerable to applied forces. And without the hex, the char layer is no longer as strongly "tied" to the virgin beneath. THAT is exactly what they did when they went to Avcoat tiles without the fiberglass hex, on Orion for the Artemis 1 and 2 capsules!

Add to that the fact that Apollo and the first Orion test flight did not attempt skip-type entries, with two heating pulses separated by a cooldown coast. Artemis 1 did! It had a weaker char layer, tied more weakly to the virgin beneath, that further had embrittled and become even more fragile, during the cooldown coast between the two entry heating pulses. This exposure was outside all previous experience ever obtained with those heat shield materials and construction methods!

So, why should ANYBODY have been surprised if it didn't act as expected? And yet they were surprised, and then took 2 years investigating what actually happened. They have decided to fly the same misbehaving heat shield on Artemis 2, primarily because it is already built and installed, not for any other valid reason! They are going to delete the skip with its cooldown coast, which is a step in the right direction.

They think it was gas pressure buildup within the material as the thermal wave works its way inward and causes more virgin to pyrolize (during the skip cooldown), that actually pushed chunks of char off the heat shield, leaving those odd craters behind. They think that was the mechanism instead of my suggestion that fluid shear forces ripped the chunks out by a severe scrubbing action. But the actual cause makes no difference, the real "fix" is EXACTLY the same: put the hex back in the material, so that the glass fibers can help tie the char to the virgin more strongly! Simple as that!

We members of the public have only recently been allowed to hear more details about how they built this thing. I would have built molds for each tile shape, and just cast them almost right to finished dimensions. That's not what they did! They made big blocks of cast balloon-filled epoxy novolac resin, and literally machined every tile shape out of those blocks!

Doesn't matter, just put the hex back into the resin that you make the big blocks from! Then machine what you want from those blocks! The easiest way to get the resin into the hex cells without manual hand-gunning, and without (fatal) voids, is to use a plastics extrusion press, and just push it right through a big hex core, to make your big blocks! What could be simpler?

Nothing about my "fix" is different, except for exactly where in their more complicated process you do it. Pushing goo with an extrusion press to where you want it to go, is a very common processing procedure in the plastics industry. There is no new technology there!

Just do it!

GW

PS -- after posting this, I noticed the logout button is unresponsive. I've often seen that bbefore, in the older system. That fault is still there in this newer one.

Last edited by GW Johnson (2024-12-08 11:39:23)

tahanson43206 · 2024-12-09 09:35:06

For all ... The initiative to try to contact folks who might be able to influence how heat shields are made for planetary/lunar missions is still active. A small hint of a possible nibble of interest just arrived. This is an opportunity for those who support GW's outreach effort to think positively. The lives of the Artemis astronauts are at risk due to cost cutting at NASA years ago.

(th)

GW Johnson · 2024-12-09 10:05:08

From AIAA’s “Daily Launch” for Monday 12-9-2024:

Here is the short item in the newsletter, which is a ink to a longer article posted on Ars Technica:

ARS TECHNICA
After critics decry Orion heat shield decision, NASA reviewer says agency is correct

Within hours of NASA announcing its decision to fly the Artemis II mission aboard an Orion spacecraft with an unmodified heat shield, critics assailed the space agency, saying it had made the wrong decision.

This is a short excerpt of 1 paragraph from the longer Ars Technica article:

"Based on the data, we have decided—NASA unanimously and our decision-makers—to move forward with the current Artemis II Orion capsule and heat shield, with a modified entry trajectory," Bill Nelson, NASA's administrator, said Thursday. The heat shield investigation and other issues with the Orion spacecraft will now delay the Artemis II launch until April 2026, a slip of seven months from the previous launch date in September 2025.

My take on it:

This is the gist of what has been going on: the investigation includes work by the regulars at NASA and an independent review team (IRT). NASA did a bunch of work, really good work, supporting flying “as-is”. The majority of the IRT agreed, although there were two who disagreed, one possibly a leader of some sort on that IRT.

I tend to think they are probably right to do this with the trajectory mod deleting the entry skip. But I also think they will see some cratering char loss anyway. How much, who knows? I think the odds of a fatal burn through are low, but not as close to zero as they should be.

The real problem is what changes will go into the heat shield for Artemis-3 landing mission? I’ve made my inputs to that, and I hope it helps them do this “right”.

-- GW

tahanson43206 · 2024-12-09 13:25:00

For GW Johnson re #44 --- best wishes for an opportunity to provide advice to the team working on the Artemis III heat shield

***
This post is about the heat shield used with success to return samples from the Moon by China. I asked Google to see what it could find, and while it did not find details, it did provide a hint or two...

AI Overview
Learn more
The Chinese lunar sample return mission, Chang'e-5, utilized a heat shield design similar to the Shenzhou capsule, featuring a modified heat shield capable of withstanding high re-entry speeds of 11 km/s upon returning to Earth's atmosphere; this design included a thruster system for active orientation control during re-entry, allowing for precise landing within a targeted zone, and employed parachutes for final descent, with a drogue parachute followed by the main chute; the heat shield was designed to be lightweight and high-temperature resistant, successfully protecting the lunar samples during re-entry.
Key points about the Chang'e-5 heat shield:
Design inspiration:
The return vehicle's design closely resembles the scaled-down Shenzhou capsule, which is also used for crewed missions.
Material and functionality:
The heat shield is specifically modified to withstand the extreme heat generated during re-entry from the Moon, allowing for safe return of the lunar samples.
Active control:
The spacecraft uses a thruster system to adjust its orientation and trajectory during re-entry, enabling precise landing.
Parachute deployment:
A drogue parachute is deployed first, followed by the main parachute, to slow the spacecraft down for a safe landing.
Design and implementation of the integrated thermal control ...
In addition, a lightweight high temperature thermal shield was designed to isolate the engine plume in confined space during power...
ScienceDirect.com
THERMAL PROTECTION SUBSYSTEM
these efforts led to the reduction in ablator material density from 66 to 31 lb/ft , but the total subsystem weight always trended...
NASA (.gov)
Chang'e-5 (China's Lunar Sample Return Mission) / CE-5
Nov 24, 2020 — The Return Vehicle closely resembles the Shenzhou-capsule design in a scaled-down version. It is equipped with a modif...
eoPortal
Show all
Generative AI is experimental.

(th)

tahanson43206 · 2024-12-09 13:30:05

The Smithsonian collection includes a section of an early Apollo heat shield used for testing prior to manned flights.

https://airandspace.si.edu/collection-o … xy%20resin.

If the link above works, you should be able to page down to see a live (sort of) example of the shield that GW Johnson has been talking about.

(th)

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