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A collection of photos in posts by Miles Lumbard there is a page titled Mercury Retro Pack Walk Around Page 1 (spotted here in reference to the thermal zebra stripes) can be found nine images with the caption:

Any questions, comments, or problems, please email me.

These pictures were taken and shared with the web by Miles Lumbard .

Photographed at the Smithsonian's National Air and Space Museum in Washington, D.C.

Below are small cropped sections from photos 1 and 3 showing the heat shield, which looks like some kind of translucent organic resin or perhaps phenolic material.

My understanding is that this is some kind of ablative material; when exposed to high temperature during reentry the surface is constantly "gassifying" producing a layer of gas of organic molecules which are opaque to the thermal infrared light radiated by the hot gas and plasma in the reentry shockwave. The gas absorbs the radiated heat and quickly moves off and is replaced by a fresh supply as the shield continues to generate more to replace it.

Question(s):

  1. What material composition(s) were used in the Mercury Heat shields?
  2. How were they manufactured in just the right shape, and without internal stresses?
  3. What are all those concentric ridges seen inside or behind the translucent shield material?

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    $\begingroup$ The first suborbital version of the Mercury capsule had no ablative heatshield. A beryllium heatsink was used. But for a reentry after an orbital flight an ablative heatshield was neccessary. See this Nasa history page : history.nasa.gov/SP-4201/ch6-4.htm. So what Mercury version is shown on the images? An early suborbital version or a later one for orbital flight? $\endgroup$
    – Uwe
    Commented Jan 8, 2021 at 0:02
  • $\begingroup$ @Uwe Good question! Since this certainly looks like an ablative organic material and doesn't look at all like beryllium and there's a "retro pack", it might be possible to figure that out. I'm no expert so I can't say for sure. $\endgroup$
    – uhoh
    Commented Jan 8, 2021 at 0:28
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    $\begingroup$ The retro pack is no proof for an orbital mission "At the top of the curve, the spacecraft's retrorockets were fired for testing purposes; they were not necessary for reentry because orbital speed had not been attained. The spacecraft landed in the Atlantic Ocean.[173] The suborbital mission took about 15 minutes, had an apogee altitude of 102–103 nautical miles (189–191 km), and a downrange distance of 262 nautical miles (485 km)" From en.wikipedia.org/wiki/Project_Mercury#Suborbital_flight testing retro packs on ballistic flight was a good idea before doing orbital flights. $\endgroup$
    – Uwe
    Commented Jan 8, 2021 at 1:01
  • $\begingroup$ @Uwe these are all very good points, thank you! $\endgroup$
    – uhoh
    Commented Jan 8, 2021 at 2:43

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Excerpt from NASA-TM-X-57097 describing the original design:

The heat shield used on Big Joe was geometrically a 74.5-inch diameter spherical segment with a radius of curvature of 80 inches. ... The heat shield consisted of two laminates: an outer ablation laminate, 1.075 inches thick and an inner structural laminate, 0.550 inch thick. The ablation laminate was made of concentric layers of fiber-glass cloth orientated so that the layers were at a 20-degree angle with the local heat-shield surface. The structural laminate was made of fiber-glass cloth orientated with the individual layers parallel to the outer surface. Both the ablation and structural laminates were made from a special finish fiber-glass cloth with a 91LD (?) phenolic resin. Resin content, by weight, of the ablation and structural laminates was 40 percent and 30 percent, respectively. A circular ring ... 5 inches high, made of fiberglass and resin, was attached to the back of the heat shield and served to bolt the heat shield to the pressurized compartment of the spacecraft.

The linked document also contains a section describing the difference made to production shields, with Appendix C providing more detail on the actual construction process.

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  • $\begingroup$ It never ceases to amaze me the quantity, quality and detail of NASA documentation. After reading Appendix C and looking at the detail in the bottom left corner of Figure 5 I think I have an fairly good idea of the construction, at least I think I do. I'm not sure yet what the rings are in the photo; they seem much too widely spaced to be individual layers of the spirally wound fiberglass cloth. Perhaps the surfaces was treaded later, or the machining of the hemispherical mold had left ridges. Thanks for your answer! $\endgroup$
    – uhoh
    Commented Jan 8, 2021 at 16:22
  • $\begingroup$ @uhoh I think the spacing is so wide as a result of the 20-degree orientation of the ablative laminate with respect to the surface. That results in the apparent rings being spaced about 3x the actual interlaminar distance. $\endgroup$
    – Tristan
    Commented Jan 8, 2021 at 16:25
  • $\begingroup$ Oh! I read "20° with the local heat shield surface" and my brain imagined it was with respect to the normal to the surface, my bad. Okay that gives a factor of almost 3, sounds close enough that it could be it. $\endgroup$
    – uhoh
    Commented Jan 8, 2021 at 16:34
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From NASA-TM-X-57097 found by Tristan:

enter image description here

enter image description here

So two manned ballistic flights were done using a heat-sink, not an ablative heat shield.

I found an image of such a heat sink.

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This is a replacement "heat sink" heatshield for the Mercury-Redstone 2 capsule. The original was lost after recovery of the capsule from the ocean and this heatshield did not fly in space. The MR-2 capsule carried Ham, a chimpanzee, on a suborbital flight in January 1961. Heat sink heatshields absorbed the tremendous heat generated during the capsule's reentry and were used only during some early Mercury test flights before they were replaced by ablative heat shields.

Image and Quote from the Smithsonian

There were 4 manned and 3 unmanned orbital flights using the Atlas rocket and 3 unmanned sub orbital flights using the Redstone rocket ( one with the chimpanzee ham).

So we got a total of 5 (potential) heat-sink and 7 heat-shield flights. Some of the sub orbital flights may have been done using a heat shield. Before the two manned sub orbital flights there should have been at least two unmanned flights using the heat sinks.

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  • $\begingroup$ Thanks for your answer! I think it's important to be reminded of this alternative technology, I'd certainly never heard of it before. I wonder if the history of use of heat sinks you raise at the end of your post should also be explored in a new question post as well. $\endgroup$
    – uhoh
    Commented Jan 8, 2021 at 23:04
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    $\begingroup$ The alternative technology heat sinks was used for sub orbital flights only. It was not sufficient for reentries from an orbital flight. If the satellite was to be destroyed during the reentry anyway, no heat sink was needed and no heat shield too. $\endgroup$
    – Uwe
    Commented Jan 8, 2021 at 23:37

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