posted 02-01-2016 10:08 AM               

It's a 1" by 1.25" plug — about as big as I have seen. It is unusual as it has a metal cap at one end with the Number 51 on the metal plate (photo).

The charred end is clearly uneven and worn. Looks like it was very much at the "business end" of the heat shield.

Most examples are much smaller in cross section. This looks as if it was lifted intact on return.

posted 02-01-2016 01:13 PM               

From the construction I've been assuming;

1. these were threaded-in assemblies for covering larger bolt and screw heads or other access points
   
2. the metal plate and O-Ring were integral to the original heat shield thread-in plug
   
3. the damage/obliteration of some of the thread was caused when coring the plug assembly out of the spacecraft.

   And to your other comment I'll add one more assumption:

4. the charred end faced re-entry heat of course and the metal cap faced toward, and perhaps bottomed out on, the bolt/screw/fastener/etc being covered and protected.

posted 02-02-2016 06:16 PM               

quote:

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Originally posted by CMikeW:
There were a large number of them that covered and closed out the bolts that attached the command module heatshield to the command module structure.

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posted 02-03-2016 02:04 PM               

See some images in this thread including the paper mentioned therein. Figures 8/9 of the paper are particularly noteworthy.

If I have my bearings straight, in all cases the heat shield attach-point ablator plugs have been cored out with something oversized to the original plug diameter; this might explain why they seem too big to be a match.

Figure 8 mentioned above doesn't show a thread on the ablator plug body but does possibly show the metal end cap.

Smaller Apollo threaded ablator plugs (with no metal cap) appear somewhat regularly, most all with the pair of holes intact. The large samples do not seem to show the twin holes or possibly they've been charred away.

My perception is the smaller plugs are generally from the hatch and panel areas on the conical CM crew compartment heat shield as John (space1) mentions. Charring is often subdued if present at all, consistent with the lower heat load.

There are also smooth-walled plugs that were probably taken most often from the expanse of the aft heat shield. Note the presence of smaller, more random, heat shield holes — often filled — in some of the photos.

I'd appreciate any improvements or corrections to this but here's a take on three primary categories of Apollo flown heat shield sample sources, whether later encased in lucite or otherwise:

1. Large Threaded: about 1-1/4" diameter threaded ablator plugs with metal cap and RTV O-ring, primarily from the aft heat shield attach points. Usually whole but here was a nice cross-sectioned version.

2. Small Threaded: about 1/2" diameter finely-threaded ablator plugs that covered screws under the crew compartment and forward heat shield. Associated with hatches, doors, and access panels.

3. Cored/Sawn: Other cored heat shield sections with smooth sides. Removed from the 'field area' of the aft heat shield (and possibly areas of the crew compartment shield?). A 1" diameter core seemed to be common for round samples, and the starting point for semi-circular or pie-shaped sub-samples. Rectangular blocks sawn from the heat shield have also appeared.

Mercury/Gemini samples seem to fall only into the latter category.

posted 02-03-2016 08:11 PM               

I noticed the significant difference in plug 'height' between our 'Type 1' Apollo 11 and Apollo 13 examples here.

Would it be a function of the variable heat shield thickness per Figure 5 of the above NASA paper and Scott's (Spaceaholic's) related discussion?