04 August 2013

Space reentry vehicles, part 4

Part 4: More on ablation


In 1907 American chemist Leo Baekeland invented Bakelite, the world's first plastic.  50 years later it would be chosen as the main ingredient in NASA's reentry capsule heat shields. 

Radio made of Bakelite 

Bakelite was a good choice for spacecraft heat shields because it's a good insulator of heat; it has high strength;  it has an abundance of carbon (a material with a high latent heat); and it has an ability to char rather than melt at high temperature.

Bakelite, or phenolic resin, is a thermoset plastic, meaning once it cures (once it's manufactured and set) it can't be uncured or remoulded.  It tends to burn before it melts, and so at high temperature it chars to leave a layer that's rich in carbon. This carbon is then supposed to melt and boil in order to carry off heat with the phase change.

The phenolic is reinforced with quartz fibres, turning it into the composite material, fibreglass.  Fibreglass is strong, but like any composite material, requires both parts to work -- both the matrix (phenolic) and the reinforcer (quartz fibres).  If the phenolic is being broken down into amorphous carbon by the heat, the quartz fibres would break due to shear stresses, thus nullifying their usefulness in strengthening the phenolic matrix.

All these fibreglass shields can really do is burn -- they can't ablate in the sense of melting or boiling as intended, and therefore they can't take advantage of a phase change to produce cooling.  NASA admits the shields burn here:
"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."
An exothermic reaction will add heat to the situation, not take heat away.  Would you put a smouldering log on the outside of your vehicle and expect it to cool down? Let's say you put burning embers all around a kombi van and drove it on the freeway at high speed.  The faster you drove the hotter you'd glow -- the faster moving air just makes it worse.

Like soot, amorphous carbon is structurally weak and would be easily swept away by the fast moving air on the outside of the vehicle. 

Cross section through heat shield.  Orientation as if the craft was moving from the bottom to the top of the page.

On Mercury and Gemini the heat shield was only at the bottom of the craft. The sides and top were made of 0.4 mm thick nickel-chrome outer shingles fastened by beryllium retaining bolts.  With no heat shield on the sides, the designers really were banking on it always pointing face down in the direction of travel, and indeed the lives of the astronauts depended on maintaining such an orientation.

Gemini heat shield was only on the bottom: better hope the capsule keeps pointing in the right direction during reentry.

(from pg 61 of Coming Home)

(from here.)

For the outer ablative heat shield the Apollo reentry capsule used fibreglass honeycomb bonded to a layer of the hull comprising stainless steel.  The cells of the honeycomb were each individually filled by hand with the phenolic epoxy fibreglass mixture.  

Apollo used a slightly upgraded version of the Mercury/Gemini fibreglass, called Avcoat 5026-39.   In addition to the quartz fibres for reinforcement, Avcoat also had phenolic microballoons.  Neither Mercury, Gemini, nor Apollo used carbon reinforcements in the phenolic, nor did they use more modern materials that have only become available in recent years such as carbon on carbon. 

The Apollo was a bigger, heavier reentry vehicle than Mercury and Gemini and was expected to have a lot more energy upon reentry, resulting from its greater weight and faster speed.  Unlike Mercury and Gemini, Apollo had phenolic epoxy ablator shields not just at the bottom, but all around the aircraft (except for the glass windows and sundry utility points such as rocket nozzles).

(figure from here)

It initially was coated in shiny aluminium:

Apollo Command Module (conical section in front)

This aluminium vaporised on the way into earth's atmosphere, to reveal the phenolic ablator underneath. 

Apollo capsule after return.  (If the aluminium vaporised, how did those handles survive the heat?)

Aside from the absence of the aluminium, notice how the sides are practically untouched by the reentry -- the ablative material is all still there -- it didn't ablate. The underside was always expected by the designers to point in the direction of travel.  How could this be assured when the craft could roll over and over again, out of control? 

When the Apollo is released from a plane it does just this: tumble and roll. Only the parachutes stop this rolling action from happening, which is why the parachutes are always deployed from the beginning on all reentry capsule launches, when they are released from the plane:

Though this rolling would be rough for the astronauts, from the point of view of ablation, the capsule could handle it to some extent because the ablative material was placed on all sides of the craft.  (Though it's not clear how the glass windows could have stood up to the heat of reentry.)

The phenolic ablative shield was thicker on the bottom than on the sides and had a total weight of 3,000 pounds (1361 kg).

In part 1 I calculated that if all the ablative heat shield on the Apollo Command Module (reentry capsule) was carbon, and boiled and melted perfectly, 2.5 metric tons would be needed to remove the necessary heat via the phase change. This is roughly double the actual total heat shield weight of 1.3 tons.  This 1.3 tons includes the top ablative panels which look unburned after reentry.  The bottom looks mostly unburned too.

(figure from here)

The figure I calculated for Apollo's reentry energy using 1/2mv^2 was 176 GJ.  I calculated this before I came across a NASA estimate, which was roughly double mine: 340 GJ. 

(From here.)

That means that instead of 2.5 tons being required, 5 tons of heat shield would have been required.

Now we factor in that not all of the phenolic heat shield is carbon:

The Bakelite (phenolic resin) molecule has a lot of benzene rings -- good for bonding and rich in carbon.

The non-carbon parts like hydrogen will volatize and escape or burn off, just like in a fire, leaving behind the silica and carbon char. According to the Wikipedia page Avcoat, regarding NASA Technical Note D-4713, only 21% by weight of the heat shield is made of carbon.

That means that you'd need 20 times the weight that the shields actually possessed, assuming perfectly melting and boiling of all the carbon on all sides of the craft.

And as I mentioned above and in part 1, it's unlikely the carbon would actually hang around to boil as well as melt, because once it melted, it would lose structural strength and fly away.  And it wouldn't melt anyway, but would burn at a lower temperature than was required to melt carbon. Carbon melts at 3550 C, but soot (amorphous carbon) burns at about 600 C.  So there's never any opportunity for the carbon in the heat shield to melt, boil or "ablate" in any way before it combusts or simply flies away.

Even assuming best case scenario (of full boiling and melting of all carbon) there's no way "ablation" as a special "fourth mode" of heat transport involving charring or phase changes could have protected these astronauts.  The only real way to spare the astronauts from frying on reentry was to have a detached shock wave.  More on that in the next part.


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