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banquo's_bumble_puppy
2015-Dec-20, 10:32 AM
I discovered a property of steel wool yesterday. I had a piece of the aforementioned that I put on a glowing hot stove burner; it was wet and I wanted to sterilize it with heat/steam. I picked it up on the hunch that it would not be hot. I was correct; it was warm to the touch and cooled quickly. So, I think that the high surface area of the intertwined steel is the cause. Am I correct in guessing that surface area similar to what I just mentioned is the property that allows heat shields to work? Warning to young folks...do not try this w/o parental supervision...not kidding. Do not try with plastic scrubbers... edit to add..I think that the silica shield used on STS worked like this.

profloater
2015-Dec-20, 11:19 AM
I think the ceramic tiles are different in that those ceramics can get red hot or hotter and stay together. they do transmit some heat through but work for long enough to keep the metal underneath below melting point. Your steel wool was protected by water evaporating off, otherwise it burns quite well. then after that yes, the surface area allowed it to cool quickly. Most insulators are in effect trapped bubbles of air in a matrix of thin solids that stop convection of the air. Other gases are even better and vacuum best of all as in thermos flasks. Then a reflective surface is used to stop radiation too. Radiation goes through these air filled insulators so domestic insulation these days includes a thin aluminium barrier. Glassy ceramic materials can be used in solid form to keep high temperatures at bay as in furnaces but without air bubbles they transmit relatively large flows of heat. So time enters the situation in a space shuttle reentry.

banquo's_bumble_puppy
2015-Dec-20, 11:45 AM
Lucky I didn't let the wool overheat.

Jens
2015-Dec-20, 12:08 PM
Yes, the high surface to volume ratio will make it cool quickly. Think of tin foil: it cooks almost instantly. It's also how radiators work.

publiusr
2015-Dec-20, 10:24 PM
Very wide aerobrake disks for OMVs were talked about for years. Lenticular designs were looked at: https://en.wikipedia.org/wiki/Lenticular_Reentry_Vehicle

from Wade's site

"Kehlet argued that a lenticular vehicle, as a manned spacecraft launched into orbit by a conventional booster, had clear advantages over ballistic, lifting body, and winged designs. At hypersonic re-entry speeds it would undergo lower heating and require less shielding. At the same time it was more maneuverable at subsonic speeds than a winged design, and could land at sea or on land without undercarriage. The symmetrical shape meant it would integrate easily into conventional booster designs, without creating excessive drag or asymmetric loads during ascent to orbit."

profloater
2015-Dec-20, 11:57 PM
Very wide aerobrake disks for OMVs were talked about for years. Lenticular designs were looked at: https://en.wikipedia.org/wiki/Lenticular_Reentry_Vehicle

from Wade's site

"Kehlet argued that a lenticular vehicle, as a manned spacecraft launched into orbit by a conventional booster, had clear advantages over ballistic, lifting body, and winged designs. At hypersonic re-entry speeds it would undergo lower heating and require less shielding. At the same time it was more maneuverable at subsonic speeds than a winged design, and could land at sea or on land without undercarriage. The symmetrical shape meant it would integrate easily into conventional booster designs, without creating excessive drag or asymmetric loads during ascent to orbit."

That's a flying saucer, right?

Noclevername
2015-Dec-21, 10:54 PM
I discovered a property of steel wool yesterday. I had a piece of the aforementioned that I put on a glowing hot stove burner; it was wet and I wanted to sterilize it with heat/steam. I picked it up on the hunch that it would not be hot. I was correct; it was warm to the touch and cooled quickly. So, I think that the high surface area of the intertwined steel is the cause. Am I correct in guessing that surface area similar to what I just mentioned is the property that allows heat shields to work? Warning to young folks...do not try this w/o parental supervision...not kidding. Do not try with plastic scrubbers... edit to add..I think that the silica shield used on STS worked like this.

IIRC re-entry heat shields are also cooled by ablation; small amounts of surface evaporation carry off the heat.

cjameshuff
2015-Dec-22, 12:29 AM
IIRC re-entry heat shields are also cooled by ablation; small amounts of surface evaporation carry off the heat.

The Shuttle tiles were a non-ablative shielding material that insulated from the high temperatures. It's not really a matter of surface area, more of the fibers having small cross sections for conducting heat and conducting the heat along indirect paths across the tile. Another approach that's been used is metallic heat shields. With both of these, the shield just absorbs and re-radiates the heat. Ablative shields must do the same when the vehicle slows enough that it can no longer cause ablation, but earlier in the reentry they dispose of most of the heat in the vaporized material.