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I'm writing a short story in which my group of character is devising the stupidest and least practical way of lighting a fireplace.

It then came to my mind that stuff that undergo reentry gets hot. I am aware that this process might disintegrate the log, making all the trouble of putting said log into outer space meaningless, and in the event of the log actually reaching the fireplace, it might disintegrate the area anyway, but that would make it stupid and unpractical, and that's exactly what I want.

The log before reentry can go any speed from orbital to stationary relative to the ground below, and the height should be no more than 150km (93,2 Miles). Concerning size and material, it's a plain old log, what did you expect.

Can the log make it to the fireplace?

It has to be on fire when it lands (or rather impacts) and must reach the fireplace with at least half of it's mass. Actually getting it up there or the trouble of accuracy is out of the scope of this question, for our sanity sake.

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  • $\begingroup$ The question is perfectly in line with what has been asked here since the changes to the rules. It is also out of the scope if a fast burning log slamming into the ground would light a fireplace, right? $\endgroup$
    – Raditz_35
    Jul 5, 2018 at 10:26
  • $\begingroup$ I am hoping for an answer with math. The log could be a green redwood trunk. $\endgroup$
    – Willk
    Jul 5, 2018 at 11:42
  • $\begingroup$ @Raditz_35 As long as the log is on fire at the moment of impact and didn't disintegrate during reentry, it's valid. What happens to the fireplace might require another question. $\endgroup$
    – Alexcommil
    Jul 5, 2018 at 11:44
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    $\begingroup$ Related: what-if.xkcd.com/28 $\endgroup$ Jul 5, 2018 at 12:06

2 Answers 2

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The wooden heat shield

Short answer

No. xkcd: what-if #28 explains why.

Long answer

As it turns out, wood paradoxically makes a good heat shield for reentry. China has actually employed wooden heat shields.

The way wood burns is not that the solid material that we all know as wood, and that gives us splinters, is burning. Instead it is gasses that are coming off of the wood is what are actually oxidising and making those pretty yellow/orange flames. To get those gasses, you need to heat the wood until it boils. (*)

This is the reason it takes so long before a bonfire/fireplace catches; why it is easier to light up thin splinters compared to thick logs: because you have to heat up the whole piece of wood so that the parts underneath the surface do not soak up the heat that is trying to boil the surface to keep the flames going. The little amount of heat caused by the first flames is not enough to heat the wood underneath to make that give off more gasses. You have to heat the whole piece of wood to near boiling temperature until the heat of the flames is enough boil more wood and make the combustion self-sustaining.

The problem — for you — is that the charring and outgassing of the surface of the wood during the fiery atmospheric re-entry is actually protecting the wood underneath. The gasses and their combustion products act like a gas shield that will protect the uncharred wood from the compression heating ahead of the wood. Also — and this is the crux — the burning gasses blow away so fast that the heat from the combusting wood gasses does not aid in heating the wood underneath. The "flame" is simply not there... it is far behind the spacecraft.

Also you have another problem in that firewood logs are very light compared to their surface area, compared to — for instance — the space shuttle. This means the firewood will lose speed a lot faster once it hits the atmosphere, meaning that the compression heating of the reentry will not be acting for as long on the firewood.

Then — as you said — there are control issues. You need to make bits of wood fly in a manner that makes them land with precision in your fireplace. Atmospheric disturbance alone is enough to make the bits rain down in a spread pattern. And any irregularity of the wood will make it veer from the ideal path you want it to take. Logs — with their long not-quite cylindrical shape — are not aerodynamically stable; they will tumble and flop all over the place. As we have seen with other things that have fallen from space; even though the parts initially had the same velocity vector, they will spread out. The tragedy of Space Shuttle Columbia, showed this even clearer.

enter image description here

Speed — as it turns out — is not that much of an issue when it comes to landing the pieces safely instead of just making a huge crater. The bits will fairly soon be subsonic and will not smack down all that hard; surviving debris from Space Shuttle Columbia showed this. However that also means that you have very effective cooling of the previously charring, smouldering wood pieces, which just compounds the issue of the logs not being hot enough to boil and burn.

So in short (again): No.... tossing down bits of firewood from orbit will probably not work. They will be charred but not on fire when they come down. And you have no way of making them come directly into your fireplace lest you make them little remote controlled vehicles.

Would be awesome though, I agree. :D

Addendum

Speaking of things that fall from space: if you could achieve such precision as to land an unguided space object in an area the size of a campfire, the military would be very interested in speaking with you.

"Say.... would you mind showing us how you did that? We have some... practical applications for that sort of thing"

This in turn risks mooting the whole thing, because you asked for a "[stupid] and [not] practical" way of doing it. Something that is stupid, but that works and that has practical applications is not wholly stupid. It is only stupid for the particular context of lighting a campfire.

(*) This is not entirely accurate, wood does not "boil" like simple substances, like water or nitrogen.

The process is called pyrolysis.

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    $\begingroup$ Would a fin stabilized trunk of Lignum Vitae (densest commercialy available wood) help maintain sufficient speed? (on top of the increased accuracy) $\endgroup$
    – Alexcommil
    Jul 5, 2018 at 13:21
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    $\begingroup$ @Alexcommil Well... if we allow for engineered pieces of wood — and not just crude and literal hatchet-jobs — then the situation is slightly better. But you still have the problem that a mere 0.01 degree deviation a the top of the atmosphere will translate to hundreds of meters on the ground. Not even the best ballistic missiles — which are marvels of precision targeting — have a CEP of better than 50-ish meters (i.e. half of the vehicles will be somewhere within a circle 50 meters across, the rest scattered outside). So: no, without active guidance, it just won't work. $\endgroup$
    – MichaelK
    Jul 5, 2018 at 13:31
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    $\begingroup$ This xkcd article might be relevant: what-if.xkcd.com/28 It actually confirms your hypothesis. With meat instead of wood, but it's likely that different kinds of organic materials won't behave too much differently under such extreme conditions. $\endgroup$
    – vsz
    Jul 5, 2018 at 13:51
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    $\begingroup$ @vsz Oh for cryin' out loud... I had forgotten about that one. Yes, yes you are entirely correct... that page is all we need to invalidate the concept. $\endgroup$
    – MichaelK
    Jul 5, 2018 at 14:02
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    $\begingroup$ I dunno, a fireplace the size of Texas seems pretty stupid and impractical to me. $\endgroup$
    – Fax
    Jul 5, 2018 at 21:13
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Your John (let's assume this is his name) is looking for

the stupidest and least practical way of lighting a fireplace.

John doesn't need to go to space, which is something any high school can achieve nowadays (staying in space is another story): just take a Lockheed SR-71 Blackbird, and start its engines.

Use the plume exiting the reactor to light the log, and put it in the fireplace. Then, if John placed the Blackbird close enough to the place where he parked the airplane, he could deliver it while still burning.

Getting such a plane will disappoint a lot of federal agencies and get John's name logged in their file, so I think it better qualifies for

the stupidest and least practical way of lighting a fireplace

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    $\begingroup$ I downvoted because in the other answer there is an explanation of why wood would not autoignite under those conditions. $\endgroup$
    – Nobody
    Jul 5, 2018 at 14:21
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    $\begingroup$ @Sean The aforementioned explanation is basically "it will be blown out before it ignites", so, yes. "blowing out" just means removing the burning gases faster than they can boil away more wood/wax/whatever to burn. $\endgroup$
    – Nobody
    Jul 6, 2018 at 12:55
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    $\begingroup$ @Nobody, point taken. See edit. $\endgroup$
    – L.Dutch
    Jul 6, 2018 at 13:23

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