SAMU: Wrong again. I wrote that it's too complicated to use as a strategy to figure out the tempreture of Apollo 13.

Okay, we're finally converging. Where I used to work we had software that would do just this. You plug in the geometry and the thermal situation and it would give you wonderful simulations that look so good on trade show posters. It's specifically made for modeling the thermal behavior of spacecraft. I'd get myself a copy of it, but it costs more than my car.

If someone handed me a pad of paper, a calculator, and a pencil and told me to compute the various thermal states of an Apollo command module based on the basic principles of heat transfer, I'd tell them to go jump in the lake. A very cold lake.

But you're throwing out very important data by saying we can just compare the steady-state temperatures of objects in space and not consider their construction characteristics.

SAMU: Well you must be a genious. To be able to teach me all I know about thermodynamics in a couple of days reading your flawed inacurate and false posts.

Um, I'm not claiming to be the one teaching you about thermodynamics. What I meant was that you seem to be doing research offline in response to questions raised here. When I say "you're learning" about thermodynamics that isn't to say I'm teaching.

SAMU: Your test is flawed by key factors being left out. I have clarified my answers by adding the key factors.

I left out key factors on purpose. People who know what they're talking about know what they need to fill in. People who are faking it don't necessarily know what's missing.

Note that I didn't specify the temperature of the coffee.

SAMU:To clarify, we are describing a room with walls at room tempreture and air at room tempreture.

Yes. The intent was to introduce hot coffee into an environment that had previously been at equilibrium.

SAMU: And we're going to eliminate " The entire assembly is suspended by a thin cable from the ceiling" and just say the whole thing, room, air, thermos and all, is in zero g.

Fine by me. The point was to eliminate the need to consider conduction through whatever was holding up the thermos.

SAMU: Unless you can show how gravity affects the thermodynamics of the situation.

As long as you know how it would affect the system I don't mind.

SAMU: Radiation and conduction if there is air at room temoreture and the walls are at room tempreture. Radiation alone if there is no air and the walls are at room tempreture.

That's what I was fishing for. You left out radiation in your example, and I just wanted to know if that was an intentional omission for the sake of simplicity. Thank you.

In gravity you have to deal with convection. It's okay with me if you want to consider convection a special case of conduction. I do it all the time for simplicity in discussion.

The coffee transfers heat to the inner vessel via conduction, or convection if you prefer, and also through radiation. The inner vessel transfers heat to the outer vessel through radiation and nothing else. The outer vessel transfers heat to the "environment" through radiation and convection (conduction, whatever).

SAMU: Sutract the amount of heat escaping to the room from the thermos from the amount of heat entering the thermos from the air and walls of the room.

What thermal gradients, if any, would there be? Would there be a thermal gradient in the coffee? Across the inner vessel wall?

SAMU:The tempreture would stabalize at 100 C if the air and walls are are 100 C.

I really only wanted to know if you had considered radiation from the environment back into the vessel. As I said, most of this series of questions was just to determine where radiation fit into your thinking.

I was hoping you'd mention the role of differential equations in the computation of steady state. For heaven's sake I sure didn't expect you solve any, but that's what I was fishing for when I asked for a description of how to deal numerically with systems progressing toward a state of equilibrium.

SAMU:Sutract the amount heat escaping from the amount of heat entering.

I was hoping for something a little more detailed. Specifically I was hoping you'd talk about what thermal gradients might exist in such a situation.

SAMU:Conduction through the insulation at whatever rate the insulation conducts to the hull where it radiates away.

Again, I wanted something more detailed. Astronauts transfer heat through convection and radiation. (I insist on convection here rather than conduction because I'm presuming the astronauts are moving about. That's effective forced convection.) Cabin atmosphere transfers to inner hull via convection and radiation. Inner hull transfers to insulation via conduction and radiation. Insulation transfers to outer hull via conduction and radiation Inner hull transfers to outer hull via conduction through attach points. (The insulation is fundamentally opaque and therefore no transfer occurs between the hulls via radiation.) Outer hull radiates.

SAMU: The astronauts would lose as much heat as they have ...

Okay, I really didn't expect a good answer to this. Those who get really excited about thermodynamics would have solved this numerically.

The answer I was fishing for was that the astronauts are the only "input" source of heat. Everything else in the system is a transfer of some kind.

SAMU: If they produce 4 watts per hour and it escapes at 3 watts per hour at a given tempreture the tempreture will rise.

And as the temperature increases, the rate of radiation increases. In fact, an increase in the temperature of the radiator produces a fourth power increase in the heat flux.

SAMU: That is the fallacy of proof by authority.

I meant "valid" in the sense "worth paying attention to," not "leading toward a true conclusion."

The problems of proof by authority do not apply to expert testimony. This caveat appears in any textbook's discussion of this particular fallacy. You characterize your line of reasoning as a simple "facts support the conclusion" scenario, when in fact that's not the case. The facts in this case require competence in thermodynamics in order to evaluate their relevancy, behavior, and therefore their degree of support for the hypothesis.

SAMU: By the way, what message did you post that tought me all that about thermodynamics?

I have not made any such post, nor have I claimed at any time to have made such a post. I was referring to the posts made early in this discussion by Bad Astronomer, David Simmons, and others. You seem to have completely ignored the implications of those posts.

SAMU: No. It's up to me to post the facts which support the assertion It's up to you to show the facts are in error.

Agreed.

Fact 1: Objects in steady sunlight in trans lunar space reach a surface tempreture of 200 degrees before they radiate as much energy as they absorb.

This is not a fact. Q.E.D.

SAMU: Factual examples already posted.

You gave two examples, lunar material (already discussed) and the space station (discussed below).

SAMU: Find some fact to refute those examples.

Already done. The descent stage of the lunar module was composed of components that had different steady-state temperatures.

SAMU: post some thermodynamic principals that apply.

Already done. Several posters early on gave you computations, examples, and qualitative discussion pertinent to this point.

Now on to your questions. I've collapsed a few of them into algebraically equivalent form for convenience.

SAMU: If an object at K degrees in sunlight radiates at the same rate that it absorbs, what is it's steady state tempreture?

Assuming no other transfer modes apply, the simple answer is K degrees. The definition of steady state in an radiation-only system is when the radiation and absorption rates are equivalent.

SAMU: 3) If an object at 38 degrees in sunlight radiates at a lower rate than it absorbs, what is it's steady state tempreture? Higher or lower than 38 degrees?

Higher. In the simple scenario the temperature of the object rises until its radiation rate is equivalent to the absorption rate.

SAMU: 4) An object in sunlight can radiate more energy than it absorbs only if what?

Only if its temperature is above the steady state temperature suggested by a radiation-only model.

SAMU: 5) Give examples of objects without active heat exchange in constant sunlight in space at a distance from the sun the same as the Apollo at a tempreture of less than 50 degrees.

I assume you mean 50 F.

I'm not going to answer that question. I'm instead going to explain what's been wrong with your questions to me, and what's been wrong with your answers to my questions.

You talk about "objects in space" as if there were no thermal gradients in any of these objects (or structures, in the case of spacecraft). You seem to believe that if you put an object in cisulanar space under constant solar illumination, that object will reach 200 degrees and that all parts of it will be that same 200 degrees.

But we know that's not true.

Say we put a hollow metal sphere filled with air out in cislunar space. We keep it from rotating. The side facing the sun will get hot. The side facing away from the sun won't get as hot. Heat will conduct through the material from the hot side to the cold side. So the cold side will be a little warmer than the temperature it would reach without being connected to the hot side.

But since the radiation rate is proportional to the fourth power of the temperature, just boosting its temperature a little will really lift its radiation rate. The question is: can it radiate faster due to the increase in temperature than it can be replaced via conduction through the material? You betcha. What's the result? A thermal gradient.

Now what happens to the air inside? Let's suppose we have a little mouse in there stirring up the air as he floats around (forced convection). And his metabolism adds to the system. If he moves around enough, we can assume the air becomes reasonably isothermal. Does that mean the outer shell is isothermal? Nope. Does that mean the air is the temperature of the mouse? Not necessarily.

Now we consider two identical spheres, one composed of a dark matte black material, and the other composed of a very bright, shiny material. The mass and geometry of the mouseships are identical.

The black ball obviously absorbs more energy than the silver ball. (The balls transmit no light, therefore what is not reflected has to be absorbed.) But the black ball radiates more energy than the silver ball. Would they then reach an equivalent state of equilibrium (200 F)?

The answer to that question lies chiefly in whether radiation and absorption behave identically in response to temperature. If differences in material properties (i.e., shininess) cause absorption and radiation to behave differently for different materials, then different materials would have different equilibrium temperatures. The equilibrium would still be the temperature at which absorption and radiation were equivalent, but it might not necessarily be the same for different materials.

There exists a quantitative solution to this problem, but we don't need it. The qualitative solution may be inferred from the fact that people who build comm satellites for a living (and who therefore ought to know what life is like for permanent cislunar residents) put matte black stuff on the parts of the spacecraft that should be warm, and shiny silver stuff on the parts of the spacecraft they'd prefer kept cool.

And if everything in cislunar was just always isothermal, the people who sell thermal modeling software to aerospace engineers ought to be the ones we lock up for fraud.

SAMU: The spacecraft cooled to 38 degrees.

The interior air temperature of the spacecraft was 38 F. Was that the temperature of the spacecraft skin? Could the skin of the spacecraft have been at a higher temperature at equilibrium? Could parts of it have been at a higher temperature, and parts of it at a lower temperature? Is an Apollo spacecraft isothermal?

SAMU: Objects in direct sunlight reach tempreture of 250 degrees. Link to example already posted.

Well, your example is for low earth orbit, not cislunar space. You still haven't explained what makes it an applicable example.

And you're only citing half your example. The sunlit side is 250 F. The shady side is -250 F. But it's the same object! Obviously this isn't an isothermal object. Half of it is really hot, and half of it is really cold. So let's fill up that spacecraft with several thousand mice all stirring up the air and making at least the air as isothermal as it can be. What would be the temperature of the air inside? +250 F? -250 F? 0 F? Something in between?

Your statement "all objects in space reach an equilibrium temperature of 200 F" just doesn't hold. Your examples don't support that. Common spacecraft design practice doesn't support that. Radiation and absorption don't support that.

What part of your argument do you believe we haven't completely shot down here?


<font size=-1>[ This Message was edited by: JayUtah on 2001-11-13 19:35 ]</font>