# B Atmospheric Burn-up During Re-Entry

#### Rensslin

You would do well to review the forum rules on personal speculation. Your rigid adherence to false personal positions in the face of careful correction is not appreciated.

The situation when releasing compressed air through a nozzle complicated. See this link for some of the complications. There is a difference between a reversible expansion and a free expansion and between a real gas and an ideal gas.

However, the conclusion in this specific case is correct. The can gets cold because the gas remaining in the can has done work, pushing exhausted gas toward the nozzle opening. The expansion of the portion of the gas that remains in the can counts as "reversible".
Thank you for your careful consideration; thank you also for your thoughtful responses. And thank you for directing me towards the forum rules. I will carefully consider them.
I don’t believe I’m being rigid. I suppose no pigheaded person does. I once argued with my high school Physics teacher about why I couldn’t make a bicycle generator that would push the bicycle until he explained the problem in a way I could understand. I appreciated my teachers patience with me, just as I appreciate you and the others now.
But I don’t see how my original idea has been addressed in a way that I can understand. If you have nine cats in a box and you decrease the size of the box you still have nine cats in that box. If you want to argue that decreasing the size of the box will require adding more cats, I will concede that. You could even argue that the reduction of the box is exactly proportional to the amount of cats being added. My point is that changing the size of the box has increased the gradient of the original amount of cats with respect to those outside the box. Again, irrespective of the cats that were added to reduce the size of the box.
I always try to be patient with people who are not as smart as me. Believe it or not there really are people not as smart as me.

#### Rensslin

Well I think entering earths atmosphere could be kind of like a bellyflop.
Here you are cruising through space with no aerodynamic drag; then all of a sudden, you have a condition that dramatically restricts your speed. Failure to consider angle could have injurious results.

#### Rensslin

They are most certainly not "stationary with respect to the planet". An object in low Earth orbit is moving at about 18,000 miles per hour relative to the ground, and basically the same speed relative to the atmosphere. this is about 12,000 26,000 feet per second, or about 20 40 times as fast as a bullet.

Also, the power dissipated by air friction is proportional to the cube of the speed through the air, so going 20 40 times faster dissipates 8000 64,000 times more power. This is more than enough to melt or even vaporize the components of the object.

Edit, reading marcusl's post I realized that I incorrectly converted from miles/hour to feet/second, so I corrected the numbers.
@phyzguy
I was told that there are communication satellites that are constantly stationary. That is how they can communicate with the ground; if they move around the planet then they quickly lose communication with their station. Is that wrong? If a stationary satellite “falls“ it must start off very slowly.
Also, I saw a video of guy back in the 60s ride a balloon all the way up to earths orbit. Yeah it was cool. He videoed himself jumping out. I wonder why he didn’t burn up. I think the guy still alive. Also is there something called maximum velocity? Or something like that?

#### phyzguy

I was told that there are communication satellites that are constantly stationary. That is how they can communicate with the ground; if they move around the planet then they quickly lose communication with their station. Is that wrong? If a stationary satellite “falls“ it must start off very slowly.
Geostationary satellites are 26,000 miles up. The atmosphere starts at about 100 miles up. If a satellite falls from 26,000 miles up to 100 miles up, it gains a huge velocity. You should be able to calculate how fast it would be going when it hit the atmosphere by calculating the change in potential energy and equating it to the gain in kinetic energy.

Also, I saw a video of guy back in the 60s ride a balloon all the way up to earths orbit. Yeah it was cool. He videoed himself jumping out. I wonder why he didn’t burn up. I think the guy still alive. Also is there something called maximum velocity? Or something like that?
He rode a balloon up to the edge of space, but he didn't ride it into orbit. In order to orbit, he would need a lateral velocity of about 18,000 miles an hour. There is no way to do this with a balloon. Most of the energy expended by a rocket putting a payload into orbit is in accelerating it laterally. This energy requirement is quite a bit larger than what is required to lift it into orbit.

Gold Member
Well I think entering earths atmosphere could be kind of like a bellyflop.
Here you are cruising through space with no aerodynamic drag; then all of a sudden, you have a condition that dramatically restricts your speed. Failure to consider angle could have injurious results.
As I previously stated, that isn't what happens, though. You don't suddenly have a condition that dramatically restricts speed. It's more like driving into the edge of a rainstorm where you get near the edge and there are a few stray raindrops hitting your windshield here and there. As you drive closer to the center, the frequency of raindrops hitting the wind increases until it's a constant pitter patter of rain on the window. That's a better illustration of molecules during reentry. The craft first encounters atmosphere that is so thin that it makes little difference to how it is flying, and as it continues on lower into the atmosphere, the frequency of molecular collisions with the vehicle increases as the density increases. It's much more gradual than you describe.

#### cjl

Thank you for your careful consideration; thank you also for your thoughtful responses. And thank you for directing me towards the forum rules. I will carefully consider them.
I don’t believe I’m being rigid. I suppose no pigheaded person does. I once argued with my high school Physics teacher about why I couldn’t make a bicycle generator that would push the bicycle until he explained the problem in a way I could understand. I appreciated my teachers patience with me, just as I appreciate you and the others now.
But I don’t see how my original idea has been addressed in a way that I can understand. If you have nine cats in a box and you decrease the size of the box you still have nine cats in that box. If you want to argue that decreasing the size of the box will require adding more cats, I will concede that. You could even argue that the reduction of the box is exactly proportional to the amount of cats being added. My point is that changing the size of the box has increased the gradient of the original amount of cats with respect to those outside the box. Again, irrespective of the cats that were added to reduce the size of the box.
I always try to be patient with people who are not as smart as me. Believe it or not there really are people not as smart as me.
Temperature isn't like "cats" in this analogy though. You have a box with 1 kg of air in it. You shrink the box. It still has 1 kg of air in it, but in the process of shrinking it, you had to apply force, so now the air that is in the box has more energy, and thus is at a higher temperature. The equivalent to the cats in your box is the quantity of air.

#### Mister T

Gold Member
If the cats are instead supposed to indicate the quantity of energy, then the only way to increase the number of cats is to increase the energy. There are two ways of doing this: work (which is a mechanical transfer of energy) and heat (which is a thermal transfer of energy).

But you can get the same result, either way. Transfer in a cat by doing work and now you have one extra cat. If instead you had transferred in a cat by transferring heat, you still end up with one extra cat. It makes no difference how you do it. Energy is energy.

#### Mister T

Gold Member
I was told that there are communication satellites that are constantly stationary.
They complete one Earth orbit every day, therefore they are not stationary. They are, however, always in the same place in the sky so that once you point your antenna at one it will stay pointed at it.

#### DaveC426913

Gold Member
I challenge the idea that compressing a gas can somehow “produce heat“. My mind simply rejects it.
Consider this: if you had a container that was 10‘ x 10‘ x 10‘ cubed; Full of air at sea level pressure and at room temperature. That container contains a certain amount of heat that can be measured. If you compress one of those thousand cubes into 1/1000 it’s volume, The amount of heat with in your thousand foot container is consistent. No heat was “magically made“.
"Amount of heat" is an absolute value, yes. But if you reduce the volume, the temperature will increase proportionally (because the same "amount of heat" is now in a smaller space).

The gas laws are specific and well understood. They show the direct and inverse correlations between volume, pressure and temperature of an ideal gas.

#### jbriggs444

Homework Helper
"Amount of heat" is an absolute value, yes. But if reduce the volume, the temperature will increase proportionally (because the same amount of heat is now in a smaller space).
This is not correct. If you reduce the volume but carefully avoid adding energy to the contents (draining as much in thermal energy as you are injecting by performing mechanical work) the result is an isothermal compression.

The same amount of thermal energy is in a smaller space, but the temperature is unchanged.

cjl

#### DaveC426913

Gold Member
This is not correct. If you reduce the volume but carefully avoid adding energy to the contents (draining as much in thermal energy as you are injecting by performing mechanical work) the result is an isothermal compression.

The same amount of thermal energy is in a smaller space, but the temperature is unchanged.
Did you not see the reference to gas laws? They don't involve bleeding off excess.

The OP believes that compressing a volume of gas will not raise its temperature. That is not true.

You can always complicate the experiment to obfuscate the principle being demonstrated if you want.

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#### jbriggs444

Homework Helper
Did you not see the reference to gas laws? They don't involve bleeding off excess.

The OP believes that compressing a volume of gas will not raise its temperature. That is not true.

You can always complicate the experiment to obfuscate the principle being demonstrated if you want.
Both your post and the point you were responding to were incorrect. It does little to help correct a misconception if you do so by promulgating a new one.

#### DaveC426913

Gold Member
Both your post and the point you were responding to were incorrect.
How is my drawing attention to the ideal gas laws incorrect? The OP seems not to be aware of them.

(OK, the first half of my post phrased it using the OP's terms, but the point is made that the ideal gas laws show that compressing a gas will raise its temperature - other factors being equal.)

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#### jbriggs444

Homework Helper
the point is made that the ideal gas laws show that compressing a gas will raise its temperature - other factors being equal.)
And that point is incorrect.

In fact, per the ideal gas law, PV=nRT, compressing a gas (reducing its volume) while holding pressure and amount of substance constant can only be achieved by reducing temperature. If you are planning to have all factors equal, you'd better spell out which ones you are holding constant and how.

#### davenn

Gold Member
They are, however, always in the same place in the sky so that once you point your antenna at one it will stay pointed at it.

not quite .... they actually move in an oscillation north and south of the equator by a small amount
an example ......

#### Rensslin

Temperature isn't like "cats" in this analogy though. You have a box with 1 kg of air in it. You shrink the box. It still has 1 kg of air in it, but in the process of shrinking it, you had to apply force, so now the air that is in the box has more energy, and thus is at a higher temperature. The equivalent to the cats in your box is the quantity of air.
Got it: that makes sense. Thanks

#### DaveC426913

Gold Member
And that point is incorrect.

In fact, per the ideal gas law, PV=nRT, compressing a gas (reducing its volume) while holding pressure and amount of substance constant can only be achieved by reducing temperature. If you are planning to have all factors equal, you'd better spell out which ones you are holding constant and how.
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OK, I take full responsibility for not grokking this - after all, as a diver, I'm supposed to know the gas laws down pat.

Yes, it is obvious (even to me) that - if you plan to keep pressure constant - you'll have to reduce the temperature to get the volume to reduce. This I know.

But if you were to take a volume of gas, in a closed container - and compress it (reduce its volume) - both temperature and pressure will go up. After all, this is how diesel engines work. (Please tell me I'm right about this, or I'm going to set my laptop on fire, and go live in a shack in the woods)

So what am I missing? Is it simply that I didn't specify that pressure doesn't have to remain constant?

< /sidebar >

#### Mister T

Gold Member
But if you were to take a volume of gas, in a closed container - and compress it (reduce its volume) - both temperature and pressure will go up. After all, this is how diesel engines work. (Please tell me I'm right about this, or I'm going to set my laptop on fire, and go live in a shack in the woods)
Well, that is certainly one of the possibilities. In fact the quantity $pT$ must go up, but there are ways of doing that that will make $p$ or $T$ go down.

#### phyzguy

@DaveC426913, I think what people are objecting to is the following statement you made:
"Amount of heat" is an absolute value, yes. But if you reduce the volume, the temperature will increase proportionally (because the same "amount of heat" is now in a smaller space).
When you compress a gas, the temperature does not increase because, "the same "amount of heat" is now in a smaller space". It increases because the act of compression does work on the gas and increases the "amount of heat" in the gas. The "amount of heat", also called the internal energy, goes up. If the internal energy is held constant, the temperature of the gas will not go up, even if it occupies a smaller volume

#### DaveC426913

Gold Member
Cool. I learned some new things today.

One of them is that I had a naive understanding of the source of the temperature rise when you compress a gas.

"Atmospheric Burn-up During Re-Entry"

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