Reentry heat - air friction?

I just finished reading The Right Stuff and was surprised to read in at least 3 places where Tom Wolfe suggested that spacecraft reentering the atmosphere require heat shields due to air friction heating up the spacecraft.

Shouldn't *somebody* have picked up that error during the editing of a book like that???

what is it then?

When anything moving that fast enters the atmosphere (spaceship, meteor, alien garbage, etc.) it pushes the air in front of it and compresses it. The air can't move out of the way fast enough to avoid being compressed.

Expanding gas cools off, and compressed gas will heat up.

The heat from the compression of air in front of the spacecraft is hot enough to melt off the heat shield. Air friction plays little to no part in the temperature rise.

ClayD said:

what is it then?

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You know those cans of compressed air you sometimes use to clean all the disgusting gunk out of your computer's keyboard? You've probably noticed how the can feels cold after you use it.

That's because the compressed gas in the can is asorbing heat as it expands.

It's just like that ... only exactly the opposite.

The air can't get out of the way of the re-entering space craft. So, it's quickly compressed which makes it really hot.

Back to the original post ... I don't think accuracy was all that important to Tom Wolfe (though, I think that the novel is more historically accurate than the film).

-- Roger

jadebox said:

Back to the original post ... I don't think accuracy was all that important to Tom Wolfe (though, I think that the novel is more historically accurate than the film).

-- Roger

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I've recently been reading a lot of books about the Apollo program while building my Estes Saturn V, and in one of the books Deke Slayton said that the book was as great as the film was horrible. So I decided to read it and he was right, it's a great book (except for the errors).

Mr. Wolfe's theory of writing historical works was to write in a "journalistic" style, where the people and their relationships are more important than cold, sterile facts. For the vast majority of consumers, and for a book to be profitable and popular, this works quite well. For scholars, this is not the case, works need to be annotated and sourced, timelines and facts that are historically correct supersede trite people and interesting exaggerations. Mr. Wolfe's book is somewhere between a scholarly work and a normal "novel", and you can see it, however it is a very good book.

Ohh, well then isn't thats friction heating? molecules get closer together and
cant disapate the heat fast enough.

if not, why isnt the compressed air can hot?

Actually has a lot more to do with the mach number than pressure...

As i read the shuttle at mach 25 on re-entry generates less aerodynamic pressure than an aricraft going 300mph at sea level...

or is that all wrong?

ClayD said:

Ohh, well then isn't thats friction heating? molecules get closer together and cant disapate the heat fast enough.

if not, why isnt the compressed air can hot?

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The compressed air can did get hot when the air was compressed in the factory, but then it cooled off.

Air conditioning works because of this. Compress the refrigerant to make it hot, blow off the heat using the outdoor coil, expand the gas to make it cold, blow air across it using the indoor coil to make the room cooler and the refrigerant picks up the room heat. Compress the refrigerant to make it hot again, blow off the head using the outdoor coil...

Actually has a lot more to do with the mach number than pressure...

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I'm sure the higher the mach number, the more the air is compressed because it can't move out of the way fast enough.

As i read the shuttle at mach 25 on re-entry generates less aerodynamic pressure than an aricraft going 300mph at sea level...

or is that all wrong?

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There is a LOT less air 50 miles up than there is at sea level, so that's probably correct. But the shuttle soon sees more heat due to air pressure than the aircraft will experience. A lot more.

Mushtang said:

The compressed air can did get hot when the air was compressed in the factory, but then it cooled off.

Air conditioning works because of this. Compress the refrigerant to make it hot, blow off the heat using the outdoor coil, expand the gas to make it cold, blow air across it using the indoor coil to make the room cooler and the refrigerant picks up the room heat. Compress the refrigerant to make it hot again, blow off the head using the outdoor coil...

I'm sure the higher the mach number, the more the air is compressed because it can't move out of the way fast enough.

There is a LOT less air 50 miles up than there is at sea level, so that's probably correct. But the shuttle soon sees more heat due to air pressure than the aircraft will experience. A lot more.

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exactly. when you compress molecules they rub together causing friction, which generates heat. Friction from the shuttles re-entry causes such a violent compression of molecules, that rub themselves so vohemently they can create a plasma, with almost no pressure involved.

Mushtang said:

When anything moving that fast enters the atmosphere (spaceship, meteor, alien garbage, etc.) it pushes the air in front of it and compresses it. The air can't move out of the way fast enough to avoid being compressed.

Expanding gas cools off, and compressed gas will heat up.

The heat from the compression of air in front of the spacecraft is hot enough to melt off the heat shield. Air friction plays little to no part in the temperature rise.

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Really... in all the years I've been reading about spaceflight and reentry and all that, this is the first time I've read this... interesting if true...

Now, I DO know that the shuttle was very carefully designed to create a shock wave that kept the hottest part of the gases actually a couple inches AHEAD (or just above the surface) of the surface of the heat shield... temperatures in the shock wave typically ran around 4,000 degrees IIRC while the temperatures on the tile/wing leading edge surface was only around 2,500 degrees... That was a big reason why the shuttle was SO vulnerable to heat shield damage-- it eliminated the formation of the shock wave in a localized area, and let the hot gases strike the surrounding surface DIRECTLY, as well as the area of the damage as well... the heat shield couldn't handle the direct heating of the hot gas striking it, and started disintegrating, leading to an "unzipping" effect where the hole gets larger and larger...

Shock wave physics are pretty interesting stuff... I was reading an interesting book a couple years ago about the difficulties involved in designing jet engine air intakes on supersonic and hypersonic aircraft, using movable cones or drooping "wedge" intakes, or movable "fences" to establish shock waves ahead of the intake, and the importance of keeping the shock wave out of the intake, since the jet engine can only use subsonic airflow-- so the shock wave has to be shaped properly to keep the supersonic airflow out of the intake, as the air drops subsonic crossing the shock wave...

If you're vehicle is "fluffy" enough (large surface area with low total vehicle weight) you don't need a heat shield, per se, at all... just a metallic structure, (called a "hot structure") to handle the heat. In the early days when the Faget shuttle was the plan, (short straight wings like a standard airplane and low vehicle weight at reentry) the plan was to just use a hot structure of Rene 41, an alloy typically used in making jet engine turbine blades (IIRC). When they went to the larger delta-wing glider for the larger payload bay and greater cross-range that the Air Force required, the increased mass of the spacecraft and higher reentry heat loads required coming up with a reusable heat shield material, as the heat loads were too much for hot structures. The rest is history...

Later! OL JR

ClayD said:

Ohh, well then isn't that's friction heating?

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No, the heat is not due to friction, though it's similar. In both cases kinetic energy is converted to heat.

Friction is a force resisting the movement of a surface relative to another. heat is produced when friction results in a loss of kinetic energy.

In the case of the re-entering spacecraft, the air isn't a surface moving against the spacecraft. The air forms a boundary layer sort of like a balloon below the spacecraft. The spacecraft compresses the "balloon," decreasing its volume and compressing the air inside. The particles of air collide with each other more often giving up some of their kinetic energy in the form of heat.

-- Roger

luke strawwalker said:

Now, I DO know that the shuttle was very carefully designed to create a shock wave that kept the hottest part of the gases actually a couple inches AHEAD (or just above the surface) of the surface of the heat shield...

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I believe that's true of the capsule designs, too. At least the Apollo capsule.

Edit: Okay ... I looked it up. Blunt shapes, like the bottom of a capsule have higher drag and, though it seems backwards, heat up less during re-entry than the pointy end would. The reason is that the hot, compressed air forms a "shock layer," but a cushion of cooler air forms on the blunt surface that keeps the hot shock layer away from the surface.

-- Roger

luke strawwalker said:

If you're vehicle is "fluffy" enough (large surface area with low total vehicle weight) you don't need a heat shield, per se, at all... just a metallic structure, (called a "hot structure") to handle the heat.

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ohh ohh ohh!!! I'll take SR-71 for $500.00

Compressable flow is a "&&&##**", and sucks when it takes off that nice paint job from your rocket!

jadebox said:

I believe that's true of the capsule designs, too. At least the Apollo capsule.

-- Roger

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Too true.

Blunt body reentry vehicles are better at shielding heat that a shape that would slice into the air.

jadebox said:

The particles of air collide with each other more often giving up some of their kinetic energy in the form of heat.

-- Roger

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My point here was not determination of the kinetic energy type, but just as you say the two are similar. No one here is saying the wing going through the air is causing the heat.

Simply put, the book is not inaccurate, in that the plasma generated is caused by the friction between the air molecules. even though an expert may call it something else, its not incorrect.

Friction is a force resisting the movement of a surface relative to another.

heat is produced when friction results in a loss of kinetic energy

The particles of air collide with each other more often giving up some of their kinetic energy in the form of heat.

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Does this not mean that the air surrounding the body, heats itself up with friction? thus, Tom Wolfe suggested that spacecraft reentering theatmosphere require heat shields due to air friction heating up the spacecraft. Means that he is correct. the airlfow around the space craft is so hot with its own friction, that it will heat the spacecraft....

ClayD said:

My point here was not determination of the kinetic energy type, but just as you say the two are similar. No one here is saying the wing going through the air is causing the heat.

Simply put, the book is not inaccurate, in that the plasma generated is caused by the friction between the air molecules. even though an expert may call it something else, its not incorrect.


Does this not mean that the air surrounding the body, heats itself up with friction? thus, Tom Wolfe suggested that spacecraft reentering theatmosphere require heat shields due to air friction heating up the spacecraft. Means that he is correct. the airlfow around the space craft is so hot with its own friction, that it will heat the spacecraft....

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No, friction between molecules does not heat up the molecules. Nor does it heat up the spacecraft. The book is wrong.

Air contains energy, and when the volume of that air is reduced (compression) the exact same amount of energy exists, Because it exists in a smaller volume than before, the temperature of that air is higher.

Something related to the discussion, the adiabatic compression of the atmosphere upon a reentry vehicle:

https://en.wikipedia.org/wiki/Adiabatic_process

https://en.wikipedia.org/wiki/Shock_wave

Greg

Additionally it should be noted that the surrounging atmosphere, like the entire worlds worth, acts as the biggest heatsink known to man. (Don't believe me just calculate the heat generated by your shock cord at full length upon ejection) The blunt end creats a shock wave, the spacecraft rides behind the shock wave. The heat has two sources, the air in front of the shockwave compresses causing it to heat up (Compression and Friction are two different things, Physics 101) and the kenetic energy from the loss of momentum presents as heat. The craft is converting speed into heat. The eariler example of the air can isn't quite right. As air, or any matter, compresses it heats up. When air rapidly expands, like quickly leaving an air can, it cools. That's why the can gets cold.
Back to the spacecraft; one additional heating element is as the craft moves through the air it ionizes the atmosphere around it, Radio blackout, thus contributing to the plasma buildup. The heatshield gets hot from thermal transfer through the shockwave. Any heat that would be contributed by friction (and this would be barely measurable) is absorbed by the surrounding atmosphere (heatsink effect). FWIW

Possibly a forgivable over-simplification considering the author and audience? I hate to admit that being a science stickler.

New Ocean said:

Possibly a forgivable over-simplification considering the author and audience? I hate to admit that being a science stickler.

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In my opinion it's not an over-simplification since it's completely wrong. Over simplifying would be to say that the air causes the temperature to rise.

I would prefer that he just skip over the explanation than to give an incorrect one.

Frankly, up until this discussion, air friction heating was the explaination I understood for a spacecraft heating up on re-entry. I am an electrical engineer by training and have little aerospace exposure. So it made sense to me on a basic level. Not to say it was the correct explaination.

Hmmm. Interesting.

So when we fly our rockets to Mach 2+ in low altitude thE heat and ablative properties are from compressing the air? My high schools project is to measure the heat "of friction"[?] as our rocket goes past Mach 2. (guess it should just be heat in general.)

When the boundary layer flow becomes supersonic don't the molecules rub and force heat to the rocket?

Interesting topic.

considering that you are moving from a frictionless environment to a frictioned envinronment, then i consider that to be much more important factor than "compression"..

this is not heating and air sysetems... its hypersonic flow!

https://www.columbiassacrifice.com/$C_hypersonic.htm

for the purposes of calculating heat transfer and thermodynamic properties, is distinctly different from 2supersonic and 2subsonic speeds.

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The major sources of increased heating during supersonic and hypersonic flight are skin and fluid friction as the atmospheric gasses pass over the surface of the aircraft and compression of the gas molecules as they pass through the boundary of the shock wave.

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bandman444 said:

Hmmm. Interesting.

So when we fly our rockets to Mach 2+ in low altitude thE heat and ablative properties are from compressing the air? ...

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As I understand it, the heat comes from the frictional element and from adiabatic compression. As far as what percentage each contributes to the thermal soak, my guess is that is a function of velocity and current atmospheric conditions (like barometric pressure) and probably a whole lot more than I am aware of. I think it would be helpful to find an aerodynamicist with an understanding of supersonic/hypersonic flight to speak about how heat is transferred.

I am sure there are lots of equations with squiggly lines involved.

Greg

If you consider the term "air friction" to be synonymous with "aerodynamic drag", then the heating is caused by air friction. However, the term "air friction" usually refers only to the skin friction component of drag, which is a function of the surface smoothness.

At a molecular level the temperature of an object is proportional to the square of the average velocity of its molecules. When an object is traveling at a high rate of speed the air molecules are hitting it at a velocity that is much higher than its average velocity due to its temperature. To the object, the air appears to be at a very high temperature. Therefore, the object heats up to match the apparent temperature of the surrounding air due to the high speed.

How about this:

The air is compressed (learn about the laws of thermodynamics and gas laws) and it gets heated - well in front of the blunt object.

The heated air can transmit some of it's heat away via several methods: directly when molecules bump each other and (my favorite) via radiation. Just like all objects do, this heated air will 'shine' and any portion of the blunt object facing this radiating hot gas will absorb energy following the laws of thermodynamics.

Attempting to teach this on a web forum is silly. There are textbooks. I'm sure some of the online MIT open courseware will cover this.

View attachment themodyn.zip

ClayD said:

Simply put, the book is not inaccurate, in that the plasma generated is caused by the friction between the air molecules. even though an expert may call it something else, its not incorrect.
.

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Friction is a force retarding the movement of two surfaces in contact with each other. It is not the collision of two particles.

-- Roger

DaveHein said:

If you consider the term "air friction" to be synonymous with "aerodynamic drag", then the heating is caused by air friction. However, the term "air friction" usually refers only to the skin friction component of drag, which is a function of the surface smoothness.

At a molecular level the temperature of an object is proportional to the square of the average velocity of its molecules. When an object is traveling at a high rate of speed the air molecules are hitting it at a velocity that is much higher than its average velocity due to its temperature. To the object, the air appears to be at a very high temperature. Therefore, the object heats up to match the apparent temperature of the surrounding air due to the high speed.

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Not really. Heat within a molecule is due to the molecule vibrating, not traveling in a straight line and bumping into an object. However, the molecular velocity that a molecule, or an atom, has when vibrating can be calculated. A Nitrogen atom at 100 degrees F vibrates with a molecular velocity of 1359 mph. At nearly 1000 degrees F it vibrates with a molecular velocity of 2140 mph. This is approximately Mach 3.5, and the speed record for the SR-71 Blackbird.

If what you say is correct, then the SR-71 should have a surface temperature of 1000 degrees F at Mach 3.5. Correct?

jadebox said:

Friction is a force retarding the movement of two surfaces in contact with each other. It is not the collision of two particles.

-- Roger

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what is the term for the energy and (thermal increase) called from the collision of particles? I would like to look up some more stuff.

after looking up some more stuff:
(Direct friction upon the reentry object is not the main cause of shock-layer heating. It is caused mainly from isentropic heating of the air molecules within the compression wave. Friction based entropy increases of the molecules within the wave also account for some heating.).

Gentelmen, We're mixxing apples and Oranges. Yes, Heating is a byproduct of both friction and compression but to say it's the same is like syaing it's the same between taking a bath and getting cought in the rain. The same result dosn't mean the same cause.
Re-ennering spacecraft (controlled re-entry) heat up from (primarily) compression of the air their falling/flying through because they're designed with a "Blunt" configuration. This creats a shockwave that the craft flys/falls behind allowing it to trade speed for heat thus slowing the craft to a managable level. Being BEHIND the shockwave means surface friction is at a bare minimum; Pictures of wind tunnel models show this.
Other craft like an SR-71 (or our rockets) are designed to cut through the air creating a minimal shockwave. The laminar flow of air over these types of craft do cause frictional heating. That's why the Black Bird grows a foot in length as it heats up. These designs are configured to minimise drag; "Blunt" shapes maximise it. A falling sheet of paper (flat side down) doesn't heat up but slows greatly due to the compressive force of the air ahead of it. This low rate of energy transfer (realitively speaking) makes it ideal for a re-entry shape. If a spacecraft had a streamlined shape for re-entry it would heat up due to friction and burn up as the thermal transfer rate would be higher.
SO, same (realative) effect, different cause. Clear as mud, and Woolfs description is wrong. Here's one to bake your noodle; A capsule going up heats due to (in part) friction, comming down it heats due to compression.