B Atmospheric Burn-up During Re-Entry

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So I’m going to have to show my dullness and ask for help.
I can’t get my mind around how things “burn-up” upon entering the atmosphere.
I envision some kind of bellyflop. Objects have no friction prior to entry. But how is this different from an acceleration within the atmosphere? Is the speed of the object faster than can be attained within the atmosphere? Can this exothermic condition be reproduced within the atmosphere? Bullets travel at 700 FPS but I would think the bullet gets cooler as it goes; which is , in fact, exothermic.

So an object (even a slow moving object) traveling within the atmosphere is causing friction. There must be a formula; Surface area(sa) * barometric pressure(p) * speed (dt) = heat (j); sa*p*dt=j


This same formula must be consistent at the top and bottom of the atmosphere. But if j and p are inverse how does the exothermic problem occur when p is minimal? Also, when old satellites and “space junk” fall out of the sky, they slowly enter the atmosphere and still burn up. If they are stationary with respect to the planet, now dt, p, and, if the item is small, sa is almost nothing. How is j anything?

I need a smart person who can explain.
 
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Summary: So I’m going to have to show my dullness and ask for help.

I envision some kind of bellyflop
Its much more gradual. More like being "sandblasted" but with air molecules. The higher the speed and the thicker the air, the more energetic the abrasion. The abrasion is so intense that it makes the object hot.
The atmosphere is very thin above 50 miles and starts to really thicken at about 10 miles. The idea is to choose the initial angle of entry so as to lose speed over a long trajectory so as not to burn up.
The formulas are quite complicated particularly if the object is supersonic. Hope that helps
 
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when old satellites and “space junk” fall out of the sky, they slowly enter the atmosphere
You have been misinformed. They start off slowly as they leave orbit but they gain so much speed that air friction makes the smaller pieces burn up and at least ablates material off of the bigger pieces.

As regards your question of can such high speeds be reached in the atmosphere, yes for sure, but comparing that to a bullet is meaningless. A bullet spends so little time traveling that it never has time to heat up much if any. There is a new generation of drawing-board supersonic transports and air friction is a VERY serious problem and design consideration for them.

EDIT: I note that my original statement above that the ablation is caused by friction is not correct. WHAT I described as happening is correct but the cause (not friction) is explained well by other posts below.
 
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phyzguy

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If they are stationary with respect to the planet...
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.
 
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How about using a new approach ... to getting a feel for understanding your problem: Thermodynamics and Chemical Engineering.

In Thermodynamics when you take a compressed gas - and cause this gas to expand, it can get really cold as it expands along a pathway of constant "Entropy".
- - - - - - - - - - - -
Likewise, in Chemical Engineering, when you 'compress' a gas to far greater pressure - you do so again along a line of Constant Entropy ... on a graph for that particular gas. You end up having a gas not only at great pressure, but more importantly, it has a great increase in temperature, too.

Now the new viewpoint. Imagine a supersonic aircraft ... especially the leading edge of its wing.

Now, imagine that the wing is not moving at all. This is your new, relative viewpoint.

Now view an air molecule approaching the leading edge of the wing of the jet - at supersonic speed. It starts decelerating/slowing down while on a collision course with the leading edge of the jet ... until the gas molecule finally just stops motion. Thus, you have 'compressed' this gas ... and it has increased in temperature moving up that line of Constant Entropy!

There's where the heat comes from ... just sort of a heat-pump effect, but you're dealing with an orbital velocity of Mach 20!

Does this help?
 
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It helps a great deal.
I had a lot of trouble getting my mind around gas compression producing heat; my physics teachers were just poor explainers for my little one-cylinder brain. I couldn’t figure out how my foot-powder can got cold when I sprayed it. I don’t believe the compression of the gas actually produces heat but displaces it from elsewhere. I knew this because of the law of the conservation of heat.

So if I’m understanding you correctly, as an object enters the earths atmosphere, it is actually accumulating the heat that is in the atmospheric gasses. So the heat that is in those gasses, when the gasses compress on the surface of the object, is transferred into all different directions one of which is the surface of the object which accumulates it.

If this is the case, is it still fair to call this, “abrasive friction“.!?
 

Nugatory

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I knew this because of the law of the conservation of heat.
There is no such thing as “conservation of heat”. There is conservation of energy, but heat is only one of many different forms of energy so we can create heat from other forms of energy: think about how an electric stove creates heat from electricity, or an automobile engine turns the heat from burning fuel into the kinetic energy of the car.
as an object enters the earth’s atmosphere, it is actually accumulating the heat that is in the atmospheric gases.
That’s not what’s happening. Instead, the kinetic energy of the object is being turned into heat. The object ends up moving more slowly (less kinetic energy) and both the atmosphere and the object end up hotter (more heat energy).
 

Filip Larsen

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Bullets travel at 700 FPS but I would think the bullet gets cooler as it goes
If, for the sake of argument, the bullet leaves the barrel being same temperature as the surrounding air then it will heat up simply by moving through the air. So will any other object.

When you say you think it may cool off you may be thinking of the effect where a hot object moving (slowly) through air will cool faster than if it remained stationary in the same volume of air simply due to air acting as a kind of heat insulation when there is no air flow over the object. That is, a stationary object heats up the surrounding air a bit which, if that air stays around the object, in turn will make the temperature gradient lesser and thus the heat flux out of the object will also be lesser making it cool slower.

As mentioned by other, bullets travel for a relative short time and also experience violent heat inducing processes both when fired and at impact that kind of dwarf any increase in temperature that happens due to air friction alone. But if you want to, you can estimate the maximum possible heating of a projectile in a nearly horizontal trajectory by considering the loss of kinetic energy from just after the barrel to just before impact and then relate that energy to the mass and heat capacity of the slug.

For something traveling fast in the atmosphere for longer time than a bullet, I suggest you read up on the now retired SR-71 Blackbird supersonic aircraft which was designed to cope and even take advantage of the significant skin heating it experienced due to air friction when cruising at Mach 3+.
 
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If this is the case, is it still fair to call this, “abrasive friction“.!?
Perhaps turbulence or aerodynamic drag sound more erudite. They are kinds of friction although some folks reserve the term friction to a narrower scope. Your choice.
 

marcusl

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A bullet at 700 FPS is a poor comparison. Bullets are shaped to generate minimal friction so as to achieve maximal range. A space capsule reenters the atmosphere at 22,000 FPS and is shaped to maximize friction so as to slow the capsule enough for safe splashdown.
 

boneh3ad

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Oh... oh my.

So there are a few larger misconceptions I feel the need to clear up here before addressing specific points. First, aerodynamic heating, whether during reentry or otherwise, is not due to "friction" as it is popularly described. That may be an easy way to try to describe it in the newspaper, but is divorced from reality. In fact, right at the surface of the vehicle, the air has zero velocity with respect to the surface (a concept called a boundary layer), so it is not a directly analogous situation.

The bottom line is that heat flows toward a colder region. If an object traveling through the air is warmer than the surrounding air, it will cool off. If it is colder than the surrounding air, it will heat up. When an object travels through a gas at high speeds, like air, it compresses some of the air in front of it. The higher the speed gets, the greater the compression. @Bruce Zerr had a pretty good description of how a gas can heat up when it compresses based on thermodynamics, though the bit about constant entropy isn't really valid here because supersonic vehicles create shock waves, which are dissipative.

When a spacecraft enters the atmosphere and compresses the air in front of it, a great deal of heat is generated. It can even get hot enough that the air becomes ionized and forms a plasma sheath around the vehicle. This immense heat can then be transferred back into the surface of the vehicle, causing major problems if it is not properly mitigated by the vehicle designer.

The actual process by which heat transfers into the surface is quite complex and not uniform over the surface. It depends on a number of factors such as the Mach number, the location in the atmosphere, the laminar/turbulent state of the boundary layer, shock wave unsteadiness around control surfaces, surface ablation/recession and the resulting chemistry, and many others, many of which are still active areas of research in the aerodynamics community.

So an object (even a slow moving object) traveling within the atmosphere is causing friction. There must be a formula; Surface area(sa) * barometric pressure(p) * speed (dt) = heat (j); sa*p*dt=j

This same formula must be consistent at the top and bottom of the atmosphere. But if j and p are inverse how does the exothermic problem occur when p is minimal?
Well first, your equation is not really meaningful in any way, so trying to draw conclusions from it is going to get you nowhere. Aerodynamic heating is a complex phenomenon that doesn't readily adhere to simple formulae like this.

More like being "sandblasted" but with air molecules. The higher the speed and the thicker the air, the more energetic the abrasion. The abrasion is so intense that it makes the object hot.
This is absolutely incorrect, but fear not, you are in good company. Isaac Newton wrongly believed this is how fluid flows work as well. The ablation is nothing like sandblasting, and the paths of air molecules bend around the body smoothly (for the most part), not abruptly after they hit the surface.

The atmosphere is very thin above 50 miles and starts to really thicken at about 10 miles. The idea is to choose the initial angle of entry so as to lose speed over a long trajectory so as not to burn up.
This is partially correct. You want to lose enough speed high in the atmosphere where density is low so that you are moving considerable slower by the time you hit the thicker atmosphere below, but if you go too shallow, you can skip off the atmosphere like a stone off the surface of a lake.

The formulas are quite complicated particularly if the object is supersonic.
All reentry problems are supersonic. All.

Perhaps turbulence or aerodynamic drag sound more erudite. They are kinds of friction although some folks reserve the term friction to a narrower scope. Your choice.
They aren't kinds of friction. Like friction, they are dissipative, but they are not "kinds" of friction, and they are no way abrasive to objects.

You have been misinformed. They start off slowly as they leave orbit but they gain so much speed that air friction makes the smaller pieces burn up and at least ablates material off of the bigger pieces.
Satellites reentering the atmosphere start out at orbital velocity, so... not slowly. They start out incredibly fast, but are moving primarily through rarefied atmosphere and so take a while to heat up and break apart.

That’s not what’s happening. Instead, the kinetic energy of the object is being turned into heat. The object ends up moving more slowly (less kinetic energy) and both the atmosphere and the object end up hotter (more heat energy).
I think it's fair to say there is a little of both going on. Kinetic energy is being converted into atmospheric heat, some of which is being transferred back into the vehicle.
 
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Satellites reentering the atmosphere start out at orbital velocity, so... not slowly. They start out incredibly fast, but are moving primarily through rarefied atmosphere and so take a while to heat up and break apart.
Good point. I was thinking of their velocity in the frame of their original orbit, which is not the right way to look at this issue.
 

Drakkith

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So if I’m understanding you correctly, as an object enters the earths atmosphere, it is actually accumulating the heat that is in the atmospheric gasses. So the heat that is in those gasses, when the gasses compress on the surface of the object, is transferred into all different directions one of which is the surface of the object which accumulates it.
Imagine a gas particle minding its own business when a giant space capsule (giant compared to the gas particle) slams into it. One single gas particle isn't going to appreciably slow down a space capsule, so the capsule retains virtually all of its speed and the gas particle gets an enormous kick out of the collision (like bouncing a super ball off the front of a moving train).

Now zoom out a bit and see that instead of one gas particle, there are gazillions of them being hit by the capsule, even in the upper atmosphere. The density of the gas may be very low when you're this high up, but it's still high enough that these gas particles can only travel a short distance before impacting another gas particle, transferring some of that initial kinetic energy to them. And then those two particles can impact more, and then those impact more, etc. And remember that the capsule is still plowing into these particles from behind. This process increases the density of the gas (i.e. it compresses it) and during these collisions the velocities of the gas particles become randomized and you get a region where the gas particles are moving very fast and in random directions. This is also known as a HOT gas!

Over several minutes (for a space capsule on a proper re-entry) the capsule gradually loses speed as its kinetic energy is transferred to the gas and it slows down. During this time the gas in front of the capsule forms a layer of highly compressed air forms known as a shock wave. The part of the shock wave directly in contact with the capsule is relatively cool, as the heat from its initial compression has already been transferred away to the capsule or the surrounding air. The part of the shock wave in contact with 'new' air (air that hasn't been plowed through yet) is where the vast majority of the heating is done since that 'new' air isn't compressed yet. This new air is compressed, heats up, and is then forced around the capsule where most of the heat is lost to the atmosphere over a short period of time. If it wasn't for the fact that most of the heat is lost to the atmosphere instead of the re-entry vehicle, returning from space would be much, much more difficult.

So as the capsule plows through the air it continually compresses a new region of air in front of it, heating this air up. Above a certain speed most of the heat transferred to the capsule is through radiation. It's only at lower speeds that direct contact with this hot gas becomes the dominant form of heat transfer.

Have a look at this article from the FAA.
 

Drakkith

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To muddy up things further, here's an excerpt from the article I linked in my previous post:

A steep re-entry causes a very high heating rate but for a brief time, so
the overall effect on the vehicle may be small. On the other hand, shallow
re-entries lead to much lower heating rates. However, because heating con-
tinues longer, the vehicle is more likely to “soak up” heat and be damaged.
Basically, shallower entries are easier on the crew and put less aerodynamic stress on the re-entry vehicle, but absorb much more heat than steeper entries. And vice versa. Here's a graph of a meteor's velocity vs time during re-entry:
246241


At steep angles, re-entry is like hitting a wall. The deceleration is so high (dozens to a hundred+ of g's) that the object risks breaking up and a person would be turned into paste. In the graph above, the meteor loses about 5,000 m/s of velocity in roughly 5 seconds. That's an average deceleration of 1000 m/s2, or 102 g's. That's not survivable by anyone. Not even close. Anything past about 15 g's and you run the risk of severe internal injuries.
 
18
3
Oh... oh my.

So there are a few larger misconceptions I feel the need to clear up here before addressing specific points. First, aerodynamic heating, whether during reentry or otherwise, is not due to "friction" as it is popularly described. That may be an easy way to try to describe it in the newspaper, but is divorced from reality. In fact, right at the surface of the vehicle, the air has zero velocity with respect to the surface (a concept called a boundary layer), so it is not a directly analogous situation.

The bottom line is that heat flows toward a colder region. If an object traveling through the air is warmer than the surrounding air, it will cool off. If it is colder than the surrounding air, it will heat up. When an object travels through a gas at high speeds, like air, it compresses some of the air in front of it. The higher the speed gets, the greater the compression. @Bruce Zerr had a pretty good description of how a gas can heat up when it compresses based on thermodynamics, though the bit about constant entropy isn't really valid here because supersonic vehicles create shock waves, which are dissipative.

When a spacecraft enters the atmosphere and compresses the air in front of it, a great deal of heat is generated. It can even get hot enough that the air becomes ionized and forms a plasma sheath around the vehicle. This immense heat can then be transferred back into the surface of the vehicle, causing major problems if it is not properly mitigated by the vehicle designer.

The actual process by which heat transfers into the surface is quite complex and not uniform over the surface. It depends on a number of factors such as the Mach number, the location in the atmosphere, the laminar/turbulent state of the boundary layer, shock wave unsteadiness around control surfaces, surface ablation/recession and the resulting chemistry, and many others, many of which are still active areas of research in the aerodynamics community.



Well first, your equation is not really meaningful in any way, so trying to draw conclusions from it is going to get you nowhere. Aerodynamic heating is a complex phenomenon that doesn't readily adhere to simple formulae like this.



This is absolutely incorrect, but fear not, you are in good company. Isaac Newton wrongly believed this is how fluid flows work as well. The ablation is nothing like sandblasting, and the paths of air molecules bend around the body smoothly (for the most part), not abruptly after they hit the surface.



This is partially correct. You want to lose enough speed high in the atmosphere where density is low so that you are moving considerable slower by the time you hit the thicker atmosphere below, but if you go too shallow, you can skip off the atmosphere like a stone off the surface of a lake.



All reentry problems are supersonic. All.



They aren't kinds of friction. Like friction, they are dissipative, but they are not "kinds" of friction, and they are no way abrasive to objects.



Satellites reentering the atmosphere start out at orbital velocity, so... not slowly. They start out incredibly fast, but are moving primarily through rarefied atmosphere and so take a while to heat up and break apart.



I think it's fair to say there is a little of both going on. Kinetic energy is being converted into atmospheric heat, some of which is being transferred back into the vehicle.
First of all thank you for validating my initial concepts. Re-entering atmosphere is like a bellyflop off of the diving board. You are moving through a gas is one condition at a speed that the water will not accommodate.
I am flattered and honored to dialogue with a mind like yours; I mean that.
The problem with smart people is that they too, like the rest of us, have egos. Please do not be offended when I question Your logic.
Smart people sit in classrooms taught by smart people who tell them something that makes no sense and then they begin to think it too.
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“.
Let’s address the concept that the kinetic energy of the moving object was changed into heat. That would mean that the kinetic energy in the fuel that was in the capsule when it was landed on the moon was sufficient to consume the entire capsule after it had propelled the capsule from the moon surface and with inconsideration of the heat that was lost in space and the heat that was lost dissipating into the earths atmosphere. I’m just not buying the idea that compressing air “produces” heat. Just because 1 million people think something doesn’t mean it’s true. It’s just not very scientifikey
 

jbriggs444

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I’m just not buying the idea that compressing air “produces” heat.
To compress air, you will have to use energy. You will have to push down on the handle of the bicycle pump or use electrical energy to drive the air compressor. Energy is conserved. That energy has to go somewhere. One of the places it goes is into the air, increasing the air temperature.

One way of thinking about it is to consider air as a swarm of tiny, rapidly moving balls (molecules) that collide elastically with each other and with the walls of a container. If you move one of the walls inward (like a piston in a cylinder), the balls that strike the moving piston will rebound from it with increased kinetic energy. It turns out that the incremental increase in kinetic energy in this manner is equal to the pressure multiplied by the incremental volume displaced by the piston. One can demonstrate this either with careful math or by applying a conservation of energy argument. The balls will wind up moving faster on average.

It is, roughly speaking(*), the average kinetic energy of the balls/molecules that we measure as the temperature of the air.

(*) It's actually kinetic energy per degree of freedom.
 

russ_watters

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I challenge the idea that compressing a gas can somehow “produce heat“. My mind simply rejects it.... I’m just not buying the idea that compressing air “produces” heat.
This is a basic thermodynamics concept that is important to the operation of internal combustion engines, is a problem for air compressors and is the key operating principle of air conditioners. You can read about it - including doing the math - in any thermodynamics book or even the wikipedia or industry articles on the subject:

But if you won't believe it without experiencing it for yourself, buy an electric tire inflator, use it to pump up a car or bike tire and touch the hose.
Just because 1 million people think something doesn’t mean it’s true.
Actually, yeah it does, when they are scientists/engineers and the concept is a key part of their work. If scientists/engineers didn't understand such basic concepts, they wouldn't be able to make modern technology work. How do you think NASA scientists calculated how hot the capsule would get before trying it?
 

256bits

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Here's a graph of a meteor's velocity vs time during re-entry:
Nice graph.
Nice write up.

Just a very rough calculation:
Average velocity 5000 m/s
duration 20 seconds
gross distance travelled = 100 kilometres

That's for a spherical-ish shape - no lift

A pointy object would travel much further, with less deceleration, but the nose would might melt off as the shock wave is 'attached' to the tip.

Blunt shaped space capsules and space shuttles have some lift to them, so would travel much further with less g's , as you pointed out. The shock wave from these objects with special curvatures is separated from the surface so the gas ( red or white hot due to the compression ) would have to radiate most of the heat absorbed by the surface rather than through conduction or convection.

It is interesting that all the engineering in the beginnings of the launching, and returns from space, was done with hand calculations, with a section of a sphere for the re-entry surface being chosen due to that being the most easiest if not just about impossible equations to manipulate with a slide rule and pencil. No computational fluid dynamics at that time.
 

cjl

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First of all thank you for validating my initial concepts. Re-entering atmosphere is like a bellyflop off of the diving board. You are moving through a gas is one condition at a speed that the water will not accommodate.
There's not really any such thing as a speed that the air (or water, in your example) "will not accommodate". Higher speeds will require higher forces to maintain, and very high speeds will also have other consequences like large amounts of heat, or the destruction of the object due to high forces, but there's no real "limit" on how fast something can move through a fluid.

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“.
Heat isn't "produced", per se, though that is a frequent simplification used by people talking about the subject casually. Energy is added to the cube as you compress it. In order to compress a gas, you must apply a force to the gas, which means that you are doing work on the gas. Because you are doing work on the gas, the energy content in the gas must rise, which is the cause of the increase in temperature.

Let’s address the concept that the kinetic energy of the moving object was changed into heat. That would mean that the kinetic energy in the fuel that was in the capsule when it was landed on the moon was sufficient to consume the entire capsule after it had propelled the capsule from the moon surface and with inconsideration of the heat that was lost in space and the heat that was lost dissipating into the earths atmosphere.
Most of the kinetic energy the craft has at reentry isn't from fuel consumed during the trip back from the moon. Most of it comes from converting potential energy to kinetic as it falls down into the earth's gravity well. This energy did also come from fuel, admittedly, but that's the fuel that was burned to get to the moon in the first place.

As for the question of whether it was enough fuel to provide that energy? It took over 200,000 gallons of kerosene and 300,000 gallons of liquid hydrogen to launch each of the Apollo missions to the moon. The return capsule was the size of a car. Do you think you could burn a car to a crisp with the energy in 200,000 gallons of kerosene?
 
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jbriggs444

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It took over 200,000 gallons of kerosene and 300,000 gallons of liquid hydrogen to launch each of the Apollo missions to the moon.
Much of the energy in the fuel and oxidizer was lost to the temperature and bulk kinetic energy of the exhaust stream and to the superstructure of the first and second stages of course. The Tsiolkovsky equation is brutal.

But enough still went into the payload to have vaporized it several times over.
 
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cjl

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Much of the energy in the fuel and oxidizer was lost to the temperature and bulk kinetic energy of the exhaust stream and to the superstructure of the first and second stages of course. The Tsiolkovsky equation is brutal.
Of course. I just wanted to point out just how much fuel energy was actually available there, and how even a tiny fraction of the chemical energy available would be more than sufficient to burn the capsule to a crisp.
 
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Oh... oh my.
 

cjl

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Despite what the Cambridge dictionary may say, boneh3ad is right - aerodynamic drag differs from friction in a number of important ways, and generally should not be considered to fall under the umbrella of "friction". In addition, even if you insist on calling drag "friction", that's still not what causes reentry heating, which is almost entirely a result of compression in the shock ahead of the object.
 

boneh3ad

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And the Oxford English Dictionary (generally considered to be the definitive source of English language orthodoxy) defines friction as:

3. Physics and Mechanics. The resistance which any body meets with in moving over another body.

This is a fun game, but it's hardly germane to the original line of questioning. The bottom line is that viscous flow resistance has a lot of differences from what we typically call friction, and calling it friction is honestly misleading when discussing this topic. Viscous dissipation of mechanical energy into heat represents a tiny fraction of the heat being transferred into a hypersonics vehicle's surface.
 
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My argument was not with your physics but your tone. You doubtless know more aerodynamics than I do.....probably I know more about atomic interactions with surfaces than you. So what? I was trying to provide a answer to the OP that would be prove useful.
 

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