Heat Shield Observation and Question

[Brian444444]

I have noticed through experimentation that the heavier the load behind a heat shield on (re)entry the more likely it is to overheat assuming the (re)entry angle is the same. I'm no physicist but should that be the case?

Edited May 9, 2016 by Brian444444

[HorusKol]

Here's a non-mathematical description of what's happening:

First - heating is from moving through the air. The amount of heating is determined from the speed of your craft through the air, and also the thickness of the air it is travelling through.

But if you're entering at the same angle and starting at the same speed, why does the heavier craft heat more? Because it takes longer for a heavier craft to slow down - it has more momentum, so needs more time or denser air to slow down.

So, your heavier craft takes longer to slow down when you first start into the atmosphere - this means more time at high speed to heat up your shield. It also means you are going faster as you pass further down through the atmosphere, so you are hitting thicker air at higher speed.

At the end of it all - more time to decelerate plus travelling faster through thicker air means more heat buildup over time.

Edited May 10, 2016 by HorusKol
removed "friction"

[FullMetalMachinist]

It's because your ballistic coefficient is higher because you're putting more mass behind the same cross section (the heat shield). 

Also, more mass at the same reentry angle (meaning the same speed) = more momentum. More momentum = more kinetic energy, which has to be bled off by turning that energy into heat, so there's going to be more heat generated. 

8 minutes ago, HorusKol said:

First - heating is from friction, and friction from the speed of your craft through the air, and also the thickness of the air it is travelling through.

Sorry, but no. The vast majority of reentry heat comes from compressing the air in front of the craft. The ship is moving so fast that the air can't move out of the way, so it gets squished so much that it heats up to the point of being plasma (the "flames"). 

[Brian444444]

Thanks...makes sense.

[bewing]

3 hours ago, FullMetalMachinist said:

Sorry, but no. The vast majority of reentry heat comes from compressing the air in front of the craft. The ship is moving so fast that the air can't move out of the way, so it gets squished so much that it heats up to the point of being plasma (the "flames"). 

Well, actually, it's basically yes. The heat that you get from air compression is thermodynamically based on friction. At its very most basic, heating = friction.

But yes, as these guys said -- the kinetic energy of the incoming craft is higher for a more massive object, and a fair fraction of that extra energy is going to get converted into heat energy in your vehicle. Since your vehicle is more massive, it has the ability to harmlessly absorb more heat energy -- but if you calculate it all out, it can't reradiate the heat as fast. So, in the end it will heat up more, and it will stay hot longer.

Edited May 10, 2016 by bewing

[Kerbart]

43 minutes ago, bewing said:

Well, actually, it's basically yes. The heat that you get from air compression is thermodynamically based on friction. At its very most basic, heating = friction.

Are you sure about that? Because a lot of people I trust about this subject tell me it's actually due to adiabatic compression. Friction has very little to do with it. It's the same effect that heats up your bicycle pump, and the same effect (in reverse) that sunk the Thresher. Friction has very little to do with it.

 

Edited May 10, 2016 by Kerbart

[bewing]

Friction, at is basis: is "molecules running into each other and bouncing off." Adiabatic compression is "gas molecules getting squished closer to each other and bouncing off." Friction is half the basis of adiabatic compression. Very far from "very little to do with it."

[p1t1o]

Eeeeh, all of this is semantics, you all know the two processes are different and that nouns can differ in their level of specificity. But for my two pence, friction involves inter-molecular forces, gases are almost defined as having very little contribution to their properties from inter-molecular forces. Adiabatic compression heating involves collisions, no inter-molecular forces required.

5 hours ago, bewing said:

At its very most basic, heating = friction.

This doesn't really hold.

[Warzouz]

11 hours ago, FullMetalMachinist said:

...

Sorry, but no. The vast majority of reentry heat comes from compressing the air in front of the craft. The ship is moving so fast that the air can't move out of the way, so it gets squished so much that it heats up to the point of being plasma (the "flames"). 

Yes. An analogy I like is a "piston" (yet less extreme)

[Kerbart]

6 hours ago, bewing said:

Friction, at is basis: is "molecules running into each other and bouncing off." Adiabatic compression is "gas molecules getting squished closer to each other and bouncing off." Friction is half the basis of adiabatic compression. Very far from "very little to do with it."

If that were the case than explain adiabatic decompression to me? Rapid expansion results in a temperature drop (this is how refrigerators work). If adiabatic compression were a friction process where does the drop in temperature come from? Anti-friction?

[bewing]

"Friction" is not a fundamental physical process. It is a convenient name for a statistical effect seen when dealing with large quantities of tiny particles. Friction is not one of Newton's 3 laws, and it is not one of the conservation laws.

Adiabatic compression is also not one of Newton's 3 laws. It is also a statistical effect.

If you go and mentally treat them as fundamental processes, then you start to get wacky answers. When you decompress a gas, the little molecules lose energy when they bump into each other -- instead of gaining. So yes, it is "anti-friction". This is another way of Nature telling you that you are not dealing with a fundamental physical process.

[Brainlord Mesomorph]

For all the semantics, you guys seem to be forgetting: its a sim!

There's a subroutine running and whether its simulating " Adiabatic compression" or simulating "friction" its still sort-of accurate and still produces "heet"   (i.e. hands a number to the thermodynamic sim subroutine)

Edited May 10, 2016 by Brainlord Mesomorph

[DancesWithSquirrels]

Adiabatic temperatures explained in short: 

Molecules bounce off of things, and that transfers energy. Expand the gas (drop the pressure), fewer molecules per unit time are bouncing, less energy comes from the gas, it's perceived as 'cooler'. Compress the gas, more molecules per volume and more molecules per unit time are bouncing off your object, higher energy transfer, it's 'hotter'. If you start with the gas and your object at the same temperature, you can modify the summed heat transfer between the two by either compressing or rarefying the gas. 

The equation used is PV = nRT; 'nR' is the number of moles and molecules per mole, P is pressure, V is volume, T is temperature. Increase pressure in front of the vehicle, and temperature goes up. 

Now, as to reentry effects - compression ahead of the vehicle greatly increases the energy per unit volume, to the point of being a plasma for a fair bit of the trip. You can see an example of this kind of heating by compressing gas in a piston. Most of the heating of the craft comes from IR from that compressed gas, not conduction, but the principle is the same. 

[hannyladom]

Thanks...makes sense.

 

[p1t1o]

40 minutes ago, bewing said:

"Friction" is not a fundamental physical process. It is a convenient name for a statistical effect seen when dealing with large quantities of tiny particles. Friction is not one of Newton's 3 laws, and it is not one of the conservation laws.

Adiabatic compression is also not one of Newton's 3 laws. It is also a statistical effect.

If you go and mentally treat them as fundamental processes, then you start to get wacky answers. When you decompress a gas, the little molecules lose energy when they bump into each other -- instead of gaining. So yes, it is "anti-friction". This is another way of Nature telling you that you are not dealing with a fundamental physical process.

I'm not even sure which way you are headed with that, but compressive heating aint friction whichever way you want to stir it.

[FullMetalMachinist]

Whatever pedantic games you want to play with "heat" "friction" "compression" and how those are all related, I think we can (hopefully) all agree that reentry heat is mostly from "a shockwave in front of the craft compresses the air and gets hot" and not "the air is rubbing against the craft and that friction makes it hot", which is the point I was trying to make. 

[Jimmy P]

9 hours ago, Kerbart said:

Are you sure about that? Because a lot of people I trust about this subject tell me it's actually due to adiabatic compression. Friction has very little to do with it. It's the same effect that heats up your bicycle pump, and the same effect (in reverse) that sunk the Thresher. Friction has very little to do with it.

 

The only thing in this statement I'm qualified to comment on is the fact that adiabatic compression (in reverse or otherwise) had no hand in the sinking of the USS Thresher. The primary cause of that was flooding due to a failed brazed joint in a main seawater line, among several other things.

edit: Well I suppose compression did cause a faulty braze to fail but it shouldn't have.

Edited May 10, 2016 by Jimmy P
Didn't consider it from all angles.

[Warzouz]

• If we say "Friction is mainly responsible for heating", we suppose it's the air against the capsule that causing the heating like when you rub your hands until you feel heating.
• If we say "Compression is mainly responsible for heating", we suppose it's the air which is heated by the pressure created by the arriving capsule and heats back the capsule because it's in contact with or irradiated by a hotter gas.

The process is not the same.

Edit : Further more

• If friction is mostly responsible, the "flames" should come from the capsule itself
• If compression is mostly responsible, the "flames" should come from the compressed air in front of the capsule.

Edited May 10, 2016 by Warzouz

[rcp27]

Friction implies a shear force, which for a fluid is an effect of viscosity. It is fundamentally and entirely irreversible, with a consequent entropy production. Compression increases the pressure (a normal, not shear force) and in principle is reversible, with no entropy rise. In practice it is impossible to achieve compression without incurring viscous losses, but they can be minimized. In a re-entry situation there will be a shock involved, which is very much irreversible, with significant entropy rise, but this is a fluid-internal effect not related to viscous shear in a solid-surface boundary layer.

[rcp27]

3 hours ago, bewing said:

"Friction" is not a fundamental physical process. It is a convenient name for a statistical effect seen when dealing with large quantities of tiny particles. Friction is not one of Newton's 3 laws, and it is not one of the conservation laws.

Adiabatic compression is also not one of Newton's 3 laws. It is also a statistical effect.

If you go and mentally treat them as fundamental processes, then you start to get wacky answers. When you decompress a gas, the little molecules lose energy when they bump into each other -- instead of gaining. So yes, it is "anti-friction". This is another way of Nature telling you that you are not dealing with a fundamental physical process.

There are more laws of physics than Newton wrote. Specifically in this case there are the laws of Thermodynamics, specifically in this case the Second Law. Friction is always fully dissipative so generates the maximum possible entropy rise that the First law allows. Ideal adiabatic compression is fully reversible, so generates no entropy (real world compression is somewhat less than ideal, so generates some entropy, depending on how close to the ideal you get).

All of this can be addressed with classical mechanics and thermodynamics without the need to consider statistical or molecular behavior.

[ferrer]

An inviscid fluid could still be compressed - compression has nothing to do with friction.

[Kerbol Macrosystems]

1 hour ago, bewing said:

"Friction" is not a fundamental physical process. It is a convenient name for a statistical effect seen when dealing with large quantities of tiny particles. Friction is not one of Newton's 3 laws, and it is not one of the conservation laws.

Adiabatic compression is also not one of Newton's 3 laws. It is also a statistical effect.

If you go and mentally treat them as fundamental processes, then you start to get wacky answers. When you decompress a gas, the little molecules lose energy when they bump into each other -- instead of gaining. So yes, it is "anti-friction". This is another way of Nature telling you that you are not dealing with a fundamental physical process.

Friction is not analogous to compression heating.  These are two very different physical phenomena and are not just statistics. Let's start with friction.

Friction is a form of energy transfer.  It involves at a minimum 1 solid surface.  Friction comes from the shear force (kind of like pressure, but parallel instead of normal to a surface) that comes up whenever two things slide across each other.  There are shear forces in flowing fluids, but this leads to the concept of viscosity. When you have 2 solid surfaces your coefficient of friction can be quite high.  We can turn the friction force into an energy by multiplying that force by a distance (E=F*d).  You'll find the mechanical energy that went into overcoming friction comes out of the process as heat.  We typically call friction between 1 surface and a fluid drag.  You'll find that drag causes very little heat in air, because there's just not that much energy required to get air out of your way.  Especially in the upper atmosphere.  You can prove this one to yourself too.  Run a re-entry with a known vessel mass (e.g. the starting capsule at 0.84 tonnes).  Watch for the fire to start.  Next note the acceleration on your vessel by timing how long it takes you to drop 1 m/s velocity.  Divide the mass of the vessel by the time to drop 1 m/s to get your force.  (a=v/t, we set v to 1 m/s and we measured t) then F=m*a

Adiabatic temperature change requires no energy transfer.  That's the definition of adiabatic; no energy or mass transfer.  Under moderate conditions this is governed by the ideal gas law, P*V=n*R*T.  One thing that is not immediately obvious is that this equation has units of energy.  P is pressure, is force / length².  V is volume, which has units of length³.  When you multiply these together you get force * distance, which is energy.  When we said we do things adiabatically our change in energy is 0 and our change in mass/matter (n) is 0 as well.  In order to satisfy these conditions P*V/(n*R*T) must remain constant.  Thus if we keep our volume the same, and drive up the pressure we will have a large increase in temperature as well, possibly to the point of generating a plasma.  

Now why is the air in front of a re-entering capsule getting heated adiabatically?  Isn't it flowing around the capsule?  This one is more of a timing kind of issue.  We have to remember that orbital velocity is ridiculously fast.  So fast, that air molecules can't get out of the way and get compressed.  They're compressed so fast so there's no time for heat transfer to dissipate the energy created by the capsule ramming into the air.  So we have the kinetic energy of the vessel being transferred to the air in front of it.  The energy went up, so P*V went up, and n*R*T went up.  Air is compressible, so we know P went up and V went down, but P*V went up.  On the n*R*T side we know that R is a measured constant.  It's not changing.  Remember we're moving really fast, so there isn't really time for heat and mass transfer in the ram area.  Also we have a detached shock cone, which will have a stagnation volume right in front of the vessel.  So if n isn't going up and R isn't changing, that means T is going up if n*R*T is going up.

 

 

 

Real gasses do depart from the ideal gas law above, but these departures are well studied as an equation of state, or other thermodynamic models.  You can think of the equation being modified to P*V=Z*n*R*T, where Z is a correction function.  The correction function has been studied for over a century, and many models are available depending on the conditions you are studying

[rcp27]

1 hour ago, Kerbol Macrosystems said:

We typically call friction between 1 surface and a fluid drag.  

Viscous surface drag is only one element of the drag on something like an aircraft. Form drag due to flow separation and wave drag due to transonic shocks are also important contributors to drag. For a bluff body like a Mk1 capsule with heat shield leading, the flow will not remain attached, so form drag will be very significant.

1 hour ago, Kerbol Macrosystems said:

That's the definition of adiabatic; no energy or mass transfer

Not quite. Adiabatic means no heat transfer. Work transfer (the other way energy can cross a system boundary) can take place in an adiabatic process. If you are taking an open system approach (aka control volume) then mass transfer is also allowed.

[HorusKol]

23 hours ago, FullMetalMachinist said:

Sorry, but no. The vast majority of reentry heat comes from compressing the air in front of the craft. The ship is moving so fast that the air can't move out of the way, so it gets squished so much that it heats up to the point of being plasma (the "flames"). 

I was trying to not get overly technical.

In any event - regardless of if the heat generated is from friction or compression, the outcome is the same: the faster your travel and the more dense the air you travel through will result in more heat being generated.

[magnemoe]

3 hours ago, HorusKol said:

I was trying to not get overly technical.

In any event - regardless of if the heat generated is from friction or compression, the outcome is the same: the faster your travel and the more dense the air you travel through will result in more heat being generated.

This, and the speed has to be converted to heat, an heavier craft will have more kinetic energy and will generate more heat slowing down. 
An secondary effect is that an heavy craft will brake slower and will have higher speed deep inside the atmosphere where the braking force is higher and you get more heating.