burn up on reentry / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

burn up on reentry

previous 1 2 3 4 next

Move Thread

LAN_403

Meatball

Thu Nov 08 2012, 11:28PM

Registered Member #2401 Joined: Mon Sept 28 2009, 04:25PM
Location:
Posts: 74

Well damn. Now I've got to kick him out. :)

I don't think I understand completely. Bear with me for a second.

So there's a small separation between the nose of the surface, and the compressed layer of super heated air? Right?

The dark lines represent pressurized, heated wavefronts right?

How does that layer end up heating the object if is not directly in contact? The heat just ends up "flowing around" does it not?

Looks like I'll have to show him up tonight.

Patrick

Fri Nov 09 2012, 04:22AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well Meatball, at least youve admitted it, even Galileo got the "ball falling in shortest time" math embarassingly wrong. now with the internet age, our blunders will be enshrined for all to see, presuming humanity exists much longer.

God knows several of mine are here on the forum.

Meatball

Fri Nov 09 2012, 09:26AM

Registered Member #2401 Joined: Mon Sept 28 2009, 04:25PM
Location:
Posts: 74

Sorry guys. But I just may have to go back to what BigBad said in post #5. I refuse to be Galileo on this one.

I talked with Charles again about it tonight. This time, he showed me his "Thermal Protection Systems Manual" for the space shuttle. Everyone on the TPS team took exams on the contents in this giant book, but anyways, he showed me some diagrams, and explained to me in a bit of detail this time.

If the heat comes from the heated compression wave, why then is the rear end of the space shuttle clearly the hottest portion of the body during re-entry? The engine flares and the REAR of the tail (catch that, the REAR of the tail, not the nosecone or underbody tiles) were often the hottest, seeing temperatures of 1400-1700 degrees.

The airflow diagrams show how the air behind the front of the shockwave becomes not just "subsonic" but fully turbulent air by the time it reaches the rear end of the vehicle.

The shockwave "line" we see in the posts above is actually called the boundary layer transition. The boundary layer transition actually acts like a "rough edge or rough surface" on the front of the vehicle, and it creates the same type of disturbances to the air than flows past it. The shuttle underbody and nose was designed to be both blunt and smooth so that as it moved the large area of air beneath it, it would move around the vehicle more smoothly. This allows the disturbances that occur near the body to be either quickly damped or not occur at all by the time that air reaches the tail end. The BLT marks the point where the air becomes outright turbulent (where disturbance goes un-damped) after the shockwave, by the time the air reaches the rear.

The more blunt and smooth the front surface of the vehicle is, the further back the turbulent is observed. You can even see in the small photos in BigBad's post how an early protrusion would let the turbulent air reach areas much closer to the front of the vehicle in general.

So the idea is, the further back the turbulent air, the less heating of the vehicle overall. It was solely the interaction of this high speed turbulence with the body of the shuttle that caused the actual vehicle heating.

Yeah, friction. The rear of the vehicle was nominally 3X hotter than the body at the nose or just under the belly. It all comes down to that turbulent air after all. That's the engineer's primary concern as the source of heat. Shaping the shuttle was about keeping as much of that turbulent air from going un-damped, or from reaching the majority of the vehicle body. So yeah, not DIRECT friction that would occur under the vehicle. That's not the friction they're talking about when they say "Its friction".

The turbulent air was actually very difficult to predict, but NASA figured it out and actually became very good at predicting where the super hot areas on the shuttle would occur. It was sometimes a matter of a couple of feet that resulted in a change of 300 degrees in difference. That's how sharp the turbulence is.

I've got the diagrams showing re-entry temps for all over the shuttle, the airflow graphs and the freakin' manual to read. And I had one of the guys on the team explain it all to me. The diagrams are something I'd be glad to show anyone if they stop by and if they're an American citizen. If it weren't for ITAR I would just post 'em here - Though No way I'm risking it.

A forum of doubters isn't worth it. Now I've got the book itself. And the guy who was trained by the man that wrote the book.

EDIT: Found this - I'm glad its already on the net.

And Patrick: Now just WTH?

Just find us another single vehicle like Discovery which made 39 trips to orbit and back, and do it without all the "kinks" that take decades of testing to work out. Find me a Russian capsule that has the loading capabilities of the flippin' space shuttle. And if you find such a vehicle, give it to me so I can laugh at Your assumed "ego and pride" - because well, you're an American.

Think of all the men and women work hours that went into the design and implementation of such an amazing machine - and you sit back and call it a crap design. Well sheesh where were you when they were hiring project managers at the time? Oh, yeah. You weren't even born yet. Give the design a break will ya.

AndrewM: I think the book itself beats out whatever wikipedia says on this one.

Pinky's Brain

Fri Nov 09 2012, 11:05AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Your conclusion is non sequitur ... from the link you posted :

"Prediction and control on boundary layer transition in
hypersonic flows are crucial to the design and safety of
planetary entry vehicles. Since turbulent heat transfer rates
can be up to five times higher than laminar heating rates"

For the heating to be from friction the AIR around the rear surfaces would have to be hotter ... the fact that the surfaces are hotter says little, the increased convective heating will shift the balance with radiative cooling on it's own.

Steve Conner

Fri Nov 09 2012, 11:48AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Meatball wrote ...

Well sheesh where were you when they were hiring project managers at the time? Oh, yeah. You weren't even born yet. Give the design a break will ya.

Actually I believe Patrick turned up for the job interview shouting abuse and brandishing an empty bottle of Tequila.

So he ended up working for Fairchild instead.

It seems to me like you're arguing over the semantics of the word "friction" rather than the design of the space shuttle. Broadly speaking, when the air rubs against a surface, that's friction. When the air molecules rub against each other, that's compression.

Meatball

Fri Nov 09 2012, 03:39PM

Registered Member #2401 Joined: Mon Sept 28 2009, 04:25PM
Location:
Posts: 74

Pinky I can think of many instances where heat is created between two surfaces which are not hot. Why would the turbulent air have to be hot? So how is air (especially turbulent) rubbing against the thermal tiles not friction?

What then is the compression based explanation for the heating on the rear of the tail?

No one has tried explaining that one yet.

Patrick

Fri Nov 09 2012, 05:05PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Steve Conner wrote ...

Actually I believe Patrick turned up for the job interview shouting abuse and brandishing an empty bottle of Tequila.

no! it was an empty bottle of jager, and i was punching kittens, kicking puppies and throwing babies off cliffs.
i guess they didnt like that, but those evil idiots at microsoft hired me right away.

Meatball wrote ...

... So how is air (especially turbulent) rubbing against the thermal tiles not friction?
...No one has tried explaining that one yet.

well the air is rubbing up on those tiles, but the rubbing itself is insuffcient to explain the heat rise, once the molecules are super hot (from that compressed bow shock), they they transfer heat to the leading surface.

The compression is where most of the significant heat rise is coming from, with minor contribuitions from adhesison, viscous forces, mice flatus, waffles, friction and peanut butter. Thus, (as per BigBad and AndrewM's points) we can presume this process to be almost perfectly isentropic.

Steve Conner

Fri Nov 09 2012, 05:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Patrick wrote ...

i guess they didnt like that, but those evil idiots at microsoft hired me right away.

:D

I think it goes like this: Compression in the supersonic shock wave heats and dissociates the air until it becomes a glowing plasma. The craft is basically flying along inside a ball of fire. The plasma might not touch the craft directly, but it still transfers heat to the craft by radiation and convection across the boundary layer. The aerodynamicists are trying to keep this boundary layer thick and laminar, to keep the fire as far away from the skin as possible as it streams backwards from the bow shock.

Pinky's Brain

Fri Nov 09 2012, 05:35PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Meatball wrote ...

Why would the turbulent air have to be hot?

The air is already hot, but if friction was the cause for the wings to get hotter then the air would have to be hotter there as well ... takes two to do the friction tango.

wrote ...
So how is air (especially turbulent) rubbing against the thermal tiles not friction?

The rubbing is friction, the words are synonymous ... but that doesn't mean it causes significant heating ...

wrote ...
What then is the compression based explanation for the heating on the rear of the tail?

No one has tried explaining that one yet.

I tried, but it didn't come across ... lets assume the insulation is 100% effective and the temperature of the surfaces are only dependent on the balance between convective heat transfer from the air to the tiles and the radiative heat loss. The air heated at the bow flows past the surfaces of the vehicle, losing some heat but conceivably can stay significantly hotter than those surfaces. Then when it gets turbulent it can transfer more heat (turbulence breaks up the boundary layers which act as insulation) thus shifting the energy balance and making the underlying surfaces hotter even if no new heating of the air occurs due to friction.

Now I'm not saying this is happening, unlike others here I don't have a PhD, I'm saying it's a potential explanation of what is happening which is compatible with what you linked ... your conclusion didn't follow, but may still be right.

Patrick

Fri Nov 09 2012, 05:39PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

MeatBall is also presuming the friction is a collision like process, i beleive he is ignoring the fluid shear mode of friction. Which would be inconcsequential anyway.

Steve Connors expalination is wordy but accurate.

careful there MeatBall, i let you slide with those first few mentions of my favorite space shuttle discovery, but remember
Columbia and its crew of seven never came back, as a direct result of poor decision making by idiots safely on the ground.
The ocean, sky and mountains can kill men easilly and without remorse.

previous 1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob