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Thread: Entering Atmosphere….

  1. #1 Entering Atmosphere…. 
    Time Lord zinjanthropos's Avatar
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    ….without burning up, can it be done? Quota and Reddit had some answers but not many others. Found this simple article..

    https://www.astronomyscope.com/why-d...he-atmosphere/

    Excerpt:
    For a spacecraft to enter the atmosphere safely, it must approach the atmosphere at a precise angle so as not to skip off it and bounce back into space or, worse yet, meet a fiery end.
    Doesn’t sound completely right to me. What about slowing down? Some might argue UFOs go in and out all the time without so much as a tiny glow but will future designs enable craft to enter without becoming a torch? What if anything are they developing while trying to beat the friction game?

    On the other side of the coin….can a spacecraft leaving Earth go so fast as to heat up from friction created? At what speed would that occur? Yet I imagine it would have something to do with type of construction material.


    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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  3. #2  
    Time Lord
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    Quote Originally Posted by zinjanthropos View Post
    ….without burning up, can it be done? Quota and Reddit had some answers but not many others. Found this simple article..

    https://www.astronomyscope.com/why-d...he-atmosphere/

    Excerpt:
    For a spacecraft to enter the atmosphere safely, it must approach the atmosphere at a precise angle so as not to skip off it and bounce back into space or, worse yet, meet a fiery end.
    Doesn’t sound completely right to me. What about slowing down? Some might argue UFOs go in and out all the time without so much as a tiny glow but will future designs enable craft to enter without becoming a torch? What if anything are they developing while trying to beat the friction game?

    On the other side of the coin….can a spacecraft leaving Earth go so fast as to heat up from friction created? At what speed would that occur? Yet I imagine it would have something to do with type of construction material.
    Think it must just be the most efficient way of doing it.It would take a lot of (too much) energy to slow the craft down under it's own "steam"


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  4. #3  
    Forum Ph.D. Double Helix's Avatar
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    Quote Originally Posted by zinjanthropos View Post
    What if anything are they developing while trying to beat the friction game?
    Don't think there is much to be done here. An interesting reference is the development of the SR-71 Blackbird, a strategic reconnaissance aircraft which could fly at 80,000 feet and was simply too fast to shoot down*. The SR-71 could fly at just greater than Mach 3, and the plane was constructed mostly of titanium because it is the only thing which could withstand the heat of friction. Even then it required considerable effort to make this work. And that is only at Mach 3!

    Materials known to humans are not likely to provide a solution for low heat entry from space. Or for anything leaving for space. The friction game is hard to beat.


    "Lockheed SR-71 Blackbird"

    * https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird
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  5. #4  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by zinjanthropos View Post
    ….without burning up, can it be done? Quota and Reddit had some answers but not many others. Found this simple article..

    https://www.astronomyscope.com/why-d...he-atmosphere/

    Excerpt:
    For a spacecraft to enter the atmosphere safely, it must approach the atmosphere at a precise angle so as not to skip off it and bounce back into space or, worse yet, meet a fiery end.
    Doesn’t sound completely right to me. What about slowing down? Some might argue UFOs go in and out all the time without so much as a tiny glow but will future designs enable craft to enter without becoming a torch? What if anything are they developing while trying to beat the friction game?

    On the other side of the coin….can a spacecraft leaving Earth go so fast as to heat up from friction created? At what speed would that occur? Yet I imagine it would have something to do with type of construction material.

    Okay, let's look at this from the idea of using rockets to do the entire return, braking the whole time. This is basically a launch in reverse, and would use as much fuel and associated boosters as a launch. This means just the payload on launch would be equal to the mass of our entire original rocket. For every kg delivered to orbit, several times as much fuel is needed. Which means a larger rocket, more superstructure, and even larger rocket engines to lift the rocket off the ground.
    So unless you are planning to build a rocket several times larger than the Saturn V, just to get your crew into orbit and back, this is a no go.

    We have an atmosphere, and it makes sense to put it to good use. Even if we don't go to the extreme of doing a full powered landing, every kg of fuel used during re-entry means several more kg of fuel needed at take off. Coming in at a shallow angle allows the craft to stay in the thinner part of the atmosphere longer and bleed off more speed before dropping into the thicker regions. You only need to burn enough fuel to lower part of your orbit to intersect the atmosphere at the right angle.

    Even if you had a craft capable of surviving a steeper approach, it isn't really worth it. A steeper approach means burning more fuel, so you drop faster, to intersect the atmosphere at a sharper angle( The most extreme example would stopping orbital velocity completely, dropping straight down. But this would require a delta v just shy of 8 km/sec, using tons of fuel). Once again, you'd be adding to the fuel needed at launch in multiples of the fuel used for the maneuver .

    As to what some might say about UFOs, let's just say that I find it problematic to base ideas on the highly improbable claim that said UFOs are spacecraft.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  6. #5  
    Time Lord zinjanthropos's Avatar
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    Are there more meteors that skip off into space than there are that hit the optimal entry angle and fall to Earth?

    Does NASA envision newer construction materials enabling craft to enter atmosphere without fear of burning up?
    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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  7. #6  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by zinjanthropos View Post
    Are there more meteors that skip off into space than there are that hit the optimal entry angle and fall to Earth?

    Does NASA envision newer construction materials enabling craft to enter atmosphere without fear of burning up?
    Again, the present materials, while they could be improved upon for safety, are sufficient. There is no real advantage to steeper entry angles. The shallower angle gives a longer path through the atmosphere in which to shed speed.
    Again, using the extreme, imagine a craft dropping straight in from a orbital altitude of 300 km. It will hit the "outer edge" of the atmosphere at ~2km/sec And will have another 100 km to slow down before hitting the ground. At that speed, over that distance, it would need to have an average deceleration of ~100g
    It doesn't really matter that the craft didn't burn up if the crew is reduced to jelly.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Genius Duck Dywyddyr's Avatar
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    Quote Originally Posted by zinjanthropos View Post
    What if anything are they developing while trying to beat the friction game?
    It's not so much friction that generates the heat but rather the stagnation layer (air trapped in front of the moving body that compresses and heats up).
    Pointier shapes reduce that layer (but see later). As far back as the 1970s Rolls-Royce were, allgedly, looking at "magnetohydrodynamic fields" to create very pointy shapes way ahead of aircraft to mininmise this layer. (And no, I have no details on how it was supposed to work).
    "In June 1952, H. Julian Allen of the NACA Ames Aeronautical Laboratory addressed the problem of structural heating during atmospheric reentry. His research led to the formulation of the "blunt-body principle," a radical departure from the streamlined aircraft of the early fifties. Allen's work indicated that a blunt shape would be most suitable for a body reentering the earth's atmosphere, since 90 percent of the friction heat would be dissipated through the bow shock wave."
    I'm not entirely sure how this works either: maybe the blunter the shape the quicker the reduction in speed, thus minimising the time spent with a hot stagnation layer, and then dissipation of any gained heat via the bow wave.
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Time Lord zinjanthropos's Avatar
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    I see two types of heat shields while googling around, Ablative and Thermal Soak. Don't quite understand either. Correct me if I'm wrong but Ablative uses gas to dissipate heat. IOW something else has to burn first to produce this gas? What would that be? No idea what is meant by Thermal Soak. Are there other types?

    Is asbestos a main component of reentry shields? It seems as if asbestos would be at least one component, so is it used, banned or unreliable? Or are heat shield materials top secret? Out of curiosity, what would happen to a chunk of 100% asbestos entering Earth's atmosphere? Would it make it to the ground practically unscathed?
    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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  10. #9  
    Genius Duck Dywyddyr's Avatar
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    Ablative heat shields "wear" away. Essentially they sacrifice themselves by degrading and carrying heat away with the bits that that get worn off by the heat.
    Thermal soaking uses materials that retain their mechanical strength at high temperatures for long periods of time, let the craft land and then simply radiate the heat away as the thing cools down "normally". Most people are unaware that lots of "usual" metals are not suited for constant high-temperature, especially when also subjected to high aerodynamic stresses, and suffer from thermal creep and/ or significant (drastic!) changes in internal structure (molecular/ crystalline composition) that would require replacement after each trip (if they actually survived that trip). Thus, specially formulated materials that can withstand those temps and stresses while retainingmechanical strength.
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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  11. #10  
    Forum Ph.D. Double Helix's Avatar
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    A new heat shield developed by NASA may change the story on the problems with heat protection during de-orbiting (1,2), especially for large payloads.

    The technology relies on an inflatable heat shield which can be stowed in a small space, and expanded when required. It is being tested for use on Mars missions, but it seems possible to make it work for Earth re-entry as well. This seems particularly true since it just passed testing for this very application.


    "NASA Heat Shield That Could Land Humans on Mars Aces Space Test Ride"

    1. https://www.cnet.com/science/space/nasa-heat-shield-that-could-land-humans-on-mars-aces-space-test-ride/


    "NASA's Inflatable Heat Shield Survives Atmospheric Trial by Fire"

    2. https://www.ign.com/articles/nasa-in...pheric-reentry
    Last edited by Double Helix; November 11th, 2022 at 08:34 PM.
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