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Thread: Entering the atmosphere with a twist.

  1. #1 Entering the atmosphere with a twist. 
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    I have a general question i cannot find an answer anywhere.
    I know objects that enter atmosphere from space burn up due to friction with the compressed air. Will objects burn up if they enter the atmosphere slowly ? If slow moving objects don't burn up why doesn't NASA use this to their advantage instead of putting ceramics on spaceships weighing them down ? I understand that NASA might have to use some thrusting to escape the orbit but do they really need to move so fast to escape the orbit ?


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  3. #2  
    Geo
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    It depends what you mean by slow.

    Orbital velocity is 7.8km/s. Escape velocity is 11km/s. Objects re-entering the atmosphere are travelling slower than 7.8km/s - maybe 7-7.5km/s?

    The Carbon and Silica heat shields do a very good job of dissipating the heat on re-entry. You can take a red hot block of this material out of an oven and pick it up with your fingers.


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    I suppose, it is just too expensive. You would need to burn a lot of fuel to keep the descend velocity low enough. To me it seems that it is easier to use the atmosphere for breaking and make sure the friction does not do any damage.
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    Quote Originally Posted by Geo
    It depends what you mean by slow.

    Orbital velocity is 7.8km/s. Escape velocity is 11km/s. Objects re-entering the atmosphere are travelling slower than 7.8km/s - maybe 7-7.5km/s?
    You got that slightly backwards, objects re-entering from orbit will be moving slightlyfaster than orbital speed.

    Here's why. You do a burn to drop your velocity to just below orbital speed. Your craft starts to drop towards the planet. But like any object that goes from a higher to lower gravity potential, it has to gain kinetic energy to compensate, so it gains speed as it falls.
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  6. #5  
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    Quote Originally Posted by Geo
    It depends what you mean by slow.

    Orbital velocity is 7.8km/s. Escape velocity is 11km/s. Objects re-entering the atmosphere are travelling slower than 7.8km/s - maybe 7-7.5km/s?

    The Carbon and Silica heat shields do a very good job of dissipating the heat on re-entry. You can take a red hot block of this material out of an oven and pick it up with your fingers.
    Thnx i guess this answers my question because i didn't know you had to travel so fast to escape the orbit. And i have seen this ceramic on some TV program were guy picked up the ceramic that was heated to 3000F with his bare fingers.
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    To go from sitting on the launchpad to orbit you need to use as much energy as it takes the rocket/shuttle/etc to takeoff...


    to go back to still on the grounud, you need to use the exact same amount of energy.

    So the cheap way is to use atmospheric drag and turn your velocity into heat.
    The expensive way is to do a Rocket launch in reverse. (with the velocity being modified by expenditure of fuel)
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    Quote Originally Posted by musicalaviator
    To go from sitting on the launchpad to orbit you need to use as much energy as it takes the rocket/shuttle/etc to takeoff...

    to go back to still on the grounud, you need to use the exact same amount of energy.
    Incorrect. A large part of the energy required to get an object to orbit is the energy required to get some of the fuel part of the way there. That fuel is no longer part of the object once it is in orbit.
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  9. #8  
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    Musicalaviator was technically correct. No fuel depots in orbit yet though.
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  10. #9 Re: Entering the atmosphere with a twist. 
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    Quote Originally Posted by webfab
    I have a general question i cannot find an answer anywhere.
    I know objects that enter atmosphere from space burn up due to friction with the compressed air. Will objects burn up if they enter the atmosphere slowly ? If slow moving objects don't burn up why doesn't NASA use this to their advantage instead of putting ceramics on spaceships weighing them down ? I understand that NASA might have to use some thrusting to escape the orbit but do they really need to move so fast to escape the orbit ?
    I was hoping you'd be talking about the possibility of using a spinning/gyroscopic affect to guide the object during re-entry. Kind of like how rifled bullets spin, or (an American) football player puts a twist on the football as they throw it. Y'know: "with a twist"?
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  11. #10  
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    I wonder if the soyuz-style landers could do that?

    Also, I understand helicopter pilots are taught that in case of engine failure they should zero rotor pitch (to keep rotating) and just before impact crank it to spend with what's left of rotor momentum. Perhaps a lander could do something similar?
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  12. #11  
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    Quote Originally Posted by Pong
    I wonder if the soyuz-style landers could do that?

    Also, I understand helicopter pilots are taught that in case of engine failure they should zero rotor pitch (to keep rotating) and just before impact crank it to spend with what's left of rotor momentum. Perhaps a lander could do something similar?
    It's hard to say whether this would be beneficial or not. The down side is that, if the entry craft is spinning and moving within its orbit, both, then its total amount of momentum, between the angular momentum and the linear momentum is greater. That should mean it experiences more heat/air friction than it would otherwise, without really slowing down any faster.

    The upside is that you can be certain of its orientation relative to the atmosphere, independent of aerodynamics. That means you can choose exactly where all the friction will occur.

    Oh... and the other downside is the effect it might have on the crew, if there are human passengers on board. .... Also I'm not sure if this is what you meant. Maybe you specifically meant there should be something similar to a helicopter rotor?
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