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Question about Space shuttle escaping Earth

  1. Feb 8, 2015 #1
    My understanding is that for space shuttle to escape earth it needs to travel at a certain high velocity. So, what happens to the space shuttle if it doesn't reach the escape velocity at edge of earths atmosphere to space? The question i'm asking and the answer i'm seeking is something like this, A bullet fired from a real gun will go through a wall, but a plastic ball bullet fired from a airsoft gun will be bounced back from the wall.
     
  2. jcsd
  3. Feb 9, 2015 #2
    It does not need (and doesn't) to reach the escape velocity.
    The shuttle does not escape Earth but moves in an orbit around the Earth. It needs to have the proper velocity for the planed orbit. If the velocity is different it will go in a different orbit. It may be an orbit intersecting the Earth (so it falls down) or not.
     
  4. Feb 9, 2015 #3

    sophiecentaur

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    The amount of energy needed to get up into an orbit that's sufficiently above the atmosphere to avoid drag is very small, compared with the energy needed to escape and never fall back. Afaik, no launches have ever carried enough fuel for this (not feasible). The Shuttle was not designed to do this and was intended to reach Low Earth Orbit, only. To get far away from Earth, it's always necessary to do a 'slingshot' path that involves passing near another planet and using some of its kinetic energy 'for free'.
     
  5. Feb 9, 2015 #4

    russ_watters

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    So, the actual answer to the corrected question is it would just follow a ballistic/parabolic trajectory back to earth, like our first spacecraft did.
     
  6. Feb 9, 2015 #5

    phinds

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    To expand on nasu's answer just in case you are not clear, "escape velocity" at the surface of the Earth is a very high velocity indeed and as far as I am aware not man made object going to space has ever had that velocity at the surface. It is a ballistic velocity. That is, if you expend all of the energy at liftoff and have no further rocket firing, then escape velocity is what you must achieve right then and there, else you will eventually fall back to Earth.

    Rockets start off REALLY slowly and use the continuing thrust to keep them moving out of Earth's gravity well. The escape velocity from the Earth's gravity well decreases as the square of the distance from the center of the Earth so when they get higher and higher it gets easier and easier to escape from where they are and that's how rockets make it to the outer planets.

    If we had to shoot a rocket with true escape velocity at the surface of the Earth, any astronauts inside would become a thin red paste, so it's good that we don't have to do that.
     
  7. Feb 9, 2015 #6

    jbriggs444

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    As sophiecentaur is surely aware... If one were concerned only with the Earth's escape velocity, such slingshot paths would not be needed. To get a craft far enough away from Earth to benefit from such a maneuver, one would have had to apply 99.99% of the energy required to escape Earth's gravity already. Escape velocity from the earth is only about 11 km/sec and is achievable. Escape velocity from the Sun (at the Earth's orbital radius) is about 42 km/sec. That is a rather more daunting challenge and one which 'slingshot' maneuvers can help with.
     
  8. Feb 9, 2015 #7

    sophiecentaur

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    Rockets are never launched off in one go. When they reach an appropriate distance, they change direction (parallel with the ground) so that they take up an orbit and don't just plough back in because they are never given the energy to 'escape' - just to reach the planned orbit height. If they are planning to go somewhere else, they leave this holding orbit at that appropriate time. But there is never enough fuel to take them away from the Sun.
    Escape Velocity is only a loose term because rockets are not fired like guns so they don't actually ever go that fast. - they don't need to because the motors work over an extended period.
     
    Last edited: Feb 9, 2015
  9. Feb 9, 2015 #8

    jbriggs444

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    Although one cannot apply all of the thrust to achieve escape velocity in a single impulsive burn at the bottom of the atmosphere, you do want to use most of your delta-V as close to the Earth as you can manage, after getting above the bulk of the atmosphere. This gives you maximum benefit from the Oberth effect. The Oberth effect is that a rocket provides energy to its payload most efficiently at high velocity. If you allow the trajectory to rise before you expend your thrust, you also allow the craft to go slower than it otherwise might and thereby lose efficiency.

    A rocket that hovered on its engines as it slowly climbed out of the Earth's gravity well would be hideously inefficient.
     
  10. Feb 9, 2015 #9

    sophiecentaur

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    Yes, of course you are right to point that out. I conflated the two quantities without thinking. The escape velocity from Earth is not a particularly useful figure, though - except when you are considering movement within the Earth's orbit. In order to 'go anywhere' you are in the ballpark of the Sun's escape velocity.
     
  11. Feb 9, 2015 #10

    phinds

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    Agreed, for sure.
     
  12. Feb 9, 2015 #11

    phinds

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    That's certainly true if they travel directly out from the sun and don't use a slingshot but we HAVE sent things out that aren't going to come back so we have ultimately escaped the sun. Whether or not Voyager 1 will escape the Milky Way (given a rather significant amount of time :smile:) I don't know.
     
  13. Feb 9, 2015 #12

    QuantumPion

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    The Milky Way escape velocity from the solar system's radius is something like ~500 km/s while the solar system is only traveling ~200 km/s. So no probes would have anywhere near enough velocity to escape the galaxy unless they managed to get an extremely lucky slingshot off another star in the distant future. It might be possible if such a maneuver was specifically targeted from the start of the mission.
     
  14. Feb 9, 2015 #13
    Thank you all for the feed back :)
     
  15. Feb 9, 2015 #14
    So at the edge of earth to space, it's less gravity? And also, Space shuttle needs approx. 11 km/sec velocity at edge of earth to space? If it doesn't have the approx. 11 km/sec velocity, would the space shuttle just stall and fall back to earth or what happens?
     
  16. Feb 9, 2015 #15

    phinds

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    It's not at all clear to me what you are asking

    What do you consider to be "the edge of space". I've seen various definitions.

    If you mean the Karman Line for example, the difference between escape velocity there and at the surface of the earth is almost negligible.

    Whether something falls back depends on whether or not it (1) has achieved a stable orbit or (2) is still under enough power to overcome gravity at its height or (3) has enough ballistic velocity to escape from its altitude.
     
  17. Feb 9, 2015 #16

    SteamKing

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    'Stall' is an aerodynamic phenomenon, which is the loss of lift generated by a wing. A rocket like the Space Shuttle does not depend on aerodynamic lift to achieve orbit; it merely needs to achieve orbital velocity by using its rocket engines. This would be true even if the earth had no atmosphere.
     
  18. Feb 9, 2015 #17
    Imagine an apple and it's skin (The skin of the apple is the edge of earth to space that i mean). For earth what is the skin like an apple's skin? Is earth's skin the Exosphere, thermosphere, mesopause, and/or stratosphere. How much gravity is on these layers of atmosphere of earth, Does the speed of space shuttle increase or decrease in these layers of the atmosphere? If the speeds of the space shuttle decreases or increases, what are the purposes of it?
     
  19. Feb 9, 2015 #18

    rcgldr

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    Note that the Apollo missions launch speed was around 10.4 km / sec to transition into lunar injection orbit. The Voyager 1 and 2 launch speeds were around 14 km / sec, well beyond Earth escape velocity, and the sling shot maneuvers have increased Voyager 1 speed to about 17 km / sec, and Voyager 2 speed to about 15.65 km / sec.

    As mentioned the Space Shuttles only achieved low earth orbit at about 7.8 km / sec, depending on the target orbit, for example the space station orbits at 7.66 km / sec, so the Shuttle went at a bit faster speed with an elliptical orbit that intercepted the space station, where a second thrust adjustment was made to sync up with the space station's orbital path. These low earth orbits are in the outer fringes of the atmosphere, with sufficient drag that would cause them to eventually re-enter the atmosphere if not for an occasional boost.
     
    Last edited: Feb 9, 2015
  20. Feb 9, 2015 #19

    Nathanael

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    The atmosphere is certainly quite massive, but I'd say it's a good approximation to say that all of the mass of the Earth is bounded by the Earth's surface.

    I'd estimate the mass of the atmosphere as something like a millionth of the mass of the Earth.
    (Since the Earth is so massive, this is still a lot of mass... but it's still only a millionth.)
     
  21. Feb 9, 2015 #20
    I understand that rockets propel something because of, "Isaac Newton's third law of motion: Every action produces an equal and opposite reaction." (Kind of away from the original question, just curious) But, how does that action produce equal and opposite reaction. What is happening? what is happening in molecular level?
    I understand that rockets propel something because of, "Isaac Newton's third law of motion: Every action produces an equal and opposite reaction." (Kind of away from the original question, just curious) But, how does that action produce equal and opposite reaction. What is happening? or what is happening in molecular level?
     
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