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I Trip to Space -- Can ship with 1g acceleration escape Earth?

  1. Mar 2, 2017 #1
    In another forum, the question was raised, "could a ship with 1G acceleration escape the gravity well of a planet with 1G gravity?"

    A popular response is, if the craft is aerodynamic, it could accelerate laterally until it reached escape velocity and then manage to get to space.

    I don't believe this is a viable response, even if friction is ignored, but I really can't think of a clear, concise way to communicate this gut-feeling.

    Am I right or is this actually a worthwhile answer?

    Thanx!
     
  2. jcsd
  3. Mar 2, 2017 #2

    A.T.

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    So the proper acceleration of the ship (measured with an on-board accelerometer) cannot be greater than 1g in magnitude?

    Do you start from the surface, or at some distance where local gravity is 1g (the surface gravity being higher than 1g)?

    If from surface, can you use it as support, while accelerating along it? Is the planet rotating?
     
  4. Mar 2, 2017 #3

    anorlunda

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    If the ship can maintain constant acceleration no matter what the gravity, then every nonzero acceleration escapes.

    Do you mean a ship with an engine or a thrown object with no engine and no acceleration after it is released?
    When we talk about escape velocity it is the object case we think of.
     
  5. Mar 2, 2017 #4

    Janus

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    Let's assume no atmosphere, a perfectly smooth and round planet, and your craft is resting on a cradle which rolls on a world circling track( to initially support the weight of your craft.)
    You fire up the engine and your craft accelerates along the track. It will continue to gain speed until eventually it reaches orbital velocity, at which point the cradle will be no longer needed. As it continues to fire its engine, it will climb to a higher and higher orbit (it will spiral away from the planet). As it does so, it will trade speed for altitude. But this is okay, because as it gains altitude, the escape velocity that it needs to achieve also decreases. Eventually it will attain an altitude where its speed exceeds the escape velocity for that altitude.
    It doesn't even matter how small an acceleration your craft has. Even if it could only generate 1/100 of a g, it still would eventually attain surface orbital speed, and then after that, slowly spiral away from the planet. The process would just be slower.
     
  6. Mar 2, 2017 #5

    russ_watters

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    As is common with these Internet questions, wording and precision of assumptions matters a lot, but I think even heavily constrained, the answer is still yes:

    If this is a craft with a thrust to weight ratio of 1.0 when standing still at sea level, as soon as it takes off and starts climbing, the thrust increases (rockets do better with less/no atmospheric backpressure) and the weight decreases (Newton's law of gravity), and vertical escape becomes possible.
     
  7. Mar 2, 2017 #6

    CWatters

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    Yes.

    If it goes up vertically accelerating at 1g (=9.8m/s/s) then passengers inside will initially experience 2g. This will reduce asymptotically towards 1g as they get far from the planet. It will remain at 1g until the craft stops accelerating.

    According to the table on Wikipedia...

    https://en.wikipedia.org/wiki/G-force#Typical_examples

    Standing on the Earth at sea level–standard = 1 g
    Saturn V moon rocket just after launch = 1.14 g

    If you can accelerate vertically from a planet then it doesn't actually matter what "g" is on that planet. For example if g was 23.572 m/s/s and the craft was able to accelerate vertically at 0.5g then it would be able to accelerate vertically at 0.5 * 23.572 = 11.786 m/s/s. Any number greater than zero will allow you to escape.
     
    Last edited: Mar 2, 2017
  8. Mar 2, 2017 #7
    Well, I guess he's right then, though it seems intuitively that the vessel could never leave the gravity well. I'm thinking vectors and the total vector going radial to the center of the planet is still 1G and the ship would never be able to get a >1G opposing vector, since the craft is limited to 1G.

    But, I must not be understanding the problem correctly - not outside the realm of possibility.

    Thanx, guys!
     
  9. Mar 2, 2017 #8
    Yes. The ship can apply a 1G vector in any direction, but no more.

    From the surface.

    Well, sorta. The idea is that the ship uses aerodynamics to stay in the air until it gets up the speed to escape - according to the other guy, about Mach 33. And, yes, the planet is rotating.
     
  10. Mar 2, 2017 #9

    CWatters

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    It's a bit of a trick question. It doesn't state under what conditions it can accelerate at 1g.

    If it can accelerate vertically at 1g from the surface of a planet then that's one thing. If they meant it can only accelerate at 1g in free space then that's another. In the latter case it wouldn't even be able to lift off a 1g planet.

    Edit: Actually that's not true. It could accelerate horizontally.
     
  11. Mar 2, 2017 #10
    Really? I'm thinking vector addition here, so if one has a ship with an acceleration vector of 0° and a magnitude of 0.5G going against a gravity vector of 180° at 1G, how does the ship get past that 1G? I mean, when you add the vectors, you always get a negative 0.5G keeping the ship firmly planted on the surface.

    This is a ship with an engine providing a constant 1G. So, I figure, the ship provides an acceleration away from the planet at 1G and the planet's gravity provides an opposite acceleration of 1G. This strikes me as providing a speed of 0 (1G-1G).
     
  12. Mar 2, 2017 #11

    CWatters

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    What do you mean by an engine providing a constant 1g? Think about the units.
     
  13. Mar 2, 2017 #12
    It would be an acceleration of 1G in space, in an essentially gravity-free environment - given that nowhere is it totally "gravity-free." So, let's put it in this manner;

    The ship has an acceleration of 9.8m/s/s.

    You give the answer that I think it should be, that the gravity of the planet of 1G would negate the 1G acceleration of the ship. Even if it were to be on some track that circled the equator enabling it to build up speed. It just seems to me that the ship still has to get an acceleration of 1G+ radially away from the planet and that is simply not happening. But, if the ship reaches escape velocity on that track, what then? It's free from the need to accelerate, since it's already at escape velocity. All it would have to do is release its grip on the track and it would fling off to space.

    But, that flies in the face of my gut saying, "that 1G acceleration is just not going to beat that 1G gravity, dadgummit!"
     
  14. Mar 2, 2017 #13
    9.8m/s/s.
     
  15. Mar 2, 2017 #14

    CWatters

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    In that case I agree with you. It can't lift off vertically from a 1g planet.
     
  16. Mar 2, 2017 #15

    CWatters

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    It would eventually be going very very fast (lot of KE) .... and then let go of the track.
     
  17. Mar 2, 2017 #16
    But, after letting go, would it fling off into space, at escape velocity+? Would this mean that a ship with <1G acceleration could also escape, it would just need to be on the track longer?

    Doesn't this conflict with your prior comment where you agreed with me? Or, is "vertically" the qualifier that makes the difference? I mean, flying off the track when the ship reaches speed seems to make sense, but I can't get the vector addition issue out of my head. The acceleration is still only 9.8m/s/s. That's the most it can put into a direction radially away from the planet. But, the planet, exerting a 1G force in the opposite direction, would negate it.

    Wouldn't it?
     
  18. Mar 2, 2017 #17

    jbriggs444

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    Yes. As @Janus pointed out in #4 above.

    However, if we require that the passengers never experience more than 1 g proper acceleration then this becomes problematic. The craft can never move from its starting point without either moving slightly downward or subjecting the passengers to a momentary acceleration slightly greater than 1 g.

    [Which, on re-reading #16, may be your point]
     
  19. Mar 3, 2017 #18

    A.T.

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    And what kind of trajectory does a mass follow when the forces on it cancel?
     
  20. Mar 3, 2017 #19
    OK, I made a graphic to illustrate to myself what was going on and I think I see my confusion - sorta. In the images, the brown circle is the hypothetical "track" the ship rides on. Light Blue arrows indicate the vector for gravity, Red arrows represent the prior path carried forward to the next time block, Green arrows represent the acceleration vector and Black arrows represent the path traveled by the ship for the current time block. It's been too long since I had Calculus, so the direction of the acceleration vectors is just the starting position tangential to the track; I wasn't about to attempt to calculate the direction over the course of the track segment during the duration of the time represented.

    Anywhoo, my intial take was to consider the Earth as a point source of gravity, so the acceleration of the ship to leave would be away from the point source and then, the gravity of Earth would cancel out the acceleration, giving a net velocity of 0;

    Escape Velocity, Point Source.png

    But, the Earth, closer up, is a sphere. And, since the ship is on a track, the upward force of the track against the ship cancels out much of the force of gravity, until the point is reached where the gravity vector's magnitude fails to bring the actual path back down into contact with the track. But, by that time, the ship's velocity is enough to allow the further application of 1G acceleration to continue off-planet;

    Escape Velocity, Spherical Source.png

    That is, if I've done this right - I think I have. But, still, my gut says it's wrong. The 1G of Earth negates the 1G of the ship, keeping the ship planet-bound. But, using the track seems to allow the ship to leave.

    Although, I note that the Black actual paths, the first couple, anyway, intersect the Brown track. Should I have considered that this represents the ship coming to a stop at the point of the first intersection? If so, then my gut ends up being right and the 1G ship ain't a-goin' nowheres...
     
  21. Mar 3, 2017 #20

    Janus

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    Look up Newton's cannon.
     
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