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Similar to pumping your feet on a child's swing can a satellite run up and down a pole to produce a forward momentum?

Red dots is the satellite's position running up and down the pole on an elliptical orbit around Earth. The pole uses solar vanes at each end to keep vertical to Earth. The pole could use magnetic entrapment to suspend the satellite not to have touching or moving parts when moving up and down the pole similar to a mono rail.

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Muze
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    Hmm... a spacecraft with a dynamic quadrupole moment... how interesting! The whole thing together (dot plus rod) is "the satellite", and the whole thing has one center of mass, but the dot-plus-rod satellite will now vary its distribution of mass around that center. Certainly there are some interesting possibilities using tidal effects and/or Earth's large J2 in some clever way; after all, Earth's one natural satellite is slowly boosting itself over time. Hmm... – uhoh Oct 26 '18 at 00:13
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    @uhoh are we looking to engineer a complete spacecraft, or to prove a theoretical concept ? If we are just assuming that the spacecraft can control his position on the rod, I don't think the implementation details have ANY consequences on the question for that particular mater. – Antzi Oct 26 '18 at 06:22
  • @uhoh The Moon's slow escape from Earth is a consequence of tidal forces; the Moon is massive enough to raise a tide; due to the phasing of the tidal peaks (mass redistribution on Earth's surface) there is a torque acting to transfer angular momentum from Earth's rotation to the Moon's orbit. It only works because of the Moon's large mass, and even then, only over astronomical time scales. Not an effect usefully transferable to an artificial satellite. – Anthony X Oct 29 '18 at 01:33
  • @uhoh my comment was in reference to yours of 2018-10-26 0:13:01Z. I may have misunderstood your point; the point I was seeking to make is that no artificial satellite is going to do what the Moon does i.e. raise its own orbit by raising a tide on Earth. – Anthony X Oct 30 '18 at 00:04
  • @AnthonyX I agree with you that the Moon's orbit and raising mechanism is a poor or at least inadequate model for this problem for sure. This one needs to be treated as a separate problem for several reasons. – uhoh Oct 30 '18 at 01:27

2 Answers2

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I am going to answer the question as asked, ignoring the comments.

The system in question is a sphere pierced by a rod. Some internal mechanism acts to move the two parts relative to each other.

Let's look at the two limiting cases - massless rod and massless sphere.

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For the massless sphere, the c.g. (red star) is in the center of the rod, it stays where it is, and the sphere slides up and down relative to the c.g. (this is what you have drawn in the question) For the massless rod, the c.g. is in the center of the sphere, and the rod slides up and down relative to the c.g.

You have provided no info about the relative masses of the objects. But any combination of massive objects will result in a case between these two. The c.g. will stay where it is, and the objects will move about it; how much depends on their relative masses.

It is the c.g. that is in orbit, and moving two objects about their c.g. can do nothing to change that orbit. Only the application of an external force can do that.

What about the swinging kid? He is pulling on the chains. That is an external force.

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I urge you to read this article about swinging (where the picture came from), it's very interesting and explains it well.

The comments talk about quadrupole moments, tidal forces, etc, but I don't think that is what you were really asking about. At least, that is not what the question itself says.

Organic Marble
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  • "Only the application of an external force can do that." Can you address why the Moon's orbit is moving farther from Earth all the time without an external force? I think this answer is an oversimplification. Also "quadrupole moment" is just a way to mention that the ball plus rod has moments. It's not something added to the problem, it's just a way to describe the problem. – uhoh Oct 29 '18 at 00:52
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    In both your limit cases, the mass distribution of the system (the rod and the sphere) relative to its center of mass is fixed and cannot be changed. But it can be changed when both the rod and the sphere have mass, and that is the whole point of moving them relative to one another. So the fact that the orbit cannot be changed in your limit cases does not mean that it cannot be changed in the general case. – Litho Nov 20 '18 at 11:01
  • @Litho is right, but still this answers the original question. I think everyone here is OK with generalizing it, in order to address all possible mechanisms that might achieve the goal of acceleration without reaction mass. – Everyday Astronaut Apr 04 '19 at 20:19
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In theory, you can change your spacecraft's orbit without expending reactive mass, just by moving its parts relative to one another. Instead of a rod and a sphere, let's consider a rod and two identical spheres which can move along it, and let us assume that the spheres always move symmetrically to each other. Instead of keeping the rod pointed to Earth, let it keep its direction in an inertial frame.

When the rod is perpendicular to the direction to Earth's center, move the spheres from the rod's center to its edges. This way, the spheres end up slightly further from Earth's center than they were, so their potential energy increases. Since the spheres move symmetrically, the rod's position does not change, so its potential energy does not change either.

Later, when the rod is pointed to Earth's center, move the spheres from the rod's edges back to its center. If each sphere's mass is $m$, the rod's length is $2l$, and the distance from the rod's center to the Earth's center is $R$ at the moment, the total potential energy of the spheres changes from $-\mu m(\frac{1}{R+l} + \frac{1}{R-l})$ to $-\frac{2\mu m}{R}$, where $\mu$ is Earth's gravitational parameter. And we can see that $$ -\mu m(\frac{1}{R+l} + \frac{1}{R-l}) = -\frac{2\mu m R}{R^2-l^2} < -\frac{2\mu m}{R}, $$ so the potential energy increases again.

By repeating this again and again, you can move your spacecraft to a higher orbit. Of course, it's going to be very slow, unless the rod's length is comparable to the orbit's radius.

Edit: uhoh has pointed out that as spacecraft's orbit rises, its orbital angular momentum increases, so this answer seems to break the law of conservation of angular momentum.

The answer assumes that the spacecraft's orientation in an inertial frame stays constant. However, the spacecraft is not spherically symmetrical, and Earth's gravity applies torque to it. For example, when the spacecraft is in the upper left or in the bottom right positions in the picture below (not to scale), the torque is in the direction opposite to the direction of the orbital rotation, since the force acting on the forward-pointing (relative to the orbital motion) sphere is smaller than the force acting on the backward-pointing sphere.

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And this torque's effect is not negated by the opposite-directed torque in other parts of the orbit: when the spacecraft is in the upper right or in the bottom left positions and the torque is in the same direction as the orbital rotation, its magnitude is smaller, since the spheres are pulled close to the center. So the torque's effect accumulates over time, and in order to keep its orientation constant in an inertial frame, the spacecraft has to have some way to compensate this torque. This compensating torque is what explains the increase in the spacecraft's total angular momentum. (Or, if there is no compensation, this increase in the orbital angular momentum happens together with the opposite change in the spacecraft's rotation around its center, so the total a.m. stays constant. I mean, the procedure described here doesn't require the spacecraft's orientation to stay constant, it just requires that sometimes the spacecraft is "horizontal", and sometimes it's "vertical". But I guess that if we don't try to compensate the torque, the spacecraft will end up always pointing to Earth, so the procedure won't be applicable anymore. On the other hand, moving the spheres along the rod changes the spacecraft's moment of inertia, and therefore changes its rotation speed, so some additional analysis is needed to figure out what happens in this case.)

How can the spacecraft compensate the torque? Well, in theory, it can do it with reaction wheels. Of course, any realistic reaction wheels would get saturated quickly, before a significant change in orbit, but I said from the beginning that this whole approach is not practical. The answer's purpose was to show that raising/lowering orbit without expending reaction mass is possible in principle, not that it's doable in practice.

Or, as Muze suggests, one can use solar vanes to keep the orientation. But in this approach one needs to make sure that the solar pressure does not negate the change of orbit.

Muze
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Litho
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    This is an interesting solution for a spherical gravity field and doesn't require $J_2$, however there may be a problem here. Moving the spheres from the center to the edges keeps the center of mass at the center of the rod, but not the center of gravity because gravity isn't uniform. The trick that makes the $(\frac{1}{R+l} + \frac{1}{R-l}) \ne \frac{2}{R}$ thing worth mathematically neglects that it's the center of gravity rather than the center of mass that would remain at a given orbit. I'm afraid that this might be a perpetual motion machine-type solution. I'm not 100% sure though. – uhoh Nov 03 '18 at 09:31
  • I still think that leveraging the Earth's strong $J_2$ with a modulated quadrupole moment may be the best mechanism, but I'm also not 100% sure of this yet... – uhoh Nov 03 '18 at 09:35
  • @uhoh About center of gravity: does it matter? During rigid body's usual orbital movement, its total energy (potential+kinetic) is constant. During these maneuvres, it increases. So it will increase over time. Or do you mean that it may increase in such a way that, for example, a circular orbit doesn't stay circular; instead, its periapsis gets lower while its apoapsis gets higher at a higher rate? I would expect that since we move the spheres to the center at two opposite poins of the orbit, the effects on the orbit's shape would negate each other. But maybe not. – Litho Nov 03 '18 at 10:18
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    @uhoh It's not a perpetuum mobile, the extra energy comes from the mechanism which moves the spheres. It perfroms work againgst Earth's gravity. About using $J_2$ being more efficient: quite possible. As I said, the procedure I suggested is very slow, unless you build a huge spacecraft. – Litho Nov 03 '18 at 10:23
  • With your spherically symmetric Earth, it is impossible to exchange angular momentum. In your scenario the satellite's angular momentum changes but the Earth's doesn't, which seems to violate conservation of angular momentum. So I don't see how your proposed mechanism can work. With $J_2$ you can exchange angular momentum with the Earth's rotation about its own axis. – uhoh Nov 03 '18 at 10:54
  • @uhoh One need to take into account Earth's angular momentum not w.r.t. its center, since its center moves slightly due to satellite's attraction, but w.r.t. of the common center of mass of Earth and the satellite, and it does change. When the spheres are extended and the rod is at an angle (neither aligned nor perpendicular) to the direction to Earth, the resulting force the satellite acts on Earth with is not quite aligned to the direction between the centers of mass of Earth and the satellite, so it changes Earth's angular momentum w.r.t. their common center of mass. – Litho Nov 03 '18 at 11:30
  • @uhoh I.e., the angular momentum of Earth's "orbit" around the common CoM of Earth and the satellite changes similarly to how the angular momentum of Moon's orbit changes due to tides on Earth. (The difference, of course, is that the Moon causes the tides on Earth, while the satellite controls its own "tides".) – Litho Nov 03 '18 at 11:33
  • No, what is happening with the Moon is different than what's happening in your answer. As soon as you used the word "tides" you've moved to a deformed, non spherically symmetric Earth. It is the torque on that quadrupole moment thereby changing the rotation rate of the Earth about its axis that allows the Moon to move. Apples and oranges. – uhoh Nov 03 '18 at 11:40
  • @Litho I think this is possible yes Earth isn't symmetric and neither is the orbit and the Moon is a driver not an inhibitor. I haven't seen any math opposing this model. In a symmetrical elliptical oblong orbit and/or using the Earth bulge on approach as the satellite as it comes closer to the Earth it would PULL the 2 weights together and PUSH as the satellite distance from the Earth may create lateral momentum, but I'm not sure that is why I asked the question. I'm sure there may be also a way to distribute the weight to use the Moon's gravity as an extra gradient as well. – Muze Nov 03 '18 at 15:43
  • @uhoh Perpetual motion is often tagged to devises that are not fully understood and is often misused. In theory the energy to move the motors to move the weights inadvertently would propel the satellite There still would be a power source like solar. – Muze Nov 03 '18 at 15:51
  • Also if you look at the linked question it has 5 weights, adjustable pole, and solar vanes. etc. I wrote this question to be a simple as possible. – Muze Nov 03 '18 at 15:59
  • Your analysis is incorrect. The spheres can't move symmetrically when the rod is vertical, since each feels a different force of gravity, so an equivalent force on both will not move the two masses the same distance. You'll end up with both at the center of gravity, not at the center of mass, and the potential energy doesn't go up. – Chris Nov 06 '18 at 22:57
  • @Chris The mechanism which moves the spheres doesn't have to apply equal forces to them. It could control the speed of their movements relative to the rod. For example, the spheres have cogwheels inside, the rod has cogs, and the mechanism controls the speed of cogwheel's rotation. – Litho Nov 07 '18 at 11:23
  • @Chris Even if the mechanism does apply equal forces to the spheres, the difference of magnitudes of total forces acting on the spheres (gravity+mechanism's) does not depend on the magnitude of the mechanism's force, so if the mechanism works fast enough, then the point where the spheres meet can be arbitrarily close to the position of the system's CoM at the moment when the mechanism starts working. (As long as we apply Newtonian mechanics, of course. If we go into relativistic speeds, it will complicate things.) It doesn't have to be the system's original CoG. – Litho Nov 07 '18 at 11:24
  • @Chris Or one can look at it like this: the movement of system's CoM follows Newton's second law, $F=Ma$, where $M$ is the total mass of the system (spheres and rod) and $F$ is the total external force (i.e., gravity) acting on the system. Since $F$ is limited, if the time of the mechanism's work can be chosen to be arbitrarily small, the vertical displacement of the CoM during this time can be made arbitrarily small as well. – Litho Nov 07 '18 at 11:24
  • @Litho It does have to apply equal forces to them, since the rod is by construction massless, and so by simple application of Newton's laws the forces are equal. If the rod is not massless, then you can move the center of gravity of the two masses, but only by moving the center of gravity of the rod in the opposite direction. The distance from the center of the earth at which the two spheres meet is, if you do the math correctly, independent of the magnitude of the force. – Chris Nov 07 '18 at 17:04
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    @Chris Please show that math, then. – Litho Nov 08 '18 at 08:24
  • @Litho There would be a series weights on a .5 km pole made of solar vanes instead of massless balls. see linked questions. – Muze Nov 15 '18 at 16:48
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    @uhoh I have been thinking on this. There is still a perigee and apogee that brings the satellite close to the Earth 2 times. As the satellite approaches the Earth the 2 weights would retract and then expand again at the pinnacle of the satellite's apogee and perigee. Energy is would be expanded and the natural orbit changed creating lateral momentum. I wish I could show some math. – Muze Nov 16 '18 at 16:06
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    @uhoh I added expanation about angular momentum conservation. – Litho Nov 17 '18 at 09:36
  • @Litho okay thank you for the ping! I'll read through it and think some more as well. I think you are talking about an exchange between orbital angular momentum of the spacecraft (around the center of the Earth) and rotational angular momentum of the spacecraft (around the center of the spacecraft) and leaving rotational angular momentum of the Earth (around the center of the Earth) unchanged. I'm not sure it works that way. I think when you are conserving angular momentum, all components all have to be defined around one center of rotation. – uhoh Nov 17 '18 at 09:45
  • Right, Earth's rotation is not affected. About the same center: there's a theorem that a system's total angular momentum w.r.t. some origin is equal to the sum of the a.m. of the system's CoM w.r.t. this origin (that is, the angular momentum the system would have if all its mass was concentrated at the CoM and it was moving with the same velocity) and the system's a.m. w.r.t. its CoM. So the total a.m. of the spacecratf w.r.t. Earth's center is the sum of its orbital a.m. w.r.t. Earth's center and its rotational a.m. w.r.t. its CoM. And this sum is what we apply the conservation law to. – Litho Nov 17 '18 at 18:24
  • So your answer is about using some existing, previously imparted rotation of the spacecraft to slightly raise it's orbit. Once exhausted, raising would cease? That's different than how pumping a swing works. Both you and your swing are initially at rest. Energy is in the lunch you just ate, and momentum is zero. You pump to "pull" momentum out of the Earth. In this answer the spacecraft has to be "wound up" ahead of time or it couldn't raise if I understand correctly. – uhoh Nov 19 '18 at 15:04
  • @uhoh The spacecraft can start with a non-rotating wheel and then use a motor to spin it up over time in order to keep its attitude. But after a while, it will spin up to the maximal speed the motor can sustain, so if the spacecraft tries to use the spheres to raise its orbit after that, it won't be able to keep its attitude. And it will happen before a significant change in orbit: changing the orbit's radius by $\Delta R$ while keeping the attitude would mean that the edges of the wheel have to move with the speed on the order of magnitude of $v\Delta R/r$, where... – Litho Nov 20 '18 at 10:45
  • (cont) ... $v$ is the spacecraft's orbital speed and $r$ is the wheel's radius. – Litho Nov 20 '18 at 10:48