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B Feasibility of a L1 Gravity Swing Cold Launch?

  1. Sep 15, 2018 at 4:27 PM #1
    At an L1 LaGrangian point between two bodies, one could - materials science notwithstanding - pit two of Newton's Laws (LM3,UG) against each other to provide thruster-free stationkeeping.

    Would it be feasible to use that to launch free from the system ? either spit out like a watermelon seed from between the two, or at least get into position to use one or the other for a slingshot manoeuver. [edit: a "cold" launch, without using rockets at all]

    Apologia for brevity:

    In case it isn't a "thing", we're talking about a space station trying to stay parked at the Earth<>Moon "null gravity" point, where there's a natural tendency to slide towards one or the other gravitational source, along the line connecting their centers of mass.

    In the case of drifting towards (say)the Moon, one could toss out a smaller mass on a tether out the side towards the Earth, letting the exponential increase in gravity increase its weight, then reeling it back in for a net position change, Earthwards.

    The same principle could be used to move a craft forwards or backwards, tossing the weight out angled more towards front or back, "poling"(one weight) creatively, or "sculling"(two weights, easier to visualize).

    L1 is of course a "trough" in the longitudinal direction, so assumed for convenience is a build-up of a back and forth reciprocation, like on a playground swing.

    Question is : could such gymnastics eventually lead to a direct or indirect escape of the system ? without jettisoning mass in any way. (Or, for that matter, use a station, to launch a spacecraft literally like a slingshot, retaining the two working masses)
    Last edited: Sep 15, 2018 at 4:42 PM
  2. jcsd
  3. Sep 15, 2018 at 4:38 PM #2


    Staff: Mentor

    It is conceivable. The Lagrange orbits are known to have trajectories that are very easy to escape on and it takes very little thrust to dramatically change escape trajectories. That is the basis of the so called interplanetary transport network.


    It could be that the orbits are so sensitive that dramatic changes in tidal forces would be enough to shift from one to another.
  4. Sep 15, 2018 at 4:52 PM #3
    Thanks for the quick reply, Dale.

    (I'll assume your answer covers using a bi/tri-partite station/spacecraft, for station-keeping and "cold" launching as well)
  5. Sep 15, 2018 at 4:58 PM #4


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    Well, I wouldn’t call it an answer, I don’t know for sure, but it doesn’t seem obviously impossible. So it’s a definite maybe.
  6. Sep 16, 2018 at 3:32 PM #5


    Staff: Mentor

    After thinking about it I am going to upgrade this from “definite maybe” to “maybe probably”. It is well known that tidal effects can be used to transfer both energy and angular momentum between gravitationally interacting objects and shift the orbit. The amounts are very small, but very small is all you need.
  7. Sep 18, 2018 at 4:22 PM #6
    Thanks for applying : I thought up the paradigms while ruminating with friends concerning the mechanics of cold-starting a regular swingset, and found it odd that somethings reasonably obvious, in the same toolbox as "orbital tether" and "slingshot", didn't appear in SF anywhere.

  8. Sep 18, 2018 at 7:59 PM #7


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    This sounds like a reactionless drive. How is this not a violation of conservation of momentum?
  9. Sep 18, 2018 at 8:57 PM #8
    Propellant-less, not reactionless.

    From a station at an L1 point, tossing a tethered small mass towards one or the other g-sources will - thanks to Newton's Third Law of Motion - result in displacements from the assemblage's center of mass (which is the zero-g point) proportional to the masses. At which time - thanks to Newton's Law of Universal Gravitation - the smaller mass will be exerting a larger force on the tether than the larger mass.

    (to be clear, I worked it out with equal gravitational sources, not the Earth<>Moon disparity)
  10. Sep 18, 2018 at 9:00 PM #9


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    I expect that the change in the position of the station (moonwards) would cancel out whatever you'd get from the tether. The Moon pulls too, and I'm betting that the increase in the gravitational pull on the station by the Moon cancels out the increase in the gravitational pull on the tether by the Earth.
  11. Sep 18, 2018 at 9:07 PM #10
    If I may... this example uses two identical gravitational sources.

    Say I want to move a station at an L1 LaGrange point closer to one or the other gravitational sources.

    - Station masses twice as much as the cannonball.
    - Station is situated halfway between two identical point sources of gravity, 4 distance units away from each.
    - Tether is conveniently 3 units long.

    So we start by tossing the cannonball at one of the grav sources. It runs away really fast then stops short. At this time, thanks to Newton's meddling, the station will be 3 units away from one source; the cannonball is 2 units away from the other.

    Before we separated them, at 4u away from each grav source, net gravity (influence of each grav source) on the station was 1/16g - 1/16g = 0g, ie: balanced: the station's not going anywhere. But now, in their new positions...

    Station: 1/9g - 1/25g
    Cannonball: 1/4g - 1/36g

    The pull of the cannonball on the tether - even though it's half the mass of the station - is still more than that of the station.

    So the system will move towards the cannonball side.
  12. Sep 18, 2018 at 9:10 PM #11


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    I haven't gone through all the math yet, but that just doesn't look right to me. The center of mass of your station-cannonball system remains the same both before and after firing, and we can model the two as if all of their mass is concentrated at the CoM. Since the CoM hasn't moved, there should be no net force on the system.
  13. Sep 18, 2018 at 9:24 PM #12
    False. Well, not false : you can model it like that, but the result would be incorrect.
    The CoM is still the same after firing, the station still masses twice as much but, in their new positions, the cannonball weighs more than 1.5x as much as the station. The cannonball will drag the station over (changing the CoM).

    I think I've accounted for everything except orbits.
    Last edited: Sep 18, 2018 at 9:45 PM
  14. Sep 18, 2018 at 9:45 PM #13


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    Hmmm. I can't find fault with your math, but I haven't done these kinds of calculations in a while, so I'm not that confident in my own results.
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