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I Changing the ISS's orbital inclination to match the Moon

  1. Feb 12, 2018 #1
    What is the practical feasibility of changing the International Space Station's orbital inclination to match the orbit of the Moon?

    Major future missions beyond the Earth-Moon system (ie: space colonization) will likely require in-orbit assembly of components from multiple launches. And anything big going to of from Earth will likely take advantage of the Moon's gravity. So it would make sense for do this assembly in orbits that match the Moons orbital inclination. It would be very handy to have the resources of a space station to do this.

    Unfortunately, the ISS was built at a 51deg inclination so that the Russians would get to it from their high latitude launch sites. This is dramatically different form the Moon's 5.14deg orbit. And the ISS is a lot of mass moving very very very fast. So changing its orbit so dramatically would require a heck of a lot of impulse, far too much than would be economical with chemical rockets.

    What I propose is to mothball and vacate the station (put it in minimal power mode) and then use the bulk of the power from the humongous solar arrays to drive a scaled-up ion thruster like what was used in the Deep Space 1 mission to gradually affect a shift in its orbit.

    How many months or years would this require? I attempted to work out an estimate, but go stick at the orbital mechanics. The biggest problem is that there is not a single correct answer for the mission profile. You could simply calculate the delta-v at the ascending/descending nodes, or you could increase the apogee, change the inclination now using less thrust, and then lower it back down. This bit is beyond by ability to analyze.

    Other parts are easy:
    - The Deep Space 1 thruster used 2100W to produce 92mN of thrust. That's 43.8mN per kilowatt.
    - The ISS solar arrays output 84 to 120 kW. Call it 100kW. Suppose that while mothballed it only needs 20% of that power to maintain itself. And suppose it's in shade about 50% of the time. The resulting budget for thrusters would be about 1.728G watt-seconds per day.
    - 43.8mN/kW * 1.728M kW-seconds = about 75,700 newton-seconds per day of operation.
    - The ISS weights about 420 tons and is moving at about 7.67km/s.
    - Ignoring added mass, daily delta-v = 75,700N-s / 420,000kg = 0.180 m/s.
    - Not I just need to figure out how much delta-v would be required for such a maneuverer.
    (Forgive me if I goofed up that math. Physics class was a long time ago)

    Obviously, this would take a good long while. Exactly how long, is the question. Since propellant mass isn't the paramount issue, as it was in Deep Space 1, and resupply launches can be done frequently it may be more practical to sacrifice some specific impulse to get more 'ommph' out of the day's energy allowance and shorten the timeframe. This may be a good project to test that VASIMR thruster that they've has been working on.

    Does this half-baked idea sound feasible to anyone?
  2. jcsd
  3. Feb 12, 2018 #2
    Assuming a circular orbit, the delta v for an orbital inclination change is-
    ##\Delta v_i = 2 v \text{ sin} \left( \frac{\Delta i}{2} \right)##
    where v is the orbital velocity and ##\Delta i## is the change in inclination.

    Using the velocity and inclinations from your post, a delta v of approximately 6 m/s would be required.
  4. Feb 12, 2018 #3
    It's current orbital velocity is 7.67km/s. Surely, 6m/s is a 2-3 orders of magnitude too low.
  5. Feb 12, 2018 #4

    stefan r

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    Gold Member

    The velocity of a spacecraft or station is a tangent to the orbital path. Any orbit angle will aimed at the moon's orbital path twice per orbit. Why not build the assembly plant at an angle that is convenient for receiving parts launched from Russian?

    A station in the Moon's plane would have more launch windows. If your Earth-Moon traffic is heavy enough that you need daily launch windows you can easily build more than one station. The ISS does would not support a dozen employees. There is no room on the ISS for a casino or tourist sporting activities (ping pong might fit).

    I think you might like tethers unlimited. [not pear reviewed journal but is a real company with excellent diagrams] They are building momentum exchange tethers and electrodynamic tethers. Neither item requires propellant.

    Should also be aware of Kessler syndrome. Instead of trying to keep mass in orbit NASA tries to launch objects in a way that will make them (or their pieces) fall and burn eventually. Satellites carry extra fuel on launch so that they can be de-orbited.
  6. Feb 12, 2018 #5
    Sorry, my bad- should have been approximately 6 km/s
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