RE: OTV Changed its inclination???

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Thu May 24 2012 - 18:48:17 UTC

  • Next message: Chris Jones: "Re: OTV Changed its inclination???"

    Art Glick asked:
    
    > doesn't it take a very large amount of energy to change the
    > inclination of an object in orbit (as opposed to orbital height)?
    
    Not necessarily. It depends on the size of the inclination change and the velocity at the time of the manoeuvre.
    
    Typically, inclination changes are made at a node, which for a circular orbit requires a velocity change of 2 * Vorbital
    * SIN (delta inc / 2). Looking at that equation, it is evident that a change of inclination of 60 deg would require a
    velocity change equal to the orbital velocity, which would indeed be prohibitive for most spacecraft, especially in LEO.
    However, OTV 2-1's inclination change was a mere 0.9 deg. Given orbital velocity of 7700 m/s, the delta-V was 121 m/s.
    
    > So much so that they almost never do this?
    
    Part of the design process of a spacecraft is to determine the total delta-V of all manoeuvres over the planned life,
    and provide sufficient fuel and propulsion systems. My recollection is that X-37B is capable of a total delta-V of very
    roughly 1000 m/s, so the recent orbit change would have been well within its capability (even after allowing for the
    routine manoeuvres it makes to maintain altitude against drag).
    
    > No doubt, being able to easily change inclination would be an
    > incredibly useful tool, but aside from perhaps the STS, has anything
    > else ever routinely changed its orbital inclination?
    
    Certain Earth observing spacecraft in sun-synchronous LEO orbit perform small routine inclination adjustments to
    maintain a precisely repeating ground track, and precise planar orientation relative the sun. The need for the
    adjustments arises from the perturbing effects of solar gravity. Sun-synchronous orbits are especially susceptible,
    because their orbital planes maintain nearly the same angle in relation to the sun for long periods, allowing the
    effects to accumulate. Examples of spacecraft that make these small corrections are Landsat, Spot and KeyHole.
    
    Geostationary satellites require a near-zero inclination, but solar and lunar gravity cause gradual long-period
    oscillations, that must be removed through periodic "north-south" station-keeping manoeuvres.
    
    Ted Molczan
    
    
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