Re: Inclination change

From: Tony Beresford (
Date: Wed Mar 10 2004 - 04:34:49 EST

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    At 16:12 10/03/04, Skywise wrote:
    >I have what seems a simple question that I know is going to have
    >a not so simple answer, but...
    >Just how difficult is it to change the inclination of an orbit?
    >I'm thinking on the order of 25 or more degrees. It doesn't have to
    >be "overnight" but could take a period of time, perhaps up to a
    >I know a lot of people asked why Columbia couldn't have rendezvoused
    >with the ISS if they knew they were in trouble and the obvious answer
    >was they were in the wrong orbit without enough fuel to change it.
    >But, I'm thinking of unmanned craft with little or no time
    >Could it be done with an ion engine?
    Brian, the formula that tells you the impulse velocity required to
    change inclination of a circular orbit is
    Delta V = (sin( DeltaI)* V)/sin( (180-deltaI)/2)
    where    V = circular orbital velocity
         DeltaI = change in inclination
            *    designates multiplication
             \   designates division
    For an LEO orbital velocity of say 7.7 Km/s this equates
    to 3.3 Km/second for the 25 degree change you mention.
    Such a journey might take 6-8 months for a reasonable payload.
    Such a plane change is routinely accomplished for payloads destined for the
    geosynchronous orbit by doing the plane change and orbit circularization
    burn at the same time, when the object is at the apogee of the transfer orbit,
    and thus moving quite slowly. 
    This is certainly in the range of ion engine capabilities.
    The current ESA , European Space agency lunar mission
    is engaged in an ion engine powered lunar journey at
    the present time.
     Tony Beresford     
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