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Retarded question raised by Disney's "Man in Space" series
So the captain of the moon craft asks his crew to start some process
which would maintain the attitude of the craft to be parallel to the tangent of the flight path. Does that not happen automatically for an orbiting craft? I mean, if the ISS is pointing parallel to the tangent of it's flight path when it passes over the KSC in Florida, would it not automatically be pointing parallel to it's flight path when it is over Europe or Africa? Or would it be pointing toward the center of the Earth? |
#2
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Dry Crackers wrote in
: So the captain of the moon craft asks his crew to start some process which would maintain the attitude of the craft to be parallel to the tangent of the flight path. Does that not happen automatically for an orbiting craft? No. An orbiting craft has to have some kind of pointing control, either passive (spin stabilization) or active (thrusters, gyros, or momentum wheels) to stay pointed in a constant direction. Otherwise, any kind of small perturbing torque will cause it to drift away from the desired attitude. For more detail, read on. I mean, if the ISS is pointing parallel to the tangent of it's flight path when it passes over the KSC in Florida, would it not automatically be pointing parallel to it's flight path when it is over Europe or Africa? Or would it be pointing toward the center of the Earth? Let's break this problem into a couple of simpler ones first. First, we'll consider the theoretical case where ISS is subject to zero perturbing torques and starts out pointed perfectly parallel to the tangent of its flight path. In the absence of disturbances, it should continue pointing in the same direction in space. However, its flight path is a circle because it is in orbit around the Earth. So a quarter orbit later, ISS will still be pointed in the same direction, but its flight path will have rotated 90 degrees, so it will be pointing perpendicular to its flight path (away from the Earth). A half orbit later, it will be pointed opposite its flight path, and so on, until one orbit later, when it will be pointing parallel to the flight path again. OK, now we throw the perturbing torques into the mix. In low Earth orbit where ISS flies, the main perturbing torques are gravity gradient torques and aerodynamic torques. Gravity gradient torques occur because gravity gets weaker with distance, so the force of gravity is stronger on the parts of a spacecraft that are closer to the Earth. Aerodynamic torques occur because the Earth's atmosphere does not end abruptly; its density exponentially decreases to zero so that even in low Earth orbit, there is still a very trace amount of residual gas molecules left. ISS has flight attitudes, called torque equilibrium attitudes (TEAs), where the gravity gradient and aero torques balance out. NASA likes to fly ISS in these attitudes because it minimizes the amount of Russian thruster firings required to desaturate ISS' gyros. And it happens that the flight attitude you describe (ISS pointed along its flight path) is close to a TEA attitude. However, not all TEAs are created equal. The TEA described above is unstable; in other words, any small perturbation away from the TEA will cause ISS' attitude to diverge. So without active attitude control, ISS could not stay pointed tangent to its flight path. It would drift out of attitude until it reached a stable TEA (in the current configuration, this would have the long axis of modules pointed at the center of the Earth, with the solar arrays edge-on to the velocity vector), and then it would slowly oscillate about that attitude. -- JRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. |
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