"Sylvia Else" wrote in message ...

On 1/07/2012 4:02 PM, Fred J. McCall wrote:
Sylvia Else wrote:

On 1/07/2012 1:46 AM, Fred J. McCall wrote:
bob haller wrote:


wonder if a work around is possible to point with just 2 reaction
wheels?


Oh, good God! THINK, Bobbert! You need pointing control in 3
dimensions. You cannot do that with only two axis control So no, you
cannot 'work around' the laws of physics.

If the axes of rotation of the three wheels are orthogonal, and one
reaction wheel rotates, it changes the axes of rotation of the other
two. Indeed, the axes of the other two can be interchanged.


Now hold it steady....

Hint: You can eventually get it pointed anywhere, but you can't KEEP
it there in order to do science.


Well, that's going to depend on what science you're trying to do, and on
whether the craft has any net angular momentum. If it has no net angular
momentum, it should be possible to get it pointed in any direction with
minimal drift, which will be good enough for many purposes.

Sylvia.


As I recall, the Hubble typically spends hours pointing at the same spot. I
don't think you can get the drift that minimal.




--
Greg D. Moore http://greenmountainsoftware.wordpress.com/
CEO QuiCR: Quick, Crowdsourced Responses. http://www.quicr.net