SPS flapping problem (was Laser SPS allows hybridization with T-solar)

--- In spacesettlers@yahoogroups.com, Al Globus wrote:
> On Mar 30, 2013, at 11:36 AM, hitssquad wrote:
> > One advantage of a passive, geo-fixed-prientation system like this is that it largely avoids the tidal-force problem that plagues typical SPS proposals.
> Could you explain the 'tidal-force problem'?

A sun-oriented SPS will nominally be rotating with respect to the earth, completing one revolution every 24 hours. As this rotation occurs, half of the SPS is periodically brought closer to the earth, while the other half of the SPS is pushed farther away. The earth gravitationally tugs on the closer half more than it tugs on the farther half (possibly causing metal-fatigue and/or requiring a stronger SPS frame in the first place), and causing it to rotate out of alignment with the sun.

For example, viewing Earthspace from above the North Pole, we would see any sun-oriented SPS at local midnight being tugged into clockwise rotation as it orbited to the local 6AM position. Then, as it orbited to the local noon position, we would observe it being tugged in a counter-clockwise rotation. Every 24-hours, we would observe the SPS flap counter-clockwise twice and clockwise twice: left-right-left-right / flap-flap-flap-flap.

The two problems this potentially causes are:

1. fatigue of the SPS frame; and

2. difficulty in keeping an SPS oriented on the sun, especially if it's a solar-concentrating kind of SPS.