[amsat-bb] AO-40: Spin Up to 5 RPM in progress..

[Peter Guelzow <peter.guelzow@xxxxxxxxxxxx>]

Dear All,

There are good news and confusing news..

The most recent magnetorqueing session finished on March 20, but will
restart soon. The spin rate is currently about 1.8 rpm, the planned
final value. The rate of reduction was about -0.7 rpm/perigee when we
began (at 17.6 rpm), and as much as -1 rpm/perigee at times later.
Overall, the rate of spin reduction was more effective than expected,
getting us from 17.6 rpm down to 1.6 rpm in 21 perigees. 

Attitude is still being measured, but gave some confusing results.
Without magnetqorqueing we observed an attitude change of about 5° per
day (or 4° per perigee), although the maximum possible, due to Sun
movement and precession, is only 0.85 deg/day! 

This much larger apparent rate of change would explain why the
magnetorques showed larger falls in sunganle (SA) than expected, since
the sessions spanned several days, and also why the hibernation was much
shorter than predicted with conventional thinking. 

After some additional WOD analysis it was found that the change in
attitude occurs only when the S/C is in perigee. Effects of drag in
perigee were taken into account, but are obviously much higher than
anticipated and calculated, perhaps due to sun activity. 

If this would continue without active attitude change, we would run out
of sun lock within 10 days or so.. 

Therefor it was decided to spin up the s/c to reduce the rapid change in
attitude due to drag. This will give us breathing space. A 5 perigee
magnetorque was initiated, starting immediately, which should raise the
spin rate to 5 rpm. 

Preparation to test the Arcjet thruster as soon as the s/c attitude 
is determined and adjusted are under way.

73s Peter DB2OS

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