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RE: Mystery effect



Greg asked:

> Just thinking about the ongoing "mystery effect"...how persistant
> it is.

At the Annual Meeting in Atlanta, Stacey Mills (W4SM) indicated that it  is
now believed that the effect is due to the permanent magnets associated with
the momentum wheels.

There are 3 wheels, each with its spin axis perpendicular to the other two
(i.e. they are orthogonal). The wheel axis of all three are at a 45 degree
angle to the s/c spin axis.

Apparently all 3 wheels were installed with their magnets pointing in the
same direction, resulting in 3/sqrt(2)=2.1 * the magnetic "pull"  of each
motor directed along the spin axis. The "mystery torque" results from the
interaction of this "pull" against the earth's field when around perigee.

> I have a theory about what it could be and how to test the theory during
the
> coming weeks of changing spacecraft orientation.
>
> Have you ever seen one of those little solar novelty toys consisting of
> diamond shaped paddles on a pinwheel balanced on a needle, and
> all contained
> inside a vacuum bottle the size of a 100 watt bulb??  When you bring a
> strong light source near the device, the paddles (black on one side and
> white on the other) begin to spin.  I'm not really sure how it works...I
> suspect it has something to do with heat absorption.  My point is that if
> one (or more) of the panels on the spacecraft were blown open and as a
> result, positioned such that the flat-black paint on the interior was
> catching full sun--it may be possible to experience the same pinwheel
> effect.

The device available in Toys R Us is called a Cooke Radiometer. The glass
bulb has air in it. The black side of the blade heats up the air on that
side of blade more than the other side. The heated air molecules on the
black side push the vane harder than the cooler molecules on the white side,
so the device spins with the BLACK vane acting as the pusher. Since the
density of air molecules at perigee is so low, this is not an effective
source of torque.

However, starting with AO-7 and -8 and continuing thru all the Microsats,
AMSAT has used a similar technique to provide a "rotisserie" spin for its
LEO satellites. Rather than using air molecules, this scheme uses photons; a
small force is generated by the radiation pressure derived from photons
coming from the sun.

The tape measure VHF/UHF turnstile antennas have one side shiny and the
other side is black.  When a photon hits the black side, it is absorbed and
the photon's momentum is passed on to the antenna blade.

On the shiny side, the photons are reflected. In the process of being
reflected, momentum must be conserved. Since the photon is bounced back from
whence it came and TWO units of momentum are imparted to the shiny side.

Thus there is a net force with the SHINY side becoming the "pusher". This
scheme is described in some detail in Marty Davidoff's (K2UBC) Satellite
Handbook (available from the AMSAT office, see
http://www.amsat.org/amsat/catalog/books.html).

73 de Tom, W3IWI


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