Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: LDEF capture time change (Forwarded)
Message-ID: <1990Jan14.234327.9907@utzoo.uucp>
Organization: U of Toronto Zoology
References: <40214@ames.arc.nasa.gov> <9090002@hp-lsd.COS.HP.COM>
Date: Sun, 14 Jan 90 23:43:27 GMT

In article <9090002@hp-lsd.COS.HP.COM> paulc@hp-lsd.COS.HP.COM (Paul Carroll) writes:
>	During the LDEF capture this morning, there were comments
>	about how stable the LDEF was (i.e., not tumbling).
>	Is there any reason why LDEF is not rotating or tumbling
>	in orbit?  ...

LDEF was designed for passive stabilization.  It was built as basically just
a platform for experiments, with essentially no intelligence or subsystems
of its own.  However, stabilization was considered desirable, as quite a
few of the experiments want a predictable orientation.  By making the thing
relatively long, you get two axes of stabilization for free:  long satellites
tend to line up with the long axis pointing toward Earth, as a result of the
slight difference in Earth's gravitational pull between the Earthward end
and the spaceward end.  This is "gravity gradient" stabilization; it's not
enormously effective but it works well enough when done carefully.  As I
recall, LDEF gets the third axis with a permanent magnet, which tends to
align one of the short axes with Earth's magnetic field.  There are also
some provisions for damping, which helps keep small perturbations under
control.  When LDEF was deployed, it was lined up in the correct orientation,
held there for a while, and then released very slowly and carefully to
minimize disturbances.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
