Newsgroups: sci.space
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Magellan Status for 07/17/89 (Forwarded)
Message-ID: <1989Jul20.155430.15289@utzoo.uucp>
Organization: U of Toronto Zoology
References: <28782@ames.arc.nasa.gov> <805@eutrc3.urc.tue.nl>
Date: Thu, 20 Jul 89 15:54:30 GMT

In article <805@eutrc3.urc.tue.nl> exiphm@eutrc3.urc.tue.nl (h.munk) writes:
>Forive me my ignorance, but what is "momentum wheel desaturation",
>and why is it done ?

There are a number of methods of attitude control for spacecraft.  Just
using control jets works, but if the spacecraft is going to have to do a
lot of turning, it uses a lot of fuel.  Magellan is going to be pivoting
around two or three times per orbit for months on end (it alternates
mapping Venus and transmitting back to Earth, through the same antenna),
so one would prefer an alternative.  If you simply put a heavy little
wheel on a motor inside, when you spin the wheel in one direction the
spacecraft turns slowly in the other.  When you stop the wheel, the
turn stops too.  In theory.  The problem is that you can get torques on
the spacecraft from outside forces, like gravity gradients and light
pressure.  Not large ones, but not insignificant.  If the momentum wheel
is used to counter those, it can end up spinning faster and faster when
the spacecraft is stationary.  So once in a while you need to put on the
brakes for the wheel to keep its speed within bounds.  (This generally
means firing control jets at the same time, to keep the spacecraft from
just picking up the wheel's rotation.)  This is momentum-wheel desaturation.
-- 
$10 million equals 18 PM       |     Henry Spencer at U of Toronto Zoology
(Pentagon-Minutes). -Tom Neff  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu