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From: mam@charm.UUCP (Matthew Marcus)
Newsgroups: net.space
Subject: Oberth Wheels
Message-ID: <304@charm.UUCP>
Date: Wed, 4-Apr-84 16:01:56 EST
Article-I.D.: charm.304
Posted: Wed Apr  4 16:01:56 1984
Date-Received: Thu, 5-Apr-84 02:32:19 EST
Organization: Physics Research - AT&T Bell Labs MH
Lines: 30

[Orbiting laser]------------------>    [space bug]

	Something I've been wondering about for a long time is why the
technology of Oberth wheels has not been much used for attitude/spin
control in space.  The idea goes like this:
	Inside your ship, you have a flywheel spun by a small motor.
If you start with both ship and wheel not spinning, then you can spin the
ship by turning the wheel the other way.  When you want to stop spinning,
you brake the wheel.  If you use regenerative braking, you recover most of
the energy you used.  Use three wheels on orthogonal axes, and you get
complete attitude control.  For satallites, you spin the wheel, launch
into correct orbit, then stop the wheel.  For torque used to stop the wheel
spins the bird.  To despin the bird, you simply start the wheel again.
If the wheel is mounted in a gimbal with magnetic bearings and a magnetic
clutch to decouple the wheel from the bird at will, you can decouple,
bring in the bird, work on it without worrying about the gyroscopic effect
of the still-rotating wheel, then recouple and despin the wheel.
	You could imagine a super-cheap (as these things go) version of the
MMU in which attitude control is done by wheels worked by hand-cranks.
If you want to make a left roll, you turn the "roll" crank until you are
faced as you want to be, then hit the "thrust" button.  To compensate
for the center of thrust not being in line with your center of gravity,
you could have booms with masses on the ends which could be extended.
This adjustment could be done manually or under control of a simple
autopilot.
	There must be something I'm missing which makes these ideas impractical,
since I know of no present uses of the Oberth wheel technology.  Could it be
that since NASA didn't invent the wheel, they won't use it :-)?  BTW, I
read a lot of Heinlein stories and I know it shows.
	{BTL}!charm!mam