DOUG@JPL-VLSI.ARPA (04/10/84)
From: Doug Freyburger <DOUG@JPL-VLSI.ARPA>
(Pre.S. Please put me on the Space-Enthusiasts mailing list. Thanks.)
Re: Oberth wheel
In fact, there has been at least one spacecraft to use the Oberth
wheel technique for attitude control. The late IRAS (Infra-Red Astromony
Satelite) had four gyros (one backup) for just this use. At launch the
satelite was put in a near polar orbit, and its spin was adjusted so that it
always pointed away from Earth. From then on, it scanned two 3/4-circles per
orbit by accelerating its gyros slowly to scan the sky at more than orbit scan
rate, from pointing almost "straight back" to pointing almost "straight up"
during each half-orbit. The gyros where quickly decelerated to slew to scope
back to the "straight back" position to start again. This trick both
maximized sky coverage, and avoided pointing the scope "forwards" in its
direction of movement. It was liquid helium cooled, and even the few air
molecules inhaled would stick in the liquid helium bath, boiling the helium,
frosting the mirror, and slowing the orbit. The usual problems of friction
had to be dealt with with attitude thrusters. I don't know if they used
magnetic bearings or what.
The most interesting feature of IRAS was not its Oberth wheel attitude
control, but its orbit. It was (is) a near polar orbit with some added bells
and whistles. Its procession-of-the-equinoxes has a one year period. It
always faces the sun at the same angle! I would like to see whoever figured
out that one give us an over-view of the math.
Re: big bang
The statement in the Scientific American article said that time blurs
at Plank Unit sizes. I had enough quantum mechanics in Phys 2 at CalTech to
keep up with that one. Time really should not be meaningfull in units less
than a Plank Time. What I was wondering about while I read the article is why
that time blurring should extend/expand linearly out into time-space. Were
they really saying that the time-bluriness at a distanse is now bigger than a
single Plank-Time? Is there some sort of proportianality vs distance from the
original center? Did anyone out there have a clear picture about this?
I once read a suggestion that this small-universe really is an
infinitely expanding big bang, but the rest of the big-universe as a whole
fits the classic steady-state model. In this case, the matter addition to
maintain an even mass distribution is supplied by occasional new big bang
small-universe creation. I guess the mass distribution just isn't either as
high or as even as the originally proposed steady-state model. I guess the
question of open vs closed universes is just an estetic one for the moment.
Most of us simply don't WANT our universe to really be facing a true entropy
heat death. Study into the missing-mass problem may someday bring the problem
into the actual detail-of-calculations mode. Let's hope.
(-)NX for the interest,
Doug
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