From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: spacecraft w/ rotating section(s)
Date: Mon, 4 Oct 1999 16:45:05 GMT

In article <37F74D75.3F054965@NOSPAM.erols.com>,
rk  <stellare@NOSPAM.erols.com> wrote:
>> I don't believe it would be unstable, I believe that the Galileo Orbiter
>> (Jupiter) has a rotating and a non-rotating section.
>
>that is correct, it's called a dual-spinner...
>of course, this is a pain, as many electrical signals need to be transferred
>cleanly between the two parts, which was difficult and has not gone all that
>well.  i haven't seen any proposals for dual spinners recently.

Galileo's problems have given the concept a bad name among people
designing planetary spacecraft, a good example of how superstition arises
in this business.  Perhaps half the geostationary comsats in the sky are
dual-spinners.  But let JPL botch the job once, and oh no, you should
avoid dual-spinners, they don't work very well...
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: spacecraft w/ rotating section(s)
Date: Mon, 4 Oct 1999 16:38:09 GMT

In article <37F510F1.558344@NOSPAMboeing.com>,
B.J. Olejnik <bernard.a.olejnik@NOSPAMboeing.com> wrote:
>...I was curious about the stability of a spacecraft with a
>rotating section, similar to the Omega in Babylon 5 or the
>Russian spacecraft in 2010.  Would some sort of odd moment be
>set up that causes the spacecraft to be slightly unstable
>unless corrected with thrusters?

Not if it's done right.  Spinning spacecraft are common; spacecraft with
spinning sections are too.  There are design details which have to be
attended to properly, and of course if you want to *turn* the thing it's
going to be hard work because of the gyroscopic stability, but it's
feasible.

>Along those same lines, how
>about two counter-rotating sections?

Nobody's done this, or anything like it, that I know of.  In practice, I
suspect it would be pretty hard to balance the spins so that the net
angular momentum of the whole spacecraft was zero.

>I'm just curious if a rotating section would cause the ship
>to want to twist or turn in a direction other than the way
>it's going.

See above.  It functions as a gyroscope, which is both good and bad.
Good in that the added stability simplifies holding a desired attitude.
Bad in that changing the attitude becomes costly in fuel.

>Just thought of another set of options:  what about if the axis
>of the rotating section(s) was in an orientation other than along
>the direction of travel, like the internal section in the
>Discovery in 2001?

Doesn't matter.  A spacecraft in free fall doesn't know or care which way
it's travelling.  (When it comes time to apply thrust, that's a different
story.)
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: spacecraft w/ rotating section(s)
Date: Wed, 6 Oct 1999 21:38:26 GMT

In article <37FA9580.B7136533@NOSPAM.erols.com>,
rk  <stellare@NOSPAM.erols.com> wrote:
>...iirc, hughes, which traditionally had used spinners (as opposed to
>rca), had a despun bottom with the antennae.  i think i remembered that right,
>it's been awhile.

It's normally considered to be the top rather than the bottom, but yes,
you have the right idea.

>i don't remember at all the relative mass of the despun
>section of the comsats as compared with galileo ...

Based on some numbers for the Intelsat 6 series, nearly half the dry mass
is in the despun section.  That goes down to maybe 1/3 at beginning of
life, because the stationkeeping fuel is in the spun section.

>but if the memory cells are
>functioning i remember controls people talking about the complexity of galileo
>and it wasn't that easy.

I suspect that part of the problem on Galileo was flexible booms on the
spinning section (the magnetometer and RTG booms are there).  The comsats
don't have anything fancy hanging off the spinning part to complicate
life.  I'd guess their fuel tanks are smaller than Galileo's, too.

>also, perhaps someone in the comsat field can comment as what sorts of signals go
>through the two sections and what they do.  would a small dropout have any
>serious consequence?  a "click" on a phone call?  crash a computer on the other
>side?

All the communications stuff is in the despun section, so you don't have
to send microwaves across the bearing.  The spinning section holds solar
cells, batteries, and stationkeeping propulsion, and I think that's it --
all the brains are in the despun section with the communications gear.

>still, i haven't seen any configurations for a dual-spinner for interplanetary
>spacecraft; has anyone else?

Giotto was a dual-spinner in a mild way -- its high-gain antenna was
despun. Can't think of any others; the planetary people prefer 3-axis
birds for imaging, and the few spinners (like Pioneer 10) have generally
been all-spinning for simplicity.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)