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```Newsgroups: sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: orbital plane changes (was Re: New Space Station)
Date: Sun, 20 Apr 1997 16:12:55 GMT

In article <335853C7.4A50@greenms.com>,
Greg d. Moore <mooregr@greenms.com> wrote:
>Basically, if I recall correctly (and I'm sure I don't) to change from
>an equatorial orbit to a polar orbit takes twice as much (actually a bit
>more I think) fuel than it took to get into orbit initially...

Actually, it's not that bad, if you're willing to be sneaky.  You can do
*any* plane change with about 0.8 times the delta-V needed to reach the
orbit in the first place.  To go from a circular orbit to an elliptical
orbit with a very high apogee takes about 0.4 times the circular orbit's
velocity.  At apogee, the orbital velocity is essentially zero, so the
difference between that orbit and a similar one in a different plane is
essentially zero, so a tiny burn there will make that change.  Then you
need another 0.4 to get back down to a circular orbit.

This is also the cheapest way to get from one circular orbit to another
in the *same* plane, if the change in orbital radius is large enough.
(Going from LEO to GSO is not quite a large enough change to make it
worthwhile, unless other constraints intervene.)

For Earth orbits in particular, further trickery is sometimes possible,
using lunar gravity assists and/or Earth aerobraking.  The Russians
estimated that they could nearly double Proton's payload to GSO using a
lunar flyby, but so far nobody has needed the extra mass badly enough to
be a guinea pig.

Typically it's a lot simpler to just launch into the desired plane to
begin with.  There are occasional exceptions.  In particular, the original
Cluster mission was going to do something very much like what I described
above, because it wanted a polar elliptical orbit, but it was getting a
cut-price launch on condition that the destination was an equatorial GTO
(since that's the main commercial destination for Ariane 5 and hence the
first test flight was going there).
--
Committees do harm merely by existing.             |       Henry Spencer
-- Freeman Dyson        |   henry@zoo.toronto.edu

```

```Newsgroups: sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: orbital plane changes (was Re: New Space Station)
Date: Mon, 21 Apr 1997 13:59:42 GMT

Greg d. Moore <mooregr@greenms.com> wrote:
>> Actually, it's not that bad, if you're willing to be sneaky.  You can do
>> *any* plane change with about 0.8 times the delta-V needed to reach the
>> orbit in the first place...
>
>	Hmm, I knew you could save some fuel by doing this trick, but I didn't
>realize that it was quite so high!
>	What is the time frame for this sort of manevour? Days, hours?  Would
>it even be practical for a manned craft?

Time frame depends on the tradeoff between patience and delta-V.  The
higher the apogee of the elliptical transfer orbits, the lower the delta-V
needed to go between them, but the longer it takes to get up to the top of
one and down from the top of another.  A day or so would be about minimum.
Plus, if you're doing this from LEO, you go through the Van Allen belts.
(Actually, that's one reason to make the apogee fairly high even if you've
got spare delta-V -- you want to pass through the belts rapidly.)  It's not
utterly impractical for a manned ship, but you'd want a really good reason
to do it.

>> ...estimated that they could nearly double Proton's payload to GSO using a
>> lunar flyby, but so far nobody has needed the extra mass badly enough to
>> be a guinea pig.
>
>	That would be an interesting experiment...

Sure would.  And of course, after you do it *once*, successfully, the
customers will be lining up to use it...

>> Cluster mission was going to do something very much like what I described
>> above, because it wanted a polar elliptical orbit, but it was getting a
>> cut-price launch on condition that the destination was an equatorial GTO...
>
>	I had forgotten that detail about Cluster.  Of course now it's not
>going much of any place. :-(

And son-of-Cluster is launching on a couple of Soyuzes, so it will probably
go directly to the desired inclination.
--
Committees do harm merely by existing.             |       Henry Spencer
-- Freeman Dyson        |   henry@zoo.toronto.edu

```

```Newsgroups: sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: orbital plane changes (was Re: New Space Station)
Date: Wed, 23 Apr 1997 12:52:10 GMT

In article <335CF6C5.5836@ricochet.net>,
cd@ricochet.net <cd@ricochet.net> wrote:
>If this is the case, then what do you do about the bodies natural desire
>to continue the orbit and return to perogee? Executing a [Northward]
>burn from a 22.5 degree orbit at apogee may make the vehicle move north
>but it does have to go downhill again and isn't that energy still at the
>orginial inclination?

Not really, no.  Potential energy -- which is what makes it go downhill
again -- has no direction.  (Alternatively, one might say that it has only
one direction:  down.)  The fact that the spacecraft is up at the top of a
very high orbit, without the energy needed to stay there, means that it is
going to go down in some fashion.  But which path it will follow downward
is determined only by its actual velocity at apogee, which is very small
and hence can be changed very easily.
--
Committees do harm merely by existing.             |       Henry Spencer
-- Freeman Dyson        |   henry@zoo.toronto.edu
```

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