From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: LOST GEO SATELLITES
Date: Thu, 17 Dec 1998 14:23:35 GMT

In article <36743C5D.C28E8DD2@bellsouth.net>, Tom  <t2jr@bellsouth.net> wrote:
>I believe that unless they boost themseleves away from GEO altitude,
>they eventually drift to one of two longitudes that represent minimum
>energy points in GEO caused by the Earth's shape. So all the GEOsatts
>that just fail eventually migrate to those longitudes.

It's not that simple.

For one thing, while non-stationkeeping satellites are *attracted* to
those two longitudes (I oversimplify slightly...), there is no effective
damping to make them *settle* there.  They oscillate in longitude, over
quite wide ranges if they were far away from the stable points to start.

For another, most stationkeeping for GSO satellites goes to maintaining
latitude, not longitude.  Uncontrolled, they will take up inclined orbits
which spend most of their time north or south of GSO, with the exact orbit
changing with time in a complex way.
--
Mass-market software technology has |  Henry Spencer   henry@spsystems.net
been deteriorating, not improving.  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Why No Direct to GTO Sea Launch Missions?
Date: Sat, 5 Aug 2000 19:05:54 GMT

In article <398B63BF.322F@dimensional.com>,
Josh Hopkins  <hopkins@dimensional.com> wrote:
>...One option is to deliver the
>spacecraft to a supersynchronous orbit with an apogee higher than GEO.
>If the spacecraft has to do a plane change, it is easier to do so at
>higher altitude.  However, satellites launched by Sea Launch don't have
>to (at least nothing significant), since the transfer orbit is nearly
>equatorial.  Therefore, they don't benefit from a supersynchronous
>orbit...

There can still be a net benefit.  Raising perigee is cheaper at higher
altitude too, although the benefits aren't as dramatic as for a plane
change, and that can make up for the need to lower the apogee afterward.
GSO is closer to infinity than to a classical GTO.

Such "bielliptic" transfers -- boost apogee up to very high altitude,
raise the perigee with a high-altitude burn, and then drop apogee down
into the destination orbit -- are better than Hohmann transfers when the
altitude change is big enough.  For going to GSO from an equatorial launch
site, normally it isn't.  But if you aren't free to slice your delta-V up
between burns precisely as you want -- for example, if a big chunk of it
is available only for the first burn -- it can still make sense.

However, the satellite does have to be capable of handling the rest of the
job.  It needs a liquid-fuel apogee engine which can do multiple burns.
It also needs attitude sensors which work at supersynchronous altitude...
and some horizon scanners etc. have altitude limits.  And various support
facilities like communications have to be up to the job.

There may be a minimum supersynchronous altitude where this actually
starts to show a gain -- I lack the time to chase through the math right
now to find out -- and that might limit things too.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)