Jorge R. Frank wrote:
"Tom Cuddihy" wrote in
ups.com:

Jorge R. Frank wrote:
(Henry Spencer) wrote in
:

It also means that the particular station partner is in the critical
path of commercial crew and cargo service to ISS. That's not to say
this is not a viable path, but it does mean that availability of
commercial services prior to 2010 is wishful thinking at best.

People are already thinking in that direction. When Boeing asked
SpaceX for a quote on freight delivery to an orbital fuel depot,
what came back was a quote for delivery to a point 1km from the
depot.

That suggests SpaceX has not thought the problem through. "1 km" is a
meaningless metric, when not tied to a particular relative *velocity*
as well.

They're using standard rendevous terms. 1 km means zero relative
velocity, in the same orbit, 1 km ahead of the spacetug. The space tug
would then perform a manuever to lower its orbit from that of the
target slightly, to catch up to the target, then phase back to the
same orbit within about 200-300 m of the target, after which it's
close enough to be 'driven in' without really worrying too much about
orbital period. (That's how shuttle rdvs happen).

Kinda. :-) Actually, that's a reasonable description of how shuttle deploy-
retrieve RNDZs happen, with the exception that shuttle pilots are taught to
make effective use of orbital mechanics throughout the manual phase. It's
not until about 10 m that they can just "drive in" and more-or-less neglect
orbital mechanics (though the effects are quite visible, even that close).

For shuttle ground-up RNDZs (which is pretty much all of them from now on),
the shuttle enters the phasing orbit from below, rather than from a
stationkeeping point with zero relative velocity. FDOs make it a point of
pride, in fact, to never waste propellant on an orbit-lowering burn during
a ground-up RNDZ.

Of course, then it has to get back to the station, which means
optimally this would all happen behind the station. The tug would then
lower its orbit to catch back up to the station with the payload 'in
hand.' Of course, all the maneuvers have to be done in a short enough
time frame that perterbations don't screw up the parameters, but it's
doable.

The doability depends on the magnitude of the relative velocity error.
Effectively, that error dictates the "retrieval window", both timeline-wise
and propellant-wise, within which the tug must operate. Therefore, if the
relative velocity is not actively controlled, the expected error must be
part of the spec for the tug.

Or at least, that was my naive assumption. :-)

Ok, obviously I'm telling the baker how to bake bread here. Sorry.
My limited knowledge of rendezvous came from a lecture my orbital
mechanics prof, Dr. Chirold Epp gave, but I thought he said you could
generally 'drive it in' within 400m or so--generally any distance you
think you can make up in less than a 1/4th of an orbit. He said he's
been a part of a few rdvs himself, but then again he's been teaching
students for the last 10 years. He's going back to Johnson this year
though--in fact, I think he might already be back there!

Anyway, you never know who you're talkin to on usenet--hope you weren't
offended by my a.s.s.umption.

tom