David.Smith@cmu-cs-ius.arpa (03/31/84)
Also, a minor point. The vast majority of all satellites go in more or less the same direction. The reason is straightforward. If you launch East you get up to 1000 miles/hour free velocity from the Earth's rotation. If you launch west you have to overcome up to 1000 miles/hour before you get any forward motion at all. This blurb is in response to someone who thought satellites go every which way and could easily be set into head on collisions. Perhaps most satellites are launched in more or less the same direction (eastward), but that doesn't mean they are travelling in more or less the same orbit. If you launch two satellites eastward from Cape Canaveral (28.5 degrees north latitude) twelve hours apart, their orbits will have a 57 degree difference in inclination. If each travels at 17,500 mph, then their closing speed is 16,700 mph. That should be enough to crack up a satellite. The fact is, satellites do go every which way. We launch satellites from 28.5 degrees north, the Soviets launch from around 55 degrees north, and we both launch into polar orbits. Maybe nobody launches straight west, but the US launches satellites south-southwest from Vandenberg. This, by the way, makes me skeptical about the claim that the space station would be useful as a base for satellite repair. David Smith
REM%MIT-MC@sri-unix.UUCP (04/01/84)
From: Robert Elton Maas <REM @ MIT-MC>
Date: Saturday, 31 March 1984 10:10:03 EST
From: David.Smith@cmu-cs-ius.arpa
[Satellite orbits go nearly every which way, even though most go
generally in the easy direction of west-to-east.]
This, by the way, makes me skeptical about the claim that the
space station would be useful as a base for satellite repair.
It takes more than a station, you also need a tug to bring the
satellite out of its former orbit into rendezvous orbit with the
station (and to put it back in proper orbit when done, although that
can be done by a permanently-mounted second-stage rocket rather than
with a true tug), an astronaut with backpack to despin the satellite,
and a grappling arm to latch onto the satellite so it won't move away
or rotate every time a tool applies force to it during repair. We've
already demonstrated the backpack and Canada-arm, so now we need the
station and tug. Note that it takes a lot less energy to tug an
orbiting object around than to re-orbit it from ground, even if it's
in a grossly different orbit, mostly because tugging can be done with
a highly efficient ion rocket or solar sail whereas lifting from
ground requires a high-impulse rocket or shuttle, which with present
technology requires highly inefficient chemical rockets which require
all their energy stored at liftoff rather than taken in from the Sun
during maneuvering.
Thus a space station is useful but only as one of the four main parts
of the system, and until the tug is developed it really won't be
directly useful for this task except where the orbit of the satellite
so nearly matches the orbit of the station that the STS might be used
as an interim tug. But I suspect by the time the station is up there
we'll be close to having a tug too. <Wishful thinking>al@ames-lm.UUCP (Al Globus) (04/04/84)
>>The fact is, satellites do go every which way. We launch satellites from >>28.5 degrees north, the Soviets launch from around 55 degrees north, and >>we both launch into polar orbits. Maybe nobody launches straight west, >>but the US launches satellites south-southwest from Vandenberg. >>This, by the way, makes me skeptical about the claim that the space station >>would be useful as a base for satellite repair. NASA plans an Orbital Transfer Vehicle (OTV) as part of the space station system. The OTV would be based at the space station base (the manned part). This vehicle would be used to place satellites in other orbits and retrieve satellites from other orbits for repair. There is a great deal of discussion over whether an OTV should be manned and/or use aerobraking. Aerobraking is particularly attractive for return from geosynchronous orbit.
karn@allegra.UUCP (Phil Karn) (04/04/84)
Dave Smith made a small error in his comments. Changing the time of launch does NOT affect the inclination, but rather the Right Ascension of the Ascending Node. Inclination is set by the "launch azimuth", ie., the compass direction in which the rocket flies after liftoff. Inclination and RAAN are the two orthogonal coordinates that determine the orientation of the orbital plane in space. Phil Karn