On Thu, 24 Jun 2004 08:33:29 GMT, Richard Lamb
wrote:

Ron Wanttaja wrote:

Anyway, you do have it backwards...orbital velocity decreases with circular
orbit altitude. ~25,200 FPS at 200 nm, ~10,100 FPS at geosynchronous
altitude (~19320 NM).

You're right about the potential energy, though. Dropping from
geosynchronous altitude to ground level, you'll hit the atmosphere at over
23,000 miles per hour. And if you're an old-timer like BOb, you'll have
the turn-signal flashing the entire way....

So?
To catch up with the guy in front of you, you first slow down?

Precisely. Think of it in terms of angular rate. If you're both in the
same 90-minute equatorial orbit, you're traveling around the Earth at an
angular rate of four degrees of longitude per minute. To catch up to the
guy in front, you need to increase your angular rate, e.g., an orbit with a
shorter period.

The angular rate is inversely proportional to orbit altitude...LEO (Low
Earth Orbit) satellites are at a few hundred miles and go around the Earth
in 90 minutes or so, but Geosynchronous satellites are at ~19300 NM and
take a full 24 hours to orbit the Earth. The velocities needed to maintain
the orbit are lower, and the circumference of the orbits is longer...you're
not only flying slower to start with, but the distance you have to travel
for one orbit is longer.

So... if your buddy is a few hundred miles ahead of you in LEO, you slow
up. This lowers your average orbit altitude and decreases your orbit
period. You've now got a period, say, of 88 minutes and an angular rate of
4.1 degrees a minute. Every minute, you're 0.1 degrees (roughly 6 nautical
miles, in LEO) closer. You're also slightly below your buddy, but when you
catch up, you increase your speed, which raises your orbit.

Mind you, if your friend had been only a hundred feet in front of you, you
just would have popped your little thrusters and flown directly to him. It
will perturb your orbit, but would be minor compared to normal orbit
maintenance maneuvers. Compare it to having to climb 10 feet in an
airplane vs. 10,000 feet. You'll just tug the stick back slightly for the
first case, but add power for the second.

Otherwise, though, orbit mechanics is *definitely* non-intuitive to an
aircraft pilot. In an airplane, we're always concerned about how far we can
fly, and can easily change directions if we desire. The situation is
exactly opposite in orbit... the vehicle has nearly unlimited range, but a
change in compass course is prohibitively expensive.

Ron Wanttaja