On 18-09-09 20:40 , JF Mezei wrote:
On 2018-09-09 04:25, Stuf4 wrote:

This is the situation you have in the lab, with the mass bobbing up
and down on the end of the spring.

If I am 10km behind the ISS in circular orbit, and I turn on the impulse
engines to try to catch up to ISS, my increased speed will also result
in my gaining altitude becase I am going faster than speed needed to
remain at that altitude. Right ?

Yes, eventually; your altitude will peak after a half-orbit, and then
start to fall again. However, if you have only 10 km to go, you may
reach the ISS, and brake to match its speed, before the gain in altitude
is very noticeable.

When a satellite dropping from 10,000 to 400 gets to 400, isn't it
correct to state that its speed is WAY higher than what is needed to
remain at 400km altitude? And like the paragraph above, with it going
faster than needed, it starts to gain altitude again.

Yes indeed, assuming that 400 km is the perigee, where the satellite's
velocity has no vertical (altitude component).

What I don't understand is that the point where the satellite starts to
go faster than needed for that altitude happens before perigee. How
come it continues to drop even if it is going faster than needed to
remain in that orbital altitude?

Because the satellite's velocity has a downward component -- the
satellite is moving in a direction that decreases altitude. The
satellite starts to gain altitude only when the satellite's velocity
vector has turned enough (relative to the local vertical, which is also
turning as the satellite orbits) to bring the downward component to
zero, and then to a positive value, in other words, when the satellite
passes its perigee.

So what is magical about perigee that causes the satellite who is
already going way faster than necessary to finally stop losing
altitude/accelerating and starts to behave normally for a satellite that
is going faster than needed at that altitude? (gain altitude, lose speed)

Perigee is the point where the satellite's vector changes from pointing
down to pointing up.

--
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
. @ .