Orbital velocity relationship to altitude

[Bill Phil]

You can do the vector math for orbits. I think it shouldn't be too hard to make an excel sheet for it... It'll tell you quite a bit about orbital mechanics, although not quite all of it.

[mikegarrison]

44 minutes ago, Fez said:

That definitely makes sense, but what about when in other parts of hr orbit, besides apoapsis? Like when you're climbing to apoapsis, but you're speed is below the amount necessary for a circular orbit. Sincerely Rita slower, shouldn't it be descending? 

As someone else said, it's like a roller coaster. You are going your fastest when you are lowest and slowest when you are highest. In between you speed up or slow down as you move either down or up the hill.

You are trading speed for altitude, or vice versa. Conservation of energy.

Edited September 24, 2016 by mikegarrison

[K^2]

Two pages and nobody mentioned vis-viva?

[mikegarrison]

2 minutes ago, K^2 said:

Two pages and nobody mentioned vis-viva?

OP said he had looked at the math. Was looking for a more intuitive explanation.

[Bill Phil]

Orbital energy is constant if you're not changing orbits (and let's just say it is, even though it isn't quite constant due to various influences). It's divided into two types of energy: potential (gets larger as you go higher) and kinetic (gets larger as you go lower). These have to sum up to the same value no matter where you are in a given orbit. So, since potential energy goes up with altitude, kinetic energy must go down with altitude, and vice versa. And the mechanism for the kinetic energy change is gravity. In an elliptical orbit your velocity vector is not perpendicular to the gravity vector, so some of that change in velocity is going to either increase or decrease your velocity, as well as change its direction.

If your velocity vector is pointing above the line perpendicular to the gravity vector, then it's going to slow down (going "up" the hill). If it's pointing below the line that's perpendicular to the gravity vector, it'll speed up (going "down" the hill).

[Fez]

According to my knowledge, a spacecraft climbs because it has enough tangential velocity, that for every x feet itbdrops due to gravity, it goes so much farther tangentially, that it misses the surface, and stays in orbit. Therefore, a faster tangential speed than that needed for a circular orbit would lead to a less curved trajectory, and thus the craft would climb. Vice versa if the craft is traveling slower than the velocity needed for a circular orbit. With that in mind, I'm confused as to how the craft that I was using in KSP was climbing when it had a velocity lower than that needed for circular orbitm. Thanks 

[andrewas]

Consider the ship at periapsis. Its moving above the velocity for circular orbit, so its radial velocity increases. Half way to apoapsis, its velocity drops below the circular orbit velocity so its radial velocity starts to decrease, but its not until apoapsis that it reaches zero radial velocity and starts to fall again. 

 

 

[Fez]

So the radial velocity outward doesn't decrease to the point that the craft starts descending as soon as the crafts tangential velocity drops below the amount needed for circular orbit? Is that because it keeps wanting to move outward due to newtons first law?