Alain Fournier wrote:

On Apr/19/2017 at 6:02 AM, Jeff Findley wrote :
In article m,
says...

Reality check question:

If you spend X energy from low Earth altitude/orbit to Moon orbit, does
Oberth effect claim you will get a value of energy greater than X
falling back from Moon to low earth altitude?

If you don't leave the earth/moon system, I don't think there is a "free
lunch" to be had here. In order for the Oberth effect to be present,
you have to do a parabolic fly-by maneuver. As in your velocity is
higher than escape going in and much higher than escape when going out.
The extra velocity you gain is "stolen" from the planet which you fly-
by. No fly-by, no Oberth effect.

No that is not what the Obert effect is. The Obert effect is due to
greater efficiency of a rocket burn deep in a gravity well than higher
in the gravity well. Suppose you are in an elliptic orbit with let's
say perigee at 200 km and apogee at 40,000 km. While you go up from
200 km to 40,000 km you lose speed. Then when you go down from 40,000
km to 200 km you gain speed. If you do a rocket burn at 200 km, you
gain yet more speed. After that burn, when you go back up, you will lose
less speed between 200 km and 40,000 km than on previous orbits because
you are going faster and therefore, you reach 40,000 km in less time
so gravity has less time to slow you down. So when you reach 40,000 km
you have the speed you had on previous orbits plus the additional
speed of your rocket burn at 200 km plus the additional speed due to
the Oberth effect, that is the additional speed due to you slowing down
less while going up. If you had done your rocket burn at 40,000 km
you would only get your speed plus your delta-v due to the rocket
burn.

You get that even if the planet was a rogue planet not around a star.
The fly-by gravity assist is a different thing. If you don't do a rocket
burn low in the gravity field of a lonely planet you don't really get
an extra push from going into the gravity field of that planet. You
come back out with the same speed you went in, just in another
direction. If the planet is around a star, you again get out with the
same speed relative to the planet, just in another direction. But that
can mean a greater speed relative to the star. For example, if you
had zero speed relative to the star, you had a large speed relative
to the planet. Now changing the direction of that large speed gives
you a large speed relative to the star. So you go from no speed relative
to the star to a large speed relative to the star with no rocket burn.

There is also another component to gravity assist, in that you actually
change very slightly the orbit of the planet, either stealing energy
from the planet or giving it some energy. That energy goes into the
spacecraft. This doesn't need any rocket burn at all. The Oberth effect
does require an acceleration deep in the gravity well.


Perhaps I'm all screwed up here (hey, I'm retired and it's early
morning), but I'm missing some things in this discussion.

1) 'Gravity slingshot' isn't just about changing the direction of your
velocity vector, is it? In a lot of cases these are designed to
'steal' orbital velocity from the planet by 'falling' from the back
side of the planet's velocity vector so you get 'dragged' along as you
fall inward, aren't they? So you get increased velocity 'free'.

2) Oberth Effect really only applies for orbits in the same plane,
doesn't it? If you want to do something like an orbital plane change,
those are actually 'easier' and 'more efficient' if you are higher up
(the opposite of Oberth Effect) and they will actually raise the
apoapsis of the orbit and do the plane change burn at maximum distance
from the planet and then recircularize.

Am I wrong?


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw