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SLS launches likely delayed



 
 
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Old April 20th 17, 01:53 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default SLS launches likely delayed

Alain Fournier wrote:

Le Apr/19/2017 à 2:07 PM, Fred J. McCall a écrit :
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'.


Changing the direction of your velocity vector relatively to one
body (let's say a planet) can be very much the same thing as
increasing your orbital velocity relatively to another body
(let's say the star). And changing the velocity vector is done by
changing (so very slightly) the velocity vector of the planet.
So the two are kind of the same thing. (In a previous post, I
was implying they are different, I was wrong.)

If you have zero velocity relatively to a star, you can have
a high velocity relatively to a planet orbiting that star in
the direction opposite to the orbital motion of the planet.
If you change that to a high velocity relatively to the planet
in the same direction as the orbital motion of the planet,
you went from zero velocity relatively to the star to twice
the orbital velocity of the planet relatively to the star.
And you are on your way to being ejected from the solar system.


I wish I could draw. The case I'm referring to is where the velocity
with regard to the planet doesn't change much (except over the short
period of the slingshot) but the velocity with regard to the star DOES
change because of the 'drag' effect of the planet moving in orbit.
It's the simplest 'gravity slingshot' maneuver with no loops around
the planet, etc.


--
"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
 




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