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SLS launches likely delayed



 
 
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  #21  
Old April 20th 17, 01:53 AM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 8,928
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
  #22  
Old April 20th 17, 02:18 AM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,174
Default SLS launches likely delayed

In article , says...
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.


Actually, it all depends what you're trying to do. If you're in an
elliptical orbit around the earth and you want to raise the furthest
distance from the earth in the orbit (or escape earth), yes you fire
your engine when you're closest to the earth to increase your velocity.

But, if you're in an elliptical orbit and you want to circularize the
orbit at the highest point, you always have to do the burn at the
highest point (increasing the speed at the highest point to match the
speed needed at that altitude for a circular orbit).

Starting from a low circular orbit and going to a higher circular orbit
involves one initial burn to increase velocity to make the orbit
elliptical, then another burn at the highest point to circularize it.
That's a Hohman Transfer, if memory serves (not sure if the spelling is
right).

So you don't always want to do your burn at the lowest point in the
orbit.

And like Fred said, if you need to do a plane change, those are best
done at a very high point in an (elliptical) orbit (theoretical maximum
efficiency is at infinite distance, which is infinite time).

https://en.wikipedia.org/wiki/Orbita...ination_change

This all gets rather complicated rather quickly, especially when you
have to take into account the moon as well as the earth and your
spacecraft (now it's a three body problem, which is hard to solve).
This is why experts in orbital mechanics (and lots of simulation time)
are required to design "grand tour" types of missions which intend to
flyby multiple targets. This essentially what my Orbital Mechanics
professor did for JPL before she became a professor at Purdue. Of
course, that's been a few decades now... ;-)

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #23  
Old April 21st 17, 01:06 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 225
Default SLS launches likely delayed

On Apr/19/2017 at 8:53 PM, Fred J. McCall wrote :
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.


It is difficult to describe a real gravity slingshot manoeuvre. It is
a three body problem so the real trajectory is complex. The example I
gave above isn't realistic, I was giving it to try to explain how it
works. I will try again. This time with something a little more
realistic.

Imagine you are in an elliptical solar orbit with apogee just a little
lower that Jupiter. When you reach apogee you are moving in parallel
to Jupiter's motion, just a little lower and slower. When I say that
you are moving slower than Jupiter, that is in the solar reference
frame. In Jupiter's reference frame, Jupiter is motion less and you
have a positive speed. So in Jupiter's reference frame you come from
outside its gravity well with positive speed and enter its gravity
well. So you will come out with positive speed, in fact with the same
speed you had coming in. You are going to have a hyperbolic trajectory
relatively to Jupiter. What we want to know is in what direction
you will come out with that positive speed.

In the solar reference frame, Jupiter is going faster than you, since
you are at the apogee of an elliptical orbit and Jupiter is in a
somewhat circular orbit. So Jupiter moves ahead while you fall into its
gravity well. Your hyperbolic trajectory will bend in the direction of
Jupiter. Since Jupiter is passing ahead of you, your trajectory will
bend in the general direction of Jupiter's orbital motion, therefore
increasing your orbital speed relatively to the Sun. By how much
depends on your original speed and how close to Jupiter you will go.
But in this case, you arrived near Jupiter with an orbital speed slower
than Jupiter and you are coming out with an orbital speed higher than
Jupiter.

You originally had a slower orbital speed than Jupiter and Jupiter
passes in front of you. But your hyperbolic trajectory brings you close
to Jupiter, so you accelerated in the direction of Jupiter, your are
going to decelerate after that, but you have a hyperbolic trajectory
so you once you caught up with Jupiter, your not going to go back
behind it. Therefore, you must have a higher orbital speed than Jupiter.


Alain Fournier

  #24  
Old April 21st 17, 01:09 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 225
Default SLS launches likely delayed

On Apr/19/2017 at 9:18 PM, Jeff Findley wrote :
In article , says...
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.


Actually, it all depends what you're trying to do. If you're in an
elliptical orbit around the earth and you want to raise the furthest
distance from the earth in the orbit (or escape earth), yes you fire
your engine when you're closest to the earth to increase your velocity.

But, if you're in an elliptical orbit and you want to circularize the
orbit at the highest point, you always have to do the burn at the
highest point (increasing the speed at the highest point to match the
speed needed at that altitude for a circular orbit).

Starting from a low circular orbit and going to a higher circular orbit
involves one initial burn to increase velocity to make the orbit
elliptical, then another burn at the highest point to circularize it.
That's a Hohman Transfer, if memory serves (not sure if the spelling is
right).

So you don't always want to do your burn at the lowest point in the
orbit.

And like Fred said, if you need to do a plane change, those are best
done at a very high point in an (elliptical) orbit (theoretical maximum
efficiency is at infinite distance, which is infinite time).

https://en.wikipedia.org/wiki/Orbita...ination_change

This all gets rather complicated rather quickly, especially when you
have to take into account the moon as well as the earth and your
spacecraft (now it's a three body problem, which is hard to solve).
This is why experts in orbital mechanics (and lots of simulation time)
are required to design "grand tour" types of missions which intend to
flyby multiple targets. This essentially what my Orbital Mechanics
professor did for JPL before she became a professor at Purdue. Of
course, that's been a few decades now... ;-)

Jeff


I totally agree with you on this.


Alain Fournier

 




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