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Old April 21st 17, 01:09 AM posted to sci.space.policy
Alain Fournier[_3_]
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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