Saveing fuel by gravatational drag?

I have heard of saving fuel by gravitational sling shots basically exchanging energy with a larger body to get an positive acceleration the smaller craft.

My question is to those who are better at this kind of math than me is if i were going to the moon say could i save fuel by choosing the right orbits maybe quite a few of them to give energy to the moon by gravitational exchange slowing my craft down.

Being of the paranoid persuasion i probably would not to orbit any lower than the highest moon mountain but intuitively i think it would save quite a bit of fuel if you only needed to jet down from mountain height if you choose the right orbit am i correct or have i missed something or do they already do this and i have missed it it the explanations or cant you get the right orbit leaving from earth with out spending more fuel in orbit changes than you would save by slowing down by gravitational drag.

I would think your decaying orbit would cause you to crash instead or you will have to burn fuel to go faster to maintain an orbit.

Ribbit

You are mixing up two spacecraft maneuvers that have been used by interplanetary probes. These are Gravity Assist or Gravity Slingshot, and Aerobraking. Neither of these will be any use on a Earth->Moon mission, because Gravity Assist uses an intermediary body to steal a bit of momentum (e=mvv/2) on the way to its final destination, and Aerobraking requires an atmosphere to slow itself down at its destination, this allows it to use less fuel for the retrorockets. I could spend hours talking about these, but I don't have any spare time today, so I suggest you read the following wikipedia articles.

http://en.wikipedia.org/wiki/Gravity_assist
http://en.wikipedia.org/wiki/Aerobraking

As always there are plenty of links to other topics if there are parts of these that you don't understand. If you have further questions, I'll try to answer them tomorrow.

johno

The closest I can think of to this, is have barely enough velocity to make it from the dominance of Earth's gravity well, to the dominance of the Moons. If you do it right then you are barely moving when you begin to head to the moon.

I was not confusing the gravity assist with areobraking what i am asking is if it is possible to do the gravity assist in reverse instead of taking energy from the larger body and giving it to the smaller body can you do it the other way say if you changed the direction of U in the diagram linked to :-


http://upload.wikimedia.org/wikipedia/c ... mple_2.gif

Would it be possible to dump energy from the smaller body to the larger one in such a way that you could do the equivalent of areobraking on airless bodys by exchanging momentum and slowing down the smaller body in relation to the larger one. And specifically even if this possible in a general sense would it save energy going from the earth to the moon or would the energy needed to get a slowing down orbit exceed the energy needed to go straight there and and down as in the way Apollo did it.

Intuitively i think a gravity assist should be reversible with the right elliptical orbit but i don't have the math to prove or disprove it. It maybe that momentum can only go from a larger body to a smaller one by gravitational exchange if so i would like to know why.

As you slingshot through the perigee of a parabolic orbit, a retro-grade burn should remove more momentum than an identical burn earlier or later.

I think.

Brain is dead.

Idiom is right - a retrograde burn at periapsis (?) will dump more momentum than any where else.

As to the original question - a gravity slingshot could be used to reduce velocity relative to the object - if you approach the body in the opposite direction to its orbit, doing a U turn around it will reduce you speed relative to its orbital velocity. I think.

I apologise, I misunderstood your question. I can't think of any reason why your proposal wouldn't work. In fact, I'd be surprised if this type of maneuver wasn't used for the Galileo and Cassini missions, although I don't remember ever reading about it. I don't have the time to search right now though, I gotta go as soon as my coffee is drank.

johno

--deleted double post--

I can understand where you misunderstood me but i am still interested if even if it works in a general sense could it be used to save fuel going from the Earth to the Moon or or if the changes needed to get the right orbits would exceed any fuel saved slowing down at the other end.

I also understand the need for more coffee thing i am thinking of having a T-Shirt made :-

If Not sufficient caffeine
Then fail Turing test = True