### Author Topic: Gravitational Assist  (Read 8721 times)

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##### Gravitational Assist
« on: 05/01/2007 04:04 pm »
All this talk about traveling to Gliese 581 has got me to thinking.  The New Horizon probe, and other spacecraft, use gravitational assist flybys of Jupiter and other planets to gain speed.  What are the  theoretical limits to this sort of thing?  How much speed could you pick up if you spent several years doing this before heading out of the solar system?

#### Jim

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##### RE: Gravitational Assist
« Reply #1 on: 05/01/2007 04:52 pm »
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bhankiii - 1/5/2007  12:04 PM

All this talk about traveling to Gliese 581 has got me to thinking.  The New Horizon probe, and other spacecraft, use gravitational assist flybys of Jupiter and other planets to gain speed.  What are the  theoretical limits to this sort of thing?  How much speed could you pick up if you spent several years doing this before heading out of the solar system?

it is a momentum exchange.  Jupiter lost the same amount of momentum that PNH gained.

The problem is get back to Jupiter or any other planet after getting the assist.  PNH wouldn't be able to do it again since it is going out of the solar system.

#### BarryKirk

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##### RE: Gravitational Assist
« Reply #2 on: 05/01/2007 04:57 pm »
Also, I think that there are limitations on the direction you end up going after the momentum exchange.

#### kevin-rf

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##### Re: Gravitational Assist
« Reply #3 on: 05/01/2007 05:27 pm »
You still woul not pick up enough speed to make interstellar travel times reasonable.

Does anyone have a good reference to the math behind gravity assists?
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#### Tom Ligon

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##### Re: Gravitational Assist
« Reply #4 on: 05/01/2007 05:29 pm »
Yes, you must work the directions of the planetary motions in.  And the effect is primarily in the ecliptic.

To reach Jupiter in the first place, the usual procedure may involve a swing past Venus, back past Earth and/or Luna, another swing past Venus, and off to Jupiter for the big boost.  The mind fairly reels at the math overload, but every pass is a slingshot designed to boost the kinetic energy of the spacecraft.

#### Jim

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##### Re: Gravitational Assist
« Reply #5 on: 05/01/2007 05:34 pm »
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Tom Ligon - 1/5/2007  1:29 PM

Yes, you must work the directions of the planetary motions in.  And the effect is primarily in the ecliptic.

To reach Jupiter in the first place, the usual procedure may involve a swing past Venus, back past Earth and/or Luna, another swing past Venus, and off to Jupiter for the big boost.  The mind fairly reels at the math overload, but every pass is a slingshot designed to boost the kinetic energy of the spacecraft.

Not necessarily true.  Depends on the launch vehicle.  PNH. Pioneers 10/11, Ulysses  and Voyager went "straight" to Jupiter.  Galileo and Cassini did intermediate flybys because their LV didn't have enough performance for the given payload mass

#### Jim

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##### Re: Gravitational Assist
« Reply #6 on: 05/01/2007 05:37 pm »
Wikipedia has a good write up

#### kevin-rf

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##### Re: Gravitational Assist
« Reply #7 on: 05/01/2007 06:06 pm »
Actually found and interesting write up from Dr. James Van Allen... http://www.dur.ac.uk/bob.johnson/SL/AJP00448.pdf

Also someone is selling matlab codes to calculate them : http://www.digibuy.com/cgi-bin/product.html?95884747868

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#### Tom Ligon

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##### Re: Gravitational Assist
« Reply #8 on: 05/01/2007 06:14 pm »
Jim,

Ah, those were the good old days!  Use a big rocket, skip the Venus/Luna slingshots.

If Dr. Bussard's idea works out, we can forget all this slingshot nonsense, stop worrying so much about waiting until the planets line up favorably, and just get in the rocket and GO!  Flash Gordon, watch out!

#### BarryKirk

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##### Re: Gravitational Assist
« Reply #9 on: 05/01/2007 07:58 pm »
Use the slingshot for really heavy payloads that don't need to arrive for a long time.

#### Jim

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##### Re: Gravitational Assist
« Reply #10 on: 05/01/2007 08:47 pm »
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Tom Ligon - 1/5/2007  2:14 PM
Luna slingshots.

The moon was never used

#### kevin-rf

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##### Re: Gravitational Assist
« Reply #11 on: 05/01/2007 08:59 pm »
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Jim - 1/5/2007  3:47 PM

Quote
Tom Ligon - 1/5/2007  2:14 PM
Luna slingshots.

The moon was never used

Jim, Actually the Wikipedia article you sited earlier reffers to the Apollo 13 free return as a form of gravity assist.

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

Though it would put an incredible set of launch window constraints on any mission to the outer/inner solar system.
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#### Jorge

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##### Re: Gravitational Assist
« Reply #12 on: 05/01/2007 09:04 pm »
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Tom Ligon - 1/5/2007  1:14 PM

Jim,

Ah, those were the good old days!  Use a big rocket, skip the Venus/Luna slingshots.

You didn't read Jim's reply carefully enough. It is not just the size of the rocket that matters, it's the size of the payload. The Titan IV rocket that required gravitational assists to get Cassini to Saturn was in fact more powerful than the Titan III rockets that sent the Voyagers directly to Jupiter. The gravity assists were required because Cassini was quite a bit heavier than the Voyagers.

Going back to the "good old days" requires smaller spacecraft, not bigger rockets. New Horizons is a good example of this; it was able to do a direct approach to Jupiter because it was quite a bit lighter than Cassini.
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#### kevin-rf

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##### Re: Gravitational Assist
« Reply #13 on: 05/01/2007 09:09 pm »
Actually the just launched Stereo mission used a lunar gravity assist.

For some reason I thought ICE (formerly ISEE-3) used a lunar assist to get to Comet Giacobini-Zimmer back in the eighties...
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#### Tom Ligon

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##### Re: Gravitational Assist
« Reply #14 on: 05/01/2007 09:10 pm »
I understand that.  But in the good old days, when we still used roman numerals, we had the Saturn V, and things like Nova were on the drawing boards.

#### mong'

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##### Re: Gravitational Assist
« Reply #15 on: 05/01/2007 11:18 pm »
you could do a powered flyby around the sun, use jupiter to send your probe to a heliocentric orbit with a very low perihelion then apply a 10km/sec DeltaV, that should send you out of the solar system at around 100 Km/s, reaching alpha centauri in about 12,000 years.
with high end chemical/low end nuclear propulsion that's as good as flybys get.

#### Jim

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##### Re: Gravitational Assist
« Reply #16 on: 05/01/2007 11:25 pm »
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kevin-rf - 1/5/2007  5:09 PM

Actually the just launched Stereo mission used a lunar gravity assist.

For some reason I thought ICE (formerly ISEE-3) used a lunar assist to get to Comet Giacobini-Zimmer back in the eighties...

I was thinking outer planets

#### Ankle-bone12

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##### Re: Gravitational Assist
« Reply #17 on: 05/03/2007 09:09 pm »
When a probe uses a gravity assist and takes a planets ( say Jupiter) gravity, does Jupiter eventually regain that lost "orbit" from when it slows down? if not, then we could theoretically send millions of Gravity Assist probes past jupiter and its rotation would slow drastically.

Sombody please correct me as i think i am wrong here.
Alex B.

#### Jim

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##### Re: Gravitational Assist
« Reply #18 on: 05/03/2007 09:45 pm »
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Ankle-bone12 - 3/5/2007  5:09 PM

When a probe uses a gravity assist and takes a planets ( say Jupiter) gravity, does Jupiter eventually regain that lost "orbit" from when it slows down? if not, then we could theoretically send millions of Gravity Assist probes past jupiter and its rotation would slow drastically.

Sombody please correct me as i think i am wrong here.

Correct

it is a momentum exchange.  The amount gained by the spacecraft is lost by the planet.  But it is not the planet rotation that gravity assist affects, it is the orbit around the sun

#### Ankle-bone12

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##### Re: Gravitational Assist
« Reply #19 on: 05/04/2007 09:30 pm »
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Jim - 3/5/2007  4:45 PM

Quote
Ankle-bone12 - 3/5/2007  5:09 PM

When a probe uses a gravity assist and takes a planets ( say Jupiter) gravity, does Jupiter eventually regain that lost "orbit" from when it slows down? if not, then we could theoretically send millions of Gravity Assist probes past jupiter and its rotation would slow drastically.

Sombody please correct me as i think i am wrong here.

Correct

it is a momentum exchange.  The amount gained by the spacecraft is lost by the planet.  But it is not the planet rotation that gravity assist affects, it is the orbit around the sun

Thanks so much for clarifying that. I can now sleep at night knowing that The planets will never stop spinning, ( though they may someday become a fixed star in are sky some day,.. if humans still around that is.
Alex B.

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