# NASASpaceFlight.com Forum

## General Discussion => Advanced Concepts => Topic started by: on 05/01/2007 04:04 pm

Title: Gravitational Assist
Post by: 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?
Title: RE: Gravitational Assist
Post by: Jim 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.
Title: RE: Gravitational Assist
Post by: BarryKirk 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.
Title: Re: Gravitational Assist
Post by: kevin-rf 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?
Title: Re: Gravitational Assist
Post by: Tom Ligon 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.
Title: Re: Gravitational Assist
Post by: Jim 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
Title: Re: Gravitational Assist
Post by: Jim on 05/01/2007 05:37 pm
Wikipedia has a good write up
Title: Re: Gravitational Assist
Post by: kevin-rf 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

Title: Re: Gravitational Assist
Post by: Tom Ligon 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!
Title: Re: Gravitational Assist
Post by: BarryKirk on 05/01/2007 07:58 pm
Use the slingshot for really heavy payloads that don't need to arrive for a long time.
Title: Re: Gravitational Assist
Post by: Jim 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
Title: Re: Gravitational Assist
Post by: kevin-rf on 05/01/2007 08:59 pm
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Jim - 1/5/2007  3:47 PM

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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.
Title: Re: Gravitational Assist
Post by: Jorge 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.
--
JRF
Title: Re: Gravitational Assist
Post by: kevin-rf 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...
Title: Re: Gravitational Assist
Post by: Tom Ligon 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.
Title: Re: Gravitational Assist
Post by: mong' 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.
Title: Re: Gravitational Assist
Post by: Jim 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
Title: Re: Gravitational Assist
Post by: Ankle-bone12 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.
Title: Re: Gravitational Assist
Post by: Jim 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
Title: Re: Gravitational Assist
Post by: Ankle-bone12 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.
Title: Re: Gravitational Assist
Post by: Jim on 05/04/2007 10:00 pm
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Ankle-bone12 - 4/5/2007  5:30 PM

Quote
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.

Actually, because of the lost of momentum, the orbital energy around the sun is decrease and the planets's orbit is smaller and the periond decreases
Title: Re: Gravitational Assist
Post by: Tom Ligon on 05/04/2007 10:20 pm
I've not worked out the math myself, but Dr. Bussard once told me there is a second way to use a gravity well to pick up energy.  If you've got to burn some rocket fuel anyway, and your trajectory takes you in to a gravity well, try to arrange to do your burns at or near the bottom, when your velocity is highest.

A given burn produces a specific delta-v.  Do that delta-v at higher velocity and the kinetic energy gain is higher than if you do the burn moving slower out in flater space.

Slingshot maneuvers take advantage of the relative velocity of the object you're using to do it ... you're doing a direction change into the orbital direction of the that object to pick up speed.  The Sun is pretty much stationary in reference to the rest of the solar system, so tends to be fairly useless for slingshots in the system.  But it sits in a heckuva deep gravity well, and doing a burn close in to it may make sense, if your trajectory is going to take you there.
Title: Re: Gravitational Assist
Post by: imfan on 05/04/2007 11:10 pm
I cannot resist to promote Orbiter space simulator. www.orbitersim.com It allows to plan also complex slingshot trajectories like Voyager, not even speaking of basic orbital maneuvers.