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In this answer to the gedankenexperiment question With current state of technology if we wanted how long would it take for newly launched probe to overtake Voyagers? I've seen the term "solar gravity well manouver" mentioned as a way to go fast and catch up to the Voyagers.

If I come up to Jupiter from behind I can pick up some energy in the heliocentric frame via a gravity assist and thereby head out of the solar system faster. But I don't see the utility of heading towards the Sun first before heading out; after a decade wouldn't I just be back at 5.5 AU going the the direction and speed that I could have been had I swung by Jupiter a different way?

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    $\begingroup$ Maybe where isn't stated clear enough. It means "Oberth maneuver" at the Sun, wuth rocket burn at perigelium. $\endgroup$
    – Heopps
    May 21, 2020 at 12:13
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    $\begingroup$ @Heopps: Would you like to write that as a proper answer? $\endgroup$
    – DrSheldon
    May 21, 2020 at 12:24
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    $\begingroup$ A vehicle barely needs any propellant to escape the solar system at a very high velocity with this technique. A solar sail that doesn't melt during perihelion passage, plus perhaps a bit of propellant for attitude control, are all that are needed. See nasa.gov/pdf/740774main_NosanovSpringSymposium2013.pdf , for example. $\endgroup$
    – David Hammen
    May 21, 2020 at 13:28
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    $\begingroup$ Also known as the Oberth-Kupiter maneuver. explainxkcd.com/wiki/index.php/1244:_Six_Words (explainxkcd link because xkcd isn't mobile friendly) $\endgroup$
    – Joshua
    May 21, 2020 at 18:35
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    $\begingroup$ It's angular momentum you have to lose to get close to the Sun, not energy. $\endgroup$
    – Steve Linton
    May 21, 2020 at 21:10

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This is an Oberth maneuver, getting the most out of your delta-v budget by adding velocity on top of an already high velocity.

Nowhere in the solar system does one achieve greater orbital velocities than during perihelion of a Sun dive, as close as thermal management allows. At that ~100km/s velocity, every km/s of velocity added corresponds to ~14km/s at solar system escape, or roughly what the Voyagers are travelling at.

This is not a gravity assist, in the usual sense, as a flyby of the Sun can not by definition increase heliocentric velocity on its own. It's a nice position to spend a propellant budget. Traditional gravity assists would still be useful for getting into the sun dive in the first place.

A related use of this maneuver is for solar sails, where near the Sun in addition to being a nice place to velocity also is the place a solar sail can gain the most velocity.

"in what cases would it be helpful?"
Pretty much never. While it allows our current propulsion technology to go a little faster into interstellar space, it's still too slow to reach neighbouring star system in any reasonable case. That thought experiment about retrieving the Voyagers is perhaps the only "practical" use case.

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  • $\begingroup$ About the "pretty much never" part, would it be helpful in building a ~10 year New Horizons-like mission to Pluto that could enter orbit there? The payload would be a lot heavier with the extra fuel, would a 3rd state that could pass near the Sun somehow help in this case? [Was there any launch vehicle possible that could have been used for a heavier New Horizons with enough fuel to enter Pluto orbit? (adding ~10 years)](space.stackexchange.com/q/53878/12102_ $\endgroup$
    – uhoh
    Jul 9, 2021 at 12:12
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    $\begingroup$ @uhoh My gut feeling is that since you would have to use Jupiter or something equivalent to even get the trajectory started my guess (not rigourious) is that just going out directly is the better option. $\endgroup$
    – SE - stop firing the good guys
    Jul 9, 2021 at 15:37

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