
Gravity assist maneuvers in space - uncertainquark
https://jatan.space/daring-gravity-assist-maneuvers/
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chupa-chups
Nice article. For even more information regarding gravity assists (and related
effects like the Oberth effect) I found these articles in my comfort zone
between too much detail and too superficial:

[https://aapt.scitation.org/doi/full/10.1119/1.2341882](https://aapt.scitation.org/doi/full/10.1119/1.2341882)
\- sorry, just learnt this is not an open access article

[https://aapt.scitation.org/doi/full/10.1119/1.5126818](https://aapt.scitation.org/doi/full/10.1119/1.5126818)
(Oberth Effect)

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TeMPOraL
> _sorry, just learnt this is not an open access article_

There is no such thing as "not open access article".

[https://sci-hub.se/https://doi.org/10.1119/1.2341882](https://sci-
hub.se/https://doi.org/10.1119/1.2341882)

(With a big thank you to Alexandra Elbakyan.)

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davidkuhta
For fans of the "Rich Purnell Maneuver" from The Martian, check out the NASA
paper:
[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201500...](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019662.pdf)

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ilogik
relevant scott manley

[https://www.youtube.com/watch?v=sF8roW6Cxm0](https://www.youtube.com/watch?v=sF8roW6Cxm0)

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ashtonkem
KSP taught me more about orbital maneuvers than I expected when I bought it.

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Ididntdothis
I find it hard to get my head around gravity assists. Whatever energy you may
be gaining from approaching a planet should be lost once you leave the planet.
What am i missing?

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gorgoiler
As well as the wrecking ball analogy there’s a second effect: if you fall in
with one mass but emerge with a lighter mass, and energy is conserved, you’ll
come out faster. So how do you lose mass at the bottom of the gravity well?
Burn some fuel!

Another good reason to burn fuel at the bottom of the gravity well is that
it’s more efficient to eject rocket gas when you are moving faster. Burning
and ejecting gas gives it energy (the exhaust velocity from your chemical
reaction) but if you do so while moving forward at the same velocity, you
effectively leave the exhaust gas behind you standing still with respect to
the universe, which is maximally efficient.

Put another way: if you propel yourself through deep space by throwing bricks
behind you at 30mph, then this method of propulsion becomes most efficient at
30mph. Each brick you throw backwards ends up standing still, as you sail off
at what is now 30.1mph.

(Presumably it’s only inefficient to burn fuel when you are moving slower than
exhaust velocity, and not if you are moving at or faster than exhaust
velocity?)

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effie
> _if you fall in with one mass but emerge with a lighter mass, and energy is
> conserved, you’ll come out faster._

It doesn't work that way. If a body loses mass, it loses also kinetic energy
that mass was carrying. To make losing mass helpful for acceleration, the mass
must be given high enough momentum, i.e. fuel must be burned, not just
ejected.

> _if you do so while moving forward at the same velocity, you effectively
> leave the exhaust gas behind you standing still with respect to the
> universe, which is maximally efficient._

That standing still happens only at one instant of an idealized scenario. It
is not the maximum fuel efficiency either. When the rocket accelerates past
that point, it gets faster and the fuel efficiency becomes even better. The
higher the speed, the better the fuel efficiency (because the same rocket
motor force acts on a faster body, generating more kinetic energy per second).

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rkagerer
_Rosetta would be in Mars’ shadow, having to rely on the limited battery
supply instead of solar panels. The risk was that the batteries weren’t
designed for the task._

Huh? This implies a change in mission parameters or doubt whether the
engineering was adequate. The link in that sentence doesn't mention either.
Just that the craft entered a "sleep" mode to conserve power. I expect the
power subsystem was designed _specifically_ for the task.

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sq_
Interestingly, that's apparently not quite correct. From the ESA link in the
article:

> Rosetta's original trajectory and engineering design did not include an
> eclipse, but unavoidable launch delays forced the trajectory to be
> replanned. Mission controllers working on Rosetta have spent months
> carefully planning and testing a low-power configuration which will allow
> the spacecraft to safely operate on batteries.

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rkagerer
That explains it. I missed that bit in the link, thanks for correcting me!

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sq_
Figured it was something like that :)

No way everyone can click through every link in every article they read. Glad
we were jointly able to bring something interesting from the article to
people's attention.

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caycep
I am just amused at all the gravity assist loop de loops portrayed in The
Expanse. That seems to take just 10 minutes at a time...

At any rate I suppose computers in the future can calculate these multiples of
assist trajectories on the fly...

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kamilner
The executive producer actually wrote a post after the episode in Season 2 as
a bit of an explanation/apology for how it ended up that way, which I thought
was a nice touch: [http://www.danielabraham.com/2017/04/04/guest-post-losing-
sc...](http://www.danielabraham.com/2017/04/04/guest-post-losing-science-
drama-finding-drama-science/)

