Using a gravity assist to circularize an orbit

[MechaLynx]

On Scott Manley's video with Ed Lu (

), when Scott asks how much fuel their craft will need to get into position, Ed Lu tells him that they won't need much because they'll use Venus's gravity to circularize.

Now, the entirety of my exposure to orbital mechanics comes from KSP, plus some extra reading here and there, but I am aware that real orbits don't work with some simplified SOI-based model. However, this still puzzles me: Whenever we enter the gravitational sphere of a celestial body in KSP, we need to circularize by adjusting our speed, hence we spend fuel doing so. Even if one considers real orbital trajectories, it still makes sense that it would work that way: if you fly by a planet close enough to use its gravity as assistance, to save fuel, it only makes sense as a maneuver if it's accelerating you and increasing your speed along your current trajectory - from what I understand, that's because your distance from the object and your speed don't correspond to a stable orbit, hence you get a gravity assist due to the force the object is applying on you (which should, by this logic, be the difference of your current speed and the speed of a stable orbit at that distance from its center). Thus, I see two ways Ed Lu's statement can reflect what they're actually doing:

1. Somehow, in the real world, you can pass by an object at lower than escape velocity and have its gravity assist increase your velocity enough to circularize your orbit (not around the object itself, I think their probe is intended to somehow get a circular orbit similar to Venus's) instead of just getting left with a sub-orbital trajectory

2. You pass by it at a high speed, but counter to its rotation, thus losing speed.

The second one is pretty much my question: I know that when you're already on an object, it's wise to use the planet's rotation to get a bit more ÃŽâ€v, but when you're passing by it, does entering a retrograde or prograde orbit matter in terms of how its gravity assist will work or the scale of its effect? As far as I understand, it shouldn't matter at all since the body is pretty much producing a gravitational field and as long as its strength is the same, the way it alters your own energy should be the same, no matter the angle of attack. Do tidal forces apply here?

As an addendum, do we see any strange effects in KSP related to this? For example, if one enters a body's SOI retrograde, prograde or in a polar trajectory, do we get a different kind of gravity assist?

Edited May 11, 2015 by MechaLynx

[Rakaydos]

Hypotheically, lets consider a rocket launched sraight up into the Mun's SoI. it's a suborbital launch, no orbital velocity at all.

But by slingshotting the Mun right, you can put the rocket into an orbit with the same semi major axis as the mun, or smaller (or larger, but limited to a bit less than twice mun's orbital velocty)

[Brotoro]

They intend to use a gravity assist from Venus to lower the aphelion of their spacecraft (making its orbit more circular and closer to the Sun than Earth's orbit). Gravity assists can be used to either speed up a spacecraft or slow it down. What matters is how the direction of the ship's incoming and outgoing velocity vectors compare to the direction Venus is moving. If Venus bends the ship's trajectory closer to the direction of Venus' orbital motion, the ship is sped up by the encounter. If Venus bends the ship's trajectory further from the direction of Venus' orbital motion, the ship is slowed down by the encounter.

[MechaLynx]

Hypotheically, lets consider a rocket launched sraight up into the Mun's SoI. it's a suborbital launch, no orbital velocity at all.

But by slingshotting the Mun right, you can put the rocket into an orbit with the same semi major axis as the mun, or smaller (or larger, but limited to a bit less than twice mun's orbital velocty)

Indeed, I didn't think of that at all.

They intend to use a gravity assist from Venus to lower the aphelion of their spacecraft (making its orbit more circular and closer to the Sun than Earth's orbit). Gravity assists can be used to either speed up a spacecraft or slow it down. What matters is how the direction of the ship's incoming and outgoing velocity vectors compare to the direction Venus is moving. If Venus bends the ship's trajectory closer to the direction of Venus' orbital motion, the ship is sped up by the encounter. If Venus bends the ship's trajectory further from the direction of Venus' orbital motion, the ship is slowed down by the encounter.

This makes perfect sense. If the object pulls the craft counter to its velocity vector it loses speed, if it pulls it prograde it gains speed.

Thanks to you both. Just to clarify, does the rotation speed/direction/tilt of the planet, relative to the craft's vector, have _any_ effect on it at all? If so, is that effect comparable to the effects of its gravity alone or can they be ignored as far as its new trajectory is concerned?

[Rakaydos]

Thanks to you both. Just to clarify, does the rotation speed/direction/tilt of the planet, relative to the craft's vector, have _any_ effect on it at all? If so, is that effect comparable to the effects of its gravity alone or can they be ignored as far as its new trajectory is concerned?

That's a relativistic effect- negligible outside relativistic velocities.

[K^2]

The only way to use an object in perfect circular orbit to gain a perfect circular orbit is lithobraking. That said, if either of these two orbits is elliptical, it's entirely achievable. And this might be what they are talking about. Using Venus to achieve an orbit closer to circular, then burn fuel the rest of the way.

[magnemoe]

They intend to use a gravity assist from Venus to lower the aphelion of their spacecraft (making its orbit more circular and closer to the Sun than Earth's orbit). Gravity assists can be used to either speed up a spacecraft or slow it down. What matters is how the direction of the ship's incoming and outgoing velocity vectors compare to the direction Venus is moving. If Venus bends the ship's trajectory closer to the direction of Venus' orbital motion, the ship is sped up by the encounter. If Venus bends the ship's trajectory further from the direction of Venus' orbital motion, the ship is slowed down by the encounter.

This, has used Jools moons many times to circulate and orbit around Jool, this does not require any fuel outside of the adjustment burns, however you don't gain energy you are raising Pe and lower Ap.

Note that you can use Laythe's atmosphere to raise Pe around Jool while keeping Ap high but in this setting you take energy from Laythe