Where the energy of gravity assist comes from ?

[Tatonf]

Hello there !

I have a simple question : when I perform a gravity assist, my ship is gaining energy. But, as stated by the 1st law of thermodynamic, the energy cannot by created nor destroyed, it can only be transferred. So, if my ship is gaining energy, it means that something else must lose energy.
What is this other thing ? The planet that gives me the GA ? Do I slow it down each time I perform one ?
I noticed that, the higher the gravity is, the more energy I can gain from a GA. That seems indepedant of it's position in space or it's rotational speed, so I don't fully understand it. Edited November 21, 2015 by Tatonf

[FancyMouse]

Yes the involved planet is losing energy, which is negligible considering its mass.

[RocketSquid]

In KSP, the energy is simply created. But in the real world, gravity works both ways. The spacecraft is using the planet's gravity to steal momentum from the planet.

[Red Iron Crown]

It's like throwing a tennis ball at the front of a moving semi truck, some momentum is transferred from the truck to the ball but only the ball's speed changes significantly. In KSP the planet's infinitesimal loss of momentum is rounded down to zero.

[Tatonf]

And what do they lose exactly ? Rotational speed around themselves ? Or around their parent body ?

[Snikersnee]

Potential energy converted to kinetic? Maybe?

[Sanic]

[quote name='Tatonf']And what do they lose exactly ? Rotational speed around themselves ? Or around their parent body ?[/QUOTE]
Nothing in KSP :D

[FancyMouse]

[quote name='Tatonf']And what do they lose exactly ? Rotational speed around themselves ? Or around their parent body ?[/QUOTE]
Nothing angular involved - you don't gain/lose angular momentum because of gravity, so the planet doesn't gain/lose angular momentum (thus rotational speed).
The speed that changed is the planet's orbital speed.

It's even a few [I]magnitudes [/I]less than the effect of n-body gravitation, so really not worth emulating at all.

[Red Iron Crown]

[quote name='Tatonf']And what do they lose exactly ? Rotational speed around themselves ? Or around their parent body ?[/QUOTE]
Around the parent body.

[xiej]

The Spacecraft is pulling on the parent body, so the parent body gets slowed down a little by the spacecraft's gravity. So, you could change the orbit of Earth by using millions of asteroids over thousands of years, each pulling on Earth just a little, but adding up over time. Also, here's how to change the thread to 'Answered'
[URL]http://forum.kerbalspaceprogram.com/threads/85063-How-to-change-Unanswered-to-Answered[/URL]

[Jovus]

[quote name='FancyMouse']Nothing angular involved - you don't gain/lose angular momentum because of gravity, so the planet doesn't gain/lose angular momentum (thus rotational speed).
The speed that changed is the planet's orbital speed.
[/QUOTE]

False. You do lose both rotational speed and orbital speed. Further, even if you only lost orbital speed, angular momentum would be lost, since the total angular momentum of a rigid body is the angular momentum of the center of mass plus the angular momentum of each piece of the body around its center of mass.

(Yes, sometimes you gain rotational angular momentum instead of lose it. Depends on the direction of rotation and the geometry of the gravity assist.)

[Tatonf]

So, question : If a planet were staying still in space (meaning it doesn't rotate around itself), would it be able to provide gravity assist ?

[FancyMouse]

[quote name='Jovus']False. You do lose both rotational speed and orbital speed. Further, even if you only lost orbital speed, angular momentum would be lost, since the total angular momentum of a rigid body is the angular momentum of the center of mass plus the angular momentum of each piece of the body around its center of mass.

(Yes, sometimes you gain rotational angular momentum instead of lose it. Depends on the direction of rotation and the geometry of the gravity assist.)[/QUOTE]
Unless you're talking about planet oblateness - otherwise (to be precise, assuming spherical symmetry of planet mass distribution) gravity force passes through both CoM of the ship and planet. How could that change angular momentum of either object?

[Red Iron Crown]

[quote name='Tatonf']So, question : If a planet were staying still in space (meaning it doesn't rotate around itself), would it be able to provide gravity assist ?[/QUOTE]
This gets a bit confusing, because there's no such thing as "staying still in space", it depends on the frame of reference.

Perhaps it's best to think of it in terms of spheres of influence: To use a gravity assist, a vessel must both enter and exit the SoI of the body. So you can't use the parent body as a gravity assist (e.g. no using Kerbin for a gravity assist when departing from the Mun or Minmus).

[Korizan]

Don't forget the planet is gravitationally locked to the sun so when you try and change the planets path you are really arguing with the sun.
And well considering how much mass the sun loses every second...
So other then math the practical answer is zero

[Combatsmithen]

For me. Gravity assist has never worked. Because you would enter the gravity field. And then on your way back out of the gravity field you would lose all the energy you gained and change your course significantly. So i never see a point in doing it anyway

[Korizan]

[quote name='Combatsmithen']For me. Gravity assist has never worked. Because you would enter the gravity field. And then on your way back out of the gravity field you would lose all the energy you gained and change your course significantly. So i never see a point in doing it anyway[/QUOTE]

Actually you have stated the exact concept of gravity assist.
We use gravity to make a CHANGE in direction to our benefit.
Something that would otherwise cost us fuel.
And yes there is a balance involved but what we are doing is more about passing the ball (ship) between friends (planets) and providing just enough force (power) to do it.

[Red Iron Crown]

[quote name='Combatsmithen']For me. Gravity assist has never worked. Because you would enter the gravity field. And then on your way back out of the gravity field you would lose all the energy you gained and change your course significantly. So i never see a point in doing it anyway[/QUOTE]
There is a net energy change in the vessel after entering and exiting a body's SoI.

[More Boosters]

[quote name='Combatsmithen']For me. Gravity assist has never worked. Because you would enter the gravity field. And then on your way back out of the gravity field you would lose all the energy you gained and change your course significantly. So i never see a point in doing it anyway[/QUOTE]

[quote name='Korizan']Actually you have stated the exact concept of gravity assist.
We use gravity to make a CHANGE in direction to our benefit.
Something that would otherwise cost us fuel.
And yes there is a balance involved but what we are doing is more about passing the ball (ship) between friends (planets) and providing just enough force (power) to do it.[/QUOTE]

I see some misconceptions here. It's not just about changing direction; you do go faster (or slower if that's what you went for), this happens because the planet is still moving with respect to the sun while it pulls you.

[EladDv]

[quote name='Red Iron Crown']There is a net energy change in the vessel after entering and exiting a body's SoI.[/QUOTE]
since energy is a scalar and gravity can only transfer energy from kinetic to potential energy and in the reverse when you get out of an SOI in KSP (which is a sphere and since that the potential energy is the same) gravity assists DON'T change your energy at all BUT you momentum does change since it is a vector it's direction is changed by the constant force towards the planet that is perpendicular to the vessel's velocity, still since the energy dosnt change your speed dosnt too. The perceived net energy gain is from the energy you'd need to expend in order to change your vector into the one you get at the end of assist (i am not a professional physicist but still a uni student in physics)

[Tatonf]

[quote name='Red Iron Crown']This gets a bit confusing, because there's no such thing as "staying still in space", it depends on the frame of reference.

Perhaps it's best to think of it in terms of spheres of influence: To use a gravity assist, a vessel must both enter and exit the SoI of the body. So you can't use the parent body as a gravity assist (e.g. no using Kerbin for a gravity assist when departing from the Mun or Minmus).[/QUOTE]

Sorry, what I meant is : what if a planet were to not rotate on itself ? That's something that happen IRL too I think (not in our solar system).
Would it still being able to provide gravity assist ?

It depends of wheter Jovus is right or not.

[Red Iron Crown]

[quote name='EladDv']since energy is a scalar and gravity can only transfer energy from kinetic to potential energy and in the reverse when you get out of an SOI in KSP (which is a sphere and since that the potential energy is the same) gravity assists DON'T change your energy at all BUT you momentum does change since it is a vector it's direction is changed by the constant force towards the planet that is perpendicular to the vessel's velocity, still since the energy dosnt change your speed dosnt too. The perceived net energy gain is from the energy you'd need to expend in order to change your vector into the one you get at the end of assist (i am not a professional physicist but still a uni student in physics)[/QUOTE]
Have to disagree here. It doesn't change your speed relative to the body being used for the slingshot, but it does relative to the parent body. After exiting the SoI of the slingshot target, there will be either an increase or decrease of the vessel's semi-major axis compared to before entering that SoI, which means an increase or decrease of the vessel's orbital energy. A gravity assist doesn't just change direction, if it did the SMA would remain the same.

[quote name='Tatonf']Sorry, what I meant is : what if a planet were to not rotate on itself ? That's something that happen IRL too I think (not in our solar system).
Would it still being able to provide gravity assist ?

It depends of wheter Jovus is right or not.[/QUOTE]
A non-rotating body can still provide a gravity assist.

For spherical bodies (not a thing IRL), no rotational momentum is exchanged between the vessel and body. For non-uniform bodies, a very small amount of the body's rotation will be lost or gained, but it is dwarfed by the already incredibly tiny change in orbital velocity. Edited November 21, 2015 by Red Iron Crown

[Tatonf]

[quote name='EladDv']since energy is a scalar and gravity can only transfer energy from kinetic to potential energy and in the reverse when you get out of an SOI in KSP (which is a sphere and since that the potential energy is the same) gravity assists DON'T change your energy at all BUT you momentum does change since it is a vector it's direction is changed by the constant force towards the planet that is perpendicular to the vessel's velocity, still since the energy dosnt change your speed dosnt too. The perceived net energy gain is from the energy you'd need to expend in order to change your vector into the one you get at the end of assist (i am not a professional physicist but still a uni student in physics)[/QUOTE]

You can't change your orbit without changing the kinetic energy of the ship, i'm pretty sure of this. What you say is very confusing.

[EladDv]

[quote name='Red Iron Crown']Have to disagree here. It doesn't change your speed relative to the body being used for the slingshot, but it does relative to the parent body. After exiting the SoI of the slingshot target, there will be either an increase or decrease of the vessel's semi-major axis compared to before entering that SoI, which means an increase or decrease of the vessel's orbital energy. A gravity assist doesn't just change direction, if it did the SMA would remain the same.[/QUOTE]
of course. but that depends on the angle of the planet in relation to the original orbital motion, my explanation came from a single body perspective and not talking about the rest of the planetary motion of the vessel, inside the SOI and relative to the used body your energy dosnt change, but in relation to another body yeah it will it's all about frames of reference as if you took the sun compared to kerbin you'd see the force affecting the trajectory and kerbin's one will change too, there is no energy created from nothing(unlike in kerbal where it IS created from nothing because of the on-rails system)
[FONT=Verdana][quote name='Tatonf']You can't change your orbit without changing the kinetic energy of the ship, i'm pretty sure of this. What you say is very confusing.[/QUOTE]
kinetic energy yes, mechanic energy no. at the same distance from a parent body(in a simple single body system like ksp RIC) your kinetic energy wont change since the work of gravity is based upon a delta in distance not the actual distance covered(cant remember english term) also any force applied perpendicular to the velocity vector wont change your kinetic energy since again it's a scalar and has no "direction" see- magnetic force always applies perpendicular to an objects speed and the magnetic field's direction and thus can never accelerate a projectile at any of those directions[/FONT] Edited November 21, 2015 by EladDv

[pellinor]

[quote name='EladDv']since energy is a scalar and gravity can only transfer energy from kinetic to potential energy and in the reverse when you get out of an SOI in KSP (which is a sphere and since that the potential energy is the same) gravity assists DON'T change your energy at all BUT you momentum does change since it is a vector it's direction is changed by the constant force towards the planet that is perpendicular to the vessel's velocity, still since the energy dosnt change your speed dosnt too. The perceived net energy gain is from the energy you'd need to expend in order to change your vector into the one you get at the end of assist (i am not a professional physicist but still a uni student in physics)[/QUOTE]

Within the reference frame/SOI of the body you are right. There is a static gravity field and conservation of energy holds, so the gravity assist can only change the direction but not the magnitude of your speed.

In the reference frame of the parent, things look different. As an example, model the gravity of Mun in the reference Frame of Kerbin. You can do this either with a time-dependent gravity field (the Mun moves), or with a rotating reference frame (coordinates rotate so that the Mun stays fixed). Both of these models break energy conservation.

Example: Say a vessel stands still at the height of the Mun orbit (zero kinetic energy). (Say it spends a short time there so that falling towards kerbin is not significant) The Mun approaches, when the SOI catches the vessel, it has a speed "v" relative to Mun which is conserved. It passes Mun's SOI on a hyperbolic trajectory, exiting in a different direction with the same relative speed. Now back in Kerbin's SOI the vessel has a speed (up to 2v), and therefore gained considerable kinetic energy. Edited November 21, 2015 by pellinor