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the Physics seems wrong to me

Question

So I went to the mun by aiming a rocket at it...It was amazing

But when it was time to come back by turning around, I was very disappointed by

the path suggested when approaching Kerbin... It should make the orbit the other direction !

Look at the image to understand what I mean:

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Here is a diagram of the situation:

The red dot is your craft.

You are forgetting that the speed relative to the Mun's SOI is not the same as the speed relative to Kerbin's SOI.  In simpler terms, you are forgetting to add the Mun's trajectory to your trajectory.

P.S. I did the same thing my first time coming back from the Mun.

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I believe what you are observing here, are the mechanics that are involved during a free-return trajectory; which when observed in relation to the the SOI currently affecting your craft, can look a little skewed: This might help:

Quote

I've catched the mun at the right time to go around it; but I arrived "under it". So I wouldn't have to go into mun's direction because "the gravity of it" would eject me at the opposite direction. Thus I would have the return vector to enter Kerbin's SOI with the "mun's anti-velocity-vector".

This is where your confusion is coming from. Yes, relative to the Mun, when you catch it 'under it', its gravitational pull takes you retrograde around it... but relative to Kerbin, you are still orbiting prograde. You would have to leave the Mun's SOI retrogrde, but with enough velocity to have cancelled out the prograde trajectory around Kerbin.

Edited by Chequers
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Your frame of reference is based on what you're orbiting at the moment, so that's all that's changing.

It just looks weird because you are viewing the orbit from two very different frames of reference when you are orbiting the Mun vs. Kerbin.

Notice how everything "jumps" when you reach your encounter? That's your reference point changing.

Edited by Rocket In My Pocket
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Mun grabbed you when you reached its gravity and is carrying you with it. When you try to leave, you still have all the speed it gave you.

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You're entering the Mun's sphere of influence so will accelerate towards it in a textbook gravity slingshot. The Mun is moving relative to Kerbin and your rocket is moving relative to the Mun, so by the time you come out the other side of the Mun's SOI, you've been accelerated relative to Kerbin. If you flew 'behind' the Mun then you'll be accelerated relative to Kerbin and if you went 'ahead' then you'll probably end up with a more elongated orbit with a lower periapsis; if you went in at an angle relative to the Mun's orbit e.g. from the 'top' or 'bottom' then you'll be accelerated in the opposite direction leading to a big increase in inclination.

Generally speaking, larger bodies have larger and more powerful gravity wells, so flying by Gilly will barely affect your speed relative to Eve, but a slingshot around Jool will hurl you clean out of the solar system if you're not careful.

Gravity slingshots are how the Voyager probes managed to visit the entire Solar system without needing vast quantities of fuel. Unfortunately it's very difficult to use them to your advantage in KSP.

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Ok, so, according to you :

it takes a weird angle approching the Kerbin's inffluence, because the ship comes from the mun

and the mun turns counter-clockwise around Kerbin, so the ship must have the same counter-clockwise velocity

even if the outer-mun direction path was made to go clockwise around Kerbin...

I understand your point, but it looks un-natural to my eyes.

Because it means that every time we go into a planet influence, the ship would always go counter-clockwise... because all planet turn the same direction.

According to me, it should be possible to enter Kerbin clockwise, because once we go outer mun, there are 5000 kms, it's far enough to get rid of the mun's velocity direction.

Edited by lopata
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11 hours ago, lopata said:

According to me, it should be possible to enter Kerbin clockwise, because once we go outer mun, there are 5000 kms, it's far enough to get rid of the mun's velocity direction.

Yes, it only takes a smallish burn from orbit of Minmus (I think about 350 m/s?) to counteract its orbital velocity and orbit Kerbin retrograde.

Edited by RoninFrog
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It's not so much about the direction of the moon's travel so much if the moon is positioned prograde/retrograde relative to the path of the ship through the parent SOI. You can get a capture while traveling in the direction of the moon's capture, and get an assist going in the opposite direction. The outcome is increased both instances by following the direction of travel because you are in the moon's SOI longer.

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It's not so much about the direction of the moon's travel so much if the moon is positioned prograde/retrograde relative to the path of the ship through the parent SOI. You can get a capture while traveling in the direction of the moon's capture, and get an assist going in the opposite direction. The outcome is increased both instances by following the direction of travel because you are in the moon's SOI longer.

Unless I misread the question, I think this is dealing with orbital mechanics in Kerbin SOI while leaving Mun orbit/flyby to return to Kerbin.

Edited by RoninFrog
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Thank you RoninFrog, I understand better,

and I am reassured that we can enter Kerbin retrograd from Minmus,
but from the mun we can't :/ ... well I think this is still wrong because:

I've catched the mun at the right time to go around it; but I arrived "under it". So I wouldn't have to go into mun's direction because "the gravity of it" would eject me at the opposite direction. Thus I would have the return vector to enter Kerbin's SOI with the "mun's anti-velocity-vector".

But anyway it's just a game, but it could be better, the rest is amazingly well done.

Edited by lopata
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On 3/24/2020 at 11:14 AM, jimmymcgoochie said:

Gravity slingshots are how the Voyager probes managed to visit the entire Solar system without needing vast quantities of fuel. Unfortunately it's very difficult to use them to your advantage in KSP.

It's even more difficult to use gravity assists in real life, but NASA and other space agencies have a bunch of PhDs available to crunch the numbers.

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16 hours ago, lopata said:

I've catched the mun at the right time to go around it; but I arrived "under it". So I wouldn't have to go into mun's direction because "the gravity of it" would eject me at the opposite direction. Thus I would have the return vector to enter Kerbin's SOI with the "mun's anti-velocity-vector".

Just because you were caught by Mun "at the right time" doesn't negate the fact that Mun is in a counterclockwise (for argument's sake "prograde") orbit. You're deep enough and long enough inside its SOI to make a semi-orbit there. From a Mun perspective you changed your velocity vector with 180°, but from a Kerbin perspective you also picked up that nearly 550 m/s orbital velocity that Mun has. Unless you are going to counteract that, leaving Mun's SOI is going to put you in that counterclockwise ("prograde") orbit around Kerbin.

So, unless your exit vector has a tangential component of at least 550 m/s you will have that prograde orbit, and from what you sketched, your exit vector is almost entirely radial towards Kerbin with virtually no tangential component.

What you should be aiming for is not a 180° trajectory around Mun, but rather a 90° retrograde trajectory, with the exit vector leaving the SOI parallel to the Mun's course, but in the opposite direction, and doing at least 550 m/s when exiting Mun SOI. At that point your return trajectory will be retrograde.

Kerbart, you don't know what you're talking about. It's impossible to return to Kerbin from a Mun flyby in a retrograde orbit. IMPOSSIBLE, I tell you. And that's why the game is broken!

There ya go.

Edited by Kerbart
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Moved to Gameplay Questions

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19 hours ago, lopata said:

But anyway it's just a game, but it could be better, the rest is amazingly well done.

If you play it enough it will start making sense to you, and you'll probably eventually realize that while some of the physics in the game is not accurate, this bit that is currently bothering you actually is accurate.

Edited by mikegarrison
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First: I studied physics and the general trajectories in KSP are doing good on physics (there are some few simplifications which are sure too subtle to notice by common sense alone).

Your problems are with the visualizations in map view, but lukcily these can be changed:

Please note: The movement is still the same, it just changes how to display them to you in respect of different world views.

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On 3/26/2020 at 6:23 PM, lopata said:

Thank you RoninFrog, I understand better,

and I am reassured that we can enter Kerbin retrograd from Minmus,
but from the mun we can't :/ ... well I think this is still wrong because:

I've catched the mun at the right time to go around it; but I arrived "under it". So I wouldn't have to go into mun's direction because "the gravity of it" would eject me at the opposite direction. Thus I would have the return vector to enter Kerbin's SOI with the "mun's anti-velocity-vector".

But anyway it's just a game, but it could be better, the rest is amazingly well done.

Think of it as if the Mun wasn't there at all. Your ship is in a circular orbit  around Kerbin at the height of the Mun. In orber to get back to Kerbin you slow down to drop your periapsis into Kerbins atmosphere. If you wanted to reverse your orbit direction you need to kill all of you forward velocity and then add more velocity to bring your periapsis up out of the planet.

Edited by Reactordrone

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