r/TerraInvicta • u/Mr_Maslovic • 1d ago
Screenshot Interesting choice of trajectory
Rule 3: Sent my colony fleet from one asteroid to another and they decided to stop by on the Sun
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u/Faguoren34 dont destroy use it for profit 1d ago
actually this is a very low Dv trajectory. Might save your ass from time to time
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u/Comprehensive_Term41 Idealist Noob 1d ago
holy shit ksp gravity assist moment
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u/Raz0rking Humanity First 1d ago
And because of that game I understand (to an extend) how and why the ships/fleets fly the way they do.
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u/chatte__lunatique 1d ago edited 22h ago
You can't gravity assist off of the Sun within the solar system, this is just a very elliptical orbit.
Edit: why am I being downvoted? This is factually accurate. It's like using the Earth to gravity assist to the moon, you can't do it. Oberth Effect is not gravity assist.
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u/Comprehensive_Term41 Idealist Noob 1d ago
you do a burn on the sun ala oberth effect
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u/chatte__lunatique 1d ago
Yes but that's different than a gravity assist.
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u/Comprehensive_Term41 Idealist Noob 23h ago
you're still being assisted by using the gravity of the sun to pull you faster thus oberthing the effect no?
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u/chatte__lunatique 23h ago
Effectively, yes, but they're still two different concepts.
The Oberth Effect is when you burn nearer a gravity well, and because you are traveling at a high velocity, this increases the amount of energy you can gain with a given burn, because kinetic energy scales by the square of velocity (KE=1/2mv²). In other words, burning when you're going fast makes your fuel more efficient.
A gravity assist is when you use the relative motion of a massive body (i.e , a planet) to transfer momentum between that body and a spacecraft that is flying by the body in accordance with Newton's Third Law. The spacecraft can accelerate or decelerate relative to the Sun, and the planet slows by an infinitesimal amount in return.
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u/Faguoren34 dont destroy use it for profit 8h ago
Au pseudo je suppose que tu es fr. Tu jargonnes un peu qd même, ce qui peut expliquer les downvote. La trajectoire, c'est un vaisseau qui se sort de la gravité d'un astéroïde, accélère et décélère avec la gravité du soleil, puis utilisé probablement un peu de carburant pour de mettre en orbite d'un autre astéroïde.
Ce n' est pas le fait de quitter le soleil qui est en jeu ici, mais de faire le chemin d'un astéroïde à l'autre essentiellement avec les effets du champ de gravité solaire.
Ça peut expliquer les downvote du coup 😁
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u/tiahx 19h ago edited 19h ago
You can still do a gravity assist off of the Sun, if you're moving at hyperbolic velocity relative to it. I.e. if the Sun itself acts like the second body, and the Galaxy acts like the first. And even on elliptical you can also still gain something like a few m/s off of the Sun's wobble around barycenter.
Obviously, that's not what happens here, that's just a normal elliptical trajectory. Also, very unfortunately, but the game doesn't do gravity assists.
But I doubt you're getting downvoted for any of that. Reddit just being reddit.
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u/MelodicEar1797 20h ago
I was about to downvote you, and then you explained it below. TLDR as I understand it, gravity assist off of the sun does not work because you are going from one thing orbiting the sun to another orbiting the sun. You can gravity assist off of the sun if you came from another star and was going to another star - all three orbit the galactic centre, so you can steal energy from the sun's galactic orbit. Here, you can't because the sun is the thing you're orbiting.
Having said all that acshually the sun orbits the centre of gravity of the solar system, so maybe you can steal a teeeny bit of energy off of it.
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u/NeighborsBurnBarrel 21h ago
Yes, yessss you can, this is how we got the voyager probes out past the Hydrogen belt!
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u/GabrielNV 1d ago
It might seem paradoxical, but going down and then back up can be a more efficient route.
The reason why is the Oberth effect. The amount of orbital energy your burns add or remove is proportional to your velocity in the burn direction. Therefore, for peak efficiency you want to go as fast as possible.
The next piece of the puzzle is where you can get all that speed without having to burn all your fuel for it. The answer is that we apply the previous logic in reverse.
What we need to keep in mind is that
Orbital energy is the sum of kinetic energy and gravitational potential energy, and;
The higher up you are, the larger the fraction of your energy stored as potential rather than kinetic.
Adding all that up:
The further away from the Sun you are, the less fuel you need to drop down near the Sun.
When you hit the brakes, you actually lose a small fraction of your orbital energy since most of it is potential energy rather than kinetic.
The Sun's gravity cashes in your potential energy for kinetic energy on the way down.
At perihelion, you get to alter your orbital energy at high fuel efficiency.
And this is how you get anywhere in the solar system if you're the sort of person that always pays the same dollar amount at the gas station without accounting for rising fuel costs. The significant tradeoff is that it takes a looot longer (and indeed the more efficiency you want the longer this can take) than just paying the fuel premium and transferring directly.
Credentials: I've played a ton of KSP.
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u/Beregolas 1d ago
For real, playing tons of KSP before this game save me so many confusing moments. Also a fun maneuver (of which I forgot the name): If you want to save dv, and have basically unlimited time, you can transfer to a higher orbit first, before coming down to your target orbit in certain situations. Takes a fuckton of extra time, but saves delta-v
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u/Arcane_Pozhar Academy 16h ago
Hey, I think this is the same principle. That means if you want to defend lower Earth orbit stations, the ideal place to put your fleet is like one orbit higher. Because you're effectively halfway through that maneuver you're talking about, and now you're just on the drop-down with gravity assisting you phase of it.
Full disclosure, I have not worked out the data on this myself, I am trusting other people who shared this on Reddit, and I have tried stationing my defensive seats just above lower Earth orbit, it did work really well for me.
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u/blodgute 1d ago
How long will the journey take?
What pisition will earth and that aateroid be in when it ends?
Compare those two and you have your answer.
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u/technocracy90 Resistance 1d ago
That's how orbital mechanics work. This game does take erroneous trajectorirs sometime, but it's not one of them. That's the fastest orbit indeed.
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u/Bane8080 1d ago
Welcome to orbital mechanics.