gravity assists are making my head hurt

[noobsrtoast]

ive been practicing for like 5 hours now and i even tried to get into the math behind it, my brain is toast, is there a good guide out there or can someone help simplify it for me, i just dont get how you can line up exactly for a gravity assist that brings you directly onto an encounter with another body, my brain is fried 

[Xyphos]

half of it is luck, the other half is pure skill.
I don't have much luck using them to accelerate, but I can definitely use skill to decelerate to orbit.

[Superfluous J]

I find it easier to visualize than math/science out.

Moons are orbiting planets. If you enter the moon's sphere of influence from the side, and exit it out forwards (these are both relative to the moon's motion around the planet), then your ship will add the speed of the moon to its own speed, when looking relative to the planet. This gives you a "boost." Likewise, if you come in at the front of the Moon's SOI and exit the side, when you exit you'll be going much slower relative to the planet.

The big idea is, relative to the moon you leave at the same speed you come in, so relative to the planet you can gain a (sometimes huge) speed boost or slowdown based on where you go into and exit from the moon. Once you have this down, the rest is just fiddling with maneuver nodes and observing how your resultant orbit (or escape) will go when you enter the moon's sphere of influence at certain points going in certain directions at certain speeds.

I should make a video about this. Or at least provide pictures.

[FancyMouse]

I thought OP is concerned about using gravity assist to encounter another planet, not gravity assist itself?

Honestly, for that I don't know other than numeric searches. There is a flyby finder tool somewhere in the forum but I'm not familiar with that. And also, given the departure time, gravity assist time, and destination time, we might be able to compute the trajectory (along with the delta V to burn in the middle planet) using the Lambert solver that computes interplanetary transfer. However I don't know enough detail of the algorithm to tell anything further.

[Plusck]

To line up for it, assuming you're in an interplanetary transfer, drop a node about an eighth of an orbit before you meet the major SOI, and use combined pro/retrograde and radial in/out to set up the encounter.

I wouldn't worry about the maths, just try to imagine the vectors: they add up or subtract as you pass from one SOI to another.

Pathetically basic diagram:

So at point A, you have a very large horizontal vector and a much lower vertical one. This combined vector lies on an escape trajectory around the planet.

You meet the SOI which is also moving, so the horizontal vector is partly cancelled out. The vertical isn't. That means you are on a more sedate escape orbit around the moon.

The moon whips you around and sends you towards the planet. It therefore adds to the vertical vector and subtracts from the horizontal vector.

When you leave the moon's SOI, the SOI's own speed vector is added to your escape velocity. That means a modest boost to your near-zero horizontal vector, and little change to the vertical one. This result (at point E) is on a completely different orbit, which is nowhere near fast enough to escape the planet's gravity. Your passage around the moon has therefore converted a fast, interplanetary escape orbit into a captured orbit around the planet.

Of course, the same principle applies the other way around or if you are using Eve to gravity-assist into a lower Kerbol orbit. It's just a question of imagining the vectors. Or imagining throwing metal balls at high speed past magnets, or whatever...

[noobsrtoast]

Just now, 5thHorseman said:

I find it easier to visualize than math/science out.

Moons are orbiting planets. If you enter the moon's sphere of influence from the side, and exit it out forwards (these are both relative to the moon's motion around the planet), then your ship will add the speed of the moon to its own speed, when looking relative to the planet. This gives you a "boost." Likewise, if you come in at the front of the Moon's SOI and exit the side, when you exit you'll be going much slower relative to the planet.

The big idea is, relative to the moon you leave at the same speed you come in, so relative to the planet you can gain a (sometimes huge) speed boost or slowdown based on where you go into and exit from the moon. Once you have this down, the rest is just fiddling with maneuver nodes and observing how your resultant orbit (or escape) will go when you enter the moon's sphere of influence at certain points going in certain directions at certain speeds.

I should make a video about this. Or at least provide pictures.

by the looks of things, unless you have a degree in orbital mechanics, its guesswork, cool ill keep that in mind, thanks

 

[Plusck]

5 minutes ago, noobsrtoast said:

by the looks of things, unless you have a degree in orbital mechanics, its guesswork, cool ill keep that in mind, thanks

 

Absolutely. You could work out all of the maths and get the right answer, but the human brain is a bit slow at doing that. However, any kid can learn to catch a ball, and any golfer can guess where a putt is going to go. Same with orbital mechanics and KSP: once you start to visualise it, you can guess pretty accurately what a planet or moon is going to do to your orbit with a gravity assist. "Just guessing" is a bit dismissive tbh.

[noobsrtoast]

Just now, Plusck said:

Absolutely. You could work out all of the maths and get the right answer, but the human brain is a bit slow at doing that. However, any kid can learn to catch a ball, and any golfer can guess where a putt is going to go. Same with orbital mechanics and KSP: once you start to visualise it, you can guess pretty accurately what a planet or moon is going to do to your orbit with a gravity assist. "Just guessing" is a bit dismissive tbh.

ah so its a bit like learning to snipe in battlefield, thats a great way of putting it, as a side note i am in the process of teaching myself the math, i decided to take it on so that i could calculate these things accurately one day, i practice for about an hour every day

Just now, Plusck said:

To line up for it, assuming you're in an interplanetary transfer, drop a node about an eighth of an orbit before you meet the major SOI, and use combined pro/retrograde and radial in/out to set up the encounter.

I wouldn't worry about the maths, just try to imagine the vectors: they add up or subtract as you pass from one SOI to another.

Pathetically basic diagram:

So at point A, you have a very large horizontal vector and a much lower vertical one. This combined vector lies on an escape trajectory around the planet.

You meet the SOI which is also moving, so the horizontal vector is partly cancelled out. The vertical isn't. That means you are on a more sedate escape orbit around the moon.

The moon whips you around and sends you towards the planet. It therefore adds to the vertical vector and subtracts from the horizontal vector.

When you leave the moon's SOI, the SOI's own speed vector is added to your escape velocity. That means a modest boost to your near-zero horizontal vector, and little change to the vertical one. This result (at point E) is on a completely different orbit, which is nowhere near fast enough to escape the planet's gravity. Your passage around the moon has therefore converted a fast, interplanetary escape orbit into a captured orbit around the planet.

Of course, the same principle applies the other way around or if you are using Eve to gravity-assist into a lower Kerbol orbit. It's just a question of imagining the vectors. Or imagining throwing metal balls at high speed past magnets, or whatever...

i dont think that diagram is pathetic, ill definitely use this, thank you

 

[Plusck]

2 minutes ago, noobsrtoast said:

ah so its a bit like learning to snipe in battlefield, thats a great way of putting it, as a side note i am in the process of teaching myself the math, i decided to take it on so that i could calculate these things accurately one day, i practice for about an hour every day

Dunno about battlefield, never played it, but generally in FPSs learning to snipe is more a social construct than a physical one: learning to lead your target is pretty easy, learning to divorce yourself from the action and get bored while waiting for easy kills that will merely annoy everyone else is a bit harder for some.

9 minutes ago, noobsrtoast said:

i dont think that diagram is pathetic, ill definitely use this, thank you

The drawing skills are pretty pathetic though. I don't have anything better (skills or software) to hand.

[Superfluous J]

1 minute ago, noobsrtoast said:

by the looks of things, unless you have a degree in orbital mechanics, its guesswork, cool ill keep that in mind, thanks

 

Not guesswork per-se. More educated stabs in the dark.

@Plusck has the right idea. That diagram shows exactly what I was talking about.

[noobsrtoast]

Just now, 5thHorseman said:

Not guesswork per-se. More educated stabs in the dark.

@Plusck has the right idea. That diagram shows exactly what I was talking about.

i just put the diagram to practice, it worked well

edit: never mind, as a reminder to everyone, you are most definitely not in a safe orbit 4 kilometres from the mun

 

Edited March 22, 2016 by noobsrtoast

[noobsrtoast]

2 hours ago, Plusck said:

Dunno about battlefield, never played it, but generally in FPSs learning to snipe is more a social construct than a physical one: learning to lead your target is pretty easy, learning to divorce yourself from the action and get bored while waiting for easy kills that will merely annoy everyone else is a bit harder for some.

The drawing skills are pretty pathetic though. I don't have anything better (skills or software) to hand.

yo man thanks for the diagram, i was able to use it as a reference for performing 2 gravity assists off kerbin and one off eve to get an encounter with jool