Useful gravity assist visualization

[kball]

I'm still working to to get an intuitive understanding of the various ways to employ gravity assists. Right now I'm wondering how to use a moon to assist an insertion. I don't even know if that's possible or practical. Maybe it's only possible if a retrograde final orbit is acceptable? Even then, does attempting a maneuver out by a moon essentially throw away the benefits from Oberth that would otherwise be had for a burn near the planet?

In any case, started poking around the interwebs to read about them and stumbled on this visualization depicting various assists which I thought some might find useful.

 

from: http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html

Edit: Realizing now I may have just been thinking about it incorrectly. I guess the trick is not to go where the moon is when you get there, but to go when the moon will happen to be in your path when you're arriving.

Edited April 25, 2017 by kball

[monstah]

That's an awesome image, I remember seeing it on Planetary.org before.

 

1 hour ago, kball said:

Right now I'm wondering how to use a moon to assist an insertion. I don't even know if that's possible or practical.

Of course it is! None of the examples here show exactly the right configuration, but figure d. is close enough (note: after typing my reply, I noticed all the images show exactly the same gravity slingshot, only the camera reference changes...) :

Look at image d. on your example, and imagine that is in Jool orbital space. Jool is toward the bottom of the image; the black dot is Laythe, orbiting Jool, and the sun and Kerbin's orbit are to the top of the image. Jool's orbital velocity (and your ship's, too) is toward the right, but we're in Jool's reference, and since it's faster than your ship at that point, from that point of view the ship is moving left (well, in the image it's also moving back towards Kerbin orbit, so it's as if you captured after going a bit out from Jool's orbit and falling back a bit, but that's not important here). Notice that before the encounter, the ship is zipping by the Jool system; after it, it stops (in Jool's reference), so it's been captured. It's just a matter of choosing: a) the right position for the moon in relation to the planet, and b) the right position for you to pass in relation to the moon.

 

In my opinion, you can practice a lot by using the Mun to help you reach Minmus, then using it again to help brake from a Minmus return trip, which is equivalent of what you want to do (pretend you're coming from interplanetary rather than Minmus).

[kball]

32 minutes ago, monstah said:

In my opinion, you can practice a lot by using the Mun to help you reach Minmus, then using it again to help brake from a Minmus return trip, which is equivalent of what you want to do (pretend you're coming from interplanetary rather than Minmus).

Thanks for the reply.

I wonder if I'm not getting as much experience with these maneuvers because I'm always over engineering rockets, taking more direct routes and relying on big insertion burns. When zipping through a system at breakneck speed the effect of any other bodies always seems negligible.

This is what I settled on for this one, which I think saved me 2-3 hundred m/s on the final burn. Far from what I was originally shooting for which was a burn at periapsis and then a subsequent assist by Mun for capture. Couldn't figure that one out.

 

[blakemw]

You can't meaningfully use the Mun to assist in capture, the reason is that when you cross the Mun's orbit like that, what you're essentially getting is a radial/anti-radial burn, which is not useful for slowing down. The problem is that when you're coming in at hyperbolic speeds, you can't both encounter Mun at a point where you'll get maximum benefit from a gravity assist, and have a low periapsis around Kerbin for maximum oberth effect. That is, if you aim for a low periapsis around Kerbin to maximize oberth effect, you'll cross Mun's orbit at 90 degrees and can't get a meaningful gravity assist. When dealing with interplanetary velocities, the oberth effect at Kerbin is much more valuable than any gravity assist the Mun can provide.

As for why it depends on velocity: Gravity is an acceleration, change in velocity over time, the more time spent near a body, the more time gravity has to bend the trajectory. When a ship is racing in like a bat out of hell it only spends a very short time near Mun and so Mun's gravity can only bend the trajectory a very small amount. Conversely, when approaching Mun at low velocities the Mun's gravity has a long time to accelerate the ship and can apply several hundred m/s of velocity. So a body either needs to have powerful gravity, or you need to approach it slowly, to get a meaningful gravity assist. The only two moons which can apply meaningful gravity assists for interplanetary transfers are Tylo and Laythe.

[monstah]

3 minutes ago, blakemw said:

You can't meaningfully use the Mun to assist in capture

Oh, no, I meant for practice. Doing that with Tylo/Jool, however, has better results

[blakemw]

That's why I qualified with "assist in capture". Mun is pretty useful in general within the Kerbin system, like to help lower heavy payloads from Minmus without having to bother with aerobraking, and when doing Kerbol "peekaboo" as Kerbal training it's handy when falling back down from the SOI edge.

[Red Iron Crown]

Images a and b look the most like using a moon to capture to me. Approaching from behind the moon and passing in front of it are the keys. 

Images c and f look like using a moon to help with an ejection. 

[kball]

12 hours ago, blakemw said:

You can't meaningfully use the Mun to assist in capture, the reason is that when you cross the Mun's orbit like that, what you're essentially getting is a radial/anti-radial burn, which is not useful for slowing down. The problem is that when you're coming in at hyperbolic speeds, you can't both encounter Mun at a point where you'll get maximum benefit from a gravity assist, and have a low periapsis around Kerbin for maximum oberth effect. That is, if you aim for a low periapsis around Kerbin to maximize oberth effect, you'll cross Mun's orbit at 90 degrees and can't get a meaningful gravity assist. When dealing with interplanetary velocities, the oberth effect at Kerbin is much more valuable than any gravity assist the Mun can provide.

As for why it depends on velocity: Gravity is an acceleration, change in velocity over time, the more time spent near a body, the more time gravity has to bend the trajectory. When a ship is racing in like a bat out of hell it only spends a very short time near Mun and so Mun's gravity can only bend the trajectory a very small amount. Conversely, when approaching Mun at low velocities the Mun's gravity has a long time to accelerate the ship and can apply several hundred m/s of velocity. So a body either needs to have powerful gravity, or you need to approach it slowly, to get a meaningful gravity assist. The only two moons which can apply meaningful gravity assists for interplanetary transfers are Tylo and Laythe.

Right. This unbalanced benefit of oberth and assist was exactly my suspicion. Your note about the radial effect of this fly by seems correct and does leave me wondering why the maneuver pictured has any effect at all. Whereas I was initially looking at burns in the 2200-2500m/s range this appeared to get me down to 1800-2100m/s. Was it just my imagination?

This is my first interplanetary return. I'm sure I'll learn a lot when I start exploring Jool.

 

[Freshmeat]

While you might not get a full benefit of a gravity assist from high velocity, you can use it in conjunction with a burn near the moon for increased effect. The ideal is to get a sling that positions you in a low periapsis of the main body for a effective capture burn afterwards. I've tried it around Jool, and while the savings was not gigantic, the maneuver itself is fun to set up and execute.

I suspect my main problem with gravity assists is that I do not trust that I can get them, and thus always bring enough fuel to do without.

[blakemw]

9 hours ago, Freshmeat said:

While you might not get a full benefit of a gravity assist from high velocity, you can use it in conjunction with a burn near the moon for increased effect. The ideal is to get a sling that positions you in a low periapsis of the main body for a effective capture burn afterwards. I've tried it around Jool, and while the savings was not gigantic, the maneuver itself is fun to set up and execute.

While this is true, it changes nothing about the Mun, because you gain much more benefit by doing the burn in LKO than while passing the Mun, because Kerbin's gravity is much stronger. It's actually often similar with Jool, if you come in hot and the moons can't capture you outright, you'll probably get more out of a burn close to Jool than close to Tylo/Laythe. About the only case where a "powered assist" might be useful, is when you're doing an Eve assist, not only does Eve have stronger gravity than Kerbin but it's also deeper in Kerbol's gravity well so you can get oberth effect from both from Eve and Kerbol. In principle it ought to be pretty useful if you want to do an extra-fast transfer (probably in outer planets mod).

Incidentally I also don't bother much with gravity assists, I usually play with a life support mod (usually Kerbalism) and like to do expedited transfers (actually Kerbalism also adds part malfunction, so even for an uncrewed probe there is benefit in getting places faster). Also IRL gravity assists are mainly only used due to tiny budgets, not really a problem in KSP.

Edited April 26, 2017 by blakemw