Advice on gravity slingshots

[Voyageur]

Evening everyone!
 

I'm currently seeking advice on one of the techniques I've never tried in KSP, and has honestly hindered my experience in interplanetary travel past Duna and Eve. I've never attempted a gravity slingshot, or using a planet's SOI to alter my trajection so that I spend less delta-v getting to a planet further out/in in the solar system. 

I understand the principle of it; make your trajection/intercept either forward or behind your direction to either slow or speed you up, allowing you to alter your course without spending excess delta-v trying to straight Hohmann Transfer your way to somewhere like Eeloo. 

Anyway, any and all advice is welcome. I'm mostly at a loss as to how you plan and fine-tune your course and know which way you're going to end up when it comes to eccentricity and inclination. I've never tried it so obviously I'm at a loss in that regard.

Teach me your ways!

[DerekL1963]

1 hour ago, Voyageur said:

I'm currently seeking advice on one of the techniques I've never tried in KSP, and has honestly hindered my experience in interplanetary travel past Duna and Eve.

I'm curious how this hinders you...  I've been all over the system and have never once used a gravity slingshot.

[A_name]

1 hour ago, Voyageur said:

I understand the principle of it; make your trajection/intercept either forward or behind your direction to either slow or speed you up, allowing you to alter your course without spending excess delta-v

This is literally all you need to know.

As for timing, I’m sure there’s ways to calculate it but honestly I don’t even bother. I do it the Kerbal way!

Suppose you want to get to Jool with an Eve assist. First set up your encounter with Eve according to the rules you mentioned. In this case since you want to boost up, you would place your Pe behind Eve along its orbit, as close as needed for your resulting orbit after the encounter to extend up to Jool’s orbit. You may need to make a small burn at Pe to make that happen. Then set Jool as your target and put a maneuver node somewhere (doesn't mater where as you will be moving it) and look at your closest approach markers. Now simply play around with the node to bring those markers closer together. Drag it up and down your orbit, pull on it form different directions, etc. until you get your Jool encounter. Don’t forget about the ‘next orbit’ button, as you won't always get a reasonable encounter on the first go around.

Edited February 16, 2017 by A_name

[Voyageur]

1 hour ago, DerekL1963 said:

I'm curious how this hinders you...  I've been all over the system and have never once used a gravity slingshot.

I mean I guess it's not impossible, I just like to make my rockets efficient and a little less kerbal than necessary. If I can use a planet's gravity as free delta-v, then I'll take it. Also because it's satisfying.

[bewing]

Well, there are two kinds of slingshot maneuvers, really. Let's call them burn and no-burn. A no-burn slingshot you can set up in advance. You leave Kerbin for CB1, and you do a mid-course correction burn to set the Pe. You can just watch the post-encounter orbit move until it intersects your target's SOI -- then you stop. This can be done with a maneuver node in LKO, and an additional maneuver node at the halfway point.

But there is a minimum Pe at any CB, so that mid-course correction has a limit. And if you hit that limit before you get an intercept at your target, then what do you do? You wait until you get to the Pe of CB1, and you burn retrograde. So you have to focus CB1 and set an extra maneuver node at its Pe.

 

Edited February 17, 2017 by bewing

[Snark]

3 hours ago, Voyageur said:

I mean I guess it's not impossible, I just like to make my rockets efficient and a little less kerbal than necessary. If I can use a planet's gravity as free delta-v, then I'll take it.

The main problem with using gravity-assist maneuvers in KSP is,

• They're really really hard to set up right-- if you get them even a little bit wrong, your post-slingshot aim will be way off.
• It can be very, very time-consuming waiting for a good window, since you need three things to line up right, instead of the usual two.
• It doesn't generally save all that much dV.

Most folks don't bother with 'em, because the potential benefit you get is small compared to the huge cost in inconvenience and difficulty-- with the added spice that if it doesn't get executed flawlessly, your aim will be off, requiring a big correction burn, and unless you're either super lucky or super skillful, there's a good chance that fixing-the-error will end up eating more dV than you supposedly saved in the first place by using the slingshot.

About the only time I've ever used a gravity assist is when I'm doing a reverse gravity assist, using Tylo or Laythe, to get captured to the Jool system.  Since the direction doesn't matter much, it's easy to set up an encounter and it saves scads of dV.

3 hours ago, Voyageur said:

Also because it's satisfying.

^ That right there would be the reason for you to do it, if you did.    Not because of the practical benefit (because for the most part, there isn't much), but simply as a fun challenge.

[PLAD]

I wrote an tool that finds flyby opportunities in KSP called Flyby Finder.  Arrowstar wrote a more detailed program called TOT that also can find them. TOT goes into much more detail and can even take control of your ship, but it is huge and only finds one flyby path at a time. FF shows a pork-chop type plot of a range of travel times and start dates but you will have to learn some tricks to get into the right start orbit around Kerbin and set up the departure burn, and figure out the flyby periapsis lattitude and longitudes by messing with a node editor. (Okder's addon, linked to in the post, makes the Kerbin departure much easier.)

Here are some flights I made using this tool, in the Imgur albums I go into detail on how I executed the flight.

Low Kerbin Orbit to Jool for 1051 m/s (later in the thread I did it for 1011m/s!)

Kerbin-Eve-Duna-land-Duna-Kerbin.   This uses an opposition-class flight to Duna similar to some proposals for a Mars mission.

A challenge to get to Moho for the least dV possible. Metaphor executes one of the most epic uses of flybys ever to win it.

Getting from LKO to Eeloo's surface and back with 1934m/s.

If you look through my Imgur page you will see a lot more flyby flights including the one where I made it from LKO to Jool orbit for 914m/s. (Note that there exists no tool for finding the Mun multi-flyby trick I used at the start, just for the encounters after that.)

These days my favorite technique when setting up a flyby is to put a node on my ship's predicted path right at the time when the next flyby is supposed to be and then working to make that node encounter the target planet's orbit at the right time. So for instance if you know you will leave Kerbin on day 10 and are supposed to fly by Eve on day 150 then put a node on your path 140 days in the future when setting up you Kerbin departure burn. Come to think of it I detailed this method in this flight using the Galileo Planets Pack.

I personally think gravity assists are awesome, it is one of the few times the laws of physics let us get something for nothing. Good luck!

 

 

 

[A_name]

1 hour ago, bewing said:

Well, there are two kinds of slingshot maneuvers, really. Let's call them burn and no-burn. A no-burn slingshot you can set up in advance. You leave Kerbin for CB1, and you do a mid-course correction burn to set the Pe. You can just watch the post-encounter orbit move until it intersects your target's SOI -- then you stop. This can be done with a maneuver node in LKO, and an additional maneuver node at the halfway point.

But there is a minimum Pe at any CB, so that mid-course correction has a limit. And if you hit that limit before you get an intercept at your target, then what do you do? You wait until you get to the Pe of CB1, and you burn retrograde. So you have to focus CB1 and set an extra maneuver node at its Pe.

 

What is CB and CB1?

41 minutes ago, Snark said:

• They're really really hard to set up right-- if you get them even a little bit wrong, your post-slingshot aim will be way off.
• It can be very, very time-consuming waiting for a good window, since you need three things to line up right, instead of the usual two.
• It doesn't generally save all that much dV.

Snark, I hesitate to contradict you but I must disagree. As long as you have a Kerbal mindset when approaching the problem, that is, the opposite of a perfectionist approach, it's not really hard at all and the DeltaV savings at least in my experience can be in the range of several thousand m/s which is substantial. Sure, timing is an issue, but that is easily remedied if you're willing to spend a few extra years/decades in space. Again, this would be unacceptable for a perfectionist but not an issue if you have a more Kerbal approach.

[bewing]

7 minutes ago, A_name said:

What is CB and CB1?

CB = Celestial body.

CB1 is Celestial Body #1 -- the first one you are flying past.