I am woefully unprepared to use gravity assists

[Reinhart Mk.1]

These are tricky

[Blaarkies]

Simple, practice on the Mun. Use it to go to Minmus and back.
Next, get really really cozy with interplanetary transfers, you need to be able to do them very confidently (since you will be doing one for every slingshot you do)

Gravity assists work best on big objects. Eve and Kerbin are good, Duna and Moho not so good
Gravity assists are like bouncing a ball (it is an exchange of momentum after all). So your entry velocity (entering the new planet's SOI) is very important, since you will hopefully be getting a 90 degree turn as you past this planet...tweak this velocity until the final result is acceptable
If the result doesn't seem good, try to cross the new planet's orbit, instead of barely touching it. This gives you more entry velocity to play around with.

Lastly, petition "Bradley Whistance" on youtube to finish their series on orbital mechanics

[The Aziz]

They are easy if you time it right. Just plan ahead. The last time I went to Sarnus (OPM) I had the opportunity to use gravity assist from Jool and not wait several years for good planet alignment. The trip was two years longer, but I saved tons of fuel (and saved even more by using Sarnus moons for a capture).

[Snark]

12 hours ago, Reinhart Mk.1 said:

These are tricky

Are you doing it for the challenge (i.e. precisely because they're hard)?  Or are you trying to use them to make life easier, i.e. "Oh, I'll use a gravity assist to save myself some dV so that building my rocket will be easier!"

If it's the former, then great-- nothing wrong with that, a big part of KSP is all about finding interesting challenges for yourself. 

If it's the latter... honestly, I gotta say that in most cases, my experience has been that they're far more trouble than they're worth.  It's pretty straightforward to build a ship in KSP to get anywhere you need to go without needing a gravity assist... and gravity assists are hard.  They're difficult and finicky to set up, they're really hard to make accurate enough to get what you want, and in most cases they honestly don't give you all that much advantage (i.e. the dV savings is minimal).

In other words:  lots of time and effort for minimal benefit.

The one place I've found where they make sense to me (i.e. provide a large benefit for minimal effort)-- in the stock solar system, anyway-- is using Tylo or Laythe to do a reverse gravity assist for capturing to Jool.  Done right, this can save several hundred m/s of dV, and they're pretty straightforward to set up.  Detailed instructions for setting up a Tylo reverse assist here:

 

[JERONIMO]

Just don't bother, after 1K hours in KSP I made only one successful gravity assist, unless if you are confident then don't bother, just build a huge stick and launch it everywhere.

[Greenfire32]

I think it would be helpful to have have a gravity assist indicator similar to the maneuver mode planner.

My problem with gravity assists is that because I'm a visual learner, if I can't SEE them, I don't KNOW that i'm using them.

Obviously over the years, I've learned that if I do X thing, Y happens and as such I can apply that to gravity assists, but it would still be nice to just KNOW that I'm using them.

[Bill Phil]

Yeah they can be tough.

Practice in the Jool system, and maybe get a mod to extend the time warp settings for interplanetary trajectories.

What a gravity assist does for you is exchange delta-v and time. You take longer to get to your destination but you can do it with less delta-v. There are tools to help you plan assists, but you can just timewarp until you get encounters, using maneuver nodes to see how many orbits you have to wait. Then you do some maneuvers to get your trajectory to be what you want (or close to what you want). The best assist targets are the biggest planets/moons - Kerbin, Eve, Jool, Tylo, and Laythe. 

There’s a series of assists called KEKKJ (Kerbin-Eve-Kerbin-Kerbin-Jool) that takes you to Jool for very little delta-v.

[Tyko]

I agree with @Snark - reverse gravity assists going into Jool can be really helpful and they’re even better because you can still plot your optimal transfer path from Kerbin to Jool first. Once you’re on your way to Jool plot a midway course correction, zoom in on the Jool system and try out different changes. You’ll find encounters with just about every moon depending on the course change. This is a great way to see how different encounters affect your course and learn which ones are helpful. 

Is it better to come in behind a moon or encounter it on the other side of its orbit when it’s heading to you? 

Do you swing in ahead of the moon or come in behind it (answer - if you want to slow down you want to come in ahead of it and swing around heading backwards relative to the moon)

[Reinhart Mk.1]

In the end I think the problem is my patience, some of these encounters take so long even at max speed. I have the general hang of them but I know i'm doing something wrong. But just to save every the frustration I'm not 100% green to gravity assists lol, right now I put jool on hold and decided to go to Dres. I think Dres in-general costs more than the average delta v because of the inclination change you have to make. Everyone's advice is very useful as always, though!

[cantab]

Research, practice, and experiment. And above all else be patient and methodical. I've been known to spend multiple play sessions just getting a single gravity assist right.

If you're trying to get gravity assists off a moon, for example using Tylo or Laythe to capture at Jool, then an important technique is a mid-course correction that balances pro/retrograde and radial burns. This allows you to keep your *trajectory* into the Jool system approximately the same, but change your arrival *time*. That's how you meet the target moon at the right time and place.

As mentioned it's not everyone's cup of tea. I for one take great pride in my gravity assists which is why I'm willing to put in the effort.

[SlinkyMcman]

Pictures are worth some words, here is an example for free insertion at Tylo. The correction burn happened as the ship crossed Dres's orbit. Tylo PE is about 1 million meters. I agree with Snark that this is easily the best use of a flyby in stock. Getting a probe to Moho/Low Solar via Eve is also a good use.

Spoiler

 

 

[Reinhart Mk.1]

6 hours ago, Snark said:

Are you doing it for the challenge (i.e. precisely because they're hard)?  Or are you trying to use them to make life easier, i.e. "Oh, I'll use a gravity assist to save myself some dV so that building my rocket will be easier!"

kinda both, i dont wanna have to build something massive if i can save fuel via gravity assists

[Snark]

4 minutes ago, Reinhart Mk.1 said:

kinda both, i dont wanna have to build something massive if i can save fuel via gravity assists

With rare exceptions, gravity assists are only going to save you a pretty small amount of dV, not thousands of m/s.  The extra hassle of trying to set them up will likely far exceed the extra hassle of putting an extra 100 m/s of dV into your rocket in the first place.  Do you really want to spend an extra hour fiddling around trying to set up a Mun slingshot so that you can save 100 m/s of dV?

Not to mention the fact that they're tricky enough to set up that there's a good chance you'll need to do a correction burn afterwards to adjust your aim, and the correction burn could easily end up being bigger than the "savings" in the first place.

The one place in the solar system that I've found that provides both a dV savings big enough to be worthwhile (i.e. several hundred meters per second) and pretty easy to set up, is using a reverse gravity assist at Tylo or Laythe in order to capture to Jool.

Other than that... seriously, it's just not worth the bother.  It's a lot of extra hassle for very little benefit.  Which is fine if you like the hassle (i.e. you're doing it for the challenge) ... but otherwise not something that's worth it.

Note that I'm speaking of the stock solar system.  Modded solar systems may provide other opportunities.

[The Aziz]

If only there was a tool for calculating such things

I'm being honest - if you really want it, spend a time checking this thing, it may take a while to set up for what you want, as you need your departure time, start planet, target, body you want to use for gravity assist (it uses your savefile for that I think?) and prepare for 100% cpu usage - it's maths after all, a lot of calculations. Now THAT is rocket science. Not recommended to use while ksp is running in the background.

[theJesuit]

On 8/3/2019 at 4:43 AM, Snark said:

The one place I've found where they make sense to me (i.e. provide a large benefit for minimal effort)-- in the stock solar system, anyway-- is using Tylo or Laythe to do a reverse gravity assist for capturing to Jool.  Done right, this can save several hundred m/s of dV, and they're pretty straightforward to set up.

This.  Especially with life support as you can push faster to get there earlier.

Peace.

[I_Killed_Jeb]

tbh the biggest barrier I have encountered in gravity assist are the small imprecisions built into KSP (possibly due to floating point errors?)

[magnemoe]

On 8/4/2019 at 8:53 PM, theJesuit said:

This.  Especially with life support as you can push faster to get there earlier.

Peace.

I wonder if doing the braking burn at low Jool orbit will be cheaper than Tylo or Laythe burn? Yes this will leave you Pe very low but it should be possible to use Tylo to lift Pe and lower Ap for an cheap encounter with Pol or Bop. 
I'm coming in a bit fast and a lit low on fuel, most nervous about how effective the long braking burn inside Tylo soi will be. 

[The Aziz]

Plan ahead then. Check all possibilities using maneuver nodes. Only then you can tell which route is cheaper.

[theJesuit]

4 hours ago, magnemoe said:

I wonder if doing the braking burn at low Jool orbit will be cheaper than Tylo or Laythe burn? Yes this will leave you Pe very low but it should be possible to use Tylo to lift Pe and lower Ap for an cheap encounter with Pol or Bop. 
I'm coming in a bit fast and a lit low on fuel, most nervous about how effective the long braking burn inside Tylo soi will be. 

Are you using Kerbalism? Inside Laythe orbit is high radiation and will kill your kerbals quickly.

Peace.

[esmenard]

9 hours ago, magnemoe said:

I wonder if doing the braking burn at low Jool orbit will be cheaper than Tylo or Laythe burn? Yes this will leave you Pe very low but it should be possible to use Tylo to lift Pe and lower Ap for an cheap encounter with Pol or Bop. 
I'm coming in a bit fast and a lit low on fuel, most nervous about how effective the long braking burn inside Tylo soi will be. 

Actually the point in Tylo or Laythe assist is not doing any capture burn. If done correctly the gravity assist alone should be enough to get anywhere in the Jool system.

[Snark]

9 hours ago, magnemoe said:

I'm coming in a bit fast and a lit low on fuel, most nervous about how effective the long braking burn inside Tylo soi will be.

Set it up properly and there's no burn needed at Tylo.  That depends on how fast you're going, though.  What's your excess hyperbolic velocity with which you're entering the Jool system?

[magnemoe]

4 minutes ago, Snark said:

Set it up properly and there's no burn needed at Tylo.  That depends on how fast you're going, though.  What's your excess hyperbolic velocity with which you're entering the Jool system?

Braking burn at Tylo to enter an fitting orbit is around 2.5 km/s, has 2.7 total on the worst case
4 bases, best has 3.8 km/s so plan to land that on Pol or Bop and then refuel the basket case. 

I forgot to factor in extra stuff like an space plane to use on Laythe in the dV calculations. Part of the fuel was carried as ore in the ore tanks so i had to calculate. 

 

[Snark]

3 minutes ago, magnemoe said:

Braking burn at Tylo to enter an fitting orbit is around 2.5 km/s, has 2.7 total on the worst case

If you say so, will just have to trust your judgment.  Effectiveness of Tylo reverse assist and/or Oberth maneuver depends a lot on the geometry, e.g. what direction you're coming from, etc.

How thermal-resistant is your craft?  Any chance of doing an aerobrake at Laythe to capture to Jool?

[magnemoe]

Just now, Snark said:

If you say so, will just have to trust your judgment.  Effectiveness of Tylo reverse assist and/or Oberth maneuver depends a lot on the geometry, e.g. what direction you're coming from, etc.

How thermal-resistant is your craft?  Any chance of doing an aerobrake at Laythe to capture to Jool?

I did aerobrake at Duna, this works well there as speed then hitting the atmosphere was just 3 km/s. 
However as I'm falling into both Jool and Laythe gravity wells it don't work well if you come in fast with an heavy base, I tested this in sandbox and hardly any gain. 


The ill fated Laythe ship getting topped up in Minmus orbit before getting dropped down to 100 km above Kerbin for the Jool burn. 
Will test then I come home. 

 

 

[ExtremeSquared]

On 8/2/2019 at 4:18 PM, Snark said:

The one place in the solar system that I've found that provides both a dV savings big enough to be worthwhile (i.e. several hundred meters per second) and pretty easy to set up, is using a reverse gravity assist at Tylo or Laythe in order to capture to Jool.

A lot of using-moon-to-capture assists are helpful for a sort of messy reason. The games use of SOI physics instead of N Body physics means that a nice little chunk of potential energy above the target planet is wiped out because you spent time crossing a secondary SOI. Obviously this is bad physics, but it is not hard to use this to your advantage even without doing traditional gravity assists.