Gravity Assists and Where to Find Them

[Aelipse]

OK, so having realised I have been performing my burns for other planets tremendously inefficiently, I decided to do some testing and compare various ways (mining included) to get from Kerbin to Jool. One of the options I heard was very efficient is to actually burn retrograde in the Sun's SoI, reach Eve and use her for a gravity slingshot. But... No matter how much I tried I haven't been successful in timing the transitions so that I would actually meet Jool, not just its orbit.

I watched Scott Manley's video on the matter and I didn't catch any particular advice on the problem, only that he spent several frustrating hours before managing to do it right.

My question is - do you go out of your way to perform gravity assists, like the one using Eve to get to Jool? And if so, is there a trick to it, or does one have to rely on luck / memorizing all the transfers?

Edited August 11, 2019 by Aelipse

[Aegolius13]

4 hours ago, Aelipse said:

do you go out of your way to perform gravity assists, like the one using Eve to get to Jool?

Nope, unless I'm looking to specifically do one for a challenge.  I find that generally they're not worth the time spent (both my time and in-game time), or the legwork to get just right.  

For example, in a good window you can do an LKO to Jool transfer burn at about 2,000 m/s.  Going from Kerbin to Eve, just to start the gravity assist process, is at least 1,000 m/s.  So the savings (is going to be under 1,000 m/s, and could be substantially less than that depending on how many correction burns are needed.

The one exception is to capture around the Jool system - I almost always do a "reverse" assist around Tylo or Laythe.  These are not nearly as picky, don't take much in-game time, and can save at least a few hundred m/s.

 

4 hours ago, Aelipse said:

K, so having realised I have been performing my burns for other planets tremendously inefficiently, I decided to do some testing and compare various ways (mining included) to get from Kerbin to Jool.

Honestly, I'd suggest mastering regular maneuvers before look at gravity assists as a way to save (other than the Jool moons as mentioned above).  To get assists to work efficiently, you need to be very precise with maneuvers and timing, and can easily end up spending net delta-v if things go amiss.  And I've found both the theory and practice of gravity assists to make more and more sense as I learned more about orbital mechanics through gameplay.  

 

4 hours ago, Aelipse said:

And if so, is there a trick to it, or does one have to rely on luck / memorizing all the transfers?

This is not my forte, but I understand there's a mod/tool called Flyby finder to help with this.  

The only time I did a gravity assist on the way to Jool, I used Kerbin (rather than Eve) to get the assist, similar to the real-life Juno probe.  This is quite a bit easier to plan, as you can launch one year prior to the Kerbin-Jool launch window, get into a different (but same period) orbit around the Sun, and then get the assist right at the launch window.

[Zhetaan]

5 hours ago, Aelipse said:

My question is - do you go out of your way to perform gravity assists, like the one using Eve to get to Jool?

Not usually, no.  Assists are fun, but KSP makes it fairly easy to add fuel for the journey--Mining and ISRU tools are part of that.  Assists may be more necessary in scaled solar systems such as RSS, where it's much more difficult to get large payloads of fuel to space.  That being said, the challenge is still fun, but I also play with life support mods, so I don't use assists for anything but probes.  I don't carry enough supplies to feed my crew for the amount of time it would take to do a crewed Grand Tour.  Perhaps someday, but not now.

5 hours ago, Aelipse said:

And if so, is there a trick to it, or does one have to rely on luck / memorizing all the transfers?

The trick is the precision.  The key to an efficient gravity assist is all in your ability to make tiny changes on one side of the sun that move your encounters by millions of kilometres on the other side.  If you're not using Precise Node or another mod that works to allow fine adjustment of your nodes (I do not know whether the stock 1.7 manoeuvre editor can do this sort of precision), then you'll need to grow very familiar with engine thrust limiters or else do most of your manoeuvres with RCS thrusters.  The reason is because, once you get close to completing a correction, you'll need to switch to map mode and make very slight adjustments in order to get the right encounter; the node won't help you at the end if it's not precise.  That's much easier to do with limited thrust.

Timing is important, but not necessarily important.  If that doesn't make much sense, what I mean is that like any other transfer, a gravity assist does depend on you getting to the right place at the right time.  However, there is a bit more flexibility for you to choose that right time, because unlike a minimum-fuel Hohmann transfer, you are not always trying to encounter your destination after exactly one-half of an orbit--once you get away from that unstated assumption, many other options become available.  You can usually afford to go round the sun a few times while you search for the encounters that you want.

One of your greatest obstacles is this:

5 hours ago, Aelipse said:

OK, so having realised I have been performing my burns for other planets tremendously inefficiently

As @Aegolius13 said above, you need to master regular transfers first.  Gravity assists are regular transfers, but multiplied so that one burn does at least two transfers.  If you have difficulty doing an efficient burn for one transfer, then you are not going to get anywhere but lost with gravity assists.

Lastly:

5 hours ago, Aelipse said:

I watched Scott Manley's video on the matter and I didn't catch any particular advice on the problem, only that he spent several frustrating hours before managing to do it right.

Try this instead:

It's an old video, so some bits are irrelevant or otherwise not accurate to the game today, but the orbital mechanics are the same.

Edited August 12, 2019 by Zhetaan