Jool's moons assists for an idiot

[paul_c]

I've been getting back into KSP for a few days/weeks now and a bunch of missions are going to Jool and its moons. The delta V for maneouvring around Jool is quite high and I'm always clashing with the moons in a fairly random way. But is there any way to sort out the intercepts to the benefit of saving the dV needed to carry (ie more fuel, bigger rocket(s) underneath etc etc)

I am guessing, you want to get the intercept with Jool as close to 250km Pe, but direct in (ie no moon intercepts beforehand) but then on the way out, get an intercept which curls you into another Jool orbit with as low an Ap as possible, but then the Pe is probably not going to stay at ~250km?

If you intercept Jool then do a manoeuvre of some kind to intercept your intended destination moon, have you saved anything/much anyway, or will whatever you haven't burned at the Jool Pe just be needed at the moon's Pe to be captured by its orbit?

[herbal space program]

The easiest way I know to get captured in the Jool system is to come in so that you encounter Tylo while going nearly tangent to its orbit and in the same direction as it is.  That should be pretty cheap to set up from >100 days out, and can be used to go  directly to a Laythe encounter that will allow you to set your Jool AP pretty much anywhere you want. 

[Snark]

On 7/7/2021 at 8:26 AM, paul_c said:

But is there any way to sort out the intercepts to the benefit of saving the dV needed to carry (ie more fuel, bigger rocket(s) underneath etc etc)

The two moons that are by far the most useful for gravity assists are Tylo and Laythe.  They both work pretty well.  Tylo has the advantage of being in a higher orbit, so using it allows establishing somewhat less-eccentric orbits around Jool.  Laythe has the advantage of being able to combine aerobraking with a gravity assist.

I find that reverse gravity assists (where I'm trying to slow down, e.g. to capture to the Jool system) are considerably easier to arrange than "positive" assists (e.g. for boosting out of the Jool system), mainly because in the latter case, it's necessary not just to get a boost but to get it in the correct ejection angle to navigate to wherever I'm going, which makes it a trickier setup.

• If you want to gain energy from an encounter, arrange an orbit so that you cross behind the moon, so that its gravity speeds you up.  Somewhat more effective if you're crossing from inside its orbit to outside.
• If you want to lose energy from an encounter, arrange an orbit so that you cross in front of the moon, so that its gravity slows you down.  Somewhat more effective if you're crossing from outside its orbit to inside.

[king of nowhere]

On 7/7/2021 at 2:26 PM, paul_c said:

I am guessing, you want to get the intercept with Jool as close to 250km Pe

 

wrong. You don't want a too low jool periapsis, unless you are trying to get atmospheric science from jool; it only causes a higher intercept speed on the moons.

The theory of gravity assist is quite simple; you pass behind a moon, you gain speed. You pass in front of it, you lose speed. All you have to do is get your trajectory to intersect either Laythe or Tylo (both are good for this) and pass in front of the moon. This will put you in jool orbit.

Here is a simple example (yes, it's simple enough if you don't get distracted by too many lines. You completed a nanocristalline diamond cavemen, you can do this)

The purple trajectory is my original trajectory. Just a jool intercept. About one year before arriving at jool, I set up a course correction - shown high on the screen, a manuever for a few m/s. That manuever results in the new path, the purple dotted line, which goes to pass directly in front of laythe. After that passage, I am on the red dotted orbit, a relatively low jool orbit. From there you can reach any moon with a mild manuever.

It also shows another reason you don't want to enter with a low jool periapsis: the graivty assist will be equivalent to a retrograde burn, and unless it is performed exactly at periapsis (which, if your periapsis is low, won't be the case) the manuever will also lower your periapsis. And set you on a collision course to jool.

There are more refined things that can be done around jool with the moons, but they are generally fiddly, hard to pull off, and dependent on not accidentally crossing the wrong moon. But getting captured around jool for free is very easy

[paul_c]

Thanks for the tips, once I have a chance to play KSP again I'll try them out!

[herbal space program]

On 7/11/2021 at 5:54 AM, Snark said:

 

• If you want to gain energy from an encounter, arrange an orbit so that you cross behind the moon, so that its gravity speeds you up.  Somewhat more effective if you're crossing from inside its orbit to outside.
• If you want to lose energy from an encounter, arrange an orbit so that you cross in front of the moon, so that its gravity slows you down.  Somewhat more effective if you're crossing from outside its orbit to inside.

This is true except with the proviso that if you come into a body's SOI parallel to its motion and with more orbital energy than it has, you can only lose energy from the encounter. Hence my recommendation above for the initial Tylo assist.