Mimnus to Kerbin via Mun

[Elroy Jetson]

Guys,

 

First, thanks for all the help!  The game would be much different without the community here!

 

my question for the day -  When I am returning from Mimnus back to Kerbin how do I use the Mun to help slow me down?  Is there a wiki topic that describes this procedure?  I have heard/read the term gravity assist but do not completely understand it.  It seems that this is going to be a technique used for much of the future missions to outer planets?

[Zhetaan]

1 hour ago, Elroy Jetson said:

When I am returning from Mimnus back to Kerbin how do I use the Mun to help slow me down?

You need to successfully use a gravity assist.

1 hour ago, Elroy Jetson said:

Is there a wiki topic that describes this procedure?

There's not a recent one, but here it is.

1 hour ago, Elroy Jetson said:

I have heard/read the term gravity assist but do not completely understand it.

Gravity assists are a way to change the orbital relationship between you and a planet by using a moon as an intermediary (or between you and the sun by using a planet).  It takes advantage of the fact that there is a difference between speed and velocity.  For our purposes, the main thing to understand is that with velocity, direction is an important component, but for speed, it is not.  For a simple illustration of that, consider two cars 100 metres apart on a narrow road.  Both are travelling at 100 kilometres per hour.  If they travel in the same direction, then the situation is perfectly safe.  Imagine now that they are travelling in opposite directions.

Direction is important.

When you enter the sphere of influence of a celestial body, assuming that you do not aerobrake, ignite your rockets or RCS thrusters, or crash into anything, then you will exit the sphere of influence with the exact same speed as you entered it.  Your speed will change as you move through the sphere because you are falling under gravity, but as soon as you pass periapsis and begin climbing out, you trade that speed for altitude until you exit.

However, fly-by orbits are not straight lines; they are special curves called hyperbolae (singular hyperbola).  Because the fly-by is not a straight line (or the original orbital track; I know those aren't straight lines either), you don't pass through the sphere of influence and exit where you would have if the celestial body was not present; your direction changes.

Imagine for a moment that you're orbiting Kerbin and your orbit takes you up to the Mun's altitude.  At that apoapsis, the Mun, being in its own orbit, catches up to you and you enter its sphere of influence.  From the Mun's perspective, you are flying more or less towards it; assuming that your apoapsis orbital speed was negligible, then on entering the Mun's sphere of influence, you are approaching the Mun (or the Mun is approaching you; it makes no difference) at about the Mun's own orbital speed, which is 542.5 m/s.  Let's also imagine that you just barely miss the Mun and that its gravity is such that your fly-by path warps around so as to exit the sphere at the same point that you entered.  That isn't actually possible for a couple of reasons, and we're also assuming that this happens quickly enough that we can neglect the Mun's orbital motion, but it's good for an illustration.  When you entered the Mun's sphere of influence, you were travelling at 542.5 m/s in.  When you exit, you're travelling at 542.5 m/s out.  When you were in Kerbin's sphere of influence and just about to enter the Mun's, you were travelling at nearly zero m/s in the Mun's prograde direction.  When you just enter Kerbin's sphere of influence after exiting the Mun's, you're travelling at 1085 m/s in the Mun's prograde direction.  That kilometre per second is the effect of the gravity assist.

In actual use, the Mun cannot provide such a good gravity assist.  Its own gravity is too low to warp your trajectory back on itself, and even if it could, it has a surface which obviously limits your closest approach.  It also orbits fairly close to Kerbin, which reduces the size of its sphere of influence.  It is best used as a helper; it can cut the fuel required for an interplanetary ejection, but it can't get you to your destination on its own.  It can also cut the fuel needed for a braking burn (though Kerbin's atmosphere does that just as well; such braking assists are much more effective at Ike, because Duna's atmosphere isn't very thick, and Tylo, because Jool's atmosphere is too thick) and it can provide an assist to Minmus.

As far as how to use a body for an assist, the key is to understand where your orbit enters and exits.  For prograde and retrograde changes, you want to exit in the Mun's own prograde or retrograde direction as shown by its orbit around Kerbin.  The ideal assist has you enter the sphere exactly where you intend to exit, but the Mun can't provide that trajectory, as I said earlier.  In your case, you'll want to catch up to the Mun as you fall from Minmus, but you'll need to hit the sphere a bit to the radial out side if you want to exit exactly retrograde.  A bit is not very exact, but you'll have to test it yourself and see.  Use the manoeuvre nodes to plan the burn (also turn up the conic patch limit in the settings if you have not already; I use six for my own games) and see what you can get.  It's actually not difficult to get the encounter; the issue is getting the encounter exactly right so you can extract the most assist from it.

If you understood none of that, don't worry.  Gravity assists are quite the advanced topic.  Consider how difficult it is to do interplanetary transfers, when you're trying to go from a moving origin to a moving target on a single burn by getting the direction and timing exactly right at the beginning.  Gravity assists complicate that exponentially by requiring you to do same thing as an interplanetary transfer and go to the second planet which is not only moving, but in such a way as to exploit that motion in order to arrive at a third planet which is also moving, and moving differently, without needing to fire the engines.

That being said, they are not out of reach.  Keep at it.

If you would like a fuller explanation, this article explains the gravity assist and gives practical examples of how Voyager 2 used them for the Grand Tour.  It includes helpful animations to show exactly what happens with different kinds of gravity assists.

1 hour ago, Elroy Jetson said:

It seems that this is going to be a technique used for much of the future missions to outer planets?

It's a help.  It is not necessary.  It can potentially save enough fuel that you can capture at Jool with zero fuel expenditure (though you'll need to burn to avoid a second assist from Tylo or whatever you use), but there's nothing to stop you from taking the fuel and doing things the brute-force way.  I will say also that you can use assists for the inner planets:  Eve has high gravity, is centrally located, is fairly easy to reach from Kerbin, has a high orbital velocity, and has a bit of inclination, as well.  There's also nothing to keep you from circling round and, with minor corrections, getting a second assist from a single celestial body, so it is actually the case that if you can get to Eve, then you can get anywhere.  The only cost is time.

Edited March 19, 2019 by Zhetaan

[Aegolius13]

I'll defer to that excellent explanation from @Zhetaan, but add that you're not going to see a huge effect using the Mun to slow down from Minmus.  Per the delta-v map, it takes about 860m/s to burn from Kerbin to the Mun, and about 930m/s to go to Minmus.  The max speed you can lose is the difference between those numbers (70m/s), which is probably not going to have a big impact on your reentry speed. If you have a craft that overheats on reentry for Minmus, it  may be more effective to shed speed by aerobraking -- i.e., setting your periapsis at Kerbin high enough that you take several passes before falling out of orbit. 

All that said, gravity assists are really helpful certain scenarios (capturing into the Jool system is number one in my book), so it's a great idea to get the concept down early.  

Edited March 19, 2019 by Aegolius13