When should I launch the rocket to arrive a planet with gravity assist ?

[berkekrkn]

Hello everyone,

I always launch my probe directly so it costs a lot of Delta-V to go outer planets (like saturn, neptune etc.) I want to use gravity assist.

For example in Juno mission. The spacecraft launched came back to earth then burned again for jupiter. I want to make same mission. When should I launch my rocket ?

Or I want to go to mars with the help of the gravity assit of venus. When should I launch my rocket to venus ?

 

Is there a specific time for that or can I launch whenever I want ?

 

I am using RSS btw.

[Hodari]

Without being able to run huge numbers of simulations to find the best possible solution(like NASA would for a real mission), I'm not sure if gravity assists are really worth it in this game.  If you really do want to try it just for fun though, first of all there's a setting somewhere for how many SoI changes will be shown on the map.  Default is 3, but you'll probably want to increase that to 5 or more so you can see the full effect of your maneuvers.

As for when to launch, the point of a gravity assist is to get an efficient launch to one planet and then pass close to it and use its gravity to redirect you towards the next planet.  So I would assume you'll need to start by launching at the same time as a normal transfer window to your first target planet.  Ideally, calculate how long this transfer will take and then make sure that when you arrive, the second planet will be in the appropriate place for a second transfer from there.  And so on, if you're planning to do more gravity assists.  Given how sensitive all of these maneuvers are to exactly where and WHEN you arrive, you'll probably have to do a few small correction burns as well(which will eat up much of the fuel you would otherwise have saved by doing the gravity assists)

[Aegolius13]

If you are using another planet to gravity assist (e.g. using Venus to get to Mars), I think you want to generally launch during the transfer window to Venus, as if you were going there.  Then you fine tune the the flyby to get a trajectory that sends you to the target.  If you don't care about game time, you could compare different Venus transfer windows to find one where Mars is in a good position.

Using Earth, I'm not sure but I think you need to go around the sun at least once.  So maybe find a good phase angle and launch one year earlier?  

 

[Geschosskopf]

2 hours ago, berkekrkn said:

Is there a specific time for that or can I launch whenever I want ?

Yes, there is a specific time to launch for a gravity assist, which depends on 3 things:  The position of Earth, the position of the assisting planet, and the position of the ultimate target.  There used to be, and maybe still is, a mod called IIRC Launch Window Planner that would tell you when various gravity assist windows open up.  I believe there's an RSS version.

The thing is, though, that such windows necessarily are few and far between.  IIRC, the Voyager probes happened when they did because another opportunity for such a grand tour wouldn't happen again for another 150+ years.  The further out the assisting and target planets are, the slower they move relative to each other, so the longer it takes for them to achieve the necessary alignment.

Now, the opportunity for a Juno-type mission would happen more frequently.  You'd expect that to happen approximately annually, every time Earth came into the right position relative to Jupiter.  However, if there's a big inclination difference between Jupiter and Earth (I really don't know if there is or not), then you might have several years where it would be more practical (Jupiter close to Earth's plane) and then several years where it wasn't (Jupiter far above or below Earth's plane).

But in any case, a general rule of spaceflight, whether in RSS or KSP, is that that you can trade time for fuel and vice versa.  If you don't care about time, then you can do gravity assists and such to save fuel,.but the missions will often take longer and you might have to wait many years for the special launch windows.  But if you want to leave this year, and/or you want to get there quickly (to save on life support, reduce the exposure to space weather, etc), then you pretty much need to pack lots of dV and go there directly.

[Vaporized Steel]

In theory and real life practice "The" launchwindow" to the outer planets is any launchwindow to mars or venus.
Both in gameplay and real life practice though, some of these times may take a while. There is a "but" though. Which is that with the better timewarp mod it doesn't matter when you launch, because you can timewarp very fast to your next gravity assist target, Even if it takes 4 orbits around the sun.

If you want to get to saturn, uranus and/or neptune with a straight gravity assist from mars > jupiter > saturn and so forth you may want to change your RSS game time to august 1977 which is when the voyager crafts were launched. This puts all the outer planets in a favourable condition to be reached with minimal delta V.

If you want a probe around all the outer planets then launch several rockets around this time in order to reach them.

If it's a Juno replication, then you'll want to use either or all gravity assists from venus, earth and mars.

I do not have the exact math! Thus, I do not have the step by step procedures or whatnot to guarantee your fail safe aproach of success. But the idea behind it is that you can use the gravity assist from venus to change your orbital time period around the sun.
Changing your orbital time period around the sun is your main focus on your first venus gravity assist. That means your main focus is not getting the fastest slingshot, and added speed from your first assists by raising your apogee as much as possible.
The trick is that your first assists has a orbital time period which equals the amount of time for your next target to meet your own as you cross it's orbit line.

The simplest example is that if your first gravity assists is Venus, then you want to gravity assist by exceeding your apogee around the sun by 1.5 times the orbital time period that Venus has orbiting the sun. This means you'll meet venus again in 2 orbits, and you can raise your apogee around the sun even further.
A third mars and/or Earth gravity assist might get you to where you want.

Remember, venus could be your second try, this can also be Earth. It can even be Mars in some cases. As Geshosskopf pointed out, it depends on the location of the planets during your game time.

In some cases it means you may want to lower your periapsis around the sun with a mars gravity assist to match your orbital time period around the sun to meet up with venus or earth again.

It could take 3, 4, 5 or more gravity assists in total to even get your apogee around the sun up to that of jupiters. By that time, it's speculative whether saturn is in the proper location to be reached at all. These are all things you have to consider in advance. So if you don't want a straight hohman transfer aproach to jupiter with the intention to reach saturn or beyond, you'll have to know in advance to use several gravity assists to reach jupiter, and by then know whether saturn and or uranus will be in the correct position when you get to jupiter. The necessity of a Nasa computer as stated is not that much of a joke. Although, I'm sure somebody could create a mod/application to calculate complex gravity assist simulations.
I don't know about mods, apps or online websites that illustrate complex rss gravity assist simulations, and I'm very sure they do not exist as I have never heard of them.
The best thing you can do is take my advice and those of others and quiksave before every maneuver and quikload if necessary.

Edited July 7, 2016 by Vaporized Steel

[Plusck]

The most massively important thing about a gravity assist from the planet you left from is what NASA cryptically calls "deep space maneuvres". Without those maneuvres, you will never gain more energy from the planet than if you made a perfectly efficient escape burn the first time.

The next important thing is: if you want to intercept the planet you came from, you need to come back to more or less the place you started. If you want to reach another planet, it will be more or less opposite (wrt the sun) where you started.

 

So if you start at Kerbin, and want a gravity assist from Kerbin, then you need to start with a multiple of 1 Kerbin year for your first sun orbit (so you can intercept Kerbin again). Copying Juno, that means a slightly-more-than-2-year orbit.

When you're at the furthest point from Kerbin (and going slowest), you make a burn that reduces Pe, makes you intercept Kerbin after your initial derparture point while rising back up after Pe, and gives you a kick forward along Kerbin's orbital path.

And then after that, you want to arrive at Jool opposite that Kerbin intercept. Journey time from Kerbin to Jool is about 3 or 3.5 years.

Therefore... if you want to copy the Juno mission profile, you need to leave Kerbin about 5.5 years prior to the time Jool will be exactly on the other side of the sun from you. This is something that happens every orbit, so you don't need to wait long to send such a mission... but it's complicated to work out exactly when you should start. Planning tools (such as maneuvre nodes) in stock are not really up to the task.