How to achieve a future orbit.

[JackAz]

I've been trying to achieve a future orbit around a planet while in interplanetary space for a while, but the only thing i can do is to change my trajectory so that i can get directly in to the planet once i've entered it's SOI.

[nhnifong]

You can't enter an SOI directly into orbit. Without slowing down somehow within the SOI you will swing by the planet and exit that SOI at the exact same speed you entered it.

You will need to slow down at your periapsis, and lower your apoapsis under the edge of the SOI.

Or you can place your periapsis in the atmosphere while you are outside the SOI, and then it will slow you down when you get there. it's difficult to predict without mechjeb though.

You could also do it with a gravitational assist using a moon of your target planet. set your trajectory so that you exit the moon's SOI going in the retrograde direction of that moon.

Edited October 3, 2012 by nhnifong

[Vanamonde]

Considering the distances involved and the precision that would be needed, that's really not bad. By "directly in" do you mean that you're hitting atmo without orbiting first? If so, try burning 90 degrees away from prograde to the east or west, and watch how your path changes. It should round into a loop around the planet. Note that this may waste fuel, if your intention is to land on the planet anyway. Also, I'm not suggesting this as a precise technique. I'm just pointing out that there are burns you can make to alter your trajectory into an orbit. Play around with it and get a feel for the processes involved.

[JackAz]

You can't enter an SOI directly into orbit. Without slowing down somehow within the SOI you will swing by the planet and exit that SOI at the exact same speed you entered it.

You will need to slow down at your periapsis, and lower your apoapsis under the edge of the SOI.

Or you can place your periapsis in the atmosphere while you are outside the SOI, and then it will slow you down when you get there. it's difficult to predict without mechjeb though.

You could also do it with a gravitational assist using a moon of your target planet. set your trajectory so that you exit the moon's SOI going in the retrograde direction of that moon.

Ok, this is the moon way.. the only problem is that i have no idea on how to get form interplanetary space into the planet's moon SOI.

http://i.imgur.com/IPEe0.png (not my screen)

Instead this is me triyng to achieve an orbit without wasting fuel. It works, nearly : ship just gets dragged in, maybe with duna atmosphere it could work.

[Peppe]

Every body in the system has an escape velocity. It is how fast you need to go to break out of orbit.

From an encounter trajectory you are in almost all cases going much faster than the targets escape velocity. So your trajectory will be a gravity slingshot, where you may get near the planet/moon, but only end up getting pulled by the body and sent in a new direction.

To orbit a body you must slow down enough for that gravity pull to form an orbit. If there is an atmosphere you can do it for free -- until we get atmosphere/heat physics. Absent an atmosphere you will have to burn retrograde ideally at the closest point to the target and turn your gravity slingshot into an orbit/capture by getting your speed below escape velocity.

[AncientAstronaut]

lol... Orbital Mech fail

[kamilDrakari]

I don't think everybody gets what the OP is asking... It seems to me that he doesn't understand what to do to get from Kerbin's SoI to some other planet's SoI.

I recommend checking out Scott Manley's videos on YouTube, he knows all the math involved and I'm pretty sure at least one of his videos explains what equations to use.

If you don't think you can handle the pure math, or don't want to spend the time learning a whole bunch of physics before having fun in the game, you can just get a ship to escape the Kerbin system so it's in an orbit around Kerbol, then use the same techniques to get an orbit that intersects the orbit of the desired planet. From there you can use warp until an encounter with the target appears, or you can adjust your orbit out more and hope you have enough fuel. I could explain it better with images, but not enough time or motivation to get those right now.

If you really were looking for ways to get into orbit from being already inside the SoI, gonna need even more luck, fuel, or math to get the moon in exactly the right spot during your approach, or rely on air braking. Really, unless you want to do a lot of math I recommend just overbuilding your spacecraft a little so you have enough fuel that you don't need to get the exact least quantity of fuel possible. It is impossible to actually enter orbit with just air braking though, no matter how you try it your periapsis will always be in the atmosphere at the end, causing your orbit to degrade over time.

Edited October 3, 2012 by kamilDrakari

[nhnifong]

Its not that hard to hit a moon of a planet several days out on interplanetary approach. Just get your trajectory in the plane of the moons orbit, and "scan" it from one end to the next.