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I'm trying to get into orbit around Duna, but I can't make it into a proper orbit.


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I have a satellite that is very close to Duna, and I am trying to put it in orbit. After a ton of tinkering with it, I eventually have my orbit like this: (it escapes Duna off the right side of the picture)

https://imgur.com/a/oM0H1C0 (For some reason the insert from URL button wont work)

The light blue arrow is the direction my thrust is going (the ship moves in the  opposite direction of the arrow when i burn)

The red arrows are the direction my orbit moves when i burn in that direction. The periapsis moves a bit and the part of the orbit past the PE moves more depending on how far away from the planet it is.

I need help getting this satellite into orbit, because i just cannot do it. I have burned in just about every possible direction to no avail.

(My engine is a Dawn, so it isnt the fastest)

Edited by FinnBot5000
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You mostly need to slow down relative to Duna, so that you no longer have enough energy to completely escape Duna's gravity so stay in orbit.  You are moving down and to the right in your image relative to Duna, so send your ion exhaust that way by pointing toward the retrograde :retrograde: marker.iBJ9egx.jpg

The green line is your communication link, so not related to your direction of motion or direction of burns. (This forum will show an image in-line if you past a URL of the image file itself, something ending in jpg like "https://i.imgur.com/iBJ9egx.jpg")

As you slow down, Duna's gravity will have more time to bend your course, so your trajectory might get too low over Duna's surface.  So  pointing a little bit 'up' :radial: from retrograde will be needed.

Most of the benefit comes from the thrusting you do near the Periapsis.  If you like, you can set a manoeuvre node at periapsis, stretch out the retrograde handle until the dotted-line orbit closes, see how long the burn will take, and start the burn at a time so half of the burn is done when you expect to reach periapsis.

Edited by OHara
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To capture into orbit of something, you need to slow down until it’s gravity can prevent you from escaping; this is called escape velocity and is the maximum speed you can travel inside a body’s sphere of influence without escaping. In reality, you can escape while going far slower if you’re at a higher altitude.

In any situation where you’re trying to capture into a body’s sphere of influence, you need to reduce your relative velocity i.e. perform a retrograde burn to lower your orbital velocity. Stick a node on your Duna periapsis and pull the retrograde handle until the projected path closes into an orbit of Duna, then start burning when the time to node is ~60% of the total burn time; this extra 20% is because ion thrusters are really weak and take a long time to do anything, so starting at 50% could mean you slow down too late and end up with a bad orbit with a high apoapsis and a periapsis inside Duna’s atmosphere or even Duna itself.

If there’s an atmosphere present, you can save some fuel by using friction from that atmosphere to slow down (aerobraking), however this is a tricky business and if misjudged even slightly can incinerate your craft, slow it down too much so that it lands/crashes or conversely not slow it down enough so that it keeps going and escapes again; Duna’s atmosphere, like Mars on which it’s based, is so think that you’ll barely slow down in it so it’s rarely worth the effort to aerobraking there, but Kerbin, Eve, Jool and Laythe can all be used for aerobraking. Another trick is to use carefully timed encounters with moons to use their gravity to slow you down into orbit, a trick which can allow a free capture around Jool with a single encounter of Tylo or Laythe but which probably won’t help you here as Ike is too small and its gravity too weak.

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