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Jovus

Launching to target inclination?

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Hi all,

I've recently discovered I'm not as clear on how to launch into a specific orbit as I thought I was.

 

I'm talking about in the simplified case provided by Kerbin; you launch from the equator into an orbit with Kerbin at the centre. I'm not concerned with off-equatorial launches, not orbits around bodies not the launch body.

What I thought would have worked would be: time warp until the longitude of your ascending node is (roughly) the same as the LAN of the target orbit. Then, offset your launch angle from the East according to the inclination of the target orbit - so if the inclination you want is 20 degrees, you launch at 90-20=70 degrees on the navball.

Unfortunately this didn't work. My orbit ended up in a vastly different orientation, though the same inclination.

What did I do wrong?

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How exactly did you get your LAN while on launchpad? If via KER, then at launchpad it shows LAN 90 degrees from where you actually are. This is technically correct, but due to vanishingly low inclination of the "orbit" you move the LAN very quickly as you start thrusting nort- or southward.

What you really need to know is where your AN would be if get a kick to orbital speed at 20 degree offset from equator. And this is conveniently more or less right where you are now. So, joining to the collective advice here - switch to map view and wait Kerbin rotates you right under the target orbit.

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Maybe you could wait at the launch pad until the target orbit is directly overhead.

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For Minmus, I typically target the orbit, then wait until the Launchpad is on the path plane (or a little before, to be exact).  Once it's lined up, I'll launch to the 95 or 85 degree target and I'll be roughly close.

I'm not too sure about the method you described to help with it.

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As Rodhern pointed out, if you are trying to get into an inclined orbit straight from the launch pad, you need to wait for the Kerbin's rotation so that the target orbit more or less crosses the KSC over the pad. Otherwise, as you noticed, your inclination will be the same in degrees, but your position will be different which results in orbits that don't match at all.

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1 hour ago, Jovus said:

What I thought would have worked would be: time warp until the longitude of your ascending node is (roughly) the same as the LAN of the target orbit.

Well, when you are sitting on the equator (waiting to launch), you don't really have an ascending node -- because your "orbit" is perfectly equatorial, and the An is defined in terms of the difference between your orbit and an equatorial orbit. So your LAN is basically undefined. I think that's the source of why it didn't work for you. The game picked an arbitrary value for your LAN.

What these guys are saying is basically right. The easiest way to know when to launch is just to eyeball it -- put some kind of marker at KSC, and in the tracking station figure out the plane of the target orbit. When the marker is on the plane, launch. I have heard that there is also a way to calculate your proper launch time via a mixture of all the orbital parameters, but I didn't pay attention.

 

 

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I launch to Minmus inclination all the time.  It's not hard, I've been meaning to write a tutorial for a while, it seems like few people know how to do it.

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