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Direct launch to inclined orbit.


qoonpooka
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I would like to put a ship into orbit around minmus with a minimum of fussing around.  I've heard that a 'direct launch' is the lowest dV way to get where you're going. Basically, skip the parking orbit and just go for what you want.

Okay. Fair enough.

 

What I want is a rendezvous orbit around Minmus with this here tanker, so that it can refuel an on-station lander.  This means that in addition to burning directly into my minmus rendezvous, I also want to have inclination.

I'm assuming, since I have zero control over where the KSC is that flying directly into the minmus encounter's no good.  But how do I know which way to point my navball to get the properly 6-degrees-inclined orbit after I circularize into LKO? (I'm aiming for an 80km parking orbit to go for my rendezvous burn from.)

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You can launch directly into a 6-degree inclined orbit to match Minmus if you launch at the right time.  These opportunities happen twice a day when the KSC passes through the plane of Minmus' orbit.  I do it it like this:

Send your rocket to the launch pad and then switch to the orbital map view.
Focus your view on Kerbin (double-click on Kerbin to change focus from rocket to planet).
Zoom out until you see the orbit of Mun.
Align the view so Mun's orbit is seen edge on.
Zoom out until you see the orbit of Minmus.
Rotate the view left-right until Minmus' orbit is seen edge on (keeping Mun's orbit edge on as well).
The orbits of Mun and Minmus will now been seen as two intersecting lines.
Time wrap ahead until the launch site reaches the point where the two orbits cross.
Check to see which direction you need to launch to match Minmus' orbital plane, i.e. north of due east or south of due east.
Switch back to the launch pad view and launch.
Immediately after beginning your ascent, rotate the rocket 6.5 degrees either north or south (as determined two steps earlier).
After rotating the rocket, begin your pitch down and fly a normal ascent to orbit.

This should get you pretty close to matching Minmus' orbital plane.  I always have some error but the correction is usually small and easy to fix.

Edited by OhioBob
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1 hour ago, qoonpooka said:

For the 'check which direction you need to launch' part... how do I know north vs. south? North is for ascending node, south for DN?

When KSC reaches the point where the two orbits cross, then the launch site is located along Minmus' line of nodes.  When zoomed out it might be hard to tell on which side of Kerbin the launch site is located, so you might have to angle the view a little bit a zoom in to see where the KSC is.  If it is on the side of Kerbin closest to Minmus' ascending node, then you want to launch along a heading that is 6.5 degrees north of due east.  If KSC is on the side of Kerbin closest the Minmus' descending node, then you want to launch along a heading that is 6.5 degrees south of due east.

The reason we use 6.5 degrees instead of 6 degrees is because we're starting out with some due east velocity because of Kerbin's rotation.  To end up in an orbit with an inclination of 6 degrees, we have to over compensate a little bit.

(edit)

Getting the heading right is usually the hardest part for me.  The NavBall just isn't precise enough to dial in on an exact heading, so I generally have to eyeball it and hope I'm about right.  If you use KER, there is an orbit inclination indicator that helps.  Once I get about halfway through the launch I can see how far off my inclination is and I can start yawing north or south to correct.
 

Edited by OhioBob
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if you launch right at the dn or an   you can launch directly into its orbit.

one way to help find this is to keep a simple sat at 0 inclination. you can switch to it and select minmusas target, then watch until the ksc reaches it(the dn or an that is).  i always just guestimate and use map view to eyeball it

Edited by DD_bwest
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1 hour ago, OhioBob said:

You can launch directly into a 6-degree inclined orbit to match Minmus if you launch at the right time.  These opportunities happen twice a day when the KSC passes through the plane of Minmus' orbit.  I do it it like this:

Send your rocket to the launch pad and then switch to the orbital map view.
Focus your view on Kerbin (double-click on Kerbin to change focus from rocket to planet).
Zoom out until you see the orbit of Mun.
Align the view so Mun's orbit is seen edge on.
Zoom out until you see the orbit of Minmus.
Rotate the view left-right until Minmus' orbit is seen edge on (keeping Mun's orbit edge on as well).
The orbits of Mun and Minmus will now been seen as to intersecting lines.
Time wrap ahead until the launch site reaches the point where the two orbits cross.
Check to see which direction you need to launch to match Minmus' orbital plane, i.e. north of due east or south of due east.
Switch back to the launch pad view and launch.
Immediately after beginning your ascent, rotate the rocket 6.5 degrees either north or south (as determined two steps earlier).
After rotating the rocket, begin your pitch down and fly a normal ascent to orbit.

This should get you pretty close to matching Minmus' orbital plane.  I always have some error but the correction is usually small and easy to fix.

 

For the 'check which direction you need to launch' part... how do I know north vs. south? North is for ascending node, south for DN?

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1 hour ago, qoonpooka said:

For the 'check which direction you need to launch' part... how do I know north vs. south? North is for ascending node, south for DN?

when the KSC is at the AN for minmus,  it means that it is passing into the part of its orbit that is above or north of its its orbital plane.   So you would want to burn a bit southerly to try and head south to match that plane.

when the KSC is at the Dn for minmus, it is heading "below" or south of minmus' orbital plane.  So you would want to go north slightly.

edit:  for clarity, mine describes the nodes for something at zero inclination,  ohiobob describes minmus's nodes to kerbins orbital plane.   they are essentially the same locations but reversed lol

Edited by DD_bwest
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12 minutes ago, OhioBob said:

When KSC reaches the point where the two orbits cross, then the launch site is located along Minmus' line of nodes.  When zoomed out it might be hard to tell on which side of Kerbin the launch site is located, so you might have to angle the view a little bit a zoom in to see where the KSC is.  If it is on the side of Kerbin closest to Minmus' ascending node, then you want to launch along a heading that is 6.5 degrees north of due east.  If KSC is on the side of Kerbin closest the Minmus' descending node, then you want to launch along a heading that is 6.5 degrees south of due east.

The reason we use 6.5 degrees instead of 6 degrees is because we're starting out with some due east velocity because of Kerbin's rotation.  To end up in an orbit with an inclination of 6 degrees, we have to over compensate a little bit.
 

  I used to do it that way  and would always get frustrated with the map until someone on this forum(forget who)  mentioned using a small sat.   If you just have a sat at 80km at 0 inclination, you can use it to set targets with dotted white lines showing you the node locations.

So you put your rocket on the pad, switch to your targeting sat, and select the target.  then time warp in map view until you see that your rocket on the pad is below one of the nodes.

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1 minute ago, DD_bwest said:

  I used to do it that way  and would always get frustrated with the map until someone on this forum(forget who)  mentioned using a small sat.   If you just have a sat at 80km at 0 inclination, you can use it to set targets with dotted white lines showing you the node locations.

So you put your rocket on the pad, switch to your targeting sat, and select the target.  then time warp in map view until you see that your rocket on the pad is below one of the nodes.

Sounds like a good idea.  I may have to try that.

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4 hours ago, DD_bwest said:

  I used to do it that way  and would always get frustrated with the map until someone on this forum(forget who)  mentioned using a small sat.   If you just have a sat at 80km at 0 inclination, you can use it to set targets with dotted white lines showing you the node locations.

So you put your rocket on the pad, switch to your targeting sat, and select the target.  then time warp in map view until you see that your rocket on the pad is below one of the nodes.

I just memorized stars in those two spots and use them to timewarp to right position. Easier than switching ships.

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