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Inclination change in Kerbol's SOI


Plain_Crazy
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Hi

I just completed my first contract to insert a satellite into a polar orbit around the sun (apoapsis 82.2Mkm, periapsis 24.8Mkm). This was actually quite difficult as it took approximately 20k delta v just to do the inclination change at the ascending node from Kerbin's orbit to the required orbit of 90 degrees. 

Normally I try and do inclination changes near the apoapsis but in this instance the required orbit was rather elliptical with the apoapsis being near the southern pole. 

Any tips on how to make inclination changes more reasonable? (assume I'm launching a new craft from Kerbin).

Thanks

Stuart

 

 

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You are correct inclination changes should be done if possible at or near Ap. Or more correctly at the point where you move slowest. If your Pe is also very high it does not really matter.

Inclination changes can be VERY expensive but there is a way to make it a bit cheaper. First you raise your Ap relatively high and perform your inclination change there, Then lower your Ap again. This may look expensive and cumbersome but the combined cost of raising and lowering your Ap is worth the reduced cost in the plane change.

But the cheapest option by far is to launch into, or very close to the correct inclination.
Before launch switch to the map screen and double click Kerbin to make it your point of focus.
Rotate the map to line up the contract AN, DN and Kerbin.
Time warp to line up KSC with the contract AN/DN and launch either north or south depending on the contract parameters. (Make VERY sure you end up orbiting in the correct direction!)
Now you're at or near the correct inclination for only a fraction more dV than a launch into an equatorial orbit.

Edit: Ignore the third part of my suggestion. I slightly misread your post and assumed a Kerbin polar orbit.
I have to agree with @Geschosskopf, an assist from Jool or another heavy body would make a plane change a lot cheaper.

Edited by Tex_NL
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1 hour ago, Plain_Crazy said:

Any tips on how to make inclination changes more reasonable? (assume I'm launching a new craft from Kerbin).

Well, a polar orbit of the sun is going to be expensive, you just have a choice of what currency you pay the price in.  In KSP, as a general rule, you can trade fuel for flight time.  IOW, if you don't care how long the trip takes, you can use less fuel by using gravity assists.  OTOH, if you need to get there quickly, then you need lots of fuel.  The problem with contracts is that they have deadlines so you might not have this choice.  Still, it doesn't hurt to at least see how much good gravity can do you.

The cheapest way to get into a solar polar orbit is to go to Jool and either fly under its south pole or over its north pole so it slings you out of its SOI perpendicular to its own orbital plane.  Jool is massive enough that this will give you a pretty highly inclined solar orbit, just be careful not to fly-by so close as to get Kerbol-escape velocity.  None of the other planets provide anywhere near Jool's amount of inclination change relative to the sun so don't bother with them.  You will still need to finalize your inclination, adjust your LAN and APe to match the target orbit's, and then adjust your solar Ap and Pe as needed, which will cost fuel.  However, these burns will be mostly happening out about at Jool's distance so won't cost all that much.

HOWEVER, this will take literally decades of game time to play out.  It's about 3 years just to get to Jool from Kerbin.  Then once you do the gravity assist there, you'll be in an orbit roughly the size of Jool's, which will mean an orbital period of 10 years or so.  And you'll probably have to make at least 1 full orbit to do all your orbit adjustments to match the contract, maybe more.  Thus, doing it this way will take a minimum of about 15 years, perhaps up to 20-30 depending on how far off you are from the contract orbit after you pass Jool.

Edited by Geschosskopf
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@Tex_NL I guessed you had misread slightly but thanks for the advice regarding raising / lowering apoapsis height. I've previously noticed this effect in Kerbin's / Eve's SOI but was unkeen to test around the Sun due to the ion engine's soul destroying burn times.

Also thanks @Geschosskopf , I assumed a gravity assist would help but was unsure if going all the way to Jool would be worth it. 

I like to play KSP fairly chronologically so launched about 3 other missions before completing this one. Took about 6 in game years to complete without the gravity assist.

I suspect if I were to launch into a polar orbit parallel with Kerbins orbit path when Kerbin was close to the ascending node of the target I could eject from kerbins SOI at 90 degrees to save a bit more fuel.

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If it saves you 5000 m/s dV by spending a few 100m/s to swing by Jool or Eve then it's definitely worth it.

Leaving Kerbins SOI from a polar orbit in order to get into a Sun polar orbit is hardly worth it. It could possible even cost you more.

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44 minutes ago, Tex_NL said:

Leaving Kerbins SOI from a polar orbit in order to get into a Sun polar orbit is hardly worth it. It could possible even cost you more.

Quite true.  None of the non-Jool planets give you enough vertical velocity to get anywhere close to a solar polar orbit.  Leaving Kerbin from a polar orbit has about the same effect as leaving Eve from a polar orbit:  you get only a very slight inclination relative to the sun when you're back out in solar orbit.

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If you wanted to launch into a Sun-polar orbit from Kerbin, you would not eject from a Kerbin-polar orbit.  It would be from a Kerbin orbit that's inclined more-than-0, less-than-90 degrees.  Exactly what the inclination angle would be would depend on what the target solar orbit is-- not just its inclination, but also on its eccentricity.

For example, if you wanted to eject from Kerbin into a Sun-polar orbit that's circular and of the same radius as Kerbin's orbit, your ejection from Kerbin would be inclined 45 degrees from equatorial.  On the other hand, if you were ejecting into a Sun-polar orbit that's highly eccentric, with an Ap at Kerbin and a Pe that's much closer, then your ejection inclination would be much less.

And any way you slice it, yeah, it's gonna cost.  If you can manage a gravity assist, that'll help.

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