Diche Bach

Inclined Retrograde Orbit: Easiest Optimized Rendezvous?

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Finally getting the tech nodes for some grabber mechanisms and I have a couple very lucrative "take tourists by an asteroid missions." The randomizer has very kindly provided me with an asteroid that is in orbit of Kerbin; it it did toss in a bit of a wrinkle: it is ~140-degree inclined orbit (or thereabouts).

J2vAi.jpg

So what is the actual method to determine the launch time and inclination for this sort of rendezvous? What tools does the game provide to allow the user to determine where the nodal line is, what the actual inclination of the target is, etc.?

I tend to approach problems like this with a "tinkerer" mindset and then use "simulation mode" (in Kerbal Construction Time) to get things figured out. In this case I've created a "Grabby Ship" that has about 10k dV at launch and launched it in a couple of different retrograde orbits. However both times, the size of the burn necessary to align the inclinations from 180 to whatever it actually is for the asteroid was gigantic: I'd have very little dV left with which to alter the asteroid's orbit and put it into a nice parking orbit of Kerbin.

I'm not even entirely sure what the damn thing's inclination IS, and apart from using a space craft that is in orbit and looking at the AN/DN divergence values I'm not sure if there is a better way to determine it.

Edited by Diche Bach
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Well no, this is the sort of data that Squad doesn't want to have stock because it would make KSP a game of numbers, instead of fun and experimentation.
The game provides exactly two tools for this sort of thing - the map mode and manoeuvre nodes.  Stock = play around until you get an encounter/rendezvous.
None of this helps unless you have a reference ship in orbit already and from which you can plot the nodes.  Launch-to-rendezvous is a whole other step.

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I'd just eyeball it. Wait until the KSC is roughly under the point where the asteroid's trajectory crosses the equator, then launch towards northwest (I think that's right) and check the mapview during the launch to adjust heading as needed to match your inclination.

It is much more efficient to launch directly into the inclined orbit than to go into an equatorial retrograde orbit and then change the inclination.

Edited by EpicSpaceTroll139
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Just now, Pecan said:

Well no, this is the sort of data that Squad doesn't want to have stock because it would make KSP a game of numbers, instead of fun and experimentation.
The game provides exactly two tools for this sort of thing - the map mode and manoeuvre nodes.  Stock = play around until you get an encounter/rendezvous.
None of this helps unless you have a reference ship in orbit already and from which you can plot the nodes.  Launch-to-rendezvous is a whole other step.

Hmmm. That is more or less what I figured. Thanks for clarifying :D

So assuming I establish exactly what the things inclination IS, then the trick is to establish "where (which is really more like "when")" its Longitude of the ascending node is?

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As far as I know there's no way to determine that in stock

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3 minutes ago, EpicSpaceTroll139 said:

I'd just eyeball it. Wait until the KSC is roughly under the point where the asteroid's trajectory crosses the equator, then launch towards northwest (I think that's right) and check the mapview during the launch to adjust heading as needed to match your inclination.

Ohh, I LOVE that method! :sticktongue:

Use the Mark II Precision Eyeball!

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1 hour ago, EpicSpaceTroll139 said:

I'd just eyeball it. Wait until the KSC is roughly under the point where the asteroid's trajectory crosses the equator, then launch towards northwest (I think that's right) and check the mapview during the launch to adjust heading as needed to match your inclination.

It is much more efficient to launch directly into the inclined orbit than to go into an equatorial retrograde orbit and then change the inclination.

Actually, it seems to be SOUTHwest.

TFzdd.jpg

This really is the tricky part: visualizing what direction to point the space craft at launch!

If you can get it into the right "ballpark" then once you get an orbit (or at lest a very long sub-orbital arc that gives you enough air time) the stock tools are sufficient (as long as you have enough dV).

But that initial "which way do I start" angle off the launch pad is a toughie.

This is about the fifth "simulation" flight I've done but I got the cost for sims turned down to 100 funds so not a big loss . . .

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This is a perfect situation for having a synchronous satellite hovering directly over KSC. If you have such a satellite, then you simply make that the active craft, and set the asteroid as its target. Wait until the AN/DN is about to touch (taking note of the inclination), then switch back to the rocket and take off with the opposite inclination. Fairly accurate even with the Mark I eyeball, trival with a mod like Gravity Turn or MechJeb.

 

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It is amazing how much dV inclination changes can eat up!

1017 dV to effect a 34.1 degree change!?

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Just now, Diche Bach said:

It is amazing how much dV inclination changes can eat up!

1017 dV to effect a 34.1 degree change!?

The further you away from the body you're orbiting, the cheaper inclination changes are. Recently, I had a rescue contract, which was of course retrograde (not that I noticed until after launch :sealed:), and by raising my apoapis to Mun-height and doing the change there, I was able to do a 180 degree change for about 12-1300 dV. Which was a lot better than the 4000ish dV it takes to do it in LKO

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6 minutes ago, Diche Bach said:

It is amazing how much dV inclination changes can eat up!

1017 dV to effect a 34.1 degree change!?

Physics can be a *bleep*.

Still peanuts compared to what that kind of thing would cost IRL

35 minutes ago, Diche Bach said:

Actually, it seems to be SOUTHwest.

Oops, yes that's right. Derp.

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1 hour ago, GluttonyReaper said:

The further you away from the body you're orbiting, the cheaper inclination changes are. Recently, I had a rescue contract, which was of course retrograde (not that I noticed until after launch :sealed:), and by raising my apoapis to Mun-height and doing the change there, I was able to do a 180 degree change for about 12-1300 dV. Which was a lot better than the 4000ish dV it takes to do it in LKO

I'll have to try that.

So I've established, it is exactly a 137-degree inclination, which seems weird somehow.

From KSC:

directly east (azimuth 090) = 000.0 - degree inclination

directly north (azimuth 000) = +090 inclination

directly west (azimuth 270) = 180 degree inclination, no?

So why is "southwest" from KSC not > 180 inclination?

 

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11 minutes ago, Diche Bach said:

I'll have to try that.

So I've established, it is exactly a 137-degree inclination, which seems weird somehow.

From KSC:

directly east (azimuth 090) = 000.0 - degree inclination

directly north (azimuth 000) = +090 inclination

directly west (azimuth 270) = 180 degree inclination, no?

So why is "southwest" from KSC not > 180 inclination?

 

Well west is also -180 degree inclination. Directly north is also -270 degree inclination. You can measure the angle going either way. KSP just decided to measure it as -137 degrees instead of 223 degrees. You could even go so far as to say any particular angle is = itself + k360   where k = any integer.

Edited by EpicSpaceTroll139

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7 minutes ago, EpicSpaceTroll139 said:

Well west is also -180 degree inclination. Directly north is also -270 degree inclination. You can measure the angle going either way. KSP just decided to measure it as -137 degrees instead of 223 degrees. You could even go so far as to say any particular angle is = itself + k360   where k = integer.

Hmm, trippy.

So 137 (not "negative 137," just straight up 137 [which is what KER is telling my my inclination is when I get my AN/DN to 0.0 relative to the target asteroid]) + 90 (which is the azimuth for "zero" inclination) = 227 (which is what I assume you meant when you wrote "223?").

So, if I launch while KSP is aligned (roughly) on the nodal line and target an azimuth of 227, that should put me in the ballpark for minimal inclination change once I achieve orbit . . . does that sound right?

Edited by Diche Bach
clarify

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OOOOHHHH.... lol this whole time I thought you had said -140 degrees, but what you really put was ~140 degrees. *facepalm*

So with proper fixes for that, yes, you're right, 227

Edited by EpicSpaceTroll139
rewriting the past
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1 hour ago, GluttonyReaper said:

The further you away from the body you're orbiting, the cheaper inclination changes are. Recently, I had a rescue contract, which was of course retrograde (not that I noticed until after launch :sealed:), and by raising my apoapis to Mun-height and doing the change there, I was able to do a 180 degree change for about 12-1300 dV. Which was a lot better than the 4000ish dV it takes to do it in LKO

Just watched a Scott Manley video. It seems that it is "velocity" that is the actual moderator of these "cheaper" inclination changes.

 

1 minute ago, EpicSpaceTroll139 said:

OOOOHHHH.... lol this whole time I thought you had said -140 degrees, but what you really put was ~140 degrees. *facepalm*

So with proper fixes for that, yes, you're right, 223

Two twenty _SEVEN_ :D

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No problem :wink:

It is a shame though that the game doesn't provide any means to determine the inclination for a celestial object the way it would be determined in real life. Kinda silly that I have to launch a satellite into an equtorial orbit and then use its "onboard computer" or whatever to determine the exact inclination of anything in the game :P

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1 minute ago, Diche Bach said:

No problem :wink:

It is a shame though that the game doesn't provide any means to determine the inclination for a celestial object the way it would be determined in real life. Kinda silly that I have to launch a satellite into an equtorial orbit and then use its "onboard computer" or whatever to determine the exact inclination of anything in the game :P

I don't know if it works for unknown objects, but often with ships I'll determine my inclination by setting the Mun as my target and looking at the ascending/descending node it gives me,

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For inclination changes, I actually have a section of my KSP spreadsheet devoted to estimating how much dV I will save by raising my orbit AP before doing the change. It is worth noticing that your PE should be as low as possible, you want all the eccentricity you can get.

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2 hours ago, Freshmeat said:

For inclination changes, I actually have a section of my KSP spreadsheet devoted to estimating how much dV I will save by raising my orbit AP before doing the change. It is worth noticing that your PE should be as low as possible, you want all the eccentricity you can get.

Yeah, I resemble that remark to the extent that . . . I sometimes wind up flying through the 65km zone. I I were quicker on the maneuver node setup and execution I wouldn't have that problem. I'll set Ap to just at 70 (and I generally strive for as platykurtic of arc as possible in my initial trajectory. But by the time I switch to map mode, setup my orbital burn turn on RemoteTech's Flight Computer, switch back to map mode and click Execute for RT FC to do it for me, I'm already PAST 70km!

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16 hours ago, Diche Bach said:

I'll have to try that.

So I've established, it is exactly a 137-degree inclination, which seems weird somehow.

From KSC:

directly east (azimuth 090) = 000.0 - degree inclination

directly north (azimuth 000) = +090 inclination

directly west (azimuth 270) = 180 degree inclination, no?

So why is "southwest" from KSC not > 180 inclination?

 

I think that by convention, inclination is measured in the range 0-180°. It's the longitude of the ascending node that tells you which way around an orbit is. If you have two orbits that are in the same orbital plane, but travelling in the opposite direction, they will have the same inclination but the longitudes of the ascending node will be 180° apart.

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18 hours ago, Diche Bach said:

No problem :wink:

It is a shame though that the game doesn't provide any means to determine the inclination for a celestial object the way it would be determined in real life. Kinda silly that I have to launch a satellite into an equtorial orbit and then use its "onboard computer" or whatever to determine the exact inclination of anything in the game :P

There are a few good reasons for having a satellite or station in equatorial orbit to help with mission planning. Not only can you determine ascending/descending nodes as discussed, but it's great for transfer window planning. Just plot a node that has the transfer you want, note how far in the future it is, then launch your actual expedition vehicle right before it's time to go. Saves a lot of life support.

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There is a simple procedure, even in stock.  You don't even need to use maneuver nodes.

 

(1) Launch, as best you can, into the plane of the target but in a much lower orbit, the lower the better.

 

(2) You now have ascending/descending nodes.  Pick the one on the dark side.  At that node, burn prograde.  The opposite node will lift.  Burn until the opposite node crosses the path of the target.  Do not attempt to match planes.

 

(3) The opposite node is where you will meet the target.  The only issue is time.  You should see one of the white tick marks at the node and the other somewhere else.  Warp to the node, to just after the intercept marker. Then burn prograde.  The other marker should rotate around the target's path.  Burn until the two markers match each other.  You now have an intercept.  Do not attempt to match planes.

 

(4) Warp to the intercept then burn to equalize speed with the target.

 

Done.  No fiddling.  No maneuver nodes.  No plane matching.  This system isn't perfect, but it is very efficient even in stock.

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7 hours ago, Sandworm said:

(4) Warp to the intercept then burn to equalize speed with the target.

Done.  No fiddling.  No maneuver nodes.  No plane matching.  This system isn't perfect, but it is very efficient even in stock.

This only works for intercepts in very high orbits.

By definition a rendesvous requires matching planes(and all other orbital parameters). Otherwise you're not in the same orbit, and will fly past at speed.

So you're making a large plane change, but at a high enough altitude (and thus low velocity) that's not too painful.

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