Krymson Skorpyon

How do you all time your launches for orbital docking?

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MechJeb makes docking pretty simple, but I have a tough time with timing my launches so that I don't have to conduct 47 orbits to close distance.

How do you all time your launches so that the two ships end up reasonably close (say, within 10 orbital degrees) to each other?

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15 minutes ago, Krymson Skorpyon said:

How do you all time your launches so that the two ships end up reasonably close (say, within 10 orbital degrees) to each other?

For a target in low orbit, I just wait until they're about 10 degrees above KSC's western horizon and then launch direct to intercept, docking before it even reaches that Korea-looking peninsula to the east.

For a target in high orbit, I launch to low orbit and then set a maneuver node, drag its :prograde: handle until Ap reaches the target's altitude, and then slide the maneuver node around until I get a good intercept with the target.

Thus, there's only ever about 1/4th of an orbit for a low-orbit target, and at most 1 orbit for a high-orbit target.

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I don't have a lot of prescision, but when the target is near the edge of the desert continent is a good benchmark for me.

Alternatively you could burn until you get a close intercept

 

 

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You don't have to, if you place your vessel (rendezvous target) in as high of an orbit as possible, then you launch your other spacecraft in a low orbit, the orbital altitude difference means you don't have to wait such a long time. This is what I do.

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Um, I don't... :D

Seriously, I'll wait to what I am wanting to rendezvous with is over the Great Pyramids in the desert, then launch the desired craft to intercept. Not very scientific and I always try to put the intercepting craft in a higher orbit than what I am attempting to dock with.

May not be perfect, may not be the NASA preferred way, but it's what I do.

 

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Same as Snark, either launch in to a higher orbit ahead of the target, or a lower orbit behind the target, and then plot an intercept.  Depending on the altitudes involved it's usually easy enough to sort it out within 1 orbit.

My standard launch profile is to 80km so I tend to put stations at 120-150ish and then come in below them.

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Posted (edited)
57 minutes ago, Krymson Skorpyon said:

MechJeb makes docking pretty simple, but I have a tough time with timing my launches so that I don't have to conduct 47 orbits to close distance.


You shouldn't ever have to make more than a few orbits unless your orbital altitudes are pretty close...  Even then, unless you've fiddled with the settings the rendezvous auto-pilot won't make more than (IIRC) 5 phasing orbits.
 

57 minutes ago, Krymson Skorpyon said:

How do you all time your launches so that the two ships end up reasonably close (say, within 10 orbital degrees) to each other?


The answer isn't launch timing, it's relative altitude. (Relative speeds.)  My initial orbit is 100km, and I put my target above 250 if I can.  (250 is where KSP switches to a lower-res engine for rendering the planet, reduces lag around space stations.)   If my target needs to stay low (initial orbit) for whatever reason, I launch into my initial orbit and then boost up to 200km and start the rendezvous autopilot from there.

Even if you manually plot and fly the rendezvous intercept, the same principles apply.  You want a significant difference between orbital periods if you want to minimize loiter/phasing time.

And yeah, time warp is a thing your friend.

Edited by DerekL1963

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Thanks for the input.  I'd been hoping that someone would say "Oh, that's XYZ feature of MechJeb" or "Use JoeBlow's LaunchToIntercept calculator addon", but I guess "eyeballs and altitude difference" it is.

BTW, 47 wasn't literal - I just meant "having to make a bunch of orbits".

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2 hours ago, Krymson Skorpyon said:

Thanks for the input.  I'd been hoping that someone would say "Oh, that's XYZ feature of MechJeb" or "Use JoeBlow's LaunchToIntercept calculator addon", but I guess "eyeballs and altitude difference" it is.


Turns out after a bit of googling, there is an "XYZ feature of MechJeb" - down at the bottom of the Ascent Guidance window, "select target for timed launch".  You'll have to launch twice, one for MJ to learn the vehicle then the second time for reals.  Got it close enough that the Rendezvous Autopilot could take over from there.  I suspect that so long as you don't change or rename your vehicle, it'll work on the first time for every subsequent launch. 

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Usually I wait until the target is passing over the ocean just west of KSC's peninsula (as long as it's in an equatorial orbit) then do a normal launch trajectory. Most of the time this gets me a fairly close intercept within an orbit, and if I time it right and fly the gravity turn accurately then I can get one in half an orbit. Sometimes, if I'm lucky, I can even intercept the target while still suborbital.

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I launch when it's about 45 degrees behind the launch site.  Then at the next ascending/descending node, match plane, and use the KAC "Closest Approach" target option to do minor RCS burns to get the intercept to within 10-20 meters.

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On 8/12/2020 at 12:38 PM, Aniruddh said:

You don't have to, if you place your vessel (rendezvous target) in as high of an orbit as possible, then you launch your other spacecraft in a low orbit, the orbital altitude difference means you don't have to wait such a long time. This is what I do.

Put it too far out there and it'll be days before you actually perform docking. With LS mods or Kerbalism, that could actually be a problem. 

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10 hours ago, James M said:

Put it too far out there and it'll be days before you actually perform docking. With LS mods or Kerbalism, that could actually be a problem. 

The ideal altitude I have found is 300-400 km, here the spacecraft in 75 km orbit can catch up in ~30 min, and the transfer to the higher orbit to intercept is only ~45 min. So not that long that your Kerbals would run out of life support supplies.

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