Establishing Synch Orbit over a Particular Longitude (+/-)

[Commander Zoom]

I'm at the point where I'd like to start putting up a few satellites in geosynchronous orbit around Kerbin. I've read the tutorial for spacing out the rest of the satellites from the first one, but I was hoping for some help with placing that first one more or less over KSC. My attempts in a "test sim" save have prompted a few questions:

What's the travel time out to 2868.75 km from, say, 100 km? (I could probably run it again and determine this myself, but still need the answers for the rest.)

How many degrees will Kerbin rotate in that time?

Thus, how many degrees off that will I need to start my burn?

The end result doesn't need to be completely exact (and I know it won't be, having seen in my tests how jittery the physics can get with such orbits), and in theory I can always adjust things by speeding up and slowing down to change my orbit (and back), but I'd like something to aim for at least.

Edited June 24, 2013 by Commander Zoom

[Mr Shifty]

If you launch straight up from KSC, you can reach geosynchronous orbit without losing line-of-sight contact. You don't need to be directly above it; you've got probably few hundred km leeway. I've done this maneuver several times, and had no issues. If you're worried about the planet rotating underneath you, then do a slight westward gravity turn during your ascent.

[maltesh]

An unorthodox method:

(BACK UP YOUR PERSISTENCE FILE IF YOU'RE GOING TO DO THIS).

Build any object, and place it on the launch pad. Call it "Target" or something.

Exit to space center.

Open your persistence file with a text editor, such as notepad (but not wordpad.)

(SERIOUSLY, BACK UP YOUR PERSISTENCE FILE BEFORE PROCEEDING.)

Find the Target object.

Find the line sit = LANDED and change it to sit = ORBITING

Change landed = True to landed = False

Change landedAt = KSC and change it to landedAt =

Make sure splashed = False

In the ORBIT section, make the following changes:

SMA = 3468751.0

ECC = 0.0

INC = 0.0

Leave the rest of the orbit parameters the same. Save.

When next the persistence file is loaded, the Target will be in a synchronous orbit over Kerbin above KSC (or close enough to not matter, because KSC is not /exactly/ on Kerbin's equator). If you screwed something up, exit back to the space center, and restore your persistence file from the Backup.

You now have a Target.

Now build your real satellite deployment craft, and fly it to rendezvous with the target.

Kill relative velocity.

Deploy your satellite.

Delete the target.

[Festivefire]

An unorthodox method:

(BACK UP YOUR PERSISTENCE FILE IF YOU'RE GOING TO DO THIS).

Build any object, and place it on the launch pad. Call it "Target" or something.

Exit to space center.

Open your persistence file with a text editor, such as notepad (but not wordpad.)

(SERIOUSLY, BACK UP YOUR PERSISTENCE FILE BEFORE PROCEEDING.)

Find the Target object.

Find the line sit = LANDED and change it to sit = ORBITING

Change landed = True to landed = False

Change landedAt = KSC and change it to landedAt =

Make sure splashed = False

In the ORBIT section, make the following changes:

SMA = 3468751.0

ECC = 0.0

INC = 0.0

Leave the rest of the orbit parameters the same. Save.

When next the persistence file is loaded, the Target will be in a synchronous orbit over Kerbin above KSC (or close enough to not matter, because KSC is not /exactly/ on Kerbin's equator). If you screwed something up, exit back to the space center, and restore your persistence file from the Backup.

You now have a Target.

Now build your real satellite deployment craft, and fly it to rendezvous with the target.

Kill relative velocity.

Deploy your satellite.

Delete the target.

Hacking in a nav beacon, that's cleaver. the ingame targeting system could use something like that, to set nav points at desired orbits.

[stupid_chris]

To the above, I would suggest a slightly less risky method using the quicksave. Just do the same as above, make a target craft, then quicksave.

Go in your quicksave file and do the same thing as above, modifying the orbit. Go back in the game, reload the quicksave and you'll be in orbit. Less risky than messing with the persistence file in case you mess up you can just dump your quicksave and reload the save file.

[stibbons]

Rather than calculating the best place to start a burn from LKO I get myself in to a circular orbit just above or below the synchronous altitude, wait until my satellite is in about the right place, and then move to a synchronous orbit. Probably slower, but 100% of the time it works every time.

[Commander Zoom]

maltesh: Unorthodox, yes, but... I can see how it would be effective.

(he said, after leaning wayyyyyy back in his chair while reading )

Ditto Mr. Shifty's "just launch straight up, do not pass go, do not establish parking orbit" suggestion.

I'll try those things out in test, though I may end up falling back on stibbons' "slow but sure" (time-consuming and fiddly, bleh) method.

Thanks to everyone who replied.

[CCKinnison]

Well, "Slow but sure" is solveable, but you left out a lot of variables.

What mass payload? What Thrust engine being used for the transfer, and ISP/fuel consumption... Needed for burn time/path calculations.

Honestly? Build the probe/sat you want. Get your starting orbit (100km, 150km, whatever). Mark your start point/time, and burn for Geosynch, noting varibles along the way (fuels in tanks at start, unless you've refuelled them, etc). Mark your final point.

Then you have your answer. If the first one is very inappropriate for where you want the array to end up, trash it. Or move it into place. But now, knowing how far everything will move relative to each other, go and place the array.

[Festivefire]

the other thing you can do, if you have mechjeb or something, is record your east/west coordinates from a parking orbit, right before you start a burn, then record the east/west coordinates once you've reached geostationary, and write down the difference between the two numbers. that's how many degrees before ksc you have to start your burn to get a geosynch orbit above the space center.