Best time to rendezvous?

[Man in the Mun]

Hi,

 

When planning to dock with my space station or fuel dump, I found some of my launches end up with a easier rendezvous in orbit than others. While some have a horrendously long burn time and still nowhere near the target. 

Does it help waiting for the target to be in a certain place in its orbit in relation to the launch pad? For example, is it more fuel (or delta V) efficient to wait for the space station to be just passing the launch pad overhead before sending the rocket up to it, compared to launching straight away when the target could be the other side of Kerbin?

What markers do people use to judge when its the right time to launch ?

Once you are in a lower orbit do you want the target slightly ahead and if you're in a higher orbit do you want the target slightly behind?

 

Thanks.

Edited September 2, 2018 by Man in the Mun

[Vanamonde]

It takes a few minutes to build up the speed to match an orbital object, so you need a bit of a head start. I launch when the target is roughly 30 degrees west of KSC, but it will also depend on the altitude of the target orbit. The higher it is, the longer the coasting phase of your Hohman transfer will take to meet it and the more you'll need to lead the target. 

[bewing]

You get the worst rendezvous burn times when you are going up and your target is coming down. So, in general, it's best to arrange your rendezvous near your target's Ap or Pe.

As Vandemonde says -- an orbital object is coming at around 2km per second. You can figure at least 100 seconds for you to get up to a reasonable altitude with a rocket. So that's 200km that your target will move. So you need to know how far 200km is from KSC. Timing the launch of your rendezvous ship does not directly reduce the deltaV cost. You can sit in LKO and wait for several hundred orbits until things line up properly before you do your rendezvous burn. However, most people are not that patient. Trying to hurry things along definitely uses up a lot more fuel.

1 hour ago, Man in the Mun said:

Once you are in a lower orbit do you want the target slightly ahead and if you're in a higher orbit do you want the target slightly behind?

Yes, that's exactly correct.

 

[Vanamonde]

Oh, by the way, this is how I settled on that 30 degrees. I saved the game, waited until the target was directly over KSC, and flew to the same orbit height. The distance I was behind the target was how much I needed to lead the target, so I loaded my save, waited until the target was that much west of KSC, and launched again.

[Zhetaan]

Spoiler

On 9/2/2018 at 2:09 PM, Vanamonde said:

Oh, by the way, this is how I settled on that 30 degrees. I saved the game, waited until the target was directly over KSC, and flew to the same orbit height. The distance I was behind the target was how much I needed to lead the target, so I loaded my save, waited until the target was that much west of KSC, and launched again.

Incidentally, this is how NASA figured out the launch windows to land on the moon.  The whole moon landing hoax thing started because of a quickload just before the landing--how else did you think Armstrong knew about that boulder field?--and the purists say that quickloads don't count for landings.

That bit about dropping the a in 'One small step for [a] man' was a different problem:  extremely low framerate.  No wonder there was some stutter.

Anyway, to seriously answer this part of the question:

On 9/2/2018 at 12:21 PM, Man in the Mun said:

Does it help waiting for the target to be in a certain place in its orbit in relation to the launch pad?

The main help is that if you time it correctly, then you can launch directly to rendezvous and save yourself both time and the little bit of fuel you'd need to use for matching orbits.  It also looks well cool when you make it work.  The drawback is that if you get it only close but not close enough, it usually takes a lot longer to achieve rendezvous:  near-identical orbits take a long time to match.  Usually, you end up boosting to a different orbit in order to match quickly, which of course uses fuel.

My usual approach is to put most things in either a 125 km or a 245 km circular orbit.  I chose 245 km because it's low enough to count as low orbit for science (the altitude for high orbit is 250 km) but high enough to get good time warp (the altitude for that is 240 km).  I chose 125 km because it gets the almost-good time warp (that altitude is 120 km) but is close enough to Kerbin that it gets short orbital periods and is easy to reach from the ground.  I chose circular orbit because it avoids the problem mentioned earlier about one vessel going up while the other is going down.  If there is no apoapsis or periapsis, then all transfers are equally ideal:  I can rendezvous anywhere.

125 km is the temporary parking orbit for most things; 245 km is the orbit for everything permanent that I want to visit repeatedly.  The best part of this system is that I can still time it so that I have a quick transfer immediately after I reach orbit, but I don't need to do so; I have lots of room under 245 km to catch up if I fall behind.  I almost never launch directly to rendezvous, but that is merely a matter of preference.

[Scarecrow]

If you find that you are doing long burns to make rendezvous then I would suggest that you are doing something wrong. Even if you find yourself on the opposite side of the planet to your target it only requires patience and a few orbits before they are much closer. No need to do any burning. 

Edited September 7, 2018 by Scarecrow

[Tyko]

Here's my simple trick - as long as the target is in a circular orbit - launch your ship into a circular orbit below the target orbit and make sure they're on the same plane, i.e. they are both in flat equatorial orbits, for example, this is the easiest.

• Set the station as Target (right click on station to pull up option).
• Drop a maneuver node ahead of your ship and raise your maneuver AP to match the altitude of your target.
• Your maneuver is now set to get you to the right altitude, but not at the right time.
• Grab the maneuver node by its center ring and drag it around your orbit line further ahead of your ship.
• Keep pulling it forward around your orbit until you get an encounter.
• The trick is to be patient and make subtle tweaks to your maneuver so your encounter is as close as possible - I usually shoot for a distance of less than 2Km.

If your ship's altitude is close in height to your station's you may have to keep going around a number of times before they line up. Because of this @Zhetaan's suggestion of using very different altitudes is a good one. If you're station is at 245Km and your ship is at 125Km you'll get an encounter sooner.

This is a lot easier than trying to launch directly to rendezvous. While it does take more time, it takes less time than reloading a half dozen times to try to get your launch timing perfect. It's also as fuel efficient as a direct rendezvous - maybe more efficient depending on how perfect your timing is. Getting into orbit and then dropping a node gives you plenty to time to tweak your maneuver versus trying to make corrections in real time during initial ascent.

Hope this helps!

Edited September 6, 2018 by Tyko