Orbital intercepts

[Lheim]

Yeah, mechjeb can do this stuff - you might actually want to install it just to see how it does things. It has helped some people learn.

That said - there's something satisfying about working through it and succeeding on your own. Your choice entirely.

The markers on the nav ball and the map are your best tools as a pilot to plan things out and execute them. I wish there was a guide to all of these things somewhere.

Basically, when you select a target and your orbits overlap you're going to get 2 purple and perhaps 2 orange tabs put on the orbits. One will represent your position at your closest approach to the target, and the other will represent the target's position. Purple is the first closest approach and orange the next. You can mouse over them for that info.

You adjust those in several ways. Higher orbits will let an object catch up underneath you, lower orbits you get closer behind them, by going faster.

Try this: If you're in equal orbits but just at different positions.. select your target, and draw out a prograde maneuver for yourself. Go as far as you need to - *eventually* you'll see that with that maneuver the two purple tabs line up within a couple kilometers of each other.

Of course you may need to go halfway to Minmus for the timing to work out in just one orbit.. horribly inefficient, as you'd be moving like a bullet and would have to slow down again as you approached your target as well. But you can do the same thing if you go less high and take a few orbits around.

So keep one side of your orbit at the target's 'height', the other higher or lower, and as you go round a few times you'll get closer and closer.

Each orbit, just keep trying to find a maneuver that will make you intercept. Start with one at the point where your orbits overlap, but you can slide that sucker around and experiment.

When you get an intercept, that's when the flying by the seat of your pants really begins. At that point it's really vital to understand the markers on your nav ball - I really recommend the video playlist I linked earlier in the thread; you don't need the whole thing; what you need are the videos on rendezvous and orbital rendezvous .

You want to be in target-relative mode - you see the speed readout on your nav ball that says orbit, you can click that to cycle through ways to display your velocity - relative to the surface, the center of the planet (orbit), and target. Relative velocity is a really difficult concept to wrap a head around, but think of it this way - say for some reason you want to come in alongside some car that's driving on the freeway to toss - I dunno, a bag of snacks to the other driver. If you're doing that the speedometer is almost irrelevant. You don't watch the speedometer; it tells you the speed you're going on the ground. You watch out the window to see how fast or slow or angled the vehicle you're trying to match up to is moving subjectively to you.

Trick to manage is to slow down gradually - relative to your target, as you get closer. You can trim the direction of your motion to be the same as the target's as well with off-angled thrusts - the video is helpful. Read the text but watch the nav ball!

[Kerba Fett]

There's been a lot of advice offered here so I figured I give some too.

First thing is that mechjebs autopilots won't do anything to help you. You mentioned being early in the career and while the basic Mechjeb unlocks pretty early on your tech tree, the autopilot functions are unlocked quite a bit later.

Still, there's 2 things that MJ can do to help you with this. The first is the SmartASS panel. This window has buttons for common orientations that you use. (prograde, retrograde, node, target +-, Rvel +-, ect) You push the button and MechJeb will turn the ship and hold it at that orientation while you're busy with other stuff.

The second thing is that you can easily make custom windows with pretty much any of MJ's info on them. The image below shows a custom window for docking info that includes my closest approach distance, relative velocity at closest approach, and current distance and time to approach.

In concept, what you're trying to do is to have your orbits meet at one point and to have your orbital period different from your target.

This picture shows a few things related to that.

Both ships are orbiting counter clockwise in the image.

My ships orbit is the blue line. The station that I want to dock with is targeted and it's orbit is the yellow line.

The station is in a circular orbit at 200 km. My ship is in an eliptical orbit. My PE is 80km and my AP is 200km. (shown in the rocket flight custom window.

Both orbit meet at my ships AP at 200km. The little pink intercept symbol on the AP marker indicates my ships position NEXT time it gets there. The one with the line going to the planet represents where my target will be next time I get to AP.

Because my target is in front of me at the intercept point and I'm in a smaller orbit, I'm orbiting in less time and will eventually catch up over a number of orbits.

Turn up time warp and sit back and wait for it to happen.

Eventually when you pass the intercept it will show you that the target will be behind you on the next orbit. At this point you want to burn prograde a little bit and the intercept markers should move closer together. By increasing that last orbit a bit you insure that rather than passing your target you arrive at the intercept point at the same time.

While you're waiting to come around on your final orbit make a maneuver node at the intercept point and adjust it so the orbits match. This allows you to match course and speed with your target when you get to your closest approach.

While the maneuver node will get you as close as you can in the map, you're still going to have some drift to deal with.

When you get close to a target the navball will switch to target mode. The yellow indicators no longer mark prograde and retrograde. They now mark your direction relative to the target. By aligning the ship with the retrograde marker you're pointing your engine in the exact direction you're moving relative to the target. If you thrust till the velocity reads 0 then you're in the EXACT same orbit as your target and not moving in relation to it.

If you want to get closer point your ship at the target and use a bit of thrust to approach. You want the yellow prograde marker right on the pink target indicator of the navball. When approaching you want to keep your speed low in the 5 to 10m/s range.

When you get within 1km bring your relative velocity to 0 and you can EVA over to your target.

While you can do all of this without MechJeb, it's ability to quickly turn and hold an orientation make things a lot easier. For this operation you'd be using the prograde, retrograde, and Node buttons in the orbit mode and the Rvel-, Tgt + in the target mode. The relative velocity indicator on the navball is not that accurate so a custom window with that function helps a lot for parking your ship before EVA without it drifting.

Of course if this all seems too complex you could just launch another probe and put parachutes on this time, but at some point you'll want to try docking and will need to learn this anyways.

[cantab]

Thanks guys, I will give it a go. I thought turning the craft around and throttling against the direction of my orbit, would essentially cause me to plummet to the ground like a rock? As I'm accelerating against the force that is dragging me around the planet.

Only if you massively overdo it. It really won't be a problem in Kerbin orbit, just set up manoeuvre nodes and make sure your projected orbit isn't hitting the planet except when you want to land.

And I concur with others. Rendezvous takes a little bit of learning and practice then it's second nature, and it's an immensely valuable skill. If you can rendezvous you can do "Apollo-style" missions where you have a separate lander and orbiter, you can do rescue and recovery missions like your current task, you can go after asteroids, and once you also learn to dock you can assemble ships in orbit and do refuelling.

[Navy2k]

Hi, maybee a simpler step by step guide could help you further:

What i do on intercepts is:

1. Get in an Orbit that intersects at anyplace with the orbit of the vehicle you want to intercept. Doesn't matter where.

2. Get in the same plane as the target via the purple ManeuverPoint pullers exactly at an AN or DN point of your orbit while having the vehicle to intercept marked as target.

3. After the maneuver set another Maneuverpoint exactly where the 2 Orbits intersect. Use the green Pullers to change the radius of your orbit and the blue ones to turn the whole orbit around the Maneuverpoint (hopefully you understand what i mean when your there). Goal is to exactly match the targets orbit but don't execute the burn jet.

4. See if your in front or behind your target. (I like in Front of because there is no risk to get to near to the athmosphere in the next step) and then change the radius of the Maneuverpoint we created in Step 3. When your in front make the radius bigger when behind make the radius smaller (But watch the Periapsis). By doing so you should get within a few kilometers of your target after exactly 1 Orbit as indicated by the 2 Red intercept arrows and when you mouse over them it should say the estimeted distance. Now execute the Maneuver.

5. All that is left is to make another maneuver after exactly this 1 Orbit (the point where your nearest to your target). All that is left to do is match the orbit of your target like in point 3. That should give you another set of intercept arrows on the other side of the orbit.

6. When within a few 1000 meters and a synced orbit all you have to do is allign all 3 markers in the nav ball: First turn your nose directly to the target(pink) an then use your RCS with the keys "J" "K" "L" and "I" to get the yellow marker to overlap with the other 2 (nose point direction and target).

7. Accelerate ether with RCS and the "H" key ("N" Key for braking) or your main engine so that your distance to your target gets smaller but stay in a save speed relative to your target. While getting nearer you have to correct the yellow marker with the "J" "K" "L" "I" Keys that it alway points to your target.

8. Break when your within the desired distance to the target and think back at all the frustration that turned into accomplishment

This is not the most efficent way to intercept but it works all the time.

A few tips:

Mark the target before liftoff so you can liftoff when its a few degrees before it is over the launch site to get in front of your target.

After burning at your maneuverpoints check the result, mostly i have to correct after the first burn for a few m/s. (simply put another maneuver ~1 Minute ahead of you) mostly i use RCS and the "H" or "N" Key for that.

'Intercept and docking is one of the most difficult things in KSP so dont get frustrated but enjoy when you achieve it.

Greetings

Ben

[Lheim]

Rendezvous I picked up relatively easily.. for me, docking was the sticker.

Finally did it after several sessions of jiggling around uselessly, with a few hints from the forum. The second time I successfully docked was actually above Duna, with a separated lander coming back to my interplanetary transfer stage. I was sweating because I didn't want to run out of monoprop a billion kilometers from a resupply

Nowadays.. I can manage docking in a minute or two with only a sprinkling of monoprop.

Anyway, yeah. Rendezvous and docking are really hard to learn but get easier with practice, and they open up SO much possibility in the game. Space stations and orbital fuel depots, reusable craft, etc, etc.

[Alshain]

The main thing that helped me with rendezvous/docking was understanding the Navball. First, figure out that burning retrograde to the orbit is not the same as burning retrograde to the target (or even the surface). Second, slow relative speed by burning retrograde (yellow/green), move toward the target by using the target's location (pink/purple)... this was my biggest mistake. Never try to slow down with the pink retro-marker and never try to approach with the yellow pro marker. Getting those backwards will basically cause you to fly circles around the target. Eventually those targets should come close together or even on top of each other but may drift apart due to minute inaccuracies in your burns. But first and foremost make sure the gimble is in Target mode, not Orbit or Surface.

Edited June 25, 2014 by Alshain

[NecroBones]

I think you've already gotten a lot of great answers on this.

I'll just throw in one of my videos on doing a rendezvous on the daytime side of the planet (it's just one of many methods for doing a rendezvous):

And the short version of that:

[jBeta]

My first orbital rendezvous was a similar situation: stranded for some reason or another. I actually just got close and EVA'd to the rescue craft I actually found it easier than matching orbits back then, but still do it on occasion. In fact not so long ago I accidently dropped a science stage on an impact trajectory toward the Mun, and Jeb went on a sub-orbital EVA to retrieve the data! (I don't recommend that though!)