Rendezvous for dummies who inexplicably don't get it

[G'th]

So as the title says, I'm having issues figuring out rendezvous. Specifically when it comes down to rendezvousing with something thats already in space, while you too are also in space. IE, transferring to Minmus to dock with a station there.

Now, I can do docking. Presuming I get close enough, docking is easy as pie. But I can't for the life of me actually set up that close approach, and especially not without being able to set up the perfect orbit for it (which I can only barely do even when I'm launching into it).

Halp!

[Tex]

Simple fix, actually, presuming you can get to Minmus. The whole operation depends on the amount of fuel you have once you enter Minmus's SOI. If you can get into a nice orbit around Minmus, orbital maneuvers are much easier due to the low velocities. Honestly, I'd recommend watching a few docking tutorials (which, more often than not, detail the rendezvous procedures) and practicing in Kerbin orbit, which is much harder than around Minmus.

Long story short; if you can dock around Kerbin, you can dock around Minmus even easier!

[Decent Weasel]

I'm similarly hopeless at rendezvous, which is probably why I haven't even tried to bring home an asteroid. One thing I did notice is that it's easier to aim for the far approach rather than the near one, I.E. don't try to make the rendezvous coming up immediately, but make lighter, gentler maneuvers and go for a rendezvous on the closest approach after that. Since it's usually at opposition or further, you can use traditional orbital maneuvers (prograde, retrograde, etc) instead of wasting significantly more fuel trying to direct-thrust to the target.

I'm sure there's a better way, but I'm waiting to hear it as well.

[Scarecrow88]

Initially I ignore the item I am trying to rendezvous with, other than trying to get my orbit in a similar inclination as the target.

When you get your Minmus encounter, while you are still some way out, adjust your approach so that when you arrive and start slowing to get captured, the resulting orbit has an inclination somewhere similar to your target. Your initial orbit after capture doesn't have to be too tidy, as that will come as you finalise your rendezvous. The first thing I do, while my initial orbit is still quite large, is to match the inclination with my target orbit at either the ascending or descending node. Once I have done that, I then adjust my orbit whilst at the apoapsis, so that my periapsis matches the altitude of my target orbit.

If the orbit at this stage is very elliptical, I will adjust it so that the apoapsis comes down a bit, but not as far as my target orbit. By leaving your incoming ship with a bigger orbit than your target ship, the target ship will eventually catch you up. Unfortunately, if the target ship is just ahead of you, it could take a few orbits to go round and come up on you.

During this process you will see the encounter markers moving on each orbit. When they get reasonably close, which should be at your periapsis, you can make a final adjustment to make the encounter as close as possible. At periapsis if the markers showing your encounter on the next orbit has your target ship close, but behind you, burn prograde slowly until the encounter markers are as close as you can get them. Conversely, if the markers show your target ship slightly ahead on the next orbit, burn retrograde slowly to close the gap. Either way, you should be able to fine tune the encounter to less than 5km.

Next time you come round, when your ships are very close to your closest approach, make sure your navball is in target mode and burn retrograde to kill your relative velocity. After this, it should be a standard approach and dock. Note, the bigger the difference between your apoapsis, and the altitude of your target orbit, the bigger the difference will be in your relative velocity at closest approach.

This isn't the most efficient way of setting up a rendezvous, but it should suffice until you recognise the mechanics involved for matching orbits, and can fine tune it yourself to make it more efficient.

HTH

[AlexinTokyo]

Rendezvous is one of the harder aspects of the game, at least in part because it is highly counter-intuitive.

That said, it's a skill, and will definitely improve with practice.

Here is my recipe for practising rendezvous:

1. Launch target
   
2. Put target in 150 km circular, equatorial orbit (eccentricity and inclination as close to 0 as possible)
   
3. Ensure target is pointing north, SAS on
   
4. Launch docker
   
5. Put into 80~100 km circular, equatorial orbit
   
6. Set target as target
   
7. Quicksave
   
8. Rendezvous - use manoeuvre node to burn up to target's altitude slightly ahead of target, use navball and main engine to equalise velocity.
   
9. Quickload either when successful (separation < 50 m, relative velocity < 0.3 m/s) or unrecoverable (de-orbiting, escape velocity exceeded, crashed into target, out of fuel, etc.)
   
10. Repeat until you get 3~5 (up to you, really) successful rendezvous in a row
    
11. Put docker into 200~250 km circular, equatorial orbit
    
12. Quicksave
    
13. Rendezvous - use manoeuvre node to burn down to target's altitude slightly behind target, use navball and main engine to equalise velocity.
    
14. Quickload either when successful (separation < 50 m, relative velocity < 0.3 m/s) or unrecoverable (de-orbiting, escape velocity exceeded, crashed into target, out of fuel, etc.)
    
15. Repeat until you get 3~5 (up to you, really) successful rendezvous in a row
    
16. Now you can rendezvous, you can start practising to dock!
    

1. Start from a good rendezvous (separation < 50 m, relative velocity < 0.3 m/s [preferably 0])
   
2. Set the target's docking port as target
   
3. Orient docker to point south, SAS on
   
4. Deactivate main engine
   
5. Quicksave
   
6. Activate RCS and use translation (IK JL HN or docking mode) to manoeuvre until you're directly in front of the target - precision control (Caps-Lock) helps
   
7. SAS off, thrust forward slowly to dock
   
8. Quickload either when successful (docked) or unrecoverable (probably either crashed or nudged target off alignment badly)
   
9. Repeat until you get 3~5 (up to you, really) successful docks in a row
   

I prefer to use quicksave/quickload to repeat, but you can just jet off into another orbit if you like.

Some things I find it helpful to bear in mind:

• If you start in a lower orbit than the target, you're going faster (catching up) but will be travelling slower when you burn up to the higher orbit (you'll be at apoapsis) - you want to intercept just in front of the target and let it catch up.
  
• If you start in a higher orbit than the target, you're going slower (falling behind) but will be travelling faster when you burn down to the lower orbit (you'll be at periapsis) - you want to intercept just behind the target and catch up to it.
  (These are both massively counter-intuitive, because (in example one) you're going faster and speed up (burn prograde) but end up going slower than the target.)
  
• When using RCS translation to dock, there's no slowing force - if you keep thrusting until you're in line you will overshoot - a couple of short thrusts to move in the right direction and a couple of counter-thrusts to stop when you arrive.
  
• If you're using the shielded docking port, remember to open the shield
  

You can, of course, practise around Minmus (or any other body) instead of Kerbin; especially if you already have a useful target in orbit there. It's much easier to start with circular, equatorial orbits, and worry about inclined or highly-eliptic orbits later.

[allmhuran]

Intercepting a craft that is orbiting the same body as you is fundamentally the same as intercepting a moon that is orbiting the same body as you. You burn appropriately to put your apoapsis or periapsis at the target's altitude, then burn again when you get there.

The additional degree of difficulty comes from the fact that a craft has no sphere of influence to assist you with the capture. This also explains why you get two closest approach markers, whereas with a planet or moon intercept you only get one: With a planet or moon intercept the intercept only happens if you enter the SOI, which generally only happens once. With a craft intercept your orbit will probably cross its orbit twice (unless you burn absolutely perfectly), so you have two closest approaches.

OK, so we set up our burn to the intercept point (either one is fine). When we get there what we want to do is match our orbit to the target's orbit. So again, this means burning prograde or retrograde (ignoring plane change corrections). It's just the same procedure to circularize at a higher or lower orbit without worrying about targets.

If you think of it in terms of "target relative velocity" (which is what the navball will show when you have a target set), here's what is happening at the closest approach in the case of intercepting a target that is higher than us. When we get to our apoapsis (at target altitude) our tangential velocity will be very slow compared to the target. So our target relative velocity will be high. When we burn to zero out target relative velocity it means (in this case) that we are burning prograde relative to our own orbit... which has the effect of circularizing at the target orbit altitude!

But once we match orbits with the target, we will still only be as close as our closest approach. So now we perform a few small burns to close that gap. This basically means pointing directly at the target and accelerating. Then turning around and decelerating when we get close, or when we start veering away from it. Depending on how far away you start you may need to repeat this process a few times, zig-zagging your way to the target. Note that this isn't the most efficient method, but it's easy.

Here's a pic describing most of this:

Edited June 2, 2014 by allmhuran

[Mighty1]

Or if all that is to compicated get mechjeb, set up rendezvous and watch what happens. After a couple of times watching mechjeb do its magic you will feel confident enough to try it out yourself. I learned that way and once you get the hang of it it's not that complicated.

[Kasuha]

My approach is:

1/ Burn at one inclination point to raise/lower the other inclination point so the two orbits intersect there (you'll see closest approach marker aiming at exactly the same place as the inclination marker)

2/ Burn at the crossing point to adjust orbital period to get an encounter in one or a few orbits (additional maneuver nodes may allow you lookup several orbits in advance)

3/ Coast till you can see your encounter

4/ Create a maneuver at the encounter point and set it up so your orbit exactly matches the target orbit

5/ Execute that maneuver. Start 60% of estimated burn time ahead.

It may not be the most optimal approach possible but it is simple. And if you do these corrections right, it is even not very far from optimal.

Edited June 2, 2014 by Kasuha

[WeirdCulture]

This biggest hint I've to remember is, when your orbits are matching, and you are at the closest point where the spacecrafts are together (if the closest point is 20 km it is OK), you have to reduce your relative speed to the target to nearly zero first THEN start to manoeuvre towards to the other one..

after a while I all ways forget this and getting bad approaches

Edited June 16, 2014 by KasperVld

[Sirrobert]

If you want some in game examples. you can either try Youtube tuturials, or download Mechjeb and let it do the rendevous for you.

Observing Mechjeb a few times is how I learned it

[allmhuran]

This biggest hint I've to remember is, when your orbits are matching, and you are at the closest point where the spacecrafts are together (if the closest point is 20 km it is OK), you have to reduce your relative speed to the target to nearly zero first THEN start to manoeuvre towards to the other one..

after a while I all ways forget this and getting bad approaches

Oddly enough, you don't actually have to do this, it's just a bit easier when starting out.

After a while you'll notice how the target indicators behave relative to the direction you burn: Your prograde market gets pulled towards your chevron* when you burn, and your retrograde marker gets pushed away from your chevron when you burn.

So what you want to do is "pull" your prograde marker towards the "directly at target" marker (which will speed up your closure rate and get you headed towards the target), and then as you get closer, "push" your retrograde marker on top of the "directly away from target marker" (which will slow down your closure rate and also get you heading towards the target).

Here's another picture to illustrate the two steps:

Now, when you perfect this technique, the next step is not to put your prograde marker directly on top of the "at target" marker when you start, but instead to watch which way the "directly at target" marker drifts, and pull your prograde marker slightly ahead of it in that direction. This will give you an even faster and more efficient intercept. (In aviation terms, this would be a "lead pursuit").

*the "chevron" being the centre of the navball, the direction the craft is pointed.

I should add that you can actually use this technique as soon as you reach the closest approach point. If you do this, your relative velocity will start out high, so you actually want to reverse the order: First burn to push your retrograde marker towards "directly away from target" and ahead of its drift, decreasing your relative velocity (and making your orbit more closely match the target's). If you do this well enough, you don't ever have to worry about "pulling" the prograde marker around, which is even more efficient again.

Edited June 16, 2014 by KasperVld

[EdFred]

This is all one needs to know.

Follow his instructions EXACTLY, and it all becomes easy. It doesn't matter if you are in orbit around Kerbin, Kerbol, Jool, Minmus, anywhere....it works. Watch it like 3-4 times, and your rendezvous problems should be over.

[allmhuran]

What Scott is demonstrating there is a bit of a hybrid approach. Notice that at about six minutes when he burns to get a closer intercept, his maneuver target is on the opposite side of the target direction from his current prograde marker. IE, he is performing step 1 in my diagram from above.

Then at about 7 minutes 15, he reverts back to the "simple" approach, setting his relative velocity to zero by burning exactly target-relative-retrograde, then turns directly towards the target and burns towards it. What he could have done at that point instead would be to burn closer to "directly at target", pushing his retrograde marker away (towards "away from target") and pulling his prograde vector towards "directly at target", which at that point is at the top right of the navball. This would have combined his "null out velocity" burn and his "head towards the target" burn into one single, more efficient burn. That would be step 2 in the above diagram.

[EdFred]

Videos are easier to follow than text and diagrams.

You can always make a Scott Manley is wrong video - if you dare.

I still do rendezvous the Manley way - and I don't even use RCS anymore. The amount of fuel used is miniscule, there's really no *need* to combine the two.

[allmhuran]

Videos are easier to follow than text and diagrams.

You can always make a Scott Manley is wrong video - if you dare.

I still do rendezvous the Manley way - and I don't even use RCS anymore. The amount of fuel used is miniscule, there's really no *need* to combine the two.

Like this one? http://youtu.be/4bX18e41OJ8 You'll have to exclude the typos and generally rushed nature, this was done quickly by request for someone asking a similar question on the forum about a year ago. Whether or not it's worth using the more advanced version depends on a lot of things... the relative velocity difference near the intercept, the weight of the vessel, the efficiency of the engines, etc.

If you try to rendezvous two large ships, you'll find that the simple way just isn't fast enough. You don't have time to keep zeroing out velocity and burning towards the target, because your TWR isn't high enough and you can't re-orient fast enough. In this case you pretty much *have* to use the more advanced version.

What Scott is doing isn't wrong, it's just not quite optimal. It's certainly better to start off the easy way, but it's also worth knowing that you can improve upon that technique.

Edited June 2, 2014 by allmhuran

[EdFred]

Like this one? http://youtu.be/4bX18e41OJ8 You'll have to exclude the typos and generally rushed nature, this was quickly by request for someone a similar question on the forum about a year ago. Whether or not it's worth using the more advanced version depends on a lot of things... the relative velocity difference near the intercept, the weight of the vessel, the efficiency of the engines, etc.

What Scott is doing isn't wrong, it's just not quite optimal. It's certainly better to start off the easy way, but it's also worth knowing that you can improve upon that technique.

True, but this was asked by someone who "inexplicably doesn't get it." I'm a flight instructor, and I don't teach my students shortcuts until they know why things work. That's a recipe for disaster. Now, in KSP no one is going to die - well no one organic - if you use shortcuts, but I guess I just fall back on making sure people have a grasp of the basics, before saying "hey you can do it this way instead" with pretty much everything.

[allmhuran]

Sure, that's why in my posts I said "here's the easy way, once you've mastered it here's a harder but better way, and once you've mastered that, here's the optimum way".

[Blind Dead McJones]

People have already given a step by step, and there are some good videos on youtube. Also, you seem to have docking down (and I agree that's the easy part). So here are some general tips:

*Remember RCS. This might seem obvious but some people don't use it apparently.

*Once you have a stable orbit, use the ascending and descending nodes to get the same inclination. Get as close to 0.0Ëš as possible, because you don't have the margin of error that a planetary SOI provides.

*Rendezvous is all about matching your target's speed. When your navball is in target mode (click the little screen on the top to cycle between orbit, surface and target) the pink markers show your direction to the target and the yellow ones show your motion relative to it. When your close, fire at the retrograde marker until you get to 0 m/s, then fire at the circular pink marker. That will make you move towards your target, although if you're far away you'll have to make a lot of corrections.

*Maneuver nodes are your friend. Learn what each direction on it does and how to move it from one spot to another (click the center circle and drag it around).

*Quicksave is also your friend. Once rendezvous becomes routine, you probably won't need it anymore. But it's a lifesaver when you're still learning.

*Small ships are easier to manipulate. When you're still practicing, make a small ship specifically for this purpose.

[LethalDose]

This is all one needs to know.

Follow his instructions EXACTLY, and it all becomes easy. It doesn't matter if you are in orbit around Kerbin, Kerbol, Jool, Minmus, anywhere....it works. Watch it like 3-4 times, and your rendezvous problems should be over.

This is how I started wrapping my mind around rendezvous. It's useful, but not the method I end up using.

This biggest hint I've to remember is, when your orbits are matching, and you are at the closest point where the spacecrafts are together (if the closest point is 20 km it is OK), you have to reduce your relative speed to the target to nearly zero first THEN start to manoeuvre towards to the other one..

after a while I all ways forget this and getting bad approaches

This advice is what I think is most important. The trick is to zero out your relative velocities somewhat near your target (within 20 km is sufficient, less is better quicker). You zero out your velocity by burning retrograde to your target's velocity. The target's retrograde velocity vector relative to your own is marked by the typical retrograde marker on the navball when you're in "target mode" (see allhuman's diagrams above).

As for new advice:

Rendezvousing is about meeting 3 criteria: Getting your ship in the

1. Same location as the target
   
2. at the same time as the target
   
3. with the same velocity as the target
   

Thinking about rendezvousing like this helped me tons. Getting to the same place at the same time as the target is done (at least by me), using maneuver nodes and the "intercept" markers. When you do get near to your target, you burn at the retrograde marker (as mentioned above) to zero out your velocity. When your velocity is zeroed out, you can burn towards your target the way Scott describes in his videos.

Everyone always seems to recommend getting into the same orbits, which I find bizarre. For example, If I'm going to rendezvous with a space station in a 150 km orbit, I won't go directly into a 150 km orbit. I'll more likely go into a circular orbit between 80 and 120 km. I will match inclination, but by staying in a different orbit, it takes less time to find a node that will allow me to get an intercept with my target. It's just a different way of making the rendezvous approach. If you think of the burn as a Hohmann transfer, it makes more sense, to me, to make the burn from a separate orbit (because you don't perform Hohmann transfers between bodies in the same orbit).

Best of luck, keep practicing, you'll get it.

Edited June 16, 2014 by KasperVld

[O-Doc]

after a while I all ways forget this and getting bad approaches

After a whie you can go pro and start combining steps into your burns.

Kashua outlined the basic step. follow these and you can't go wrong.

1. Get into orbit of the same planetary body.

2. Make sure you are tracking in the same direction as your target

3. Match inclination of your target at the ascending/descending nodes

4. Burn prograde/retrograde until your orbit path touches or crosses your targets'

5. Read space magazines and eat space cookies waiting for a closest approach(< 10km will do)

6. Match orbit at closest approach by burning to retrograde in target mode(you switch from orbital)

7. Burn prograde to target and close in to docking distance

When you get pro, you will get all of these steps done within two to three burns. Leave Mchjeb in the VAB, you'll have a better experience without it and internalise orbital mechanics enough to change your decision making and build optimisation at a mission level. Win, win.

Edited June 16, 2014 by KasperVld

[Mr Shifty]

If you try to rendezvous two large ships, you'll find that the simple way just isn't fast enough. You don't have time to keep zeroing out velocity and burning towards the target, because your TWR isn't high enough and you can't re-orient fast enough. In this case you pretty much *have* to use the more advanced version.

This. Every time you zero out your velocity and then burn towards the target again, you have to flip your vessel 180 degrees twice. Much easier to simply slow down as you approach and nudge the velocity vectors so that you intercept.

[Pecan]

Now' date=' I can do docking. Presuming I get close enough, docking is easy as pie.[/quote']

Oh yeah, totally sympathise with that. Rendezvous and docking are different skills/procedures and rendezvous is definitely the one I suck at (most), while I can also understand people who find docking harder. 0.23.5 made a lot of difference for me with the 'add one orbit' buttons on the manoeuvre nodes. Depending on your phasing orbit it can take a while to get a good intercept and I usually ran out of patience before finding it.

[michaelhester07]

This. Every time you zero out your velocity and then burn towards the target again, you have to flip your vessel 180 degrees twice. Much easier to simply slow down as you approach and nudge the velocity vectors so that you intercept.

Either that or you load up with enough RCS thrusters that the TWR off the RCS is greater than your main engines (i have a few ships like this).

Some tips I'll cover that may have been missed:

1. Practice the "advanced" way of nudging your retrograde to the other side of the target's retrograde drift. This saves gas and you'll wonder why you ever did it the "easy" way.

2. If your target has a high part count that slows your computer down... drop your target speed to below 50m/s before you get closer than 2.3km. You may otherwise miss your target while trying to decelerate on a low framerate.

3. Did you ever plan a maneuver to go to Mun? Minmus? Duna? While doing so did you notice the little white notches on the orbit path of your target? The one on top is where you'll be when you cross the target's orbit path. The one on the bottom is where the target will be. When these line up you'll get an intercept.

4. Did you notice the little green notches labled "AN" and "DN"? These are where your orbit's plane intersects with the target's plane. If you thrust "south" at AN and "north" at DN you can match your orbit plane to your target's plane. This makes it a LOT easier to intercept the target. Plan burns accordingly. The quotes are there because the direction for "north" and "south" may vary. Plan the maneuver with the purple nodes.

5. The smaller the object you're orbiting the easier it is to rendezvous with something else orbiting it. If you can't get a rendezvous in low kerbin orbit try it over the Mun instead. Or over Minmus. The low orbit speeds make it super easy to Rendezvous with something there.

6. When docking two ships with a single docking point control each ship from the docking point and turn them to point at each other's prograde target (purple) vector. This makes docking the ships much easier.

[Skorpychan]

It's actually ridiculously simple. You need to get as close as possible to the target, then match velocities. Just like encounters with planets and muns.

[G'th]

Well I've been playing with it, and I'm starting to notice that trying to rendezvous in a 75km orbit around Kerbin isn't something for the faint of heart. So I moved my parking orbits up to 125km and I've had more luck.

But how would you set yourself up to catch up with your target when you manage to get your orbits matching? Been having issues with that.