Low orbit rendezvous assistance

[mabarry3]

[if this kind of thing has been discussed before, a link would work fine; I'm new here and haven't looked into all of the forum features]

So, I accepted some contracts I haven't tried before after a Minmus trip that got me a lot of new tech and made me want to try doing harder contracts because I wanted to attempt some more difficult things to get the full game experience.

Anyway, I accepted two contracts to rendezvous with and save two kerbals stuck in low Kerbin orbit. Turns out they both have identical nearly circular orbits (see the picture) that spans 80-85km. After a few mishaps such as discovering that they had no docking port (since I knew of no other way at the time to save someone), I eventually unlocked the Claw and tried again. Now, I can't seem to rendezvous with them at all. No matter where in my orbit I move the maneuver node, or adjust my [anti]normal or pro/retrograde, I can't get anywhere close to the target without having a periapsis of <40kish. I can either maintain orbit, or get close to the target. I sat on high time warp and let myself orbit a few times to change our relative positions, but it doesn't seem to matter since no matter what it seems like I can't get anywhere close to them without de-orbiting.

My last attempt ran around Kerbin for 3 in-game days of trying to get a maneuver node to reach them, adjusting & moving it around, etc, before giving it up and just orbiting again a few times to have another go. I eventually gave up and I can't seem for the life of me to get a rendezvous with them.

Anyone have any tips on how to rendezvous with someone in such low orbit?

Edited October 18, 2015 by mabarry3

[FancyMouse]

If you do it first time, do this. Start with any position. Pick your favorite desired rendezvous point (daytime, at least). Then make your periapsis there, and increase apoapsis until you get a intercept at the next period of the orbit.

Once you feel good about it, you can try raise apoapsis less so that intercept happens one peruod later - but saves fuel.

[Snark]

By the way, unless the contract actually says that you need to recover the stranded kerbal's ship, you don't need the klaw. Just send up a ship with an empty command pod and pull up alongside. As soon as you get within 2.3 km, you can switch to the stranded kerbal's ship, take the kerbal EVA, and fly him/her to your rescue ship.

No need to make it harder than it is.

[The_Rocketeer]

Welcome to the forums!

If you do it first time, do this. Start with any position. Pick your favorite desired rendezvous point (daytime, at least). Then make your periapsis there, and increase apoapsis until you get a intercept at the next period of the orbit.

Once you feel good about it, you can try raise apoapsis less so that intercept happens one peruod later - but saves fuel.

^^ This.

Assuming your altitude crosses the target's altitude somewhere, getting an intercept is just about timing. Bigger orbits take longer, smaller orbits are quicker. So, to get a rendezvous you can either pull in to catch up, or push out to let the target catch you. You can save a lot of fuel by not trying to make the rendezvous in one loop - plan to make 4 orbits before the rendezvous and you'll only need 1/4 of the fuel.

By the way, unless the contract actually says that you need to recover the stranded kerbal's ship, you don't need the klaw. Just send up a ship with an empty command pod and pull up alongside. As soon as you get within 2.3 km, you can switch to the stranded kerbal's ship, take the kerbal EVA, and fly him/her to your rescue ship.

No need to make it harder than it is.

Yup, use the [ ] keys to switch vessel. This 'unlocks' the craft, but only works when you've got another craft already in range.

[mabarry3]

Didn't know you could EVA from a ship & a kerbel you didn't control - my only previous experience was being unable to EVA tourists, and I figured the same logic applied. I feel like there should have been a bit of additional explanation on that sort of thing in the in-game rendezvous tutorial (unless I missed it?), that would have helped greatly to not need to figure that part out when it comes to rescuing anything. I had already figured out that I could just EVA my own kerbal if during a rendezvous if I didn't need the ship/wanted to rescue it later, but never knew how to switch to the ship if it wasn't under my control to begin with. It does explain why I got rescue missions well before I ever had access to a docking port/claw, though...

Thanks a lot guys. I'll try increasing apoapsis to increase orbital period so we catch up to each other, that makes perfect sense actually. I presume I still need to burn retrograde with respect to target once both my ship and theirs are close to each other to slow the relative velocity down to ~0 so I can EVA over, right?

[Streetwind]

The "push apoapsis out until you get an intercept" thing works best if you first match orbits with your target. As in, getting the same apoapsis height, the same periapsis height. Also, positioning apoapsis and periapsis in the same spots (this is called having the same "argument of periapsis", which is a term that sounds a lot more confusing than it actually is. Welcome to orbital mechanics!). Matching orbits is a great piloting exercise that I recommend doing a few times until you can do it in only 2-3 burns. You'll feel like an ace pilot, I guarantee you Again, it's not strictly required, but it makes it easier.

When your orbit is completely matched, you are still not meeting the target, because the target is very likely to be somewhere else along the orbital path. Both of you are running the same loop around Kerbin, but neither can catch the other. This is called the "phasing" of the two orbits. To rendezvous, not only does your orbit need to be identical to that of your target, but the phasing between the orbits also needs to be identical! Thankfully, changing the phasing is probably the easiest part of the whole affair. It works based on a simple principle:

A higher orbit moves slower than a lower one. Therefore, an object in a lower orbit will catch up to, pass, and run away from an object in a higher orbit. You can see this in the solar system - just check how many times Moho goes around the sun for each time Jool does. If the sun is Kerbin instead, and the planets are you and your rendezvous target instead, you have the exact same effect. If your orbit is perfectly matched, both you and your target move at exactly the same speed around Kerbin; neither will catch the other. But if your orbits differ, then your relative position changes each time you go around the planet.

This is why matching orbits and then burning to raise your apoapsis guarantees that you will eventually see a close encounter pop up: what you are actually doing here is a phasing maneuver. You make your orbit so much larger that it takes you so much time to go around Kerbin that your target has time to catch up so much that it just happens to arrive at your periapsis the same moment you arrive. Now, how much dV that takes will depend on where exactly your target is relative to you on the orbit. If it is a little bit behind you, it will take very little fuel; if it is on the other side of the planet, it will take a moderately large amount; and if it is slightly ahead of you, it will take absurd amounts of fuel and it's probably better to lower your periapsis instead if the atmosphere doesn't get in the way, so you will do the catching up instead of your target.

However, you usually do not need to rush a phasing maneuver - you do not need to meet the target on your first orbit around. If you change your orbit only a little bit, you can then wait loop after loop after loop as your relative position to your target changes a bit every time. Then, when the target is suitably close, you make another adjustment to get your final close encounter. This patient method is recommended so that you get your target close to you without spending much fuel. If you do the single-orbit rendezvous outlined above, not only will your initial burn potentially cost a lot of dV if the target isn't already close to you, but the velocity burn you need to perform at the close encounter will also be very large! (In effect, you are re-matching orbits again during close approach, and the more you changed your orbit away from that of your target for phasing, the more you have to work to change it back.)

To memorize all of this, just remember this simple rule of orbital mechanics: "If you want to be faster, you need to go slower!"

Edited October 15, 2015 by Streetwind

[Stoney3K]

Possibly a quite stupid question here, but...

Are you orbiting in the same direction as your target?

[Kryxal]

If that were the problem, it wouldn't be a matter of never intercepting the target, but of blowing by them at upwards of 4 km/s...

Oh, and for letting a target catch up to you, I prefer to have my periapsis just enough lower to get a pair of intercept points. It makes life easier for me when planning a periapsis close-approach burn.

[arise257]

Another tip not mentioned is to delay launching until the target ship in orbit is near the shoreline of the ocean to the west of the mountains near KSP. By the time you ascend, perform your orbital insertion, and circularize, the target ship should be VERY close to your rescue ship. I'm talking under 50km away, though I've come as close as 7km -- it depends on the TWR and efficiency of ascent, but is very doable. If you're within 30km, and not burning in a direction that drops your periapsis, you can literally fly straight to the stranded ship with no delay. I take these types of missions early on (even before the RCS tech node, I rendezvous like a m'fckn BOSS) because it's a cheap way to build your roster of Kerbonauts.

[GarrisonChisholm]

The "push apoapsis out until you get an intercept" thing works best if you first match orbits with your target. As in, getting the same apoapsis height, the same periapsis height. Also, positioning apoapsis and periapsis in the same spots (this is called having the same "argument of periapsis", which is a term that sounds a lot more confusing than it actually is. Welcome to orbital mechanics!). Matching orbits is a great piloting exercise that I recommend doing a few times until you can do it in only 2-3 burns. You'll feel like an ace pilot, I guarantee you Again, it's not strictly required, but it makes it easier.

When your orbit is completely matched, you are still not meeting the target, because the target is very likely to be somewhere else along the orbital path. Both of you are running the same loop around Kerbin, but neither can catch the other. This is called the "phasing" of the two orbits. To rendezvous, not only does your orbit need to be identical to that of your target, but the phasing between the orbits also needs to be identical! Thankfully, changing the phasing is probably the easiest part of the whole affair. It works based on a simple principle:

A higher orbit moves slower than a lower one. Therefore, an object in a lower orbit will catch up to, pass, and run away from an object in a higher orbit. You can see this in the solar system - just check how many times Moho goes around the sun for each time Jool does. If the sun is Kerbin instead, and the planets are you and your rendezvous target instead, you have the exact same effect. If your orbit is perfectly matched, both you and your target move at exactly the same speed around Kerbin; neither will catch the other. But if your orbits differ, then your relative position changes each time you go around the planet.

This is why matching orbits and then burning to raise your apoapsis guarantees that you will eventually see a close encounter pop up: what you are actually doing here is a phasing maneuver. You make your orbit so much larger that it takes you so much time to go around Kerbin that your target has time to catch up so much that it just happens to arrive at your periapsis the same moment you arrive. Now, how much dV that takes will depend on where exactly your target is relative to you on the orbit. If it is a little bit behind you, it will take very little fuel; if it is on the other side of the planet, it will take a moderately large amount; and if it is slightly ahead of you, it will take absurd amounts of fuel and it's probably better to lower your periapsis instead if the atmosphere doesn't get in the way, so you will do the catching up instead of your target.

However, you usually do not need to rush a phasing maneuver - you do not need to meet the target on your first orbit around. If you change your orbit only a little bit, you can then wait loop after loop after loop as your relative position to your target changes a bit every time. Then, when the target is suitably close, you make another adjustment to get your final close encounter. This patient method is recommended so that you get your target close to you without spending much fuel. If you do the single-orbit rendezvous outlined above, not only will your initial burn potentially cost a lot of dV if the target isn't already close to you, but the velocity burn you need to perform at the close encounter will also be very large! (In effect, you are re-matching orbits again during close approach, and the more you changed your orbit away from that of your target for phasing, the more you have to work to change it back.)

To memorize all of this, just remember this simple rule of orbital mechanics: "If you want to be faster, you need to go slower!"

I have just today tried to undertake my first orbital rendezvous, as in the first 5 months of playing it seemed absurdly intimidating. I have met with a gross lack of success.

I know this is written clearly and as an (allegedly) intelligent person it should suffice to explain it to me, but is there a "Rendezvous for Dummys", or "Rendezvous 101" tutorial somewhere?.. :\

[Waxing_Kibbous]

This is an interesting approach that I've used successfully a few times, the tricky part is using a retro burn to achieve orbit.

The second part, approaching the vessel, can be done easier. Instead of pushing or pulling the target markers around on the navball, simply only fire when approaching the target, on the round pink one, then firing retrograde to slow down, your yellow one. Simply by using these two you can move towards it, slow down, move towards it again, slow down etc. Do this gradually and you can get really close quite comfortably. Here is my craft- it has a spark engine in the last stage which I find works well, I usually ditch a half tank of fuel but it's nice to have a cushion, and besides kerbals cost a lot to hire so a bit of waste is OK! Also notice the ladder down the side since I'm still bad at EVA- aligning the rescue ship so the ladder is facing the other craft and straight up before EVAing the stranded kerbal gives you a big target to grab on to. Edit: don't pay attention to the Kerbal engineer readout, it's set on Minmus

Edited October 21, 2015 by Waxing_Kibbous

[CrashTestDanny]

I have just today tried to undertake my first orbital rendezvous, as in the first 5 months of playing it seemed absurdly intimidating. I have met with a gross lack of success.

I know this is written clearly and as an (allegedly) intelligent person it should suffice to explain it to me, but is there a "Rendezvous for Dummys", or "Rendezvous 101" tutorial somewhere?.. :\

I actually studied this stuff for 7 years at the University of Arizona. It's hard and taking some time to get it doesn't make you a dummy. Especially considering we're talking about a game, not a college program or a job where you would probably have more time to spend on it and more motivation to complete the hard stuff.

That said, here's what I think you should probably understand first about orbits and orbital mechanics. First, you should know that every orbit is an ellipse. That's not the three dots you put in a sentence to indicate a pause or an unfinished thought, it's what happens to a circle when one side of it is longer than the other . Where a circle has a single point as its center, an ellipse has two foci (foci is the plural of focus) which are sort of analogous to a circle's center. In fact, a circle is a special case of an ellipse where both foci are at the same location. The defining characteristic of an ellipse is that from any given point, the sum of the distances to the foci is constant. There's a whole lot more to know about ellipses, but that's probably nearly all you need, but maybe some trivia; ever wonder why the game talks about "conics" or what a conic is? It's because every ellipse (and therefore every orbit) is a slice of a cone, hence the term conic. Trivia for you and me, bread and butter for someone who writes software that deals with orbital mechanics. Every orbit is an ellipse and the body that is being orbited is located at one of the foci of that ellipse. That's it for orbits. Kepler's laws will give you some more detail and plenty of math, if you want to look them up. They were originally published in 1609 and 1619 and were meant to explain observations of the motion of planets, but they apply to spacecraft as well.

Now on to orbital mechanics, which is the term I am using to describe the manipulation of a spacecraft's orbit. There are three axes along which you can apply a force to change an orbit. Each has a positive and negative direction. The first is "prograde" and its negative would be "retrograde". The second is "normal" (not an observation of its mental state) and its negative is "anti-normal". The third is "radial" and it's negative is "anti-radial". While you can burn in a direction that is not exactly one of these, the result of any burn will be a vector which can be broken down into components that are entirely in one of these three directions. "Prograde" means in the direction of movement, that is to say tangential to the current orbit at the position of the burn (we're kind of assuming that burns take no time, which we all know is not true, but it's easier to get us close to the right answer by making that assumption). When you burn prograde or retrograde, the effect is always to change the orbital altitude at the opposite side of the orbit. If you burn pro/retro at one of the two apses (periapsis and apoapsis), then only the opposite apsis will be changed, increasing if your burn was prograde, and decreasing if your burn was retro. If you burn between the two apses, then the apsis ahead of you will increase while the apsis behind you will decrease. This will also roll the orbit, bringing the periapsis closer and pushing the apoapsis further away. Burning normal or anti-normal is used for changing the plane or inclination of your orbit. When you burn normal the point on the opposite side of your orbit from the burn stays the same, the points halfway from your burn to the opposite point will raise (prograde side) and lower (retro side), changing the inclination. You really need to know that 1 - the best point to change planes is either the AN or DN and 2 - the cost at AN and the cost at DN are only the same if your orbit is perfectly circular; if you are returning from a moon (like minimus) and want to enter a 0-inclination orbit of kerbin, you want to do it as far from Kerbin as you can, and that means you do it as soon as you enter Kerbin's SOI - you could probably do it by planning your ejection angle from Min perfectly, but when you're in Kerbin's SOI, you can see it, which makes a world of difference when planning. Finally, a radial burn will push your orbit toward the apsis ahead of you (anti-radial pushes it toward the other apsis). If you are burning at an apsis, the radial burn will first rotate the two apses 90 degrees and then push toward the apsis ahead or behind. If you push far enough, your trajectory can become sub-orbital.

Now what you (and the rest of us) have to do is learn these types of burns and the impacts they have on orbits in order to combine them to achieve our desired outcomes. Again, this is not easy; if it was it would be boring, wouldn't it?

Also note, that sometimes (often) you have to wait for the right opportunity to do what you want. For example, Minimus' orbit is inclined 6 degrees, or -6 degrees depending upon which side of the planet she is on in her upward side of her orbit. If you are in Kerbin orbit, simply changing your inclination does not match the plane. Matching the plane can only be done at the AN or DN. Of course, if you match planes with Min from a circular Kerbin orbit, you are wasting a lot of delta-V on that maneuver. The much less expensive way to do that is to perform your transfer burn and then match planes at the AN on your outward leg, then fine-tune your transfer to Min and your insertion burn after the plane correction is completed.

Eventually in this game you will need to have an understanding of these things, IMO, and a better understanding than I have, even. I'm sure I'll get corrections about some things here... However, I found that MechJeb's Maneuver Planner does an excellent job with most things here and will easily get you anywhere within Kerbin's SOI as well as Duna, Eve, and Moho. Catching asteroids is still a little tough with MechJeb. I highly recomment MechJeb as the essential mod for anyone who doesn't care to learn this much about orbital mechanics. But having studied it for so long, having dreamed of it my entire childhood and pined for it since the day I left the aerospace industry (22 years ago), I think everyone should love doing this stuff!

Hope this helps - feel free to reach out to me if you need any clarification.

Danny

- - - Updated - - -

This is an interesting approach that I've used successfully a few times, the tricky part is using a retro burn to achieve orbit.

The second part, approaching the vessel, can be done easier. Instead of pushing or pulling the target markers around on the navball, simply only fire when approaching the target, on the round pink one, then firing retrograde to slow down, your yellow one. Simply by using these two you can move towards it, slow down, move towards it again, slow down etc. Do this gradually and you can get really close quite comfortably. Here is my craft- it has a spark engine in the last stage which I find works well, I usually ditch a half tank of fuel but it's nice to have a cushion, and besides kerbals cost a lot to hire so a bit of waste is OK! Also notice the ladder down the side since I'm still bad at EVA- aligning the rescue ship so the ladder is facing the other craft and straight up before EVAing the stranded kerbal gives you a big target to grab on to. Edit: don't pay attention to the Kerbal engineer readout, it's set on Minmus

https://i.imgur.com/JU5zjal.jpg

This is very similar to the way Gene Kerman walks you through the rendezvous section of the docking tutorial. I highly recommend the docking tutorial - you will come out of it knowing exactly how to rendezvous - and dock!

The ship I use for these rescue missions now is actually just a klaw on a probe core with enough engine to orbit it. I don't even use RCS cause you don't need to translate when klawing. The return section is fitted with a bunch of airbrakes and chutes. I've used it to land 20t+ successfully. I bring it down pointing retro as long as possible, but it's usually a little too top heavy to maintain that attitude in the lower atmosphere. This rig has not failed a single rescue or recovery/salvage mission yet.

Danny.

[GarrisonChisholm]

...I think everyone should love doing this stuff!

Hope this helps - feel free to reach out to me if you need any clarification.

Danny

Wow, thank you. Now I'm going to be embarrassed if such a comprehensive response to my issue doesn't lead to a successful resolution. We'll find out in about 5 hours when I am off work.

[Streetwind]

I have just today tried to undertake my first orbital rendezvous, as in the first 5 months of playing it seemed absurdly intimidating. I have met with a gross lack of success.

I know this is written clearly and as an (allegedly) intelligent person it should suffice to explain it to me, but is there a "Rendezvous for Dummys", or "Rendezvous 101" tutorial somewhere?.. :\

The best way for us to answer your questions is... for you to ask them

If you are still struggling later today after having digested Danny's information dump, then I recommend you describe to us exactly where it goes wrong - where the results you are seeing start to deviate from what the explanations in this thread state should be happening. Often it can be a tiny little thing that for us veterans is so natural that we don't even think about it when writing up guides, because it never occurs to us that someone might not be familiar with them. A thing such as "you need to actually click the target craft and choose 'set as target' or you will not see any close approach markers". (Mea culpa!)

Perhaps even include a screenshot showing the orbit of your craft and the orbit of your target craft, and explain where you struggle with matching them up.

Edited October 21, 2015 by Streetwind

[CrashTestDanny]

enquiring minds want to know... did it help?

Danny

[GarrisonChisholm]

Ordred & Mind are safely back at KSP concluding their quarantine period. It helped indeed. Both Eva's were nearly a kilometer, but they were accomplished.

- not that I'm eager to try one again very soon. Thank you very much, one and all!

[CrashTestDanny]

Awesome! Great Job!

Danny