Orbital intercepts

[hiddenevil]

Hi there everyone

I'm sorry for asking this, I feel extremely stupid to be honest. I've watched several youtube movies and still I'm not getting how the heck I intercept another object in orbit of kerbal.

I'm still in the early stages of research and made an unmanned satellite, with a jr science lab. When I got the thing in orbit I realised I'd made a massive mistake, I'd forgotten to fit a parachute DOH

So I thinks to myself, no problem. Send up a kerbal to intercept the satellite and retrieve the science from the unit and return it to earth. Oh lord has this led me to pulling my hair out, the make "target" function leaves me scratching my head. Today I had to take a step away from KSP as no game should leave the player stressed, games are meant to be fun!

I can get my space craft in a relatively identical orbit to the satellite, but I'm always behind or infront of it and I can't figure out how to reduce the distance between the two objects without altering the orbit of the space craft. Pushing me out of sync with my target.

Any help would be appreciated (an overly stressed gamer)

[Moonfrog]

Alright. Basic concept is first getting an intercept with your target. It sounds like you already know this and so i'll leave that step out.

Next step is, to reduce your velocity relative to the target. On the navball, it will change automatically to Target 230m/s from orbit 2300 m/s or whatever it happens to be. You can change this manually by clicking on this part of the nav ball.

Assuming u have a good intercept, when u get within a good distance that u feel confident you can decelerate fast enough, change direction so your facing the marker that is a circle with a cross through it, yellow one. This is so you decrease your relative velocity towards the target to 0.

After it becomes 0, your distance should change minimally. At this point, u want to face in the direction of the target, I think its the purple/pink marker with the circle. Might be wrong. Basically u want to point yourself towards your target. Then increase your velocity. This will decrease the gap between you and the target. You may want to slow down as you get close, so u don't crash into it. It helps if you have a good intercept with a close distance and your both traveling in the same rotational direction. It is possible if your a good pilot to do it from other ways, just be harder.

That is basically what you do. After you have a successful intercept point with the target from those manouver nodes.

Hope that helps.

If you have trouble getting an intercept, one method is to adjust your alighment with your target to 0 degrees, the two green markers on your orbit, like arrows. Then increase your velocity so your orbit loops round and intercepts your targets orbit. Then follow what I said at the beginning.

Edited June 23, 2014 by Moonfrog

[Lheim]

If you're significantly behind or in front, you have to change your orbit to get closer.

The key thing: higher orbits move slower, lower orbits move faster. To catch up to an object ahead of you, drop below it; to let an object catch up to you, go above.

Let's say it's behind you and you want it to catch up. Select it as a target, draw a maneuver to thrust prograde, and get a good bit above it. Depending on how far behind you it is, you may be able to get an intercept immediately sometime up ahead - or you may just want to go 20-50 kilometers higher and go around a few orbits as it creeps up a little closer each time.

Let's say you've gone around a couple times and it's gotten close enough that you have a maneuver you can do that will bring you within a couple kilometers: fair enough, execute it.

As you're approaching, switch the velocity read-out to target-relative mode.

You can, when you get close, burn full retrograde (relative to target, remember), to still yourself.. then just a bit prograde towards them, and slowly approach and then slow down again. Or you can manage it without the intermediate stop - you will find you can trim the position of your retrograde vector overtop theirs as you approach with off-angled thrusts. It's a bit difficult to describe, but basically any off-angled thrusts you do will 'push' the retograde vector, so you can use that to fine tune your approach. With timing, you can reduce speed to zero just as you glide in alongside.

Edit: video series that helped me learn:

Edited June 23, 2014 by Lheim

[hiddenevil]

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.

I'll give it a shot

[Tortoise]

[EdFred]

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.

I'll give it a shot

It does if you accelerate enough, but when you readjust to match speeds when you are within a few meters of your target, you essentially end up re-establishing a stable orbit. And remember in LKO, you're moving at about 2300m/s. To rendezvous you're only making changes up to about 10m/s, so you have quite a bit of time before you drop like a rock.

[hiddenevil]

Tortoise thanks for the video, but i'm afraid there is way to much going on there for me to follow what the heck the person is trying to show me.

I'm close to giving up at this point as I just had another crack as it and I can't seem to get close enough. I have to two orbits very close together so when i try to adjust my orbit, the two a meshing together and i'm having trouble knowing one from the other. I might go back and watch some more orbital tutorials.

Perhaps it's just me, but I felt the tutorials for KSP took a bit of a jump from learning orbits to going to the mun, something I've yet to achieve. Everyone's learning curve is different and some take to plotting orbits easier then others :/

[Lheim]

Yeah, the changes you're making are usually pretty minor. Even if they're huge, and even if you're approaching your target in a way that would send you into the drink without intervention.. well, sure, okay, but you're going to be matching velocity vectors with the target when you approach, right? You maneuver yourself into the same position, same direction and speed of movement - you end up in the same orbit automatically.

Edited June 23, 2014 by Lheim

[Pecan]

Games are meant to be fun but simulators are meant to be realistic. KSP is a space simulator game 8~0

Rendezvous and docking ARE hard for most people to learn, so don't let it stress you out. Keep reading/watching the tutorials and keep practicing. You will get it eventually. I usually use MechJeb for rendezvous anyway, simply because of the number of orbits it usually takes to get a good intercept.

ETA: Practicing rendezvous & docking with small ships that are designed for it can help a lot, as can using a docking-alignment mod. Chapter 5 of my tutorial starts with a practice docking drone, if it'll help (http://forum.kerbalspaceprogram.com/threads/79658-Exploring-The-System-A-design-tutorial-campaign?p=1183116&viewfull=1#post1183116)

Edited June 23, 2014 by Pecan

[Taki117]

Even NASA had issues with Docking...So don't feel bad. Just keep at it and soak up the information.

[weezl]

Tortoise thanks for the video, but i'm afraid there is way to much going on there for me to follow what the heck the person is trying to show me.

I'm close to giving up at this point as I just had another crack as it and I can't seem to get close enough. I have to two orbits very close together so when i try to adjust my orbit, the two a meshing together and i'm having trouble knowing one from the other. I might go back and watch some more orbital tutorials.

Perhaps it's just me, but I felt the tutorials for KSP took a bit of a jump from learning orbits to going to the mun, something I've yet to achieve. Everyone's learning curve is different and some take to plotting orbits easier then others :/

Do a search on this site for 'Hohmann transfer' that is a good start..

Some things to keep in mind..

Since you wish to rendezvous.. you can just point at your target and fire your booster.. but it isn't very efficient of fuel (as in you may need more than you can reasonable lift into orbit).. the most efficient is to transfer from your own lower/higher orbit to the target's orbit so that they both meet a the same point in time at near the same place.. this is called a Hohmann transfer.

If you disregard target position and just think about orbits.. you can burn prograde from a lower orbit (or retrograde from a higher orbit) and adjust the OTHER side of your orbit so its apoapsis (or periapsis from a higher orbit) is at the same altitude as the target's orbit. YOUR orbit should now be elliptical... with its periapsis (or apoapsis from a higher orbit) at your original orbit's altitude and the apoapsis (or periapsis from a higher orbit) at the target orbit's altitude... That point where the two orbits meet is where you want to rendezvous with your target.. but since you can't just floor the accelerator like you would on earth (since changing speed = changing orbit), you must time when you do your burn so that when those two orbits meet, the target is in exactly the right position NOW to meet your craft when you get to apoapsis (your rendezvous point) at the exact other half of the orbit you are on.

Another point to consider is that both your orbit and the target's orbit must have exactly the same inclination. That is easy to determine in KSP since once you mark a target it shows the inclination difference (they show up as two dart markers on your orbit after you've selected a target.. or one if the target is on an escape trajectory.. like an asteroid).. and you can use maneuver nodes to adjust that. This is important because if the orbits DON'T have the same inclination, then there are only TWO points your orbit where it will cross the other orbit's path (when looking at them side on).. You want those suckers to be aligned at ALL points so that when you adjust your orbit you have the minimum distance between your apoapsis and the target orbit, no matter where your maneuver node is placed in your own orbit.

Another point to keep in mind is that as much as possible both orbits should be as circular as you can make them (apoapsis ~= periapsis)... that is important for what comes below..

That is the 2nd point of Hohmann transfers. Its not just the touching orbits.. its waiting until the target is just in the correct position in its own orbit so that given its speed and YOUR speed you both meet when the orbits meet. If you are below your target (in altitude), then generally you'll be behind your target when starting your burn (since you are currently going FASTER than your target). Planning Hohmann's is easy if both orbits are fully circular.. wait until your target is up orbit of you if you are lower (i.e faster).. back orbit of you if you are higher (i.e. slower).. then lay a maneuver point a bit ahead of your craft and adjust the orbit until its apoapsis (if you are lower) or periapsis (if you are higher) just touches the other one.. arrows should appear showing the target's and your relative position at the point the orbits meet (as well as their distance at rendezvous.. you must hover over the arrows to see this). Now slide the maneuver node around until you've minimized the rendezvous distance (this is where its important to have circular orbits.. if you DON'T, then you must adjust the apoapsis/periapsis as you drag the node around..).

Once you've mastered this doing circular orbits.. you will have a basic understanding of orbital intercept mechanics for interplanetary travel.. since those are just hohmann transfers done over months rather than minutes. Further.. once you get to an intuitive understanding of all this.. you'll understand how to match orbits under less than ideal circumstances (i.e. one or both orbits are non circular).. which is important for doing things like asteroid intercepts or intercepting objects in elliptical orbits.

There are some video explanations of this.. as well as one that uses pictures taken at points in the process.. but its not the pictures that are important.. its the concepts.. if you have more questions.. ask..

Edited June 23, 2014 by weezl

[Red Iron Crown]

Since you wish to rendezvous.. you can just point at your target and fire your booster.

That's singularly bad advice. It doesn't work that way at all.

[Lheim]

Two questions, OP:

Are you selecting the target ship as your target? The map will provide 'closest approach' markers when you do.

Do you understand how to make maneuver nodes?

[weezl]

That's singularly bad advice. It doesn't work that way at all.

Didn't say it was good advice.... you did read the rest of my post, didn't you? You can actually radially burn from Kerbin to Mun if you want.. you'll need horrific amounts of fuel to do it.. but its possible. More to the point.. because of gravity interfering.. you'll have to learn how to make it work no matter HOW you choose to do it.. which is what folks are faced with anyways.. its why my second statement is about fuel efficiency.

At the end of the day if you don't learn Hohmann transfers, then most of the solar system is closed to you (because of the extreme fuel requirements). I get it Red.. please take the whole post in context, not just one sentence.

[weezl]

Two questions, OP:

Are you selecting the target ship as your target? The map will provide 'closest approach' markers when you do.

Do you understand how to make maneuver nodes?

..will provide 'closest approach' markers when your orbits overlap.. and may create two sets of markers when your orbit intersects with the target orbit more than once.

[hiddenevil]

Two questions, OP:

Are you selecting the target ship as your target? The map will provide 'closest approach' markers when you do.

Do you understand how to make maneuver nodes?

Answer to the first, yes I am setting the satellite to be my target.

I have not yet researched RCS thrusters. My plan was to get within a reasonable distance of the target and EVA across to it, retrieve the data, get back to my craft and go home.

What is Mechjeb? As reluctant as i am do modding, if there is something out there that will make this easier..Well hell I'm inclined to use it

I know how to alter my orbit and plan manoeuvres, but as this is the first time i've used target, I dont fully understand the additional markers that appear when you set something as your target. I dont know what they are telling me, which is part of the problem i guess.

[Red Iron Crown]

MechJeb is a mod that adds autopilot and informational displays. If you think you'd learn from watching the autopilot do the rendezvous for you (it describes a little bit of what it's doing), by all means try it. Be careful though, if piloting is a big part of your enjoyment of the game using the autopilot functions will reduce that satisfaction.

[blizzy78]

but as this is the first time i've used target, I dont fully understand the additional markers that appear when you set something as your target. I dont know what they are telling me, which is part of the problem i guess.

There are usually two sets of markers, purple and orange. They are telling you the closest distance you get to your target, and where. Just hover over them for information. If your orbit is intersecting your target's orbit more than once, you'll get the purple and orange markers, if not, you'll only get purple.

Ideally you will want to plan a Hohmann transfer so that one set of markers comes very close to one another. That means your closest approach distance is short.

It is actually pretty easy to do. Suppose your target is in a 200 km orbit, and you are in a 100 km orbit. Now plan a maneuver node, and simply drag prograde until your projected orbit just intersects your target's orbit. The purple and orange markers should appear. (*) Now grab the maneuver node by its central ring, and drag it along your current orbit, until one set of markers gets really close (it doesn't matter which one.) Hover over them to see the actual projected approach distance. The closer it gets to zero, the better. Then execute that maneuver. Voila, you're halfway there.

(*) Note that you might need to kill relative inclination between the two orbits first. Too big an inclination can prevent the close approach markers from appearing at all.

Also, as much as I like MechJeb, I'd advise against it when learning the ropes. The maneuver is actually pretty easy to learn and execute, you just need to know how.

Edited June 24, 2014 by blizzy78

[Moonfrog]

hmmm, there is another option, on Skype there is a option to let someone see your screen while your doing it. Would it help if you streamed your game to someone through Skype they could give u pointers and tell u what u need to know in real time? As sometimes there be miscommunications in forums.

My Skype is moonofsilverdragon if u need a little help with it. I'm in the uk, incase theres another moonofsilverdragon.

[hiddenevil]

I'll add you to my skype thanks

Piloting the vessel is enjoyable, however i'm happy to yield control to the computer if the alternative is a lot of stress. Making the crafts, doing the science is what I really love. If I had the option to let my Kerbals pilot their own craft, then I'd like that option. As I like managing the agency and developing the tech most of all

I think the issue here is that I obviously have gabs in my understanding of orbiting procedure and how to read the markers correctly. I can get my craft close, but not close enough for an intercept.

I didn't even know you could move the manoeuvre node around, I thought it was anchored to that one spot when you created it.

hmmm, there is another option, on Skype there is a option to let someone see your screen while your doing it. Would it help if you streamed your game to someone through Skype they could give u pointers and tell u what u need to know in real time? As sometimes there be miscommunications in forums.

My Skype is moonofsilverdragon if u need a little help with it. I'm in the uk, incase theres another moonofsilverdragon.

[Sky_walker]

If it's going to encourage you: First docking is always the most difficult one. After that you'll become so good in it that you'll be able to build multi-component ships on an orbit through docking procedures with sub-meter precision.

[Oafman]

Firstly, stick at it. RV is very hard at first, but once you crack it you'll find it gets easier and easier.

I first learned when I had Jeb stuck in EVA orbit around the Mun. I sent a rescue craft, got it into a similar orbit, and then spent hours (literally) tweaking at maneuver nodes, making tiny burns. Eventually I got the hang of it, in the process learning a couple of key principles, and I've been able to do it ever since.

Secondly, I'm not against MechJeb at all; I use it all the time. But I do believe you should learn how to do something before becoming reliant on an autopilot.

So I would urge you not to fall back to MJ quite yet. I'm also quite critical of how MJ makes RVs. It does things in quite an inefficient manner, and there will come a time when you absolutely have to make a fuel-efficient RV, and you need to know how.

Thirdly, if you're able to match orbits (albeit with your craft at different points on that orbit) then you've already won half the battle.

I actually think talk of Hohmann transfers isn't that helpful at this stage. I think the best thing is to get into a very similar orbit, then tweak it so it's either slightly higher or slightly lower than your target. If the target is less than half an orbit ahead of you, you want to be at a slightly lower altitude, so you'll circle the planet faster and catch it up. If it's less than half an orbit behind you, you want to be slightly higher altitude, so it circles faster and catches up with you.

I think it's easiest if you just make the orbits slightly different. Maybe just 5km or 10km difference in altitude, instead of burning out to a much higher orbit and then trying to Hohmann your way back in to the target. That works fine, and is much quicker (in terms of in game time), but it's more complicated. Admittedly, what I am suggesting is basically a mini-Hohmann, but you don't need to think of it like that.

Once you have a similar orbit, but a slightly different altitude, just time warp until you're close to the target (ie close behind or in front of it, don't keep warping until you are exactly above or below it, you want to make your correction burn before that point). This warping might take ages (many orbits, possibly many days) but what's the hurry? If your altitude prevents you from time warping quickly, then just go to the VAB, put a lander can on the launch pad (just a can, it's not going anywhere). Then switch to map, where you'll be able to see your two ships in orbit, and now you're on the ground you'll be able to time warp very very quickly, and days or even weeks can pass in no time.

So, once you do this, you'll have a close encounter (say 20km or less). You can then very gently burn prograde (if you're at a lower altitude) or retro (if you're higher) so your orbit meets that of your target somewhere on the other side of the planet. This will be a very small burn, so be extra gentle with it. Once done, warp to just before the point where your orbits meet (map will show it as the closest approach), and it's time to start using the navball with the target marker to make your very small corrections, to perfectly align your orbits, and to move very close. That's a separate skill, which people here can also help you with, but the first challenge is to get the two ships close to each other, on a very similar orbit.

[orangewarning]

Don't give up!!! watch scott manleys tutorial, it helps you to know what to watch out for. I was so bad at my first docking that by the time I got close it was at night!! That was just a couple of days ago... Now I have made about 5 more dockings and only failed once... I believe there is a bug as even after I gave up that attempt, mechjeb cant dock as well... Out of so many possible achievements in KSP, I feel that docking for the first time is the single most sastifying feeling in the game ever.

Rendevous I think is the killer for most people so don't get stressed out over it.

[RoboRay]

I can get my space craft in a relatively identical orbit to the satellite, but I'm always behind or infront of it and I can't figure out how to reduce the distance between the two objects without altering the orbit of the space craft.

Altering the orbit of your spacecraft is how you reduce the distance between them. It doesn't matter if they are in different orbits; it only matters that at some time in the future they will get very close to each other. When that happens, then you match velocities to get them into the same orbit.

If two people standing on the opposite sides of a room want to shake hands, one of them has to start walking.

Edited June 24, 2014 by RoboRay

[blizzy78]

If two people standing on the opposite sides of a room want to shake hands, one of them has to start walking.

And if both of them walk in circles in the same direction at the same speed, they'll never meet. That's the important lesson to learn here.