How to get near target on orbit

[martafix]

Hi, I've accepted a contract to save a kerbal from vessel on orbit. I've upgraded tracking station and mission control, so I can do maneuvers, but i don't understadn logic of getting close to target when on orbit. There are some A,D nodes and some Intersect points, but i don't know how to use them :(.

Pls help, Martafix

[StarStreak2109]

1. When the Kerbal in question is in an equatorial orbit, wait until he/she is over the KSC, then launch. While ascending (and high enough), try shifting your angle of attack in such a way that the close approach markers move closer to each other. Once in a stable orbit, if you are behind your target, move to a lower (ie. faster) orbit, if you are in front of it, move to a higher (ie. slower) orbit.
2. If the Kerbal is in an inclined orbit, wait till the plane of your target's orbit is over the KSC, then launch into the inclination of your target orbit. Proceed according to lit. 1. If your relative incliniation is off, burn normal (happy triangle) or antinormal (angry triangle) at the ascending/descending nodes, to align your orbits.

Also, to learn this, you can install Mechjeb and use the Rendezvous Planner, which has some useful readouts and helps you create/execute maneouver nodes.

HTH

Edited April 3, 2016 by StarStreak2109
Completeness

[Wanderfound]

I'd recommend you stay away from Mechjeb until you've learnt how to do it yourself; MJ isn't that great at rendezvous anyway.

The first thing you need to do is make sure that your orbits are matched in inclination (i.e., both on an equatorial orbit, or both on the same polar orbit, etc.). For a rescue mission, this is easy; the target is probably in a routine equatorial orbit, just launch east as usual.

Once your orbital inclinations are matched, you need to either speed up or slow down in order to catch up to the target or let it catch up to you.

This is where it gets weird: lower orbits move faster than higher ones. If your orbit is the same height as the target, you'll hold position with it; if your orbit is higher, it will get farther ahead of you; if your orbit is lower, you'll catch up on it. So, if you need to catch up with the target, you actually need to lower your orbit by burning retrograde. Conversely, if you need to slow down, burn prograde to raise your orbit.

In terms or reading the map screen: first, set the rescuee as a target. After you've done that, the ascending and descending nodes are where the plane of your orbit crosses the plane of the target orbit on the way north (ascending) and south (descending). So long as your orbital plane is close to that of the target, these don't matter. If you do need to correct your orbital plane, the AN or DN is where you'd make the normal/antinormal burn.

The intercept points show your closest approaches to the target in each orbit. Fiddle about with the height of your orbit until you get these down to docking range (the closer the better, but anything under a kilometre will do), then time accelerate to the rendezvous and dock (or just get close, match speeds and spacewalk if it's a rescue mission). If you're starting a long distance from the target, it may require several orbits for you to achieve a close enough intercept.

However, for a rescue mission, there's no need to start a long distance away; just wait until the rescue target is a little bit short of passing over KSC, then launch direct to an orbit of the same altitude and inclination as it. That should start you off fairly close together, but you'll likely need a bit of fine tuning. So, if the target is ahead of you, burn retrograde a bit to lower and compress your orbit; if the target is behind you, burn prograde a bit to raise and expand your orbit.

The last complication is that you need to be matching the targets altitude when you meet up. This gets problematic when you're using the raising and lowering of your orbit to control your speed. So, what you do is make use of elliptical orbits; have one side of the orbit at the same altitude as the target, while the other side is higher or lower depending upon what is required. Careful tweaking of the orbit height allows you to arrange it so that the rendezvous happens on the equal-altitude side.

Like a lot of Kerbal, it's really simple once you're used to it and really difficult when you're not. But just try to keep in mind this basic point: big orbits are slower than little orbits.

 

And for the obligatory Manley:

 

 

The version of the game in that vid is outdated, but nothing has changed in the basic methodology of rendezvous.

Edited April 3, 2016 by Wanderfound

[Plusck]

Specifically on the point of "There are ... some Intersect points, but i don't know how to use them", and after reading through Wanderfound's post above, you might find the following useful:

You might have one (orange) or two (orange and purple) sets of intersect points. If two, it's because you cross the target orbit twice per orbit. If you have the option, choose the intercept point that is in daylight... it's so much easier.

The easiest way I've found to get a very close approach in a minimum time and with minimum fuel, is to use two maneuvre nodes.

First, drop a node by the next intersect chevron (the one that says where you will be, not the target, since it is the actual point the orbits cross). Leave it at 0 m/s.

Drop a second node about a quarter of an orbit later. This is to make sure that you always see the intersect point one full orbit later.

Now you need to judge whether it is reasonable to try to get an intercept within a single orbit. If the chevrons are far apart at this stage, it might be very expensive, so in this case right-click the second node, and click on the "+1 orbit" button on the bottom right.

Now go back to the first node and add a small amount of prograde/retrograde to move the intersect chevrons closer together.

If you can get them to line up with a minimal burn, then you're done, and you can burn at that first node, delete it and warp forward until you pass the second node, then delete that and warp to about 3 minutes before the actual intersect.

If they are still quite far apart, go back to your second node, right-click and add an orbit or two. Basically, at this stage, it's up to you to determine how much fuel you're wanting to use. If you're in LKO, you can always get a perfect intersect within one orbit by burning hard to a much higher orbit - but to slow down to meet your target you're always going to have to make an equal and opposite burn so this can be very expensive. Likewise you can catch up on a distant target by going into a much lower orbit but you could easily end up in the atmosphere... 

By going back and forward between these two nodes, you will always be able to get an optimal intercept ... where "optimal" means whatever porportions of expending fuel or warping forward you are prepared to suffer 

Since you have maneuvre nodes set, you can use the "warp to next node" option when you click on your orbit. This takes away some of the dull and boring part of the process...

And finally, once you've burned the first node, deleted it then warped forward to the second node, and your chevrons are aligned for the next pass, warp forward to a point a few minutes before the intercept. This is where you should switch to target mode, and push the retrograde marker over antitarget. If you watch the map view while doing this you should see the intercept chevrons move closer together, but also move further away along the orbit. If they stop closing, stop burning and wait to get closer, rince and repeat. However, this is a whole different subject it it's own right, so I'll stop here. 

 

edit: one final thing I forgot to say: if there is any significant difference in inclination, then you should (a) always choose to make your burns at or very close to AN/DN, (b) add a normal / antinormal component to every burn (you don't have to fix it all at once, because you'll have to match speeds with the target later, so spreading the normal / antinormal component over your burns costs less than trying to get inclination perfect with a single burn) and (c) always make bigger normal/antinormal burns at the most distant point in your orbit since it's cheaper out there (and if there is a severe difference in inclination, you might want to add more nodes to deal with this problem).

Edited April 3, 2016 by Plusck

[N_Danger]

 I've found this chart helpful to get a visual idea of what the steps are. Since you do not have to dock you can ignore the part about getting a mod.

I will second the warnings about MecJeb not being efficient. I'm a long time user of it but you need to learn to do stuff without it. If but to recognize when it is screwing up. 

Spoiler

Edited April 3, 2016 by N_Danger

[Plusck]

1 hour ago, N_Danger said:

 I've found this chart helpful to get a visual idea of what the steps are. Since you do not have to dock you can ignore the part about getting a mod.

I will second the warnings about MecJeb not being efficient. I'm a long time user of it but you need to learn to do stuff without it. If but to recognize when it is screwing up. 

  Reveal hidden contents

I agree that the chart helps to visualise the steps... just as long as you don't actually keep doing the steps one at a time because it's horribly wasteful. Fixing inclination first, then doing a radial burn, then burning prograde massively to get an encounter within one orbit... rather than burning along two sides of a triangle, this is equivalent to burning along three edges of a cube (or whatever you call a cube cut in half diagonally - a right-angled tetrahedron?). If you're in LKO this could easily end up in the high hundreds or even thousands of m/s. 

IMO you can forget about radial since that will be efficiently sorted (together with any remaining inclination) when you kill relative velocity to your target at the end. You could go halfway on your "intercept matching" burn but that adds further complication if you're still getting your head around the idea.

So yes, good chart for descriptive purposes, bad for efficiency (and for implying that a mod is needed to dock properly...).

[Snark]

Just gonna toss this in here, as a helpful thread on the topic:

 

[martafix]

So, if I understand it right, i need first to make my orbit same as target, then if target is in front of me, i need to make my Ap lower than target, if is it behind me, i need to make it higher. If is targets orbit degree higher then my , i have to burn normal or antionormal on AN or DN. But when I want to get close to target, and i have selected "Target" on Navball, should i have burn prograde or at target? Also, does exist any trick, to know when to put Ap back in normal position?, becouse when I see next Intersect point, its too late to edit it.

Anyway thx, Martafix

[Gojira1000]

I feel bad for always posting this one, but it does help:

 

 

[Plusck]

2 hours ago, martafix said:

...But when I want to get close to target, and i have selected "Target" on Navball, should i have burn prograde or at target? Also, does exist any trick, to know when to put Ap back in normal position?, becouse when I see next Intersect point, its too late to edit it.

When you are nearly at the point where the chevrons intersect (say about 1/16th of an orbit before - the ships should look like they're almost flying parallel), turn on target mode and point to the retrograde marker. If it's perfectly aligned to the ani-target marker (pink dotted cross) then great - that means you're headed directly for the target - so stay on map view and warp forward a bit closer.

If the retrograde marker isn't exactly on anti-target, then point a little way to the other side to push retrograde over anti-target.

For example (this is around Gilly, so it's kinda in slo-mo but the principle is the same, further away and faster, anywhere else):

Spoiler

In the first pic, I'm pointing directly at retrograde-to-target. At this rate, I'll pass the target a hundred metres away or so. I've also set up a maneuvre node to imitate the target's orbit, but this is actually pointless*.

In the second pic, I point to the opposite side of retrograde and burn. This reduces my speed, but at the same time it pushes the retrograde marker towards the target, as seen in pic three. I'll now pass it about 20m away.

 

If you do this when you're a few dozen kilometres away from the target, you can bring a "decent" intercept of a couple of kilometres down to a hundred metres or so. The closer you burn to retrograde, the slower your approach and the longer you'll have to wait. In that second pic, for example, it would have been more time-efficient to point further away - just left of the 135° / 45° point in the brown, say - to change direction without losing much velocity.

When you are very close, switch to external view and burn pure retrograde so that you come to a stop (or almost a stop - again so that you can push retrograde over anti-target so that you can approach at a crawl) about a hundred metres or so from the target.

* It's pointless trying to plot a node to match orbits because by killing relative velocity to target, you are by definition making the most efficient possible burn to match orbits with it.

** however it can help if you have a large burn because the maneuvre node will tell you how long you have to burn for, and how much time you have until you meet the target. Starting the burn about 2/3 of that time away from target will make sure you slow down in time without crashing into it and without stopping kilometres off.

[Wemb]

On 04/04/2016 at 4:13 PM, martafix said:

So, if I understand it right, i need first to make my orbit same as target, then if target is in front of me, i need to make my Ap lower than target, if is it behind me, i need to make it higher. If is targets orbit degree higher then my , i have to burn normal or antionormal on AN or DN. But when I want to get close to target, and i have selected "Target" on Navball, should i have burn prograde or at target? Also, does exist any trick, to know when to put Ap back in normal position?, becouse when I see next Intersect point, its too late to edit it.

Anyway thx, Martafix

Yup, more or less.  The lower your orbit, the fast you move - so to catch up you have to lower your orbit.  If your orbit isn't entirely circular you'll be at different speeds at different points so may not always be catching up, but if your Ap's are the same but you lower your Pe below the target's your orbital period will be lower and you'll catch up.  The reverse is true if you need to slow down and let the target catch up with you.  

The counter-intuitive bit is that to 'catch up' you have to burn away from the target (which lowers your orbit, which speeds you up) - Even the Gemini IV astronauts got that wrong, so it really is rocket science.

Your right about the plane adjustment - do it at the AN or DN  - the points at which you cross your target's orbit.  Also, be aware that the further you are away from the body you're orbiting the cheaper this will be (because the further away you are, the slower you are, and therefore the less fuel you will burn to change velocity).  This won't matter ina near circular orbit, but can be important in an elliptical orbit.

Finally, when you're on final approach in Target Mode burning prograde will increase your relative velocity tot he target, retro grade will slow it - so to rendevous, you ideally want to burn retrograde when you're at the closest approach until your velocity in Target Mode is zero  - you will then be parked relative to the target.

Not sure what you mean by 'putting Ap back'.  I would suggest that you concenrate on getting your orbit's co-planar first - that is, get the AN and DN to zero first, and then once you're in the same plane, you only have to worry about 2 dimensions instead of 3.

Wemb

[martafix]

I will ask you one more time :), before I'll do something wrong, i have Intersect points 3,3Km far from each other. What i have to do than? Should I leave the cabin and just go ahead to "KSC(Kerbal-Saving-Craft)" or is it better to do this with "KSC"(I dont have RCS on it)? If is my "Burn directly to target" realy wrong pls tell me better, I'm too noby in it :D.

Thx

Edited April 5, 2016 by martafix

[Wemb]

 Nice - that's not too bad - if you have time, fiddle with a maneouver and see if you can get it closer than 3.3km before you get there  - but 3km is okay too.  What you should do is when to get to 3.3 km away, and just before your distance then starts to go back up, that is the point when you should reduce your relative velocity to zero.

Once that relative velocity is zero you will be more or less at the same place as the target. (well, 3.3km away) , at the same point in time and, importantly, going in the same direction at the same speed - that means you're in the same orbit and you can relax a bit.  

At that point, you need to get closer - either you point one ship at the other, and fly (relatively slowly) toward it - when you do this, your prograde marker will be in the same direction as the target marker - they will probably diverge a bit as you fly, as your orbit will be changing compared to his because you're accelerating again - but just remember to stop use the retrograde marker don't eyeball it - or you could eva your kerbal and fly him 3km using his jetpack.  It's not a lot different, in principal - you're just more limited by fuel in the spacesuit - but it might be easier depending on how manoeuvrable your ships are. On the other hand, the spaceships will be easier to see if you get them closer.  

You don't need RCS unless you're trying to dock.- the Kerbals' jetpack is a mini-RCS system, so you can fly him in circles round your spaceship to find the door. 

Good luck!

Wemb

Edited April 5, 2016 by Wemb

[Plusck]

3 hours ago, martafix said:

I will ask you one more time :), before I'll do something wrong, i have Intersect points 3,3Km far from each other. What i have to do than? Should I leave the cabin and just go ahead to "KSC(Kerbal-Saving-Craft)" or is it better to do this with "KSC"(I dont have RCS on it)? If is my "Burn directly to target" realy wrong pls tell me better, I'm too noby in it :D.

Thx

Really, what I said yesterday is (IMHO) exactly what you need to do: wait until your a short way before meeting the target (about 1/16th of an orbit), then in target mode, aim to the side and push retrograde over antitarget, while watching the map and mousing over the intersect chevrons: they should decrease.

If for any reason they don't decrease while doing this (i.e. you are just 2-3 minutes away, you have moved the to-target yellow retrograde directly over the anti-target pink cross, but your closest approach is still more than 2 km or so) then it's probably because your orbits never actually cross at all (i.e. your orbit is always higher or lower than the target). Rotate the map view around quickly to check that this is the case. If so, you do have to point to target and thrust in that direction some, which will force your orbits to intersect. Unfortunately it's a loss of fuel because you're going to have to slow down again relative to target when you reach it, so next time get a better approach to start with : p

Once you've done this a few times, you'll also start to get a feel for narrowing down your approach manually, even if you start from a much bigger gap.

[martafix]

Thx a lot guys, before saving, I have maked  Intersect points distance 1,1km, so it was easy. Kerbal was succesfuly loaded in "KSC", and I had very lucky landing too i have landed on end of runway :D. Again thx a lot.

[Wemb]

2 hours ago, martafix said:

Thx a lot guys, before saving, I have maked  Intersect points distance 1,1km, so it was easy. Kerbal was succesfuly loaded in "KSC", and I had very lucky landing too i have landed on end of runway 

Doesn't get much better than that - well done!

Wemb

[martafix]

Ok, last  two questions . 1)I have heard about something called DelaV. Does anyone know what does it mean?

                                          2)I have misson to put some kind of satelite on orbit:

but i dont understand longtitude of Ascending node. What is An i know from before, but i dont know, how to get it into this place

(btw, how can I insert image here?)

 

Edited April 6, 2016 by martafix

[Plusck]

1 hour ago, martafix said:

Ok, last  two questions . 1)I have heard about something called DelaV. Does anyone know what does it mean?

                                          2)I have misson to put some kind of satelite on orbit:

but i dont understand longtitude of Ascending node. What is An i know from before, but i dont know, how to get it into this place

(btw, how can I insert image here?)

 

de la vie means "of life" in French.

delta v (often shortened to dv) is how much change in velocity you need to do something, or how much change in velocity your craft can make.

A rocket's delta-v is absolutely fixed by its mass, propellant mass and engine efficiency. It can use that dv more effectively or less effectively (such as burning low in a gravity well at high speed) but it cannot alter its total dv. The dv for an individual maneuvre is the figure on the right of the navball.

The second question should really be in its own thread but the answer is simple: you need to eyeball it.

To insert image, you need to host it somewhere public (so not on a Google drive, for example). Then get the actual URL of the image (and NOT what the image hosting site tells you is a link to the image: instead right-click to view the image and then copy it's address). Then click on the "Insert other media" button on the bottom right of the posting box.

Edited April 6, 2016 by Plusck

[Wanderfound]

16 hours ago, martafix said:

Ok, last  two questions . 1)I have heard about something called DelaV. Does anyone know what does it mean?

Δ = delta = change.

V = velocity.

ΔV = change in velocity.

Any given manoeuvre will require you to alter your velocity in some way. This is the ΔV number that appears to the right of the navball when you plan a manoeuvre.

ΔV is also used to describe the manoeuvring capacity of a ship. If you have 1,000m/s worth of ΔV available, you could accelerate by 1,000m/s, or cancel out 500m/s of velocity and then accelerate to 500m/s in a different direction. Etc.

[Wemb]

And to clarify a little more, the reason we talk about delta-V rather than needing some amount of fuel, is that every movement you make Kerbin's solar system involves changing your velocity (which is a measure of speed and direction) - usually this happens for free when gravity is pulling your arround an orbit - but, regardless, any change in where you're going or what you're doing is, in esscense, a change in your velocity.  As such, it makes more sense to describe the capability of your vessel not in terms of the amount of fuel it carries or how big it is, but the amount of change of velocity it is capable of. This is also the figure that's displayed on the Navball when you plan and execute a burn.

This becomes really important when you want to do more advanced missions which you will need to budget your delta-V for.  A consequence of your ship's being (in relative terms) a so much smaller than the planets they orbit, is that the size/shape and of it's engines of the rocket is largely immaterial in calculating the amount of delta-v needed to get it from one particular situation to the other - and as such we can produce maps that describe the dV needed to get somewhere. See here

 

The final step in planning your missions is having the tools needed to work out how much dV any rocket (or stages of a rocket) you build has - you can do this with a spreadsheet and the appropriate formulae, but there are some handy mods - such as Kerbal Engineering Redux - which will tell you in the VAB, and when you fly, how much dV your currently have.

Getting to grips with this will enable you to plan and make much more complex and wide-ranging missions, but also to trim your existing rockets down to the much smaller sizes as you're (probably) building rockets that have way more dV than you need for whatever it is you're doing with them. (Unless you're already runnign out of fuel right at the last moment).

[Edit] - here's a link to an excellent thread describing this from a year or two back.

Wemb

 

 

 

Edited April 7, 2016 by Wemb