Rendevouz in orbit won't work ...

[dubAUT]

Hi guys,

since days I try to perform a rendevouz in Kerbal's orbit. What ever I do, the closest intersection is always between 1.000km and 1.300km and I do not know what I do wrong.

I tried to do as in some YT tutorials and as well what was written in the Wiki, but still - something does not work.

Any ideas what else I could try?

[heng]

would be helpful to know what you where doing in the first place...

you do know, for example, that you can set the maneuver node to next orbital pass?

that, if your current intersection ist at 1000km, you can just wait and see what happens after that? wait one orbit and look again, maybe now it's only 750km? wait one more... this is slow, but sometimes necessariy due to d/v restrictions...

are you in the same plane?

are you circular, is your target?

are you ahead or behind your target?

and don't despair, docking/rendevouz is one (if not the most) difficult maneuver to master early on.

-

not necessarily the most efficient, but a working method:

a) circularize your orbit slightly below (or above) the target's PE (or AP)

(the closer you are, the easier it gets, but the more time you have to spend waiting in step d))

go co-planar, i.e. set inclination to 0°

(burn at the ascending/descending nodes down/up)

c) make your orbit elliptical, so that your PE (or AP) matches up with targets AP (or PE)

(the more precisely you time this (e.g. maybe now, maybe one orbit later), the less time is spend waiting in the next step...)

d) watch the closest encounter markers for a while.

- say it starts at 1000km and goes down to 700 after one revolution

- wait... it will be 400km... then 100km.

- the very next pass it would overshoot to 200km, the other direction. we do not want that to happen.

- hence now is the time to burn at your PE (or AP) (the one i meant in c))

- burn slowly to adjust your AP (or PE) to match the target's orbit

- keep watching the encounter markers!!!

e) you should be able to setup an encounter as close as you like. <1km if needed.

this encounter will happen roughly one orbital revolution after your burn!

once you are at the encounter, you need to match velocities... not going to write that up here...

and docking is a whole new can of worms...

Edited May 21, 2015 by heng
style

[mckamx]

Actually, 1.0 - 1.3 km separation after doing a Hohmann transfer and velocity matching isn't too bad. Once you get to this separation, point one ship at the other and burn about 2 m/s. wait until your separation hits a minimum, then match velocities again. Repeat until you get to within docking range.

[Streetwind]

wait one orbit and look again, maybe now it's only 750km? wait one more...

^-- This.

For rendezvous, doing the maneuver correct is not always enough. You also need to be in the correct spot relative to your target. Think of it as two people running towards each other trying to high-five. If they make the arm movement too soon, or too late, they will miss each other.

If your orbit and the orbit of your target are not 100% identical, then each loop around the planet changes your relative position to one another. Either you will get closer, in which case, great! Or you will get further away, in which case you have two options: change your orbit so that you get closer instead, or simply wait. Any target that moves away from you in one direction will eventually come around the planet and come closer to you from the other direction.

Actually, 1.0 - 1.3 km separation after doing a Hohmann transfer and velocity matching isn't too bad. Once you get to this separation, point one ship at the other and burn about 2 m/s. wait until your separation hits a minimum, then match velocities again. Repeat until you get to within docking range.

He has 1000 to 13000 kilometers, not meters, separation.

[Starwhip]

I wrote a tutorial a while ago for rendezvous and docking, the link is in my signature. It has more on the subject than I would be able to tell you offhand.

[dubAUT]

would be helpful to know what you where doing in the first place...

I am doing it around Kerbal. My craft flying in an 80km orbit, the target space ship in a 90/92km orbit.

you do know, for example, that you can set the maneuver node to next orbital pass?

What is the next orbital pass and how do I set a MN there?

are you in the same plane?

I do not understand?

are you circular, is your target?

I try to be as circular as possible; however the target is PE 90km, AP 92km

are you ahead or behind your target?

Both already happened at several attempts.

Thanks so far!

[heng]

My craft flying in an 80km orbit, the target space ship in a 90/92km orbit.

ok, so you are faster than your target.

What is the next orbital pass and how do I set a MN there?

your ship goes around Kerbin in a circle... one round is one orbital revolution. two rounds are two orbital revolutions, the second pass

set a regular maneuver node. right (left? left, then right? dunno right now) click on it. you should get a display with three buttons on the node. upper right: close, lower left: minus one revolution (initially greyed out), lower right: plus one revolution. watch the time next to your navball telling you when the node will be reached. this should change accordingly... i.e. initial time to node: 1min. click "plus" and the time changes to 1min + 1h (if you need 1h to do a full orbit)

if you set a node and do absolutely nothing, you will just pass it, and then pass by it again. and again, as you keep going round in circles (or ellipses). with the buttons described above, you can tell KSP to calculate everything as if you execute your burn at the next pass. or as many passes in the future as you like.

I try to be as circular as possible; however the target is PE 90km, AP 92km

that's absolutely fine. your orbit being circular just makes it easier for the basic method (above post) to do. i.e. you can just set a maneuver anywhere and do not have to worry about efficiency and Oberth and stuff... by no means requirement, just easier.

setting up your encounter at PE/AP only makes it easier to match velocities later on. recommended, but not required.

Starwhip's tutorial is brilliant. if you can spare a couple of minutes, go read it!

PS:

"same plane" - "plane" as in geometrical plane spanned by three points, not the avionic plane with wings...

your orbit lies in a plane. ideal launches will end up in the equatorial plane. seldomly do so for me

your target's orbit is another plane, you should aim to be in the same.

if not, your orbit will intersect the target's plane in two points. going up, and going down again.

easily seen if you tilt your camera so that your orbit is a straight line. try it.

if your target's orbit is the same straight line, you are coplanar. if not, you should see the other orbit being tilted, relative to yours. can you see it? it is inclined.

there are to markers once you select a target with an orbit. the ascending node and the descending node. the values given are in angular degrees. if you burn up or down there (purple navball markers) you can change your inclination to match. and only at these points you can do so!

PPS: sorry for laggy response time, at the office right now, boss comes around every now and then :-)

Edited May 21, 2015 by heng

[cicatrix]

1. The inclinations of both orbits (of the craft you are piloting and of the craft you're trying to rendezvous to) should be nearly the same (lie in the same plane). You can assess that by looking at the AN and DN markers (Ascending and Descending node) - they should be within 1 degree difference (the closer to 0 - the better). If not - do a plane change by putting a maneuver node in either AN or DN and try to match the planes as close as you can. (It doesn't matter for nearly circular orbits but in elliptical ones the dV difference between choosing either AN or DN can be quite significant).

2. Once you have both orbits in the same plane see for the phasing rate (the frequency at which both crafts appear at the closest range to each other). At this point you might consider raising or lowering your orbit to increase the phasing rate (if both orbits are very close to each other and you are completely out of phase with another craft - i.e. it's on the other side of Kerbin). The closer the orbits are the longer you would have to wait for the correct moment to make a Hohmann's transfer. I usually set the ratio to 1:1.2 - 1:1.5 (meaning that if your target's orbit height is 100km yours should be either 120-130 km or 70-80 km).

3. Now, place a maneuver node on your path and adjust it so that your final trajectory was tangential (appear to touch) the target's one. Two markers will appear displaying the resulting separation. Now slide your maneuver node along your orbit to see how those markers behave. This might take a while or you might even want to wait a few more orbits for the conditions to improve. Once you have successfully aligned your separation markers, execute the maneuver node. Naturally, the less the separation the better. Still, even 20-30 km will be normal.

4. Once your Hohmann's transfer burn is complete, place your maneuver node on the point your trajectories intersect and adjust your final orbit so that it matched perfectly the orbit of the target. Wait for it and burn.

5. You should be pretty close to your target. Orient your craft to point towards it and switch (click) the speed indicator to be relative to the speed of the target. (should read 'Target', not 'Orbit' or 'Surface'). Divide the distance to your target (in m/s) by 1000 and burn towards it so that the relative speed was about that figure (for example, if the distance is 20 km, your approach speed should be about 20 m/s). This ratio is not mandatory byt don't get too fast. You will need to turn your craft away from the target and burn again as you approach it to minimize the relative speed (ideally it should be 0 m/s). Now switch to your RCS and proceed with docking if necessary.

[Chaos_Klaus]

You have to align your orbital plane to match the plane the other orbit is in. This is easy if both craft are in the equatorial plane already.

The angle between the two orbital planes is called "relative inclination". You can see this value if you target the other craft and then mouse over either the ascending node (AN) or descending node (DN). This value needs to be 0°. If it is not, you will most likely pass "above" or "below" your target.

I put up and down in quotes, because there is no real up and down in space.

To match inclination, you need to burn radial ("up") at the descending node, or anti-radial ("down") at the ascending node

[KerbMav]

I am doing it around Kerbal. My craft flying in an 80km orbit, the target space ship in a 90/92km orbit.

What is the next orbital pass and how do I set a MN there?

I do not understand?

I try to be as circular as possible; however the target is PE 90km, AP 92km

Both already happened at several attempts.

Thanks so far!

You are orbiting lower, so your ship moves faster and will come up from behind the other vessel, but change your orbit to have one/two points close to the other orbit, meaning raise your AP up to the orbit of your target. There are two indicators on both orbits, position of your vessel and your target when they are closest next and the next time after that. Your goal is to change your orbit that at some time two indicators of the same color are very close, showing you a distance from your target at ideally under 10km.

Next orbital pass means the next round around the planet/moon, the next time you arrive at a certain point on your orbit again.

The maneuver node should give you two buttons, they enable you to look into the future, so to speak, and change the time of your burn to the next time you come around to this point. This also gives you the opportunity to see if your vessel comes close to your target in x orbits and how far in the future this will happen, the indicators for the position of your vessel and your target will update to the future orbit.

The same plane means, both orbits have the same inclination (they are parallel so to speak). You can burn up and down on your AN/DN (ascending/descending node) to get to the same inlination. Mousing over these points on your orbit (visible while having a target selected) shows you by how many degrees you are deviating.

[dubAUT]

if you set a node and do absolutely nothing, you will just pass it, and then pass by it again. and again, as you keep going round in circles (or ellipses). with the buttons described above, you can tell KSP to calculate everything as if you execute your burn at the next pass. or as many passes in the future as you like.

I now managed to find a very close approach with that, but even with the fastes Wrap wich is in the low orbit possible it takes ages, due to the fact I have to forward over 1d. Do I just have to wait or did I something wrong?

Thanks again!

[cicatrix]

I now managed to find a very close approach with that, but even with the fastes Wrap wich is in the low orbit possible it takes ages, due to the fact I have to forward over 1d. Do I just have to wait or did I something wrong?

Thanks again!

Switch back to KSC and time warp there. Personally I use Kerbal Alarm Clock which makes it easier not to miss the right time, but if you are completely against all mods just wait a day at the KSC.

This is what I described as 'low phasing rate' your orbits are too close to each other so the changes take time.

[heng]

No, you didn't do anything 'wrong'. It just comes out that way. If you keep playing a while and do this more often, you will soon learn to eyeball your initial orbit so that the waiting time gets reduced (if possible). Your orbit is 80km, to ~90km of your target. that is not much... you could in theory go up to - say for example - 150km and let the target catch up with you. this might be faster... but will cost more d/v.

For now i recommend playing this through.

Plant a flag at the KSC, switch to it. Then you can timewarp as fast as you like without having to go to the astronaut complex at all (and thus reducing loading times).

[JoCRaM]

you can make the rendezvous happen in fewer orbits by changing the orbit of one of your - if it's catching up with the other, then burn retrograde, if the other ship is catching up burn prograde. A few m/s makes a big difference.

[cephalo]

I find that rendezvous with something in low orbit is much faster if you start with an orbit that is significantly bigger. It might take more fuel but it's worth it in my opinion if you have fuel to spare. To intercept at 90k start at 140k. You can always catch it in one or two orbits.

Also, know that you can drag the maneuver node along the orbital path. That takes a ton of work out of planning rendezvous. I didn't know that initially and was frustrated having to guess where to place my nodes.

[xtoro]

What you can do is install MechJeb if you're still having problems, and just watch how it does it a few times. That might help you out, then you can remove it.