[Updated] New player! Need help about tuning the interception orbit for docking

[uboats]

Hi, I just started to try interception and docking for low Kerbin orbit (95km-120km) vessels.

The status of two vessels are shown in the fig.1

A (blue) is in red orbit 105km, and is after B.

B (red) is in blue orbit 110km, and is befow A.

There are two ways to let them close:

1. lower PE of A

2. rise AP of B

But none of above methods can create interception nodes.

Ignore this issue, I find that if greatly lower/rise PE/AP of A/B, maybe only one cycle will be used before they reach the closest points.

However, in this case, PE of A or AP of B could be very low/high.

For A, it's not safe since it could return the atmosphere of Kerbin

For B, it may cost hundreds of dV to rise AP, and will also cost hundreds of dV to lower to circle orbit.

But if the PE/AP is not that low/high, it requires many cycles, while for low orbit, the time warp is limited

And we have to wait a long time.

Do we have to make two orbits having intersection nodes in order to monitor the interception distance?

So what is the fastest way to intercept?

What is the way of smallest fuel cost?

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Is it a good choice that to make the two orbits very close at the very beginning?

Or make them a large difference (100km and 150km) would be better?

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Another question is that if the two orbits do not have intersection nodes. How can I know where is the closest position?

I know that if two orbits intersect, then there will be marks showing interception nodes with the (I think) closest distance. Correct me if I'm wrong.

Or intercept nodes do not show the closest position but just tell us if one craft is at the intercept node, where is another one?

Edited November 9, 2013 by uboats

[blizzy78]

So what is the fastest way to intercept?

What is the way of smallest fuel cost?

Well you have figured it out yourself. The fastest way is to burn fuel to decrease the synodic period below the orbital period. The cost-efficient way is to just wait one or more orbits.

Or just don't start with nearly similar orbits to begin with. For example, have a station at 200-250 km, and bring visitor crafts to only 100. That will give you a nice low synodic period where you don't need to wait for as long.

Edited November 7, 2013 by blizzy78

[700NitroXpress]

Watch the first 20 minutes of this for interception and docking an engine to a orbiting base.

[Spyritdragon]

Im not sure which is most efficient, its really a tradeoff between how much delta-V you're willing to use and how long you're willing to take. With timewarp, however, it shouldn't take long to catch up. Not as long as it would've taken to post this .

If i were in your position, i'd lower the periapsis of A to aroundabouts 80 km. Kerbins atmosphere ends just below 70km up, so you won't have any problem with encountering the atmosphere. That way A can catch up to B reasonably quickly. When you estimate you have less than one orbit left to catch up to B, create a maneuver node, move it around your orbit a bit while pulling the prograde and retrograde markers back and forth a bit (its really a bit of a feel you gotta get for this - usually just pull it prograde untill your orbit goes slightly above the item you're trying to RV with) untill you get an intercept of under 5 km. Preform your maneuver, then wait untill you get to your closest approach. Then cancel out all target-relative velocity, burn towars your target a bit. Then burn retro to stop when you're within about 100 metres, and dock with RCS from there on. Beware that when you burn retro with main engines, your thrust can push your target around, so avoid using your main engines when you are very close to your target.

[capi3101]

Here's how I'd handle that situation: assuming A is the source/control craft and B is the target craft. First, I'm assuming the nodes are zeroed out - that's very important. If the ascending/descending node is not zero, a burn needs to take place to make it that way. Assuming both craft are on a course where east (090) is prograde, a northward burn at the descending node or a southward burn at the ascending node will do the trick. Zero is where you want it; if you can manage to get it to say "NaN", you're right on the money.

I'd then push A's apoapsis on up to 110k (i.e. roughly the same altitude as B's orbit). You should then get the intercept chevrons. Plan at the apoapsis next to push the periapsis down closer to Kerbin and watch what that does to the intercept point. If it shortens it, great - just make sure you haven't pushed your periapsis below 70k (the edge of the atmosphere). If it lengthens it, then go the other direction (I'll tell you right now that that shouldn't happen). Do the same thing at periapsis - adjust the apoapsis. At some point while you're doing all this you'll get close and find a "break even point", where it won't decrease any more; that's where you plan to burn. Expect to muck it up - H/N on RCS does wonders for fixing that sort of thing.

If after a while you find planning nodes at neither the apoapsis or periapsis helps, try one somewhere in between.

[uboats]

Thank you.

Another question is that if the two orbits do not have intersection nodes. How can I know where is the closest position?

I know that if two orbits intersect, then there will be marks showing interception nodes with the (I think) closest distance. Correct me if I'm wrong.

Or intercept nodes do not show the closest position but just tell us if one craft is at the intercept node, where is another one?

[phunk]

The intercept nodes show where the orbits are closest together, but they only show up if the orbits are within a certain threshold (maybe 5k?)

With non-crossing orbits, I'd assume that either AP or PE of the lower orbit would be the closest point, depending on the orientation of the orbits. Whichever one is closer to the PE of the higher orbit should be the closest point between the orbits.

[Geschosskopf]

Rather than put 1 ship into an egg-shaped orbit hoping to meet the other where their paths cross, I find it a LOT easier to do docking rendezvous the same way as interplanetary transfers. 2 ships orbiting Kerbin at different altitudes, in orbits that don't cross, are the same as 2 planets orbiting the sun so the same method can be applied to both situations. So what I do is let them go around until the angle between the ships looks pretty much like the angle between Kerbin and Duna (if the lower ship is moving to the higher ship) or between Kerbin and Eve (if the higher ship is moving to the lower ship) that I'd use to transfer between those planets. Then I plunk down a node in front of the ship that will move and fiddle with it until I get an intercept I like.

I do it this way because I lack the brainpower to remember more than 1 way of doing things

[capi3101]

The intercept nodes indicate the positions of the controlled craft and its target at the point at which they will have their closest approach to one another in their respective orbits. They will only show up if the orbital paths of the two craft come within close proximity to one another, which is why you need an -apsis of the control craft to be somewhere in the neighborhood of the orbit of the target craft. Now (and here's the kicker), it doesn't really matter where along the target craft's path the control craft's -apsis is, it just matters that it's there. You can't adjust the intercept nodes if you can't see them or if the game doesn't recognize them.

[Smidge204]

Have orbits that intersect... or at least a maneuver node that causes them to intersect. If you have a maneuver node in place you will see, within the next two orbits of your predicted post-maneuver orbit, where your ship and the target will be. (Assuming, of course, you have actually set the target!) You can now fiddle with the maneuver node, dragging it around etc, until you get a satisfactorily close encounter. If you're patient you can get it within a km easy, but 1-2km is more than good enough.

Perform the maneuver and verify the encounter. Wait until the encounter and set your navball velocity to "Target" - it should now be showing your m/s and the pro/retrograde markers relative to the target, as well as pink circle (prograde) and cross (retrodrade) direction to target. Manually burn to get your prograde vector aligned with the target prograde vector, and you are now headed directly towards the target.

Go slow and take it easy, especially as you get close (within a few hundred meters) as it's really easy to whoosh right past it. constantly doing small burns to adjust your speed down as you get closer and keep your prograde marker and target prograde marker aligned. Note that if you have them perfectly aligned you're on a collision course!

Switch to RCS if you got it, since the blast from a larger engine will push the target if you're too close. Reduce approach speed to 1-2m/s max once you're under 100m to target. Be patient! Zero out your velocity entirely once you're within 30 meters or so, and do some gentle maneuvering to align the docking ports.

Click on your docking port and select "Control from here" so your navball is as accurate and relevant as possible.

Very gently approach (under 1m/s!) keeping the prograde marker, target prograde marker and center of the navball all aligned.

With a gentle bump, and maybe some last bit of nuzzling the target with your ship, you should dock.

=Smidge=