Pointing 'UP' when orbit is inclined

[Callmedave]

Hello, When im on an orbit around Kerbin, I point North/up so that when i come to dock, the craft dont rotate around each other, ie. one points 'up' one points 'down' and as we travel around the planet, we dont go 'around each other'

But which way do i point my craft when the orbit is inclined? How do i work out where this theoretical line is? Im not even sure of the correct term for this position. I use KER, is there a number on here i should be looking for?

If any understands me and can help, please let me know.

Edited July 31, 2014 by Callmedave

[EdFred]

You can still leave it pointed due north, and it won't rotate. However, the equation is quite simple if you want to point normal or perpendicular to your orbital plane. Subtract your inclination from 360. So if you have an inclination of 15°, 360° -15° = 345°.

Edited July 30, 2014 by EdFred

[Sirrobert]

Make a manouver node, pull on the purple ones.

Point your rocket at the manouver marker. That's where you need to point to stop it from rotating (like you'd do north when equatorial)

[Callmedave]

You can still leave it pointed due north, and it won't rotate. However, the equation is quite simple if you want to point normal or perpendicular to your orbital plane. Subtract your inclination from 360. So if you have an inclination of 15°, 360° -15° = 345°.

Ok thanks, il try this, I think the term is 'normal'

I.e. i want to be pointing 90 'up' from my orbit vector

[StuntMax]

Wat?

....crafts in orbit DO NOT rotate.

Better: they DO rotate if your reference point is the surface of the planet, but if your reference is the centre of mass of the orbited body (or another orbiting object for what it matters) it does not rotate at all.

Demonstration: if you point your nose towards the sun, it will keep pointing the sun fot the entire orbit (better: it will move soooo slowly you probably don't even notice).

SO... if you are RV'ing and there is a relative rotation between the vessels, and no thrust is applied, then the rotation is uniform (you can stop it with SAS or manually, and it won't restart, or if you have no time to waste just kick in timewarp for a second: it kills any rotational movement).

Then install the navball docking alignment indicator, and live happily ever after.

[Callmedave]

Make a manouver node, pull on the purple ones.

Point your rocket at the manouver marker. That's where you need to point to stop it from rotating (like you'd do north when equatorial)

Hi, Thanks for the answer, but thats not what im trying to acheive, Im in an inclined orbit on purpose, Im landing a Science lander on minmus, gathering science and then docking with the lab in orbit. Its a lot easier to dock, when i can just float above the lab and line up the docking ports, as opposed to going round it as our obits cross. The orbit is inclined so that i can easily reach all the biomes.

[Callmedave]

Wat?

....crafts in orbit DO NOT rotate.

Better: they DO rotate if your reference point is the surface of the planet, but if your reference is the centre of mass of the orbited body (or another orbiting object for what it matters) it does not rotate at all.

Demonstration: if you point your nose towards the sun, it will keep pointing the sun fot the entire orbit (better: it will move soooo slowly you probably don't even notice).

SO... if you are RV'ing and there is a relative rotation between the vessels, and no thrust is applied, then the rotation is uniform (you can stop it with SAS or manually, and it won't restart, or if you have no time to waste just kick in timewarp for a second: it kills any rotational movement).

Then install the navball docking alignment indicator, and live happily ever after.

Rotate in reference to each other i mean, sorry i dont know the term.

[AlexinTokyo]

I think you've got the wrong end of the stick there.

What Sirrobert is saying is that creating a maneuver node with only a normal (or anti-normal) component will by definition give you the direction to point in. Once you're pointed in the right direction, delete the node; not saying you should actually make any burns on that node.

[StuntMax]

Rotate in reference to each other i mean, sorry i dont know the term.

Well, they shouldn't.

If engines are off and no RCS is used, then there's no force applied on one of the crafts that is not applied to the other.

All the relative movements are uniform, and if you stop them, they won't "restart".

Try this: from craft A, set target on craft B, then turn unitl you face the purple navball marker (target prograde). Engage SAS, make sure you aren't moving.

Then switch to craft B and do the same.

Now you can wait as long as you want, the crafts WILL keep facing. Even if they are on completely different orbits.

[Mr Shifty]

....crafts in orbit DO NOT rotate.

Better: they DO rotate if your reference point is the surface of the planet, but if your reference is the centre of mass of the orbited body (or another orbiting object for what it matters) it does not rotate at all.

They don't rotate relative to the Sun, but they do rotate relative to each other. Put two craft in a circular orbit, one behind the other with zero relative velocity, both pointed prograde. After 90° of orbit they'll both be pointed radially outward; that is, they'll be side-by-side instead of in-line. This is why people recommend orienting the docking port so that it points normal to your orbit. That way the relative rotation is around the docking port axis.

[Diazo]

Actually, it does not work like that in game.

SAS keeps a vessel's rotation relative to the 'universal' co-ordinate system, not relative to what the vessel is currently orbiting.

Think of the Kerbol system in game as being inside a big box with all the orbits happening inside this box.

When you turn SAS on, it keeps a vessel pointed in the same direction, relative to the big box. So as you orbit the planet, your craft is "turning".

Another example it to take two craft, both in a perfectly circular 100km equalatorial orbit above kerbin, 50m apart. Point the two craft at each other and then timewarp until the craft complete half an orbit around kerbin. The craft will now be pointing directly away from each other.

This is because each craft keeps its orientation relative to the universal coordinates, but by completing half an orbit they have swapped their relative positions. Timewarp again for another half orbit (so one full orbit) and the two craft will be pointing at each other again.

This is why it is suggested for easier docking that you point north or south to dock. While the craft is turning, the bottom (pointed south) part of the craft stays pointing south throughout the entire orbit.

D.

[Wanderfound]

They don't rotate relative to the Sun, but they do rotate relative to each other. Put two craft in a circular orbit, one behind the other with zero relative velocity, both pointed prograde. After 90° of orbit they'll both be pointed radially outward; that is, they'll be side-by-side instead of in-line. This is why people recommend orienting the docking port so that it points normal to your orbit. That way the relative rotation is around the docking port axis.

OTOH, if your docking takes so long that this sort of orbital effect becomes an issue, you really need to polish up the speed of your docking. It should only take a minute or two.

[StuntMax]

They don't rotate relative to the Sun, but they do rotate relative to each other. Put two craft in a circular orbit, one behind the other with zero relative velocity, both pointed prograde. After 90° of orbit they'll both be pointed radially outward; that is, they'll be side-by-side instead of in-line. This is why people recommend orienting the docking port so that it points normal to your orbit. That way the relative rotation is around the docking port axis.

I don't agree.

They only rotate relative to the surface, and this won't be a problem (at least if you aren't trying to dock with the surface...)

But they DON'T rotate relative to each other.

Try this: two ships in identical LKO. A is 90° ahead of B.

Both pointing prograde. Then wait... after 1/2 of orbit, both of them will be pointing retrograde. They are rotating ath the same identical speed, and this means that tey are perfectly still relative to the other.

Early players are advised to point normal simply because it is a fixed reference point that is always available, does not change in all the orbit, and doesn't require a "target".

If you point both ships at it they WILL be aligned even if they are on different orbits; for all that matters, you could use ANYTHING as a reference (the Sun, Moho, a third craft... not Mun, it moves too quickly) and obtain the exact same result. Try it.

[Alshain]

Rotate in reference to each other i mean, sorry i dont know the term.

They shouldn't rotate at all. If both craft are aimed at the same point, they should stay that way. This of course assumes you don't have any rotational momentum. If you do, turn on SAS to halt it.

However, of your using markers, that could move. Prograde and Retrograde change position as you orbit. Technically the craft isn't moving, the marker is. Normal and Antinormal don't change position during your orbit so another way is to install Enhanched Navball. This will add additional markers to the navball. With this, simply aim your craft at one of the pink triangles (Normal/Antinormal markers) and any craft you want to dock can be done the same way to align them.

Finally, the real easy way is to use Docking Port Alignment Indicator, with this orientation doesn't matter at all because the indicator will help you orient the docking craft to the docking port.

Edited July 30, 2014 by Alshain

[Mr Shifty]

Try this: two ships in identical LKO. A is 90° ahead of B.

Both pointing prograde. Then wait... after 1/2 of orbit, both of them will be pointing retrograde. They are rotating ath the same identical speed, and this means that tey are perfectly still relative to the other.

Here is a crude MS Paint illustration of your scheme. Notice that the crafts do not change orientation: B is always pointed up and A is always pointed to the right. In the first drawing, A is pointed in the direction of B's tail. In the second B is pointed in the direction of A's tail. They rotate relative to each other. If they both always point toward the Sun, they must rotate relative to each other, since as their orbit circles around Kerbin, the relative position of the Sun changes.

[StuntMax]

Actually, it does not work like that in game.

Crap, this happens when you forget switching on your brains before posting.

You're right (...and even in RL, not only in game): I was (wrongly) limiting the issue to a docking problem, that should take a short time compared to an orbit.

Edited July 30, 2014 by StuntMax
typos... damn you, autocorrect!

[KSK]

This sounds like a question of stellar inertial vs orbital rate. This page has a nice explanation. Check out the diagrams about a quarter of the way down the page at time stamp 000:30:28.

In stellar inertial, your spacecraft is in a fixed attitude relative to the stars (the universal coordinates that Diazo is talking about). Which means that your attitude relative to the planet you're orbiting changes throughout the orbit. In orbital rate, you put your spacecraft into a slow rotation, to keep it 'facing forward' in its orbit.

Docking in KSP is generally thought to be easier in orbital rate. However, setting up orb-rate can be tricky, not least because going into time-warp automatically puts you into stellar inertial. The way to side step the issue altogether (at least if you have a nice symmetric docking port) is to have both spacecraft pointing normal to their orbital vectors, as others have pointed out in this thread.

Edited July 30, 2014 by KSK

[Mr Shifty]

OTOH, if your docking takes so long that this sort of orbital effect becomes an issue, you really need to polish up the speed of your docking. It should only take a minute or two.

Crafts rotate around the center of mass. Really long crafts with a docking port far from the center of mass can move a surprising amount. Orbits in LKO are only 30 minutes, so if a dock takes, say 2 minutes, that's a full 24° of rotation, which can be quite significant when trying to dock.

[RexKramer]

I think the simple solution here is to use the ] or [ keys to switch between vessels, and keep them pointed towards each other.

Keep in mind that you may need to go to map view, and select the other vessel as a 'target', so you get a magenta indicator on your navball, to indicate the direction of the other vessel.

Doing this is much easier than trying to maneuver around a vessel, trying to get into position to approach a docking port. It also helps to zero-out your relative motion before switching vessels. That gives you time to target the other ship, if needed, without drifting past while you fumble with the controls (at least that's been my experience).

The method of pointing one ship 'north' and the other ship 'south' also works, but adds confusion when the orbits are inclined. Using the [ and ] keys to switch between ships, keeping them pointed at each other, works really well, no matter what type of orbit you are in.

[RexKramer]

Also, using the yellow indicators on the navball in target mode is very helpful to zero out your relative motion. While you are getting the hang of docking, I think it's common to get frustrated when you end up with unwanted motion. Instead of getting closer to the target, you find yourself just going around it. Using the yellow indicators in target mode really helps.

To zero out your motion relative to the target vessel, point your ship so it's aimed at the yellow circle with an 'X' in it. Then, thrust (or RCS forward) until your speed is at or near zero. The speed indicated in target mode is your speed relative to your target. Making that relative speed zero gives you time to line the ships up, so you can start bringing them together- slowly.

Once you have zeroed out your relative motions with the yellow indicator, align both ships so they are facing each other. To do that, use the magenta (pink) indicators. You want to point towards the magenta indicator which is a circle with a dot in the center. Thrust or RCS forward a little.

[Callmedave]

Hi all. I understand (now) about setting up a node to give me the direction. Thank you. Sorry I can't dock in two minutes. The quickest is maybe 4/5 mins. But if I can figure this out it should make me quicker.

[Sirrobert]

Hi, Thanks for the answer, but thats not what im trying to acheive, Im in an inclined orbit on purpose, Im landing a Science lander on minmus, gathering science and then docking with the lab in orbit. Its a lot easier to dock, when i can just float above the lab and line up the docking ports, as opposed to going round it as our obits cross. The orbit is inclined so that i can easily reach all the biomes.

I know

Point towards the purple marker IS the equivilant of pointing north on an equatorial orbit. I'm not talking about changing your inclination.

The marker is simply a way to find the correct direction

OTOH, if your docking takes so long that this sort of orbital effect becomes an issue, you really need to polish up the speed of your docking. It should only take a minute or two.

It's not about how long it takes to dock.

If you park your craft along the normal/antinormal vector, it will always stay pointed in that direction, while it'll rotate relative to the planet in any other direction.

So if you park your craft along the normal/antinormal vector, you always know which way the docking port is pointing when you aproch

Edited July 30, 2014 by Sirrobert

[StuntMax]

Hi all. I understand (now) about setting up a node to give me the direction. Thank you. Sorry I can't dock in two minutes. The quickest is maybe 4/5 mins. But if I can figure this out it should make me quicker.

How close can you get to your orbital lab?

I don't mean docking, just RV. Can you easily get at 300-500 m?

[Callmedave]

How close can you get to your orbital lab?

I don't mean docking, just RV. Can you easily get at 300-500 m?

I can rendezvous to less than 1km quite easily. And then I spend around another 10 mins closing the gap and docking.

[StuntMax]

I can rendezvous to less than 1km quite easily. And then I spend around another 10 mins closing the gap and docking.

Ok, then try this.

When you are at 1 km, switch the navball to target mode. Burn to green retrograde marker to kill relative speed.

When your relative speed is about 10-15 m/s, your target is to "drag" the green prograde marker directly over the target prograde marker: remember that when you burn, the prograde marker is "pulled" towards the center of the navball. If your relative speed rises, slow it down with a retrograde burn.

When the two markers are aligned, it means that you are pointing straight to your target; gradually reduce your relative speed, at about 200 m you can switch to the target craft to align the docking port, and you're done.