Docking and relative velocity

[Broetchenholer]

So, i am trying to create a mun orbiter stage where i can unduck the lander, fly to the Mun, come back, rendezvous and fly back to Kerbin. To do that, i am first practising my docking and how to effectively rendezvous at all. And the game is different from the tutorial videos i have seen. I am undocking switching control to the lander, wait till i am 100 meters away and then try to dock back up. What i understand i need to do to succeed is this:

1) set the docking clamp as a a target.

2) switch to docking mode.

3. Switch to target mode for velocity

4.) point towards target and burn a little.

5.) once i am close, burn retrograde to kill off velocity

6.) use rcs to do the last steps by going normal and then do just simpy x y z axis manoevery with rcs until i dock.

The problem however starts when i am doing step 4 and 5. even without rcs on and sas only on stabilize the target speed and direction of it's speed is wobbling, so the direction is circling around as if one of the capsules was rolling somehow, but both are stable with sas. also, apparently the relative speed is also varying between 1.5 and 4. So the game is clearly having a stroke, but at some point it will stabilize. Trying to do any of that stuff with RCS seems like the worst idea ever, because RCS seems to not be able at all to keep anything stable. When it's on together with SAS, the vehicle will always start to become unstable in relation to the target. 

Second, going normal does not do what has been advertised. It doesn't put the two vessels in parallel mode. i have a very very small relative velocity but i am gliding next to the other vessel and there is no easy way to set me up. prograde is just spinning me for some reason with the prograde indicator always being really far away from the nose, retrograde is wobbling again normal puts me, it could be orthagonal, i would say it's not, but i can's really see a real direction as well, anti normal is the opposite direction, radial is the center of the red  half and the degrees are just floating as if i was spinning and anti radial is the opposite. And target of course puts me directly at the target. What i probably could to is cheat, switch to the other vessel, and set the target there as well to point at it, but there has to be a real way to do it. Why does normal not work for me?

[Vanamonde]

When you are orbiting close to another object, the two of you will seem to be circling each other. It's a trick of perspective. How close are you when this is happening? 

Incidentally, docking mode is an alternative control system but it's not required. In fact, I find it confusing. I just stay in normal control mode. 

[Broetchenholer]

Hey, thanks for taking your time to answer

 

The relative speed being all over the place happens as soon as you turn SAS on to align towards the target. So, i undock, i set the target to the other docking ring, i switch to relative velocity and to docking mode, everyhting is fine and stable. As soon as i turn my vessel towards target, the speed starts to quickly cycle from, +2 to what the actual relative velocity is to -2 of that and then rinse repeat forever. I did it at 100m, it stayed that way until 1100m . Then i started to burn towards it and the speed alternated between 5 and 20 m/s, and everyhting in between. I knew i was closing in, because the distance got smaller, but apart from that, no clue. When i got closer, the relative speed went to 3-6 m/s, with no change in velocity.  But then it was pretty quickly back up again. All this is done just with SAS corrections and main thrust towards target and then retrograde to get closer.

At the end, i was at 12 meter disctance and had killed most of my velocity towards the target and now the velocity at the navbal was switching between 0 and ~1 m/s and the retrograte marker on the navball is moving around like it has a spasm, and as a consequence the active SAS is moving the shhip all the time to keep retrograde. If i switch to stability of docking mode for SAS, the vessel keeps twitching, only when it is really close, and you actually managed to kill almost alle of relative velocity, both the orietation of the vessel udner SAS and the display of relative speed calms down and remains stable enough to do precision manoevers. This is with a MK1 landing craft with no extra SAS modules and 4 symmetrical attached rv-105 rcs thrusters.

[Vanamonde]

Hmm. That's close enough for docking. Sounds like a bug? 

[Broetchenholer]

Oh, by the way, don't wanna create a different topic for it, i try to use the mentioned formula for an efficient ascend from Kerbin, so, trying to ai for 45° at ~ 10km height and then set to prograde and keep the AP at 50 seconds away. Doing that, my current AP is around 50k, so i think i am too shallow.  My first stage has a TWR of 1.68 with 850 dV and the second stage kicks in at around 5k with 1.38 TWR and 3.2 kdV. If i let SAS Prograde do the gravity turn, i end up at 25000 meter, if i do it manually, i get to 50k, so i think the drag is too high on the vehicule? Would a good or efficient launcher go to 80k with that kind of ascend procedure?

[The Aziz]

Likely you've encountered one or more of these 

 

 

It's best to not use RCS except for forward/backward movement. After initial encounter and velocity match, point at target, and crawl your way towards it. Stop close to it, set the docking ports to target each other, set SAS to target and finish the job.

That's of course theory.

 

 

[Broetchenholer]

Oh yeah, the second one is what i am experiencing. I am getting better at it and have now worked around it, but it's definately not helping either

[stephensmat]

A quick-fix that has worked for me? When the ports are right up next to each other, and should be connecting, speed up the clock to 2x. I don't know why, but that works.

[Probus]

Step 2 is <DEL> to enter docking mode.  What is the key to enter target mode, step 3?  I've totally forgotten.

EDIT: Click the yellow tab next to the nav ball...

Edited February 3 by Probus

[BechMeister]

On 1/22/2024 at 3:11 PM, Broetchenholer said:

The problem however starts when i am doing step 4 and 5. even without rcs on and sas only on stabilize the target speed and direction of it's speed is wobbling, so the direction is circling around as if one of the capsules was rolling somehow, but both are stable with sas. also, apparently the relative speed is also varying between 1.5 and 4.

Do you set the target docking port to target at your crafts docking port and ask it to point at target?

If the target is moving - your relative speed will be moving too. which will make the prograde ocolate.

I often find that the best thing you can do is, make it point at target, then hit the lock so it stops correcting itself and then I just fly closer to the target with RCS thrusters - once I am in a better position I kill the relative speed, hit "point at target" and then lock and rins and repeat. 

I find that if SAS is on and you set the craft to point at target.. the fact that It uses the RCS to fix the aiming means it often spirals its self out of control.

I made a guide myself at some point to a similar question... I can try see if I can find it?

Edited February 4 by BechMeister