Trouble Matching Docking Velocities

[Apollo13]

After dozens and dozens of manual docking failures, all ending with the QuickLoad key to try again, my frustration meter is pegged to max. I've watched a half-dozen videos on docking...I use NavyFish's mod...all to no avail.

Nearly all docking begins with the premise that the docking vehicle has matched velocity vectors (speed and direction) with the target vehicle. That's where I'm having problems. I have SAS on. I may use main engine to catch-up. I use RCS to try to match velocities. Just when I get close to matching (I think), the target slips away or drifts because the docker's X-Y-Z vectors does not exactly match the target's.

Once you can match velocities, such that docker and target are at zero relative velocity to each other, then docking is rather straight forward.

Is there some secret I'm missing? I could use MechJeb, but I want to do without.

Edited August 17, 2013 by Apollo13

[Deejay2000]

I think this should help:

-Deejay

[MrKicker]

If you set the target ship as the target. Once you get close, the speed will show the speed relative to the target. And the navball will show prograde/retrograde vectors relative to the target. So once you get close enuf, you just point retrograde and burn untill the relative velocity is 0.

[blizzy78]

Exactly what MrKicker says. In my "Launch, rendezvous, and docking tutorial" I'm elaborating a bit more on that, in text form. (see signature)

[Scarecrow88]

The important thing with rendezvous manoeuvres is that you try not to rush it. If your target is 20km or less away, and if you are approaching at more than 100m/s, chances are that it will flash past before you have registered how close you are getting or had a chance to slow down.

When you are closing with your target, keep an eye on the target and prograde markers on your nav ball. If the prograde marker is not on the target marker, it means that you are going to drift by and that at some point you will need to kill your closing speed and then burn again towards your target. You may need to repeat this process in decreasing increments until you get close enough to switch to RCS for final docking.

[Apollo13]

DeeJay: thanks for the video. I've seen many of Manley's tutorials, but I missed this one.

Kicker and Scarecrow: I appreciate that info. I'll put it to use.

blizzy: I previously downloaded your video mod, but hadn't tried it yet.

Edited August 9, 2013 by Apollo13

[blizzy78]

blizzy: I previously downloaded your mod, but hadn't tried it yet.

Actually I was talking about the other tutorial this time, second link from above in the signature. The one with video and text.

[Apollo13]

Actually I was talking about the other tutorial this time, second link from above in the signature. The one with video and text.

Oh. I'll have a look at that. Thanks.

[Anton P. Nym]

A rule of thumb I use in rendezvous is the "100 Second rule"; I burn at a target until the relative velocity is 1/100th the distance shown to the target. That gives me a good minute and a half to do the flip-over and orient for a braking burn without taking so long that a) the target has moved so much along its own orbit that the velocity estimate is badly off, and I run out of patience.

Once the 100 Second rule dictates a speed of 1m/s or less, I switch to using RCS and that's when I do my docking approach.

It's probably not optimal, but it's easy to calculate on the fly and it works fairly well.

-- Steve

[Apollo13]

The frustration continues. Over 80+ attempts to dock and I can't match velocities, all ending in QuickLoads. If I start to slow down too early, I drop out of orbit with the target. Sometimes into the atmosphere. Slow too late or off by a degree or two, and the target whooshes by.

The game has become unfun..............

[stringyquark]

Try matching orbits of the start, then if the target is ahead of you go to a slightly lower orbit to catch up, if behind a slightly higher orbit so it can catch up. Keep it targeted the whole time, watch for the encounters to get under 5km. I had allot of issues at first also. Found that my orbit was to far off of my targets, then my burns to match would wildly change my orbit before I got close. Good luck, took me a while but once you get it it's like riding a bike! :-)

[Apollo13]

Here's a short video of my latest failure. You'll note that at closest approach, I'll be at 550 meters. However, I'm speeding towards the target at over 900 m/s. I turn to slow my approach speed. That is, I goto retrograde to the target. As I apply thrust, my periapsis drops and my closest distance rises dramatically. Were I to apply more thrust, I would enter the atmosphere.

What do I need to do? I can't burn towards the target, because that would only exacerbate the approach speed problem.

http://www.rickdomain.com/ksp/ksp_whatnow.mpg

[Read have Read]

Here's a short video of my latest failure. You'll note that at closest approach, I'll be at 550 meters. However, I'm speeding towards the target at over 900 m/s. I turn to slow my approach speed. That is, I goto retrograde to the target. As I apply thrust, my periapsis drops and my closest distance rises dramatically. Were I to apply more thrust, I would enter the atmosphere.

What do I need to do? I can't burn towards the target, because that would only exacerbate the approach speed problem.

http://www.rickdomain.com/ksp/ksp_whatnow.mpg

No no nooooo! Don't burn there! That isn't Target Retrograde, that is Pro-Target Position! Burn at the Triangular YELLOW marker - the opposite side of the Navball.

And since you have MJ2, set up a "Target Info" Window, put Target Distance, Relative Vel. to Target, Relative Vel. at closest approach , Time to closest Approach, Separation distance at closest approach on one window. When you are burning right your closest approach and Relative Velocity at closest approach will decrease (but your time to rendev will increase as you are going slower).

Don't feel bad though, even NASA had trouble with rendezvous mechanics during Gemini 4

Edited August 13, 2013 by Read have Read

[Apollo13]

Read have read: thank you!!!

I've also revisited blizzy's tutorial on the forum and understand considerable more(since my previous post). I actually closed in properly on my target. Still lost it and could not get close enough to dock, though.

BTW. I'm using MJ2 Rendezvous Planner/Interception to help plan, but not execute/autopilot.

[Mr Shifty]

Here's a short video of my latest failure. You'll note that at closest approach, I'll be at 550 meters. However, I'm speeding towards the target at over 900 m/s. I turn to slow my approach speed. That is, I goto retrograde to the target. As I apply thrust, my periapsis drops and my closest distance rises dramatically. Were I to apply more thrust, I would enter the atmosphere.

What do I need to do? I can't burn towards the target, because that would only exacerbate the approach speed problem.

Yeah, you're burning toward the target in that video. The anti-target marker looks like a triangle cross-hairs. It's useful to understand when you burn your engines, you are going to "pull" your prograde marker toward where you're pointing and "push" your retrograde marker away from where you're pointing. To rendezvous:

1) First get an intercept to within 20 km or so. To do this, first get into an orbit that is the same as your target's, most importantly with the same orbital inclination. Then, if your target is "ahead" (counter-clockwise) of you, you need to go faster to catch up, so lower one side of your orbit slightly. Conversely, if your target is behind you, raise one side slightly. You're looking to get those closest approach markers to within 20km. It might take a few orbits, but you'll get there. Be careful not to dip into the atmosphere.

2) Once you've approached within 20 km or so, make sure your NavBall is in target mode and check your relative speed. I use the 100x rule as well, so relative speed at 20km should be 200 m/s. If you're going too slow, you should point near prograde to speed up and if you're going too fast you should point near retrograde to slow down. Note that after you establish the initial speed, you'll probably be slowing down for the rest of the approach.

3) What you want to do is either "pull" your prograde marker onto the pro-target marker or "push" your retrograde marker onto the anti-target marker. You'll have to continuously adjust as you approach because orbits are round.

4) If you get going too fast or the rendezvous gets out of control, just point directly at your retrograde marker and fire your engines until your target relative velocity is zero. Then point at the target marker and burn toward it to start the process again.

Once you're within 200 meters, using the docking alignment indicator.

[Read have Read]

Read have read: thank you!!!

I've also revisited blizzy's tutorial on the forum and understand considerable more(since my previous post). I actually closed in properly on my target. Still lost it and could not get close enough to dock, though.

BTW. I'm using MJ2 Rendezvous Planner/Interception to help plan, but not execute/autopilot.

What I mean is the information that mechjeb could be more useful. You can create new info panels with whatever information you want, and there are quite nice readouts under the "Target" category yet there is no default Target Info panel. I find it very useful to know my speed and distance to target now, and my speed and distance at my closest approach as well as how long before I get there. With a custom Target Info window and SASS set to TGT mode to point the craft at the markers it makes docking much less tedious.

I don't trust the Automated Rendezvous and Automated Docking however. I used the former once and was quite silly and I used the latter once and it was decent, but I could have done better.

[HeadHunter67]

The frustration continues. Over 80+ attempts to dock and I can't match velocities, all ending in QuickLoads. If I start to slow down too early, I drop out of orbit with the target. Sometimes into the atmosphere. Slow too late or off by a degree or two, and the target whooshes by.

The game has become unfun..............

I feel your frustration. I too have tried getting the hang of rendezvous - my only success wound up using far too much fuel to return safely. I know how to match relative velocities, but I usually wind up with the same issues. I'm not about to give up on the game because of it, but I think manual rendezvous will have to wait.

[HeadHunter67]

first get into an orbit that is the same as your target's, most importantly with the same orbital inclination. Then, if your target is "ahead" (counter-clockwise) of you, you need to go faster to catch up, so lower one side of your orbit slightly. Conversely, if your target is behind you, raise one side slightly.

I was with you right up until this point. Could you elaborate? Because I'm not understanding what you mean by "raise" or "lower" when you speak of the "same orbit".

If you get going too fast or the rendezvous gets out of control, just point directly at your retrograde marker and fire your engines until your target relative velocity is zero. Then point at the target marker and burn toward it to start the process again.

...Aaaaand, this is how you wind up with an "orbit" that terminates below atmosphere. I managed to turn a 300km circular orbit into a sub-atmo orbit this way yesterday. I call this process "jousting" - it's horribly wasteful of fuel and is guaranteed to drop your orbit below the target.

Docking's not a problem for me - trying to rectify all the (sometimes contradictory) advice I'm given is what's hanging me up. I'm sure this all seems clear to people who think rendezvous is easy (just as I think docking is easy)... but there has got to be a more sensible and fuel-efficient way to cancel relative velocity and still end up close enough to the target to finish via RCS. I know where to point and what to do - the problem seems to be in the "when", and nothing I've seen anywhere has answered that question for me.

[MockKnizzle]

I feel like your problem is rendezvous, not docking.

So, first question: Do you know how do a Hohmann transfer?

[Specialist290]

Docking's not a problem for me - trying to rectify all the (sometimes contradictory) advice I'm given is what's hanging me up. I'm sure this all seems clear to people who think rendezvous is easy (just as I think docking is easy)... but there has got to be a more sensible and fuel-efficient way to cancel relative velocity and still end up close enough to the target to finish via RCS. I know where to point and what to do - the problem seems to be in the "when", and nothing I've seen anywhere has answered that question for me.

Well, if you're talking about theory, then the ideal rendezvous approach would essentially be a Hohmann transfer between your current orbit and the target orbit such that you arrive within spitting distance of the target itself at the moment you do your orbital insertion burn. It's a bit hard to pull off just by eyeballing, though.

Then again, I'm not exactly an expert myself.

EDIT: ...Aaaand I'm ninja'd yet again.

[Anton P. Nym]

I was with you right up until this point. Could you elaborate? Because I'm not understanding what you mean by "raise" or "lower" when you speak of the "same orbit".

Start by adopting roughly the same orbit as the target; same Ap/Pe, same inclination, same direction. (Needs saying...) Then figure out whether you're ahead or behind your target; if you're ahead and the target's behind, raise your Ap a couple of kilometers; if you're behind and the target's ahead, lower your Pe a couple of kilometers. After that, it's a matter of waiting until you get an encounter of a few kilometers showing up on the map.

...Aaaaand, this is how you wind up with an "orbit" that terminates below atmosphere. I managed to turn a 300km circular orbit into a sub-atmo orbit this way yesterday. I call this process "jousting" - it's horribly wasteful of fuel and is guaranteed to drop your orbit below the target.

You may be trying to rendezvous on too distant an encounter, then. Burns shouldn't be hard enough to de-orbit, particularly from a fairly safe 300km orbit. If you've got an encounter off by more than about 20km, I'd recommend just playing with small changes in your Ap/Pe in Maneuver nodes to nudge the encounter closer before doing a direct approach.

-- Steve

[HeadHunter67]

the ideal rendezvous approach would essentially be a Hohmann transfer between your current orbit and the target orbit such that you arrive within spitting distance of the target itself at the moment you do your orbital insertion burn. It's a bit hard to pull off just by eyeballing, though.

It's a bit hard to pull off even with the maneuver nodes - since they're so finicky that if your mouse pointer's off by a pixel when you slide them, it deforms the thrust vector in an undesired way - or closes the node... neither of which will help when time is tight.

I understand the basics of a Hohmann transfer - accelerating to raise your orbit to a higher one or decelerating to lower it, that little arc that connects your previous orbit to the new one essentially. That's not the problem. The problem, as I've said before, is knowing when to shed the sudden difference in relative velocity that got you to the new orbit in the first place. This is why Apollo13 and I are having the same issue - too soon and we can never close the gap (because chasing the target would alter the orbit), too late and it sails right by, scant meters away, never to be seen again.

When. When?

[HeadHunter67]

Start by adopting roughly the same orbit as the target; same Ap/Pe, same inclination, same direction. (Needs saying...)

Meaning, an orbit at which an intercept can never occur.

Then figure out whether you're ahead or behind your target; if you're ahead and the target's behind, raise your Ap a couple of kilometers; if you're behind and the target's ahead, lower your Pe a couple of kilometers. After that, it's a matter of waiting until you get an encounter of a few kilometers showing up on the map.

I guess I'm not understanding how this helps - it seems that would defeat the whole purpose of going to the effort of matching the orbit, for the sake of one shot when your orbits intersect again.

You may be trying to rendezvous on too distant an encounter, then. Burns shouldn't be hard enough to de-orbit, particularly from a fairly safe 300km orbit.

No sir. I am talking about encounters at 400 meters or less. I will reiterate that my problem is not an issue of getting close enough to the target - it's a matter of killing the RVEL at the right time so as not to widen the distance (if too soon) or watch the target sail by into the distance (if too late). All the toying around with orbital arcs isn't answering the one sticking point for me in this whole thing. But I thank you for your assistance.

[Anton P. Nym]

Meaning, an orbit at which an intercept can never occur.

Yes, but it has exactly the same dynamics as the orbit you're trying to match. From that basis it's much easier to make the slight changes in orbit that make rendezvous humanly possible, as opposed to a task that'd trouble computers. (Ballistic missile defense still ends more in "miss" than "defense", even 20 years after Patriot.) Ideally your encounter should end up with a relative velocity in the tens of meters/s.

No sir. I am talking about encounters at 400 meters or less. I will reiterate that my problem is not an issue of getting close enough to the target - it's a matter of killing the RVEL at the right time so as not to widen the distance (if too soon) or watch the target sail by into the distance (if too late).

How much RVEL are we talking about? As I noted before, we should be talking double-digit m/s needing only short burns. If we're talking much higher RVEL then I can certainly understand why you're having the same trouble SDI/"Star Wars" did. Matching orbits, then skewing them slightly for intercept, will take longer but in the end make that "one shot when your orbits intersect again" a much, much easier shot to hit with. (And it makes a miss a lot easier to recover from, because a difference of a few meters per second is easy to fix.)

-- Steve

[MockKnizzle]

What exactly is your relative velocity when you're at closest approach?