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[support] Win64 / v1.6.1 - Delta-V to Target in Orbit is wrong !


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KSP Version: 1.6.1.2401 Windows64 - build id = 02401 - 2019.01.08 at 12:56:12

What Happens: I try to rescue some Kerbals from Orbit. So i must at last make some maneuver to set the Delta-V between the Vessels to zero.

This is (most times) not possible, because the Delta-V shown on the nav-ball is wrong. For example : If delts-V = 11 it shows 20 ... if its 100 it shows 500 or 600

  Its complete impossible to handle a rendezvous, if i cant match their speed-difference.

Mods / Add-Ons: playing vanilla

Steps to Replicate: check my savegame and vessel

Result: KSP is at this time not playable for me

Fixes/Workarounds: found nothing

Other Notes/Pictures/Log Files: https://workupload.com/file/73FnzTPU (Delta-V-Error.zip (10.51 MB))

contains:

- output_log.txt , buildID.txt , DxDiag.txt , KSP.log

- Delta-V-Error.jpg ( edited Screenshot )

- Delta-V-Error.avi & Delta-V-Error2.avi ( shows the complete situation )

- Rettung-1.craft (my Vessel) & persistent.sfs (situation in Orbit)

Edited by Randalf
forgotten 2 Files zu explain
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Hi @Randalf, and welcome to the forums!  :)

Sorry to be a bother, but perchance could you post any screenshots here, directly in this thread?  (Just upload them to some public image-hosting site such as imgur.com, then right-click on the image on its hosted page, choose "Copy Image Location", and paste that URL here.)

Would make it easier for those of us who aren't in a position to download your zip file (e.g. on mobile) to look at what's happening and to help you.

It's hard for me to picture what your problem is because I'm having trouble understanding what exactly you mean.  For example,

2 hours ago, Randalf said:

So i must at last make some maneuver to set the Delta-V between the Vessels to zero.

That may be correct, or it may be very very wrong, depending on exactly how you mean it.  Yes, you need to make their relative velocity zero, when they're next to each other.  (Not "delta-V", that means something different.)  But you need to do that at the point where they're right next to each other-- if you're trying to do that when they're not together, it won't solve anything and may very well make matters worse by spending fuel in a way that doesn't help you.

TL;DR:  orbital rendezvous is a skill that involves several different steps-- for people who are new to KSP, it has a bit of a learning curve.  I'm not sure how much experience you have, here.  If you could show some screenshots of what you're trying to do, we could probably offer better advice.  ;)

2 hours ago, Randalf said:

This is (most times) not possible, because the Delta-V shown on the nav-ball is wrong. For example : If delts-V = 11 it shows 20 ... if its 100 it shows 500 or 600

Again... not quite sure what you mean.  When you say "Delta-V shown on the navball", what exactly do you mean?  Which number, shown in which place?  (There are a lot of numbers there, just want to make sure we're talking about the same thing.)  Also, how do you know it's "wrong"?  i.e. when you say "if it's 100 it shows 500 or 600" ... what do you mean by that?  It shows 500 or 600 where, exactly?  And when you sauy "it's 100", that sounds like you think you know the actual correct number-- where are you getting that from?

TL;DR:  it's easy to misinterpret what the navball is actually saying, for folks who are new to KSP.  Just want to make sure we're on the same page.  ;)

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Ah, okay.  Yes, this isn't a technical-support issue, it's a gameplay question.  Moving to the right forum section.  ;)

RlYsx2k.jpg

So, there are a couple of problems here.

Problem #1:  That's not what the readout means on the navball.  It means something different than you think.

The first part is that you're trying to do scalar arithmetic, but velocity is a vector quantity.

If I say "I'm going 2000 m/s, and you're going 1990 m/s, what's our relative velocity?" the answer could be anywhere from "10 m/s" to "3990 m/s", depending on what direction we're going.

Those two ships aren't moving in a straight line.  They're going in circles (well, ellipses actually, but you know what I mean).  That means that two ships that are traveling at the same speed will have very different relative velocities.

So there's absolutely nothing wrong with the navball display-- it's showing correct information.  It's just that you're misinterpreting what it means.

The "relative velocity" display (not "delta-V", that's something else) is showing your velocity relative to the target.  Since you're both zooming along at over 2000 m/s, that means even a fairly small difference in their direction (i.e. one is slightly farther along the curving path than the other) will translate to a pretty big difference in the relative velocity.

Executive summary:  The navball's "relative velocity to target" display isn't useful when you're far apart, so don't use it then.  You care about the relative velocity when you're close, e.g. just a few kilometers, not 57 km apart as in your screenshot.

 

Problem #2:  That's not how you do an orbital rendezvous.

Yes, to rendezvous with someone for a rescue, you do in fact need to make your relative velocity zero (or something pretty low, anyway) so that you don't just go zooming past each other.  But you need to do that when you're right next to each other.  Zero relative velocity when you're still many kilometers apart won't really help you at all (in fact, quite the opposite).

That's not how orbital mechanics work.

 

In short:  There's nothing wrong with your KSP, everything appears to be functioning correctly and as designed.  Rather, the problem is that you're still learning how to do an orbital rendezvous (and it sounds like you might be going about it the wrong way).  That's a whole topic that we can discuss at length, if you like.  ;)  Or you can search the forum for "rendezvous" (lots of people ask about this, since it's such an important skill and has some learning curve).

If you're experimenting by tweaking a maneuver node, don't try to make your relative velocity zero at the node.  That's, 1. virtually impossible, and 2. wouldn't help you even if you succeeded, because that's not what you need to aim for.  Rather, what you're trying to do when you work with a maneuver node is to get a close encounter with the target at some future point (the little orange "target intercept" markers that appear)-- make the encounter distance as close as you can, e.g. under 1 kilometer if possible.  Then, wait until you get next to the target at closest approach, then just set your SAS to :retrograde: relative to target and thrust to zero relative velocity then.

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Thanks for your quick answer. First i must apologise my week english, its 40 jears ago i learnt the basics, but i see i must better explain what i mean.

Before i bought KSP, i was watching the Youtuber wintermancer , he made a tutorial with 30 episodes - from nothing to Minmas. Otherwise KSP would be more "hard Work" than fun. He also made at episode#5 a tutorial about space-recue KSP Karrieremodus - Tutorial - #05 SOS Rescue  . I've seen more than 15 times, because i feared to be confused with the symbols.

Overall i have ~50h playing KSP and i went on Mun and had dozens of missions, tourists on Kerbal, tourists to Mun, satellite and something else.

But now im stuck, because i've 9 open contracts for recue. 6 ( from 8 ) on Kerbal, 3 on Mun, funnily enough i could rescue 2 Kerbals from orbit of Kerbal. So you may ask - Hey Guy, whats your Problem? Two times it worked ...

... and that exactly is the problem. Two times everything worked exactly as described in the video ( and you explained at the end of your post ), but more than 20 times since there, it doesnt. Each time i start, i do the same. Build a similar orbit with the target, but a little bit deeper. Then i start the approach. Two times it worked as discribed, 20 times the navball shown funny numbers and the orbit i created with this missinformation was "interesting-looking".

So i was asking myself ... whats going on here?

Edited by Randalf
typing errors
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One thing that would make this a whole lot easier is an "overhead" shot, from above Kerbin's pole so that the orbit is better-displayed.  As-is, you're not close enough and need to catch up more, beyond that's difficult to say.

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Hello @Randalf welcome!

How is your trigonometry? I admit that I'm not skilled in the mathematics, but I can help explain why this problem is more complex than you are imagining.

Sometimes it's helpful to take an example and push it to the most extreme limit to see where the underlying problem is.

Imagine that you have two ships orbiting the planet in coplanar (parallel) equatorial orbits at the exact same altitude. Because their orbits are identical they are going exactly the same speed, let's say 2000 m/s. If they were traveling in the same direction their relative speed would obviously be zero (0). If they were traveling in opposite directions their velocities would combine so you add them together for a relative velocity of 4000 m/s.

Now, imagine that one of the ships is on an equatorial orbit going 2000m/s. And the other ship has the same orbital altitude and speed but its orbit is inclined by 90 degrees and is on a polar orbit. They both have the same shaped orbits, the same altitude, and the same velocity. But you can NOT simply add or subtract their velocities to find the relative velocity. It's somewhere in between.

v5dx2dd.png

Now what about this case?

gJzQD6g.png

In this case, the data shown in the orbital information is the same: 2000m/s for both ships, same apoapsis, same periapsis. The orbits intersect and you can see that the ships will be in the same place when they converge so they could hit each other. There must be some positive relative velocity; it will be lower, but not zero. But you can't subtract 2000 from 2000 and find the actual relative velocity for orbits that are inclined in respect to each other. You need more information than just what is presented on the screen.

In your case, there will always be some small difference in inclination or some small elliptical difference, and the point where those ships are in their orbits also changes the outcome.

Edited by HvP
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  • 3 weeks later...
On 3/5/2019 at 11:03 PM, Randalf said:

Thanks for your quick answer. First i must apologise my week english, its 40 jears ago i learnt the basics, but i see i must better explain what i mean.

Before i bought KSP, i was watching the Youtuber wintermancer , he made a tutorial with 30 episodes - from nothing to Minmas. Otherwise KSP would be more "hard Work" than fun. He also made at episode#5 a tutorial about space-recue KSP Karrieremodus - Tutorial - #05 SOS Rescue  . I've seen more than 15 times, because i feared to be confused with the symbols.

Overall i have ~50h playing KSP and i went on Mun and had dozens of missions, tourists on Kerbal, tourists to Mun, satellite and something else.

But now im stuck, because i've 9 open contracts for recue. 6 ( from 8 ) on Kerbal, 3 on Mun, funnily enough i could rescue 2 Kerbals from orbit of Kerbal. So you may ask - Hey Guy, whats your Problem? Two times it worked ...

... and that exactly is the problem. Two times everything worked exactly as described in the video ( and you explained at the end of your post ), but more than 20 times since there, it doesnt. Each time i start, i do the same. Build a similar orbit with the target, but a little bit deeper. Then i start the approach. Two times it worked as discribed, 20 times the navball shown funny numbers and the orbit i created with this missinformation was "interesting-looking".

So i was asking myself ... whats going on here?

Hi Randalf,

happy to hear my tutorials helped you so far. If you are stuck and not getting any further feel free to contact me either here or directly over YouTube. I'm always happy to help - and also happy to help in your native language 

 

Best

Winter

 

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