relays constellation drift over time

[antipro]

hi, I have to splash an unmanned probe on eve.
First I need a complete signal coverage, so I'm planning to try to launch a 3 relays constellation into an equatorial 40000Km orbit.
Before I embark myself into this complicated(at least for me) mission, I'd like to know what accuracy must I have,
in order to avoid that after many years the constellation no longer works due to differences in its 3 orbital periods.
If, at that altitude, the minimum possible RCS burn, changes the orbital period by 200ms,
is this difference enough to prevent orbital drift from making the constellation unusable after, for example 400years or more?

[Blaarkies]

- Use adjusted throttle settings
- Point the craft half way into any of these directions () to get smaller changes in orbital period. It will affect your inclination/eccentricity of course, but only by a small amount.

So point forward() and then point some 60° away, and make a tiny burn

[Zhetaan]

@antipro:

The short answer is yes.  The longer answer is that it depends.  For extremely high-period orbits, the amount of 'perturbation' (it isn't, really) is lessened when the 200 milliseconds is such a tiny fraction of the overall orbit.

A 40,000,000 metre circular orbit of Eve has a semi-major axis of 40,700,000, for a period of

2π * √ [(40,700,000³) / (8.1717302 x 10¹²)] = 570,708.6729 seconds, or 26 days, 2 hours, 31 minutes, 48.6729 seconds.

This is over the circumference of 255,725,642 metres.  If we make a pessimistic assumption that one of your satellites is exactly correct and the other is exactly 200 milliseconds less, that gives a distance loss of 89.6 centimetres per orbit.  If we assume that the distance to the coverage overlap is also the minimum distance for loss-of-signal, then that means that you need to lose one sixth of the orbit, or 42,620,940.3 metres, before you have a signal loss.  That requires 47.5 million orbits.  At 26 days (or a little over) per orbit ...

...

...

I don't know about you, but I am going to make a snack while we wait.

Edited November 3, 2020 by Zhetaan

[Vanamonde]

Since KSP is a simulation, there is a tiny bit of imprecision in KSP orbits which cannot be eliminated. Every orbit in KSP will alter slowly over time. The only remedy would be to use a mod which performs corrections in an on-going way. 

[Caerfinon]

There is a mod called Station keeping that helps with drift reduction

 

[Spricigo]

Mind you it may as well be a moot point.

The thing is that even if you get the orbital period synchronized to such precision, it may suffer from rounding errors when the orbital parameter values get registered into the save file.

This is often the reason why player go for a "good enough" and,  if they really want to maintain the synchronicity, use a mod or save edit to zero out the final difference, while others just send redundant satellites (me included).

[antipro]

ty all, I was trying to perform the constellation and I've done it with a lot of patience.
It isn't perfect, I stll have to modify inclinations and orbit periods.
I'll try to redo it better tomorrow or so.
some errors have occurred especially with Phase Angles. it's very difficult if not impossible to match 120,000°.
also orbit periods are a bit out of phase.
after 5 or 6 hours I was so tired that I close ksp with alt+f4 in the middle of the game and I didn't finish to adjust them.


anyway I would like to know a thing:
is there a simple way to reduce the thrust power of rcs thruster rv-105?
cause I have 3 on each relay that is pretty light and even if I set them up to 0.5 value and I use the CAPS LOCK key, I mean "Toggle precision ("softer") controls",
the total thrust is always too much for precision adjustments.

this is the relay


 

Edited November 4, 2020 by antipro

[The Flying Kerbal]

I see you have Kerbal Engineer installed.  You can have it set up to show you the exact period of a vessel's orbit, down to - I think - 1/1000th of a second.  It's a great tool making what you're trying to do so very much easier.

To calculate the correct orbits to aim for, I can't do any better than recommend a video by Mike Aben, here's a link to it on Youtube: https://www.youtube.com/watch?v=xNiFcI-fcmA.

As for further reducing the power of your RCS thrusters, have you tried disabling two of them?  With the thrust set to 0.5 coupled with CAPS LOCK on, there's a very good chance the reaction wheel will be able to keep your satellites pointing in the right direction.

Edited November 4, 2020 by The Flying Kerbal
Spelling Mistake

[HansAcker]

9 hours ago, antipro said:

some errors have occurred especially with Phase Angles. it's very difficult if not impossible to match 120,000°.
also orbit periods are a bit out of phase.

The orbital details don't have to match exactly. The most important value is the "orbital period". As long as those are mostly the same, the satellites should stay in their relative positions over many orbits.

9 hours ago, antipro said:

after 5 or 6 hours I was so tired that I close ksp with alt+f4 in the middle of the game and I didn't finish to adjust them.

What I do for important constellations outside Kerbin (and don't consider cheaty) is to synchronize the sat's periods as close as possible, then edit the save file to give them all the same SMA.
There's also a mod for that:

 

[antipro]

1 hour ago, The Flying Kerbal said:

I see you have Kerbal Engineer installed.  You can have it set up to show you the exact period of a vessel's orbit, down to - I think - 1/1000th of a second.  It's a great tool making what you're trying to do so very much easier.

well ok.. the penultimate ss I've sent was to show Orbital Periods that vary between 655ms and 691ms and Phase Angles.

 

1 hour ago, The Flying Kerbal said:

To calculate the correct orbits to aim for, I can't do any better than recommend a video by Mike Aben, here's a link to it on Youtube: https://www.youtube.com/watch?v=xNiFcI-fcmA.

thx for that video but for me is like hieroglyphs, I suck with both Math and English.
what I do is to use MJ resonant orbit maneuver planner for an approximate 2/3 orbit.
MJ is precise when creates the node, a bit less when burns so after the MJ burn,
I slightly adjust the Pe with RCS to the exact value taken from MJ node and double checked on this website:
https://meyerweb.com/eric/ksp/resonant-orbits/
that is Eve (KSP Stock) 3-satellite constellation
Ap 40.000.000 m  Pe 20.719.826,2 m Δv 75.98 m/s

 

1 hour ago, The Flying Kerbal said:

As for further reducing the power of your RCS thrusters, have you tried disabling two of them?  With the thrust set to 0.5 coupled with CAPS LOCK on, there's a very good chance the reaction wheel will be able to keep your satellites pointing in the right direction.

thx for the tip, I though about it: to use 1/3 of the thrusters makes every adjustment 66% more precise.
I have no doubts that Caps Lock and Gyroscopes will stabilize the sat but it's bad looking to see 1 of 3 rcs burns, it's asymmetric isn't it?
 

thanks  @Caerfinon and @HansAcker for that mod, I'll take a look at it.

Edited November 4, 2020 by antipro

[The Flying Kerbal]

I've never used MJ so can't comment on it.

The maths in the video looks absolutely terrifying, I nearly ran a mile when I saw it!   However it got to the stage where I had to just sit down and tackle it.  It took me a little while and many mistakes were made, but I finally got the grasp of it, now I can figure out how to calculate what orbit I need for a specific reason.  I highly recommend you give it a try, believe me, if I can get it, anyone can!

As for the solution to reducing the power of your RCS, actually it won't be noticed unless you really go out of your way to spot it.  The thrust is set so low there is almost nothing to be seen coming out of the RCS nozzle.  And anyway... you're in space - there's nobody there to see it!

Well that's how I would do it, but I suspect there are people here who are WAY better than me at KSP, maybe they have a more elegant solution for you.

[antipro]

I think I found a workaround for reduce the RCS Thrust less than 1.0KN.
Obviously with "Toggle precision ("softer") controls" and the slider on 0,5 it seems it works,
I mean it's possible to match a 40M*40M eve orbit with few lucky tries.
The trick is to add another 2 rv-105, over the probe core, on the adapter, that is already inclined by about 60°.
I think the inclination reduces the thrust by a small amount, I still didn't reach the accuracy of 1m, but is better than before:
in that 40M orbit a fast H/N keys pressure move the Pe by 40m or less.

So I start with the 3 rcs enabled at 100% to make circularization fast, and the other 2 disabled at 0.5%,
after circularized I just press an Action Group key that toggle both the rcs groups. this way only 1 of 2 rcs group is enabled at time.
after an orbit when I'm again on the Ap I do the fine tuning with the 2 rcs group.
isn't so nice to see it, of course absolutely inefficient and maybe a bit insane but at least it works.
this image explain it well:

Edited November 5, 2020 by antipro

[The Flying Kerbal]

Nothing's insane if it works! 

[Snark]

On 11/4/2020 at 9:28 PM, antipro said:

I think I found a workaround for reduce the RCS Thrust less than 1.0KN

For making extremely fine-grained adjustments to orbital period, of course it's good to have the thrust as tiny as possible.  However, you can also reduce your effective thrust by a factor of 10 or more just by pointing your ship in the correct direction. 

The idea is to leverage cosine losses, same as you've done with your angled-mount RCS thrusters here.  But instead of angling the thrusters on the ship, you simply angle the whole ship.

Longer explanation is below, but the TL;DR is, don't point your ship or .  Instead, point it just barely on the or side of (or one of the other three cardinal directions).  This will give you a massive cosine loss that allows making very fine-grained adjustments to your orbital period.

Lengthy explanation in spoiler.

Spoiler

Here's the deal:

The most efficient way to adjust the size of an orbit (i.e. get the biggest change for the least amount of thrust) is to point perfectly to make the orbit bigger, or perfectly to make it smaller.

The least efficient way is to point 90 degrees away from or .  If you were pointing perfectly or or or , then your effect on the orbit period would be basically zero.

So, if your goal is to make a super-tiny adjustment to the period of your orbit, you can leverage this.  Deliberately make your burn extremely inefficient by pointing almost-but-not-quite in one of those 90-degrees-off directions.

For example, suppose your current orbital period is a few milliseconds too long, and you want to make it shorter.  If you pointed your craft directly , then your thrust will be 100%, which is bad for you because that's likely too powerful and you'll overshoot-- it's hard to make an adjustment that's fine enough.

So you don't.

Instead of pointing ... you point nearly (or or or , but let's say for this example).  You don't point exactly .  Instead, you point juuuuust slightly to the side of , like by a degree or two.

Suppose you can manage to point about 2 degrees on the side of .  That means you're 88 degrees off .  The cosine of 88 degrees is about 0.035, or pretty close to 1/30th.  Bingo, you just effectively reduced your period-changing thrust by a factor of 30! 

And if you can manage to get even closer to than 2 degrees, the granularity gets even better.

 

I use this method all the time, and it works great.  I don't use KER myself, but I do use BetterBurnTime, which provides a feature that shows (to the millisecond) how far off your orbital time is from another target's.  My comsats usually have some little engine on them like a Terrier or Spark.  I just reduce the thrust limiter down to the lowest it'll go above zero (0.5%, IIRC), then use this cosine method I describe here.  Works like a charm, and I can pretty easily adjust the orbital period to within 1 millisecond.

[antipro]

ah! ok.. "cosine losses", that's what @Blaarkies was talking about in his first answer, now I've got it, very well explained.
Thanks you both!

also, what I have noticed in my last try is, like @Vanamondeand @Spricigo said:
whatever thing I do, i.e., to warp or save/load, " there is a tiny bit of imprecision in KSP orbits which cannot be eliminated" and
"it may suffer from rounding errors when the orbital parameter values get registered into the save file. "

In short there is always a small inaccuracy, inherent in the game itself.
Got it. Thanks both!

Edited November 7, 2020 by antipro

[Blaarkies]

It seems like 1.11 will solve your problems with 1/10th size RCS blocks  
 

 

Edited November 9, 2020 by Blaarkies