Is it possible to keep relative distance between two objects in the same orbit?

[AceMgy]

What I'm trying to do is to build two space stations, one for fuel storage with minimal amount of crew, and then my main space station for long-term habitation. They'll ideally share the same orbit but 10 km apart for safety reasons in case something goes wrong on the fuel station (in reality I'm trying to reduce lag). Such a distance requires a minimal amount of delta-v and time to move between them, and I'll be shuttling between them a lot as the habitation station will be the launching-off point for all my interplanetary missions. Tugs will ferry fuel to my habitation station (KSS) from my fuel depot (DVS), then kerbals will load up into the ship and go.

Trouble is I can't accurately match orbits. Best I've done is 10 km apart and 27 m/s relative velocity between them. This just won't work because time-warping for any significant amount of time will separate them quite quickly. To aggravate the situation, floating point errors in the orbits shift the apoapsis and periapsis by more than 1 m.

Is there a better way to arrange these stations in KSP? Perhaps some resonance so there'd be a transfer window between them every 2 to 3 orbits or so? I'm just brainstorming here.

Or is the simple answer that this just isn't really possible without some sort of active orbit correction in KSP?

Edited June 29, 2013 by AceMgy
Answered

[capi3101]

To get those to stay relatively close to one another, their orbital characteristics would have to math exactly (apoapsis and periapsis in the exact same spot and the exact same altitude, both orbiting at exactly the same speed). That would be ridiculously difficult to do by hand - not impossible, mind you, just really, really hard.

Here's an instance where I'd advocate the use of Mechjeb. And I don't use Mechjeb, so I don't know if it could actually do the job or not.

[duckofthemoon]

Actually, it's nearly impossible. when you timewarp, conversion of units means there is always a slight jump. this mean they aren't rock solid, but they won't drift apart very fast. try using rcs to reduce relative speed.

[AceMgy]

Here's an instance where I'd advocate the use of Mechjeb. And I don't use Mechjeb, so I don't know if it could actually do the job or not.

I only use Mechjeb for numbers now on this playthrough (last save I let it do everything, not as fun). I did resort to trying it to see if it could get the 0.01 km accuracy I needed. Nope, it did worse.

Regardless, I don't think it would be stable even if you got the perfect numbers, you can see your apoapsis and periapsis shift a little bit when you're focused on a craft out of timewarp. I think this is another one of those instances where we could wish for 64-bit floating point values in KSP but it'll never happen.

I was just wondering if anybody managed to do something similar, either with a mod or just through some clever trick...

[Mr Shifty]

I'd guess this is extremely difficult to do in LKO because of the high speeds there. You're going 1km/s, so even a 1 second difference in the orbital periods between the two would result in an extra kilometer of separation per orbit. With MechJeb, you can see your relative velocity down to tenths of a millimeter per second, but you can realistically only adjust it down to 10 mm/s or so. Even with that relative speed, you're looking at a kilometer per day or so.

Edited June 28, 2013 by Mr Shifty

[capi3101]

Hmm.......only other thing I can think to do then is to do a manual edit of the persistence file. Somebody with more expertise than I have would have to walk you through the specific changes that would need to be made.

[AceMgy]

Just edit the persistence file when you notice they're getting a tad too far apart? You know I like that idea, it's not really cheating because real spacecraft would have active orbital correction like the ISS does, and this would be the closest that KSP could emulate that.

But I'd have to be careful with it, as it seems it would be really easy to screw up your save file.

Edited June 28, 2013 by AceMgy

[N_Molson]

This is theorically possible if you match exactly the orbit of your target. It means an Rinc of 0° and the exact same orbital elements (Apa, PeA, Period...). In the real world it is impossible, because the Earth is not a perfect sphere and has not a perfect mass distribution. Even worse, the Moon has a noticeable tidal effect. In KSP, timewarp and precision limits will probably make it very difficult.

[Sirrobert]

I love your idea, but other than editting the save file to get theyr orbits to be exact, I can't see it happening.

Or maybe use tons of RCS fuel to constandly adjust the orbit, but than your game would turn into a station maintenance job

[Wjolcz]

Naaaah. Mods and cfg editing teach you how to be lazy and you'll simply get bored without any satisfaction of making something yourself. You should do everything stock just as I do. Simply put the stations in the same orbit but in different places and you'll be fine. You can always use RCS to correct the orbits.

It's not that hard as some people say.

OR: You can put the stations into such orbits it will be easy to move between them with minimum dV. One should be on a lower orbit, another on higher.

Edited June 28, 2013 by Veeltch

[Sirrobert]

Naaaah. Mods and cfg editing teach you how to be lazy and you'll simply get bored without any satisfaction of making something yourself. You should do everything stock just as I do. Simply put the stations in the same orbit but in different places and you'll be fine. You can always use RCS to correct the orbits.

It's not that hard as some people say.

OR: You can put the stations into such orbits it will be easy to move between them with minimum dV. One should be on a lower orbit, another on higher.

Really? So I asume you reguarly finetune your apoaps/periapses to within 1m than? Cause that's the level of precision you need if you want 2 crafts to stay close to eachother.

If you put them in DIFFERENT orbits, they are eventually going to drift apart. Won't be long until they are on opposit ends of the planet. You call that easy to move between?

And you know what's boring? Launching the same ship 20 times over. I'll just let mechjeb to do that for me, thank you ver much

[AceMgy]

Anybody know how to safely change the persistence file to do this? That sounds like the best plan.

(I'll ignore the stock vs. mods debate and just say I'm playing the game how I want. I'm having lots more fun Kethane mining and mapping than I ever did when I was all-stock like you Veeltch, to each their own. )

[capi3101]

Here, this might be helpful. The Google Doc it links to looks promising...especially the "Placing Object near Other Objects in orbit" bit.

Might want to do more than just skim that my own self...

[SDIR]

In the persistence file, there are sections labelled "vessel" and what ever your vessel is named. Now, locate both your vessel names within the persistence file and try to find the orbital characteristics of one (should have a line in each vessel for the apoapsisl periapsis, inclination etc.) now this is the tricky part, and the part I haven't done before. Copy the apoapsis, periapsis and inclination and what ever is needed to match the orbits, but do not change what ever determines the position of the two vessels relative to Kerbin. If the two values indicating their positions are the same, then they will clip inside one another and bad things happen. The lines telling you of the apoapsis ect. should be a few lines below the "vessel" line. After that, good luck, tell us if you successfully do it.

[Greywind]

Even if you edit the persistence file to get two stations in the same orbit, the moment you dock with one of them it's going to make small changes in the orbit, eventually leading to them drifting apart. Is it absolutely necessary to have them separate? Although i suppose you could make some giant truss and dock them together, so it'd be station-long(long)truss-station, assuming it doesn't lag something horrible and/or explode. Anyway, back to the point: yes you can edit the file to make them exact, but that is only exact until you start doing things to them, which is playing the game haha. Keep it simple, make one station, but protect the crew habitation somehow.

[adjit]

If I were you, I would either use the persistence file and give them the same orbit, but Greywind is right. Any slight movement of the craft, even if you tap RCS the orbit will ultimately be doomed to get misaligned. Now, if you were okay with consistently editing your persistence file then by all means.

However, if you didn't want to "cheat" I would get the crafts together as if I were docking, and slow the relative speed to 0 m/s, then little by little I would back away from it(retrograde) and then cancel out the relative speed(prograde) until I was 10km away from the craft.

[SDIR]

That is very true Greywind. More than once I've wanted to dock two ships to the same space station. After getting an inclination or 0 for both and docking one, the inclination almost always changes, meaning the orbit has also changed. Your best bet if you want to switch between two stations is what I do: have one station orbit 100 km above the other, and undock and transfer directly between the two. Docking between two stations in the same orbit is more difficult.

[AceMgy]

Even if you edit the persistence file to get two stations in the same orbit, the moment you dock with one of them it's going to make small changes in the orbit, eventually leading to them drifting apart. Is it absolutely necessary to have them separate? Although i suppose you could make some giant truss and dock them together, so it'd be station-long(long)truss-station, assuming it doesn't lag something horrible and/or explode. Anyway, back to the point: yes you can edit the file to make them exact, but that is only exact until you start doing things to them, which is playing the game haha. Keep it simple, make one station, but protect the crew habitation somehow.

Yeah I must admit much of the reason I'm doing this (besides lag and my tendancy to build big stations) is sentimental. Have you ever read "Privateers" by Ben Bova? It's an entertaining hard sci-fi novel which features two stations very close together, it's what inspired me to get around this lag problem. I'm going to try the persistence file trick and see how often I have to update it (after the first time I imagine it'd be just a simple cut/paste job). If it's too much of a bother I'll just pretend that my fuel station can hold 9 kerbals comfortably (sigh).

[purpletarget]

It's completely possible to get accurate orbits manually, it just requires a bit of patience and forethought. The Ap/Pe's don't need to be exact, just close enough, and mostly it's just that you need the Semi-Major axis to be the same. Of course at 10km separation, that's only about 5 second apart from one station passing overhead to the next, and any orbit will eventually drift a little.

If you really want to avoid the lag, then I'd put them in different orbits, that are at least 4-5km apart in their Ap/Pe, as then you'll never be in range of both at the same time. You can adjust the orbital period to something in resonance, like 40 minutes and 60 minutes, which would give you 2 & 3 orbit windows, but really, how often do you need the windows to happen? 35 & 42 minutes should get a sync every 7 or so.

But as long as your transit bus has a few hundred delta-V for the transfer, even a station on the other side of Kerbin is only one outside orbit to an RV away, allowing a transfer at anytime within an hour or for a daily commute time, in which case all you need is the altitude separation to avoid the lag, and the timing and resonance becomes moot.

[AlexanderB]

If you put them in DIFFERENT orbits, they are eventually going to drift apart. Won't be long until they are on opposit ends of the planet. You call that easy to move between?

Wait some more and they drift back together. In fact, the more space you put between their orbits, the more often they pass each other per 'day', so an opportunity for a transfer burn comes along soon and often enough.

[Spock_Kerbin]

You could put them in exact same orbits but use mechjeb to change the inclination a degree,

then as long as both are circularised correctly and are at the same speed they shouldn't crash and should be minimal distance away

[Wjolcz]

(I'll ignore the stock vs. mods debate and just say I'm playing the game how I want. I'm having lots more fun Kethane mining and mapping than I ever did when I was all-stock like you Veeltch, to each their own. )

It's not that I hate mods or something. I just had a lot of screwing around with them back in the old versions when we had only few parts in the game. I got so bored with abusing those mods that I stopped playing for 2 months or so. Now I set challanges like colonizing Duna or building big interplanetary ships with resources I have in-game only. To me it's simply more satysfying.

[AceMgy]

It's completely possible to get accurate orbits manually, it just requires a bit of patience and forethought. The Ap/Pe's don't need to be exact, just close enough, and mostly it's just that you need the Semi-Major axis to be the same. Of course at 10km separation, that's only about 5 second apart from one station passing overhead to the next, and any orbit will eventually drift a little.

If you really want to avoid the lag, then I'd put them in different orbits, that are at least 4-5km apart in their Ap/Pe, as then you'll never be in range of both at the same time. You can adjust the orbital period to something in resonance, like 40 minutes and 60 minutes, which would give you 2 & 3 orbit windows, but really, how often do you need the windows to happen? 35 & 42 minutes should get a sync every 7 or so.

But as long as your transit bus has a few hundred delta-V for the transfer, even a station on the other side of Kerbin is only one outside orbit to an RV away, allowing a transfer at anytime within an hour or for a daily commute time, in which case all you need is the altitude separation to avoid the lag, and the timing and resonance becomes moot.

This is what I thought of first, and it's an elegant solution, but there's a tradeoff: the less delta-v you make for the transfer burn, the less transfer windows available to you. The delta-v needed to go between a 35 period and 42 period orbit is 129 m/s (appreciate that number: I used windows calculator to get it). That may not seem like much, but considering the sheer amount of fuel I'm going to transfer between the stations, this will add up. You can see why two stations 10km apart would be appealing to me: at that range one just needs to point and burn essentially, you can get there as slow and efficient as you want. I could have two stations at 80km and 85km, but then the transfer windows are four hours apart or so. That wouldn't be a problem if only KSP would let me fast-forward more than 50x that close! And it has to be that close because of the Oberth effect, moving the stations higher would cost me a lot more fuel in the long run than it would save in LKO maneuvers.

[Wjolcz]

Why don't you use Zenith Rockets Family? They are stock and have plenty of dV.

[AceMgy]

It's not that I hate mods or something. I just had a lot of screwing around with them back in the old versions when we had only few parts in the game. I got so bored with abusing those mods that I stopped playing for 2 months or so. Now I set challanges like colonizing Duna or building big interplanetary ships with resources I have in-game only. To me it's simply more satysfying.

Believe me Veeltch I get that, I wasn't attacking you. But right now I'm more satisfied with emulating a space program as accurately as I can in KSP than I am with difficult challenges. I start with atmospheric rockets, move through the historic missions like Vostok, Mercury, Gemini, then up to Apollo and beyond. Takes a damn long time but is very satisfying seeing a whole space program grow like that under your maintenance. Sometimes KSP has trouble doing things that would be pretty trivial in real life (like having two space stations near each other) and I have no problems using mods to overcome these limitations.