Lowest Stable Orbit for Large Space Station

[Castille7]

What's the Lowest Stable Orbit around Kerbin or your best guess for a large Space Station?

Edited July 8, 2016 by castille7
Question Answered

[Superfluous J]

Absolute lowest is 70km. But practically that's not very good as it'd be hard to approach. More practical is 80km or more, and a lot of people like it to remain above 100km. I personally like my stations in the 80-90 range.

Note that atmospheres in the game have hard limits. If you're over that limit you are in vacuum and never experience any drag. That's why 70km (the listed height of Kerbin's atmosphere) is the absolute lowest you can go and be stable.

Unless you do a trick like getting you station at 30km altitude going orbital velocity, and then deatch from the station and quickly get more than 22km away. But that's not practical and is just abusing shortcuts in the simulation.

[Castille7]

Man I am on fumes right now and running on Gas only as we speak, my Apoapsis is great but concerned about my Periapsis right now, it's creeping near 50km and I a hoping to get it to 70 on my first pass around Kerbin. Thanks for the info and I will let you know in just a few minutes what happen. The trajectory is taking a while and I hope not to launch again if I don't have to. Thanks again and I'll report back asap.

Update: 54m was the best I could do with the fuel I had left and the craft did not make Orbit. Next attempt will be posted on the Space Craft Exchange Thread.

@5thHorseman Your answer is accurate according to my findings here after my craft would not hold at 54m. 

Edited July 8, 2016 by castille7

[Stargate525]

Depends on what I use it for. It's more expensive to get to from orbit, but I like sticking mine in the 250km range to take advantage of the timewarp.

[Castille7]

Just now, Stargate525 said:

Depends on what I use it for. It's more expensive to get to from orbit, but I like sticking mine in the 250km range to take advantage of the timewarp.

Yes I agree, it is nice to have the timewarp at full speeds.

[Reactordrone]

1 hour ago, castille7 said:

Man I am on fumes right now and running on Gas only as we speak, my Apoapsis is great but concerned about my Periapsis right now, it's creeping near 50km and I a hoping to get it to 70 on my first pass around Kerbin. Thanks for the info and I will let you know in just a few minutes what happen. The trajectory is taking a while and I hope not to launch again if I don't have to. Thanks again and I'll report back asap.

Update: 54m was the best I could do with the fuel I had left and the craft did not make Orbit. Next attempt will be posted on the Space Craft Exchange Thread.

 

If you're not in control of the ship (or have another ship within physics range of it) it will happily orbit on rails without any drag effects when it dips down to 54km so it is possible to abuse that little game quirk to get a rescue ship to dock with it at apoapsis and boost up to a stable orbit.

[Claw]

I'll echo that 70km is the absolute minimum, if you ever plan on visiting it.

I would highly recommend against putting it below 100km around Kerbin. At 100km, there is a shift in the reference frame that KSP uses, which can cause a variety minor issues. The biggest disadvantage is that Oberth effect is actually reduced below 100km due to physics and reference frames. So if you're using the station as any sort of refueling post, then put it above 100km.

Personally I aim for 150km to 250km (usually up higher for time warp). At those altitudes, you get better time warp and I find the rendezvous for craft taking off out of Kerbin to be easier to manage (less compressed to a narrow altitude band, faster to catch the station, and higher time warps available).

Cheers,
~Claw

[Tatonf]

3 hours ago, Claw said:

I'll echo that 70km is the absolute minimum, if you ever plan on visiting it.

I would highly recommend against putting it below 100km around Kerbin. At 100km, there is a shift in the reference frame that KSP uses, which can cause a variety minor issues. The biggest disadvantage is that Oberth effect is actually reduced below 100km due to physics and reference frames. So if you're using the station as any sort of refueling post, then put it above 100km.

Personally I aim for 150km to 250km (usually up higher for time warp). At those altitudes, you get better time warp and I find the rendezvous for craft taking off out of Kerbin to be easier to manage (less compressed to a narrow altitude band, faster to catch the station, and higher time warps available).

Cheers,
~Claw

HOLY SMOKES WHAAAAAAAAT ?

I've been playing KSP for 3 years, always putting my spacecraft at the absolute minimum altitude (~71 km, 1 km margin in case of), and you're telling me it's not the most efficient way to do things ?

I refuse that.

[Superfluous J]

36 minutes ago, Tatonf said:

HOLY SMOKES WHAAAAAAAAT ?

I've been playing KSP for 3 years, always putting my spacecraft at the absolute minimum altitude (~71 km, 1 km margin in case of), and you're telling me it's not the most efficient way to do things ?

I refuse that.

Going from the surface to LKO to other places (From Mun to Eeloo) it's better to be as low as possible for your LKO.

If you're refueling in orbit, though, it's actually cheaper to eject from higher, because while you lose some to Oberth, you gain more from being further from the gravity well, so leaving from there with full tanks is better than leaving from LKO with full tanks. I don't remember the exact height but it IS over 100km.

[Castille7]

14 hours ago, Reactordrone said:

If you're not in control of the ship (or have another ship within physics range of it) it will happily orbit on rails without any drag effects when it dips down to 54km so it is possible to abuse that little game quirk to get a rescue ship to dock with it at apoapsis and boost up to a stable orbit.

This would have been a nice last minute save but I just started 1.1.3 and the skies are clear right now. I will make another attempt this weekend. This latest version has me pumped up about getting some use of an old space station that was too large for the earlier versions.

Omega 7 is the station I was referring to, I'll post a few pictures of my situation last night.

Omega 7 

 

These pictures are showing my near Orbit attempt, I only had 6 Xenon Gas Engines running and losing Electric Power because I launched late in the evening. The purpose of these were to use them for positioning the craft. It was a last resort.

 

Edited July 8, 2016 by castille7

[Andem]

Safely? 75km minimum, 80km is best in my experience.

[Omegagoldfish]

The lowest safe altitude is at seventy kilometers, but I prefer my stations to be about two hundred kilometers. 

[adsii1970]

I have mine between 80k to 125k. For some reason, rendezvous and docking maneuvers are a lot easier at that altitude.

54 minutes ago, 5thHorseman said:

Going from the surface to LKO to other places (From Mun to Eeloo) it's better to be as low as possible for your LKO.

If you're refueling in orbit, though, it's actually cheaper to eject from higher, because while you lose some to Oberth, you gain more from being further from the gravity well, so leaving from there with full tanks is better than leaving from LKO with full tanks. I don't remember the exact height but it IS over 100km.

Exactly, I have a refueling platform at an orbit of 800k and it seems to fit the bill nicely.

[Claw]

5 hours ago, Tatonf said:

I've been playing KSP for 3 years, always putting my spacecraft at the absolute minimum altitude (~71 km, 1 km margin in case of), and you're telling me it's not the most efficient way to do things ?

Nope, it's not. That's because there are some discrete breaks in KSP's physics, and one of them happens to be at 100km around Kerbin. You actually get a little bit better Oberth response if you have your lowest point just above 100km rather than 70km. I'm not sure where that reverses itself (i.e. is that still the case at 50km).

 

5 hours ago, 5thHorseman said:

If you're refueling in orbit, though, it's actually cheaper to eject from higher, because while you lose some to Oberth, you gain more from being further from the gravity well, so leaving from there with full tanks is better than leaving from LKO with full tanks.

This is also a valid approach, though it wasn't what I was referring to in my post. If the station is primarily for refueling options, then in my opinion it's actually better to put the station up higher for refueling (no higher than the Mun's orbit). After refueling, do a Kerbin dive (drop the PE down to just above 100km) as you eject out of the solar system, taking the relevant ejection parameters into account. This is sort of like doing a split burn (which better harnesses Oberth) except that the fuel tanks are more full since the AP was raised for the station rendezvous (prior to refueling).

[Superfluous J]

3 hours ago, Claw said:

Nope, it's not. That's because there are some discrete breaks in KSP's physics, and one of them happens to be at 100km around Kerbin. You actually get a little bit better Oberth response if you have your lowest point just above 100km rather than 70km. I'm not sure where that reverses itself (i.e. is that still the case at 50km).

 

This is also a valid approach, though it wasn't what I was referring to in my post. If the station is primarily for refueling options, then in my opinion it's actually better to put the station up higher for refueling (no higher than the Mun's orbit). After refueling, do a Kerbin dive (drop the PE down to just above 100km) as you eject out of the solar system, taking the relevant ejection parameters into account. This is sort of like doing a split burn (which better harnesses Oberth) except that the fuel tanks are more full since the AP was raised for the station rendezvous (prior to refueling).

Do you really save that much, though, from such a low orbit? It seems sometimes even dropping from Mun isn't worth the effort, and there you get the benefit of Oberth leaving the Mun to drop back to Kerbin.

I've considered the idea of setting up 12 fueling stations in a clock pattern around Kerbin. My idea was that they'd be in polar orbits that reach out just past Minmus' orbit, inclined and placed so that they are guaranteed to never encounter Mun or Minmus. They could be refilled from a Minmus mining base and be available at all times for any mission, and with 30 degrees between them, you'd never be more than 15 degrees off of ideal on your ejection burn which should still save quite a bit of fuel. Plus, you'd leave Kerbin with as close to full tanks as you could manage.

The only reason I've not tried it is it'd be 1 interesting (though not difficult) challenge followed by 11 boring repetitions. And then I'd have to - after each mission, plan a refueling mission for the station. Not worth the effort. If I'm putting all that work into a video game to set up an infrastructure, I want it to reward me by working automatically.

[OhioBob]

9 hours ago, 5thHorseman said:

Going from the surface to LKO to other places (From Mun to Eeloo) it's better to be as low as possible for your LKO.

If you're refueling in orbit, though, it's actually cheaper to eject from higher, because while you lose some to Oberth, you gain more from being further from the gravity well, so leaving from there with full tanks is better than leaving from LKO with full tanks. I don't remember the exact height but it IS over 100km.

The ideal altitude for ejection from orbit is called the gate orbit.  It's altitude depends on the ejection Δv.  For a high Δv ejection the gate orbit may be a few hundred kilometers, while for a low Δv ejection it might be several thousand kilometers.  For example, for transfers to Jool the gate orbit is about 300 km, but for transfers to Duna, it ranges about 6500-10000 km with a median of about 7000 km.

[Castille7]

A huge thanks to everyone for sharing their knowledge on this subject, I now have my station in LKO. After I correct it's Orbit to something more satisfying I will see if refueling won't be too time consuming due to lag and if not I will post it on the spacecraft exchange at a later date. Again thank you all for the help!  

The lag was not bad when I arrived with another craft to refuel, if you would like to follow this project I will have it on the Spacecraft Exchange Forum - WIP Thread. I will provide a link at a later date when I begin the Thread.

Edited July 9, 2016 by castille7

[Blaarkies]

On 7/8/2016 at 9:06 AM, Claw said:

I'll echo that 70km is the absolute minimum, if you ever plan on visiting it.

I would highly recommend against putting it below 100km around Kerbin. At 100km, there is a shift in the reference frame that KSP uses, which can cause a variety minor issues. The biggest disadvantage is that Oberth effect is actually reduced below 100km due to physics and reference frames. So if you're using the station as any sort of refueling post, then put it above 100km.

Personally I aim for 150km to 250km (usually up higher for time warp). At those altitudes, you get better time warp and I find the rendezvous for craft taking off out of Kerbin to be easier to manage (less compressed to a narrow altitude band, faster to catch the station, and higher time warps available).

Cheers,
~Claw

"Nope, it's not. That's because there are some discrete breaks in KSP's physics, and one of them happens to be at 100km around Kerbin. You actually get a little bit better Oberth response if you have your lowest point just above 100km rather than 70km. I'm not sure where that reverses itself (i.e. is that still the case at 50km)."

Are you sure about the Oberth effect not active under 100km thing? I just tested it with a tiny probe:
-Probe has an oscar tanks and a swivel engine(waaayyy overpowered, but a super short burn time is good for such tests)

-Put probe in 71 000m orbit, use 5% throttle limit. Burn at Pe to get Ap up to 103 700m, burn again at Ap until Pe is 103 700m(+-100m). Turn off the limiter, and full thrust prograde with the remaining fuel
> Ap increased to 16 798 000m

-Quickload, use the exact same probe in the same 71 000m orbit. Burn full throttle prograde with all fuel
> Ap increased to 33 223 000m

Does this maybe only happen after scene changes from KSC back to orbital stations?
* maybe all Oberth is reduced <100km, but because I did it at 71km it was strong enough to overcome the disadvantage: A test at 95km vs 101km might show a different result?

Edited July 9, 2016 by Blaarkies
problems with quote boxes

[bewing]

Well, I'm glad you did the test -- but Claw has to be wrong on this one. Oberth is just based on your velocity times the thrust of your engine. Engine thrust is constant at all altitudes in space. And your velocity increases as you go to lower orbits. There really is no way around this, no matter how you calculate the reference frame -- since the reference frames have to match at the boundary (and there has to be a boundary). Even if there is some funny reference frame calculation issue, it can't amount to more than a fraction of a percent. And the velocity difference between 100km and 71km is a lot more than just a fraction of a percent.

[Tatonf]

4 hours ago, Blaarkies said:

-Put probe in 71 000m orbit, use 5% throttle limit. Burn at Pe to get Ap up to 103 700m, burn again at Ap until Pe is 103 700m(+-100m). Turn off the limiter, and full thrust prograde with the remaining fuel
> Ap increased to 16 798 000m

-Quickload, use the exact same probe in the same 71 000m orbit. Burn full throttle prograde with all fuel
> Ap increased to 33 223 000m

Does this maybe only happen after scene changes from KSC back to orbital stations?
* maybe all Oberth is reduced <100km, but because I did it at 71km it was strong enough to overcome the disadvantage: A test at 95km vs 101km might show a different result?

I tried to replicate your experiment but I can't get the same results as you. My spacecraft is composed of 3 parts : the HECS probe, the oscar fuel tank and the swivel engine. I used Hyperedit to put it in the following circular orbit (and to refuel it between each experiment) :

71 km : Ap raised to 771 262m

95 km : Ap raised to 841 500m

101 km : Ap raised to 859 380m

103.7 km : Ap raised to 867 420m

But I don't think this experimental protocol will lead us anywhere, in order to check the influence of the Oberth effect we should use non-circular orbit, with a fixed apoapsis and a variable periapsis.

So, I replaced the swivel engine by the vector engine (so I could get a shorter burn time), and I used a constant 5 000 km apoapsis so I can have a great change despite my small dV (this is what we call a "big response") :

71 km : Speed at Pe is 3066.0 m/s ; Ap raised to 32 787 857m

85 km : Speed at Pe is 3031.1 m/s ; Ap raised to 31 556 426m

99 km : Speed at Pe is 2997.2 m/s ; Ap raised to 30 443 993m

101 km : Speed at Pe is 2992.5 m/s ; Ap raised to 30 299 403m

110 km : Speed at Pe is 2971.3 m/s ; Ap raised to 29 648 300m

As we expect, the faster I go, the more my apoapsis is raised. According to my results, the Oberth effect is functioning totally fine. Or maybe the use of Hyperedit fixed it ?

Edited July 9, 2016 by Tatonf

[linuxgurugamer]

20 hours ago, Claw said:

Nope, it's not. That's because there are some discrete breaks in KSP's physics, and one of them happens to be at 100km around Kerbin. You actually get a little bit better Oberth response if you have your lowest point just above 100km rather than 70km. I'm not sure where that reverses itself (i.e. is that still the case at 50km).

@Claw

After reading the tests above, I'm wondering if the breaks in physics changed in 1.1.3.  From what the tests showed, there doesn't seem to be a break

[Blaarkies]

1 hour ago, Tatonf said:

I tried to replicate your experiment but I can't get the same results as you. My spacecraft is composed of 3 parts : the HECS probe, the oscar fuel tank and the swivel engine. I used Hyperedit to put it in the following circular orbit (and to refuel it between each experiment) :

Remember I did not start with a full tank at 103km.
I started both times at 71km full tank, but for one of these I used the crafts fuel to get into a 103km circular orbit. I did this to counter the obvious influence of starting at a higher orbit(if we are in a 10Mm orbit and burn just a little bit prograde, we can reach Minmus) I deemed it unfair to compare the higher against the lower orbit when the 103km orbit starts with more potential energy

I like your elliptical test, it is way more conclusive than mine.
edit: i also used hyperedit 

Edited July 9, 2016 by Blaarkies

[Tatonf]

Yes but starting from a 71x71 km orbit you go up to 33 223 000m while I only go up to 771 262m, meaning you had way more delta-V than I had (or way more buggued Oberth effect - which I doubt).

[Candlelight]

I tend to put my fuel stations sub-100km for quick and easy refuelling and my commercial station above 600km, just works really.

[klesh]

The higher the orbit of your station, the more forgiving a target it is for rendezvous.  You want plenty of vaccum space below the station, so you dont have to approach the station while still in the atmosphere.