Best orbit for Mün/Minmus station?

[Musil]

What do you think is the best orbit for a station on a Mün or Minmus? Keeping in mind that the idea is to use the station as a lab to reset the experiments on a lander. An equatorial orbit will make it really easy to build, since all the parts are going to be coming from the same angle, but that means that if you go to the poles with the lander, you will then have to use a lot of fuel to get into the same orbit, and most of the times that fuel is needed on the lander for other things. I have tried two times (on the Mün and on Minmus) to build stations on a polar/high inclination orbit, and it works really well with the lander: no matter where I have landed, it's really easy to get back (just have to wait for the rotation to bring me under the station before taking off), but bringing new parts to the station is a nightmare. I am either very lucky and come into the moon's SOI with the right angle, or I come in with a 90º difference and have to kill all my speed to get into the right orbit.

My latter experiments seem to indicate that I can adjust the angle of entry by putting my Kerbin apoapsis over the orbit of the moon, wait for it there and burn a little, prograde or retro, to come in from the front or the side of the SOI.

What do you think?

Edited December 22, 2014 by Musil

[Anquietas314]

Your best bet is probably to use a polar orbit, yeah. As for getting new parts, you should make your Kerbin orbit slightly inclined long before you get to Mun - with a bit of tweaking that will make you arrive in a polar orbit around Mun. If you make it so your periapsis is very near either the north or south pole, you can burn at periapsis to bring your apoapsis only just inside the Mun's SoI, and then do the plane change at apoapsis for a tiny fraction of the deltaV.

Edited December 22, 2014 by armagheddonsgw

[Musil]

Yeah, sure, getting the inclination is no problem:just burn very little while still near Kerbin. The problem is getting the same ​polar orbit, and not one that intersects in a 90º angle with the first one.

Edit: aaah... I see what you mean... Will try that.

Edited December 22, 2014 by Musil

[Anquietas314]

That's why you do the plane change at apoapsis on the edge of Mun's SoI. It's dirt cheap. Read the last part of my previous post; it covers how to make sure you can do that.

[Musil]

Yeah, sorry, understood that a bit latter (read edit ). Thanks, btw!

[Anquietas314]

Yeah, sorry, understood that a bit latter (read edit ). Thanks, btw!

No problem Don't worry if your apoapsis doesn't match up with the AN/DN perfectly though - just burn wherever those are instead; as long as they're pretty far out it won't matter much. Of course you can always do a radial burn about half way to apoapsis to correct that if it's really bad.

[KingPhantom]

i set my stations every time on a stable equatorial orbit with 180° at moons and 0° at planets.

[Anquietas314]

i set my stations every time on a stable equatorial orbit with 180° at moons and 0° at planets.

I usually do that too, but after thinking about it a polar orbit (except perhaps on planets/moons with very long days) is much easier; No matter where you are on the planet, you can just wait for the orbit to line up with where you are before launching. On higher-speed rotation planets (kerbin and such) you obviously don't get the little boost from the planet's rotation, but being able to reach the poles is probably worth it.

[Kesa]

Establishing orbit at the edge of the moon then changing plane is reasonable, but you can be much more efficient by directely getting the good orbit.

For this purpose one need to know that the direction of an orbit is constant (at least in KSP).

That's why you will match your station's orbit if you send a ship at the same moment (eg a multiple of 6 days later for the Mun) or the same place (eg having an intercept with Minmus at its ascending node (hard with Mun since its orbit is perfectly Round with no equatorial inclinaison, but it has an inclinaison with Minmus, which can serves as a landmark)).

But you don't want to count how many days have past since you launched your station (except if you can have Kerbal Alarm Clock doing it for you) to know when you can send things to it. So here is the way to generally match a desired inclined munar orbit (also works for contracts).

When you send a spacecraft to the mun and establish a polar orbit around the mun, you maybe have noticed that, just after the orbit is made, it should be roughly facing Kerbin, ie from any point of the orbit, you can see Kerbin (if you have not, launch a probe to see!). That is because the mun has a nice circular orbit and you intercept it at your apoapsis (-> no radial velocity relatively to kerbin).

If not so, your probably overshooting, or the time to circularize make that your obrit have already changed. Anyway, the position relative to Kerbin/ parent body should be the same for similar launches.

So you have to make sure that when your vessel will intercept the moon's SOI, the target orbit will be facing Kerbin. You can figure that out by yourself, or follow these steps :

(1) look the plane that curently contains the target orbit. It won't change orientation, it will only translate as the mun goes. Eyeball the position of the moon for which this plane will be tangent to the mun orbit.

(2) this position will be your intersect. Set up a manoeuver node to have an intercept with the moon orbit (don't bother the normal burn for now). The moon should not be there at the good time.

(3) Timing. Right click your manoeuver node, and set it several orbits later, until you have a good intersect.

(4) Focus view on the moon, your predicted trajectory is coherent with the present target trajectory. Set the normal burn to have a ppolar orbit with no inclination.

(5) Fine tunes your nodes : would it be better if I wait one more orbit? one less? If I have an intercept slightly sooner/later (draging node)? Is my PE close enough to those of my target?

With a good definition of "facing Kerbin", this method apply to any inclined orbit around a moon you want to catch.

IMHO, this is the most efficient way to catch an orbit, but also restrcit you to 2 launches / moon's revolution. For the mun, 3 days are not that much to wait and if you have big playload to deliver, it may worth optimize the path and transfer vahecle, but for Minmus or distant small Muns, it maybe simpler to orbit at SOI edge, then change plane + lower PE, then lower AP.

If you have any questions, or need pictures, feel free to ask

I may put that in a tuto, if it does not already exist.

[Anquietas314]

Establishing orbit at the edge of the moon then changing plane is reasonable, but you can be much more efficient by directely getting the good orbit.

We're talking a matter of 200m/s deltaV tops - for a full orbit reversal. Considering it takes approximately 6-6.5km/s to get there at all (in stock), it's peanuts and not worth caring about. You should generally have a few hundred m/s spare anyway for a safety margin.

Edited December 22, 2014 by armagheddonsgw

[Kesa]

As for the choice between Polar/equatorial, I think they are both valuable :

a polar station can reach any spot on ground, wich is very good for exploration, but there are only 2 windows from and to parent planet per orbital rotation (even though windows are not as much important as interplanetary windows), and 2 windows from and to ground per moon's day.

An equtorial station is good if you want to use the spin of the planet, and are easier to reach. Also, It can only reach equatorial sites of the planet, but it does it far more often (passes over a specific location ~every 20 min for kerbin to lko), and, if it is a moon station, it can be reach from parent body at any time

[Kesa]

We're talking a matter of 200m/s deltaV tops - for a full orbit reversal. Considering it takes approximately 6-6.5km/s to get there at all (in stock), it's peanuts and not worth caring about. You should generally have a few hundred m/s spare anyway for a safety margin.

It's peanuts if you go from Kerbin's ground. If you want to use Thug/Bus/Cargo that stay in space, it is 200 m/s out of 1000 or 2000m/s...

Well, if you just want to have an station that can land rover/ miscelanous funny thing on the planet, it do not worth care, but if you like optimization, searching the better path is part of the fun

Edited December 22, 2014 by Kesa

[Anquietas314]

It's peanuts if you go from Kerbin's ground. If you want to use Thug/Bus/Cargo that stay in space, it is 200 m/s out of 1000 or 2000m/s...

Since we're talking about launching station parts, in most cases you'll either have to use a new transfer stage every time, or you'll need quite a large reusable transfer stage, in which case you may as well give it plenty of fuel anyway - it's already going to be heavy.

Edited December 22, 2014 by armagheddonsgw

[Kesa]

You should generally have a few hundred m/s spare anyway for a safety margin.

If you choose a pth that cost 1500 m/s, you will take, say 300 m/s of margin, so yes, you are able to take a road that costs 1700 m/s, at your own risk. If you wnted to go for the 1700m/s road, you would have also take a 300m/s margin, so you would had have a heavier design.

Since we're talking about launching station parts, in most cases you'll either have to use a new transfer stage every time, or you'll need quite a large reusable transfer stage, in which case you may as well give it plenty of fuel anyway - it's already going to be heavy.

Spare fuel can be left in the station for the landers to be refurbished, sparing the time of a refuelling trip. And Heavy often goes exponentially in rocket science (exponentiallly. You seem to like Bold font ). Saying 200 m/s of dv is nothing worth for small crafts, not for big ones!

My point was to show the op and the readers something they could have not known.

Your way to go is good, and it's what I do most of the time, but it's a solution that anybody can figure out (that's alredy almost what Musil did before opening this thread).

Think of it as aiming for the 4550 m/s orbital insertion. All my 5-min created rockets have 5000-5500m/s to go into orbit and spend them. Nontheless, I'm sometime interested in making better rockets and ascent path that aim the 4550 m/s milestone.

Edited December 22, 2014 by Kesa

[Anquietas314]

Even for large craft, 200m/s is nothing during the launch stage. KSP simply doesn't offer that level of granularity in the tanks. Sure you can always take some out before launching, but why bother? Fuel is dirt cheap. If you're making a reusable transfer stage, chances are it'll have nuclear engines, which means 2 things: 1) you don't need as big a margin - the lower thrust means you're much less likely to screw up in a big way, never mind the fact 300 m/s of margin is mainly for getting to orbit - there's a huge amount of variance in delta-v requirement for getting to orbit depending on subtle variations in your flight path. 2) 2-300m/s of delta-v with a nuclear engine, even for quite substantial loads, is not a lot of fuel. The engines themselves will probably cost you more in terms of mass.

The thing about the fuel requirement being exponential only really matters when you have a large difference between full and dry mass - that is what the exponential affects. You don't have that when you're shifting a station from LKO to LMO.

Edited December 22, 2014 by armagheddonsgw
derp

[Janos1986]

What about a 45° orbit then? It should cover a greater portion of the moon's biomes even if it means spending a little more deltaV for docking when off course (i.e. poles).

[Anquietas314]

What about a 45° orbit then? It should cover a greater portion of the moon's biomes even if it means spending a little more deltaV for docking when off course (i.e. poles).

The advantage of a polar orbit is that you don't have to do a plane change at all when launching from the surface - you can just wait for the orbit to pass over you, then launch into it.

[Musil]

We're talking a matter of 200m/s deltaV tops - for a full orbit reversal. Considering it takes approximately 6-6.5km/s to get there at all (in stock), it's peanuts and not worth caring about. You should generally have a few hundred m/s spare anyway for a safety margin.

I think this might actually work better for smaller craft that carry less fuel, yes, but are also lighter and get more dV for less fuel. Changing a probe's plane is no big deal, but when you are hauling a very large fuel tank to refuel a lander, you might end up wasting all the fuel just to get those 200 dV (I once did this, as a matter of fact, and felt very stupid after).

Also, I don't know about planing with 300 dV as a safety margin. In my current career, my second Mün lander made it back to Kerbin's surface with something like 10 dV remaining, and that only after I spent 20 minutes optimizing my escape burn.

So you have to make sure that when your vessel will intercept the moon's SOI, the target orbit will be facing Kerbin. You can figure that out by yourself, or follow these steps

Yeah, that was the kind of answer I was hoping for. There had to be a way... Seems to requiere a lot of careful eyeballing and time warp waiting, but that's the fun of it, of course. As I said, with lighter craft, it may not be worth it, but definitely with heavier payloads.

Also, it's more important on the Mün than Minmus. Orbital velocity on Minmus is so low you can affort to do a complete plane change, but on the Mün...

Edited December 22, 2014 by Musil

[Musil]

What about a 45° orbit then? It should cover a greater portion of the moon's biomes even if it means spending a little more deltaV for docking when off course (i.e. poles).

Actually seems like the worst option to me. You not only get the problem of having to get into the right plane, you dont get the full advantage of a polar orbit.

[Anquietas314]

Musil: Well, if you want to do things that close to the metal, I guess that's your choice I've always found it really doesn't make a huge difference in launch costs or total fuel requirement to budget ~300m/s over - and that's after 3 years of playing the game for at least a week pretty much every release! I've tried a lot of variations on things in that time.

Edit: just checked, I just passed 500 hours logged on KSP in steam, and that's not counting the non-steam modded versions (edit 2: steam versions that I copied to a different directory to prevent things being broken by updates) I've used, the time shortly after it went on steam that it reset my hours to 0 (it was something like 150 at the time), or the time spent playing the game before it was on steam.

Edited December 22, 2014 by armagheddonsgw

[Guest]

Polar will be nice and easy. Minmus allows for incredibly cheap inclination changes anyway.

[Anquietas314]

Polar will be nice and easy. Minmus allows for incredibly cheap inclination changes anyway.

This is also a good point

[Musil]

Musil: Well, if you want to do things that close to the metal, I guess that's your choice

Actually, it was not so much by choice... I wanted to pass over some contract fly by check points before landing, and wasted a lot of fuel doing that. Also, my lander had 2 stages (it was the best lander I have used so far, really elegant Apollo style, easy to make now thanks to the gizmos) and some got left behind on the surface of the Mün, so I am not going to claim I did a perfect calculation. Once the lander was re-docked with the transfer craft, I discovered in horror I didn't have enough fuel to go back. My orbit was in something of a 70º angle, because of the fly bys, and it was not just a matter of burning prograde on the usual place... I had to search for the best spot to start my burn and then adjust the burn very carefully so that Kerbin periapsis was at 40k.

All of this confirms how awesome KSP is, and makes me hope that I am still playing it in 2 years as well (I am a newbie, started on 24.2)

[Anquietas314]

Musil: There is also a fun little exploit where you can get out on EVA and literally push your ship home. I've done that a few times - it takes a certain amount of skill to do without making your ship spin out of control though. If you have RCS left over you can of course use that first - it's a lot easier to control that

[LethalDose]

I would vote for an inclined orbit over a polar orbit. Even an orbit inclined only 30 degrees will cover about 50% of the body's surface. A 45 degree inclination will cover ~71%, and 60 degrees will cover ~87%. (Based on some quick math derived from equations here.)

IMO, it's easier and cheaper to rendezvous with these lower inclination orbits from both the surface and incoming from outside the SoI.

On the Mun, a 30 degree inclination should give good coverage of all the biomes except polar lowlands, polar crater, the poles.

On Minmus, orbital velocities are so low that inclination changes aren't a big deal, so it's dealer's choice. If memory serves, 7 of Minmus' 8 biomes occur on or very, very near to the equator.

Edited December 22, 2014 by LethalDose