Most Efficient Orbit Altitudes

[CtrlAltEL1TE]

Hey Gents

Excuse me if this makes no sense but my brain is fried from working in the sun all day.

So I just started my brand new save file and Im in the process of launching my Mün and Minmus refueling stations and in the past i have always placed these at 30km and 15km respectively. However well I was transistioning between kerbin and the Mün a thought hit me, Is there a more efficient altitude delta V to place them at (i think as it stands the 30km Mün orbit takes about 800m/s delta V to launch to). However im not the best at space fysiks so i couldnt even think of where to start on calculating acurate launch altitudes, although in my mind the lowest orbit possible without scraping mountains would be best so around 4k for the Mün and 6k for Minmus, however this may lead to some real interesting launches of kethane tankers.

Has anyone worked this out / is there a way to easily calculate this?

Any input apreciated

Ctrl

PS: Literally every single one of my Mün orbits since 0.12 have been at 30-50km with the exception of comm and mapping satelites. So if someone tells me there is a far more efficient way i might have to rage ;D

[sporkafife]

Lower orbits are more efficient in terms of Oberth effect for transfers away from the Mun/Minmus, and it takes much less delta-v to reach orbit from the surface of a body if the orbit you're trying to reach is as low as possible. I personally think it's quite a good idea for the orbits to be pretty low, but not too low you risk smacking into the surface. 30-50k is probably fine for the Mun, and I'd personally go for 15-20k around Minmus

[astropapi1]

In theory, the most efficent orbit is the lowest one , as you can take full advantage of the Oberth effect.

Go as low as you can.

[BeefTenderloin]

"fysiks"

That's a very creative spelling

Physics, for further use.

[Spatzimaus]

It depends on what you want to use the orbit for. In general, lower orbits are best, but there are a few notable exceptions:

1> Things you want to dock with on the way DOWN into the gravity well, when you might not be able to reach a lower orbit safely. For instance, if you've got an interplanetary vessel that gets back to Kerbin but doesn't have enough fuel to land; you'll want to dock with something to refuel, and a station in high orbit is less fuel to do that with than one in low orbit. This is especially useful for Jool, where you'll need fuel to do a rocket landing on four of the five moons; placing a fuel bunker out by Pol is a good idea.

2> A station you want to use as a refueling waypoint for something coming from outside the gravity well that intends to again leave the gravity well afterwards without stopping on the actual body you orbit. Take a Minmus station as an example; if your intention is to have a place for a vessel to refuel before proceeding on out of Kerbin's SOI, then the option that uses the least fuel is to not even put the station in Minmus' SOI; you'd be better off putting it in a Kerbin orbit. Putting it in Minmus orbit takes a little more fuel, but makes refueling the station from the surface MUCH easier, so you'll do that instead. But if you're trying to minimize the amount of fuel a vessel requires to enter the Minmus well, slow down, meet the fuel station, and then leave again, then a higher orbit is better.

3> When you're flying a vessel that's extremely hard to maneuver. The more massive the body, the more "curved" a low orbit is. If you try docking in very low Kerbin orbit (or better yet, Eve) you'll find that your approach vector "drifts" to the side; a straight burn towards the target just won't work well; you'll end up wasting a lot of RCS fuel to compenstate. Higher orbits are much better for this. (Also, less chance of clipping any atmosphere or terrain if you screw up your approach.)

4> There's probably something involving bi-elliptic transfers, where you'd want a station at a far-out radius just to be used for these transfers, but the bi-elliptic doesn't involve circularizing at the outer radius, so I'm not sure how you could use this to your advantage.

In general, though, you want to go as low as you possibly can, unless you're trying for a geosynchronous orbit of some kind.

[CtrlAltEL1TE]

Cheers guys, Just leaving my orbits at 30km (Mün) and 15km (Minmus) for now. Done some experimenting on the moon and your orbital velocity at 30km(ok 29km close enough ^^) is 533m/s versus a 550m/s at 2km, Wouldnt recommend the 2km orbit either for obvious reasons, those being that A. Its extremely hard to dock incoming from Kerbin and B. It went on rails when i warped focused on something else and decided to lower its orbit by ~500m thus smashing into a mountain. I doubt ~20m/s would make a massive difference for exploiting the Oberth Effect but I rarely go interplanatery from my Mün base anyway, I got 2 refueling depot's over kerbin for that (one at 80km another at 100km).

Thanks for the help guys

[Spatzimaus]

It went on rails when i warped focused on something else and decided to lower its orbit by ~500m thus smashing into a mountain.

Oh, this brings up another one: planets have limits on what sort of time acceleration you can use at a given altitude. For instance, Gilly's gravity is so low that orbits around it are going MAYBE 20m/s, which means it takes days to do a single orbit, but if your orbit is below 8km then you can't use any kind of time acceleration. (Given that the highest terrain is something like 7km up, you wouldn't want to put a station that low anyway, but you never know.) In most planets' cases, the comfortable altitudes allow 50x acceleration, but sometimes you want to go more than that and it won't let you. Higher orbits avoid this.

The most notable example is that on Kerbin, a station below ~120km is much more limited on acceleration than a higher one, so even if the lower orbit is better for Oberth work, you're usually better off putting your big space station above that line for practical reasons.

[Fyrem]

As Spatzimaus said, .. your desired orbital alt will depend on what the station is FOR, and who is intended to visit it.

[BeefTenderloin]

double post, derp.

[tavert]

You can always switch craft and warp to pass the time in something else. Kerbal Alarm Clock is really useful for this.

The orbital speeds aren't much different between 30 km and just scraping the mountains, but you will lose more speed to gravity in getting up to 30 km. Compare your speed when you shut off the engines to when you get to apoapsis. Burning horizontal as soon as possible to get a high periapsis before your apoapsis reaches your target altitude will also reduce your gravity losses.

[CtrlAltEL1TE]

Hehe ive got a Geostationary relay sat network up and running, usually i switch to one of those or one of my many random rovers dotted around KSC for fast warps... so thats never been an issue for me.

I am assembling my Kerbal "research station" which is big and decorative at 125km. A bit high to be practical for me though im afraid since most of my SSTO's are designed to handle 80-100km orbits maximum

That network might not be geostationary for long though. Tempted to rip it down and put up a Molniya orbital relay network just for the giggles.

[darkstrike]

I'm about to re-launch my lander from the surface of Dres....anybody know the lowest stable orbit I can launch to safely? (Not much fuel left!)

[Catullus]

Nice to see Dres getting some love. I suggest trying for a 10 k orbit, although 6 k should theoretically be safe.

[darkstrike]

Thanks! My first time sending any Kerbals there!