What's the best orbit?

[SpacedCowboy]

I know there's about a million tutes and vids on this subject but I want to hear it directly from the general population, (Yeah, KSP is like a prison, Once you get in, you can't get out!) BUT,,,What's the best orbital altitude when doing the Apollo style thing? I know a circularized orbit is good, but how high or low? Where does that command mod need to be? Thanks for any input. Yes, I'm talking about a munar orbit

Edited July 11, 2014 by SpacedCowboy
I forgot

[Wanderfound]

I'd expect the lowest possible orbit to be most efficient for the lander. Anything 8km or higher should be safe from intercepting a mountain top.

How long are you planning on leaving them up there? If it's a one shot, you probably want equatorial, but a polar orbit may be worthwhile for a long-term orbiting lander base.

[G'th]

Low Kerbin Orbit (LKO) is always a standard, and that's normally considered anywhere from 70km to 100km, and its where most people park before going off to the Mun or elsewhere. The reason for this is because at those altitudes, the Oberth Effect will have the strongest effect on your craft, and if you can time your burn to be as close to your periapse as possible, you'll save a bit of Delta-V during the burn. When going to one of the Mun's it doesn't make that much of a difference, but there's no reason not to take advantage of it anyway.

EDIT: Ah, you may have been talking about Mun orbit. Personally, I prefer a 50km orbit myself, but this is because I'm still practicing rendezvous so I need that kind of wiggle room.

[LethalDose]

Originally, I was going to say "it depends", based on whether we're talking about a kerbin orbit or a Mun orbit.

But it doesn't. You want both to be as low as possible without hitting anything (atmo or a large hill).

Keep your Kerbin parking orbit as low as possible. You asked for the "best", which I assume means "most efficient" and would be 70 km. Anything under 100 km should be fine though.

On the Mun, the lower orbit will require you to slow down less to get captured, and then require less speeding up to escape. As pointed out above, if you're over 8 km on the Mun, you won't hit anything.

Lower orbits are faster, and in general, going straight into those orbits is more efficient because it allows you keep that lovely speed.

Now, the caveat to all this, is that you absolutely should not waste lots of time and fuel to reach these "ideal" orbits if the ones you're in are fine. After you get a Mun capture, if all youre doing is an Apollo style mission and you don't care about where you're landing, don't waste fuel on correcting your inclination. If you get captured into a 25 km circular orbit, just go with it. The fuel you'll spend trying to get into the "best" orbit will cost you more than that orbit will save you, except in some extreme circumstances.

Probably the best way you can prepare for a Kerpollo mission is to practice docking and redezvousing in LKO and fly a Munar orbital mission (see Apollo 8). If you've got those skills pretty well honed, then you'll have at least some idea of what altitudes your orbits should be at, or understand why they don't matter all that much.

Edited July 10, 2014 by LethalDose

[Taki117]

Generally speaking for Kerbin I use a 100km orbit, unless I have a specific reason for another. (I have a COMSAT network at KSO, a space station at 200km, and a space telescope at 350km)

For the Mun Almost all of my orbits are 20km because I feel like if I go much lower I run the risk of slamming into things.

[SpacedCowboy]

Yes, I am talking about a munar orbit. Lethal, lots of great advice there, and yeah I don't have my "Dockturate" in docking, sure need to work on that. Taki, I like 20km. , think I'll start with that. BTW you got a screen shot of that "Space telescope at 350km" Would love to see that.

[Xavven]

I'm going to go against the grain and say that I like going into 50-60km orbits around airless bodies. I had a bad experience on Tylo once where I didn't have enough thrust to kill my horizontal velocity before I slammed into a mountainside. My post-crash analysis revealed that I needed bigger engines, but also a higher orbit, in order to ensure I could slow down in time to come down a little bit more vertically.

[Aethon]

Not to ninja Taki there but strangely, I also have a space telescope in a 350 km orbit . I'ts just parked for now, robobooster still attached, on it's way to high solar orbit. The slower orbital speed will allow better objective tracking.

http://steamcommunity.com/sharedfiles/filedetails/?id=268848668

http://steamcommunity.com/sharedfiles/filedetails/?id=274895486

[Moss]

Well, as long as you don't end up like this guy:

[Taki117]

Yes, I am talking about a munar orbit. Lethal, lots of great advice there, and yeah I don't have my "Dockturate" in docking, sure need to work on that. Taki, I like 20km. , think I'll start with that. BTW you got a screen shot of that "Space telescope at 350km" Would love to see that.

I'll do you one better. Here's the Mission Report where I put it into orbit. Please don't ask for the Craft File, not only has it been lost to the ages, but it has tons of mods (There's not a single stock part on it)

[bsalis]

Side note: If you're actually wanting to do a genuine Apollo style mission. Then you setup a free return trajectory. Then retro-burn into a clockwise orbit around the Mun.

[John FX]

On thing to add, the highest point on the equator of Mun is 6.2Km so if your Pe is 6.25 or higher, you will not hit anything when you orbit. I usually split my lander from my Cm when I`m on a 6.3x25km orbit (mostly circularised but not all the way) then I burn to keep my lander at 6.3km and then switch and burn to keep the Cm at 25km. Then I land my lander like this, very calmly.

Side note: If you're actually wanting to do a genuine Apollo style mission. Then you setup a free return trajectory. Then retro-burn into a clockwise orbit around the Mun.

If you want to do a FRT burn then set up a node with 872 Dv prograde then slide it around your orbit until it looks like this. With practice, you can have your Munar Pe under 10km as you leave LKO.

Edited July 11, 2014 by John FX

[cantab]

The other thing to bear in mind is time warp limits. Too low and you'll be hanging around waiting.

[NecroBones]

The other thing to bear in mind is time warp limits. Too low and you'll be hanging around waiting.

Also, too low, and sometimes the 100,000x timewarp considers a low-orbit (for a craft that is not the current focus) to be an impact on the surface, and you can lose your vehicle. I've had space stations and ships disappear just because they were in 20km orbits and I hit a high timewarp for an interplanetary mission.

Another thing worth noting, is that if you're visiting a moon (say one of Jool's moons, for instance), and have other destinations after that (such as in the Jool-5 challenge), and your orbiter has a detachable lander, it can be far better to park the orbiter in a highly eccentric orbit. That is, a low periapsis, for a nice Oberth Effect in your capture burn, but only execute the burn long enough to make your apoapsis manageable. No need to waste fuel to completely circularize, unless you really need it for a fast rendezvous after your landing. If you don't care about the orbital period much, you can save fuel by keeping the apoapsis high.

[Kryxal]

Everybody seems to like the retrograde Free Return Trajectory ... note that there's a prograde one too, basically you pass apoapsis, start your return to Kerbin, enter the Mun's SOI, get tossed BACK up, and return that way.

[technion]

The answer actually depends on your TWR. If you have a very low TWR, 8km may not be enough to shave off your speed before landing.

[Pecan]

I know there's about a million tutes and vids on this subject but I want to hear it directly from the general population, (Yeah, KSP is like a prison, Once you get in, you can't get out!) BUT,,,What's the best orbital altitude when doing the Apollo style thing? I know a circularized orbit is good, but how high or low? Where does that command mod need to be? Thanks for any input. Yes, I'm talking about a munar orbit

If you want a sample vehicle and walkthrough section 6 of chapter 6 of the tutorial in my signature has all you need for Kerpollo.

[NecroBones]

Everybody seems to like the retrograde Free Return Trajectory ... note that there's a prograde one too, basically you pass apoapsis, start your return to Kerbin, enter the Mun's SOI, get tossed BACK up, and return that way.

The retrograde one is the easiest to find information on, since it's what was used in the Apollo program. It also costs less dV, and doesn't take as long to travel through, as the prograde version. So I think there are several reasons why the retrograde version is more popular.

[John FX]

Apollo used a low Pe orbit and followed pretty much the flight path I described (which is a good one for low TWR landers)

The did a hohmann trajectory to a low Pe

then did this

they took 250 miles to stop and adjusted their angle to control their descent. Being that they were real people and wanted a bit more margin of error and orbiting a larger body, they started out from a higher orbit than we should. I would suggest as low as possible but no lower than 6200m

The answer actually depends on your TWR. If you have a very low TWR, 8km may not be enough to shave off your speed before landing.

Actually, if are in a low TWR craft you are safer if you fly the profile shown. My lander has a kerbin TWR of 0.3 (2 on Mun) and the only way it`s really safe to land is this way. you get a LOT more control over your descent (and landing point) and it removes the urgency and precision needed compared to things closer to suicide burns.

Edited July 13, 2014 by John FX

[KerikBalm]

Another thing worth noting, is that if you're visiting a moon (say one of Jool's moons, for instance), and have other destinations after that (such as in the Jool-5 challenge), and your orbiter has a detachable lander, it can be far better to park the orbiter in a highly eccentric orbit. That is, a low periapsis, for a nice Oberth Effect in your capture burn, but only execute the burn long enough to make your apoapsis manageable. No need to waste fuel to completely circularize, unless you really need it for a fast rendezvous after your landing. If you don't care about the orbital period much, you can save fuel by keeping the apoapsis high.

I'm not sure this is ever a good idea, and if so, it is only in really limited circumstances.

The problem with this highly eccentric rendevous (aside for the more challenging rendevous), is that you shift the dV requirements from the mothership to the lander

Lets assume your "highly eccentric" orbit is going right to the edge of the SOI. In this case your lander will need enough dV to basically acheive orbit + escape velocity... which laregly negates the point in doing a rendevous in the first place (if you're only doing a single destination, and its near kerbin, such as duna).

Such an eccentric orbit means the lander needs more dV capability, which means it must be bigger.

I suppose if the orbiting "mothership" is much more massive (suppose it has habitats, life support for very long durations, etc), then it makes sense (similar to an aldrin cycler almost). With multiple destinations in mind, the mothership will have a lot of mass for fuel, and you probably have a lander intended for multiple moons, with large excesses for some.

However, it is almost always a good idea if you add a 2nd rendevous*:

Your mothership arrives in a highly eccentric orbit, and detaches a lander and a fuel depot (fuel tank+ docking ports). The lander engine places the fuel tank in low orbit around the moon, and then descends to the surface, then ascends, and rendevous with the fuel tank. The fuel tank then supplies enough dV to make the rendevous with the mothership that is in the highly eccentric orbit.

No sense in carrying the fuel that you need to get that eccentricity down to the surface just to lift it up again.

Of course, there is also no sense in slowing all your fuel down into a circular orbit, just to accelerate it again.

You can save a lot of mass by avoiding situations where you decelerate something, only to accelerate it again

*excluding cases where the savings are small, and the dV used to dock is likely to surpass the savings.

[NecroBones]

While that's true that it shifts a lot of the burden to the lander, your lander will also use less fuel since it's moving less mass. If you're doing a long duration mission, and need to save as much fuel as possible, your larger mothership may have to consume quite a bit more fuel to achieve the same dV. YMMV.

Having said that, I prefer the "tug" method, of having a second stage that lowers the lander's orbit, and carries it back up again as well (and admittedly this is the scenario I had in mind). Not everyone may want to do that though.

Edited July 14, 2014 by NecroBones