how to perform orbital maneuvers?

[plazmaXIII]

Hi all.

Just wondering how you guys do certain maneuvers in KSP.

i can orbit very well and do hohmann transfer orbits etc.

the more advanced moves like orbital plane changes and landing over the launch pad however are mysterys to me.

is it possible to change your orbital plane by 45 degrees or so mid flight? or is the fuel needed too high?

[semininja]

Hi all.

Just wondering how you guys do certain maneuvers in KSP.

i can orbit very well and do hohmann transfer orbits etc.

the more advanced moves like orbital plane changes and landing over the launch pad however are mysterys to me.

is it possible to change your orbital plane by 45 degrees or so mid flight? or is the fuel needed too high?

Orbital plane changes are possible, if you build a rocket that can carry enough delta-vee into orbit, but accurate (that is to say, specific) plane changes are nigh-impossible, because there is no accurate way to tell what orbit you're in.

As far as landing at the pad, it's really a matter of trial and error (and more error, and more error). Been there, haven't done it quite yet.

[DoctorEvo]

I managed to muscle my way into an orbit around Kearth's terminator, which as you probably know is NOT conveniently-located. To achieve this, I needed to make a near-90-degree plane change - no small task, mind you.

I realized that to maximize my chances of success, I would need to build a rocket with a massive amount of delta-V, and then follow a somewhat unorthodox trajectory to make it without running out of fuel. The first step was easy; the rest was a bit harder.

You see, a plane-change maneuver requires less delta-V when you're going slowly. Typically, the slowest point in your orbit is at apokee. A good procedure for rendezvousing with an orbital target, as I have learnt from playing Orbiter, is to launch at the right time so that your plane is nearly aligned, perform a prograde orbit-sync burn to let the target catch up to you, and while you're waiting, make the final plane-change correction at apogee.

Seeing as Kearth doesn't rotate, waiting for a launch window was not an option for me - I had to fly there and make a HUGE dogleg one way or another. Thus, I considered which trajectory would leave me nice and slow when the time came to change planes. Obviously, launching into an orbit such that apokee was near the descending node (where my maneuver would occur) would be the way to go; but then I realized, since I'm making a 90-degree turn before I circle Kearth, I don't NEED my preliminary trajectory to be orbital, and a suborbital launch would both save fuel and leave me with less velocity at the descending node. I thus took off and launched into a steeply-climbing, high-altitude suborbital trajectory.

After waiting for the opportune moment, I finally turned Northwest and made my plane-change maneuver just after Apokee. After I got my velocity pointed in the right heading (within the proper plane), I made a few quick corrections then burned the last of my fuel trying to circularize/stabilize my orbit so I wouldn't come crashing down. I made it, but just barely.

Moral of the story: Massive plane-changes can be difficult.

[foamyesque]

I managed to muscle my way into an orbit around Kearth's terminator, which as you probably know is NOT conveniently-located. To achieve this, I needed to make a near-90-degree plane change - no small task, mind you.

Why would you need to make such a large plane change? You can launch from KSP in any direction you choose, and Kerbin doesn't rotate. The terminator can therefore intersect your flight path at any angle you wish, right? That should mean your plane change need be only a few degrees, if you get your timings correct.

[semininja]

Why would you need to make such a large plane change? You can launch from KSP in any direction you choose, and Kerbin doesn't rotate. The terminator can therefore intersect your flight path at any angle you wish, right? That should mean your plane change need be only a few degrees, if you get your timings correct.

The launch pad is pretty well right in the center of the bright side of Kerbin, so no matter which direction you launch from, you're still going to have to turn quite a bit to get a successful plane change. It's a pain no matter how you try to do it.

[plazmaXIII]

i managed to change my plane of orbit by about 15 - 20 degrees by expending a whole tank of fuel. however doing this increased my speed no matter how hard i tried to keep the rocket at 60 degrees it took me out of my stable orbit . . . is there a way of doing it without causing this to happen?

my orginal process was to get orbit at around 150,000m and just fire my rocket perfectly sideways until i could see a change of direction on navball. is that correct?

so evo, would a highly eliptic orbit be a good idea to save fuel on plane changes?

[foamyesque]

The launch pad is pretty well right in the center of the bright side of Kerbin, so no matter which direction you launch from, you're still going to have to turn quite a bit to get a successful plane change. It's a pain no matter how you try to do it.

After thinking about it, yes, you and Evo are right. I was thinking about the problem wrong.

[DoctorEvo]

i managed to change my plane of orbit by about 15 - 20 degrees by expending a whole tank of fuel. however doing this increased my speed no matter how hard i tried to keep the rocket at 60 degrees it took me out of my stable orbit . . . is there a way of doing it without causing this to happen?

You should orient yourself orbit-normal, or 90 degrees to your flight path. Sounds to me like you were less than that.

An easy way to tell is by turning until the moment the little yellow velocity vector jumps from one (prograde) side to the other (retrograde), and holding it right where this transition occurs.

What I did, since MY plane change was so huge, is I actually turned PAST it to about 120 degrees and then held my heading. This caused my velocity to dip, then rise again as I accelerated in the new direction. You don't NEED to do this, but it does save a bit of propellant.

so evo, would a highly eliptic orbit be a good idea to save fuel on plane changes?

The answer is 'sometimes.' If your trajectory requires (or allows) you to be in a highly-elliptical orbit anyways without wasting excess propellant, then that's a good time to do it. If your target orbit is circular and there's no orbit-syncing to be done, then MAYBE it'd be better to do it down low (I haven't yet done the math to say for certain).

Since I didn't care about the eccentricity of my orbit, I figured an elliptical one would be the easiest to get to within the terminator's plane, so I aimed high.

[plazmaXIII]

Just tried that technique you said about lining up the yellow markers and it was very effective at changing my flight plane. shifted about 20 degrees with half a tank of fuel the without drasticly affecting my circular orbit.

ps. my prograde and retrograde markers keep fluxing back and forth. whats the explanation for this?

[plazmaXIII]

update. its true that higher orbits will allow you to change your flight plane with less fuel. just tested it now on a eliptic orbit. made 45 degree change with the same amount of fuel. thanks!

[DoctorEvo]

Just tried that technique you said about lining up the yellow markers and it was very effective at changing my flight plane. shifted about 20 degrees with half a tank of fuel the without drasticly affecting my circular orbit.

update. its true that higher orbits will allow you to change your flight plane with less fuel. just tested it now on a eliptic orbit. made 45 degree change with the same amount of fuel. thanks!

Glad to help.

ps. my prograde and retrograde markers keep fluxing back and forth. whats the explanation for this?

Well, they're going to spin a little bit in whichever direction you're turning...

Also, on the edge of the meatball, it's hard to judge motion, so if you're swinging back and forth a little bit you might not notice except for the flashing of the markers. To steady this, it's a good idea to watch your heading, and try to hold it fairly steady (remember to spin with the markers though as you go around the turn, or to split the difference like I do, or else your orbit-normal heading will slowly become a prograde heading!).

[plazmaXIII]

yeah i noticed that. my procedure was to burn until it started to affect my speed. then stop and reset using the pro and retrograde markers. 8)

when i mean the prograde markers moving. i dont mean between left and right when your making a burn . . . when i fly over certain parts of kearth i think the markers move from the 45 degree angle to 0 then back to 45 :S crossing a pole prehaps?

chase cam is quite good for this as well. itl follow your spaceship as your turning so u can get a better view than you would with free cam.

just read somewhere that a lot of the time the fuel needed to put a craft into an high eliptic orbit, plane change up high and then come back down often uses less fuel than plane changing at a lower altitude.

[DoctorEvo]

when i mean the prograde markers moving. i dont mean between left and right when your making a burn . . . when i fly over certain parts of kearth i think the markers move from the 45 degree angle to 0 then back to 45 :S crossing a pole prehaps?

Perhaps. Radical reference-frame changes occur over the poles, though it has been resolved that it is ONLY the reference frame and not the ship itself. It does funny things to the camera, though.

chase cam is quite good for this as well. itl follow your spaceship as your turning so u can get a better view than you would with free cam.

Ooh... interesting. I've pretty much given up on using chase cam because I always end up inadvertently click-dragging and then I can't get it back, but I may have to try that.

just read somewhere that a lot of the time the fuel needed to put a craft into an high eliptic orbit, plane change up high and then come back down often uses less fuel than plane changing at a lower altitude.

Sweet. Guess that answers that.

[frenchie16]

On the subject of landing on the pad: What I've been trying, without much success, is waiting until I'm on the dark side of Kerbin with the launch pad directly 'below' me (with the purple marker in the dead center of the orange hemisphere on the gimbal indicator thing) and entering a hohmann transfer orbit to 34500m, while leaving enough fuel leftover that I can maneuver to some degree within the atmosphere. I have yet to actually hit the pad, or even the large green grassy area around it, but it gets me pretty close every time I try.

[DoctorEvo]

On the subject of landing on the pad: What I've been trying, without much success, is waiting until I'm on the dark side of Kerbin with the launch pad directly 'below' me (with the purple marker in the dead center of the orange hemisphere on the gimbal indicator thing) and entering a hohmann transfer orbit to 34500m, while leaving enough fuel leftover that I can maneuver to some degree within the atmosphere. I have yet to actually hit the pad, or even the large green grassy area around it, but it gets me pretty close every time I try.

In an ideal world, this would be the most efficient way to reenter.

Unfortunately, due to the limitations of KSP's instrumentation, it isn't very precise. The way most players have achieved precise reentries is by reducing their altitude (a similar Hohmann transfer to about 35-37 km should do nicely), and then waiting until they can actually SEE the launch center coming over the horizon to actually deorbit. I've landed on the little peninsula just North of the pad using this method, and that was without maneuvering within the atmosphere (save a bit just seconds before landing). Being able to SEE the pad allows you to sorta gauge how much of a burn you should make in order to come down where you want to, rather than blindly hoping that your Hohmann transfer sets you down at the right spot halfway around the planet. And if you're that low, it won't be that long/far of a drop after deorbit before you actually begin reentry, which certainly helps with the timing. Just note that the harder you deorbit, the faster you're going to fall into the atmosphere and the shorter your reentry will be. In theory, if you had enough delta-V, you could deorbit by simply killing ALL your velocity right over the pad and then just dropping straight down.... but this is rarely the case.

[Xenoc]

I've not managed to land on the pad yet, but I can reliably land within spitting distance of it simply by decellerating as the pad approaches over the horizon. I've done this repeatedly from 140km-ish orbits without problems. The trick is that once you've lost most of your horizontal movement use any remaing thrust to keep the vector indicator slightly ahead of the pad marker - you're going to be coming in slightly obliquely, and you need to account of the loss of lateral movement as you enter the atmosphere.

Basically it comes down to practice.

[frenchie16]

(in response to DoctorEvo)

That's roughly what I end up doing, anyway: the hohmann transfer gets me to roughly the right place and then I try to do things manually by guesswork from there. Unfortunately, I think things'd turn out better if I just left it alone and let the hohmann transfer take me in; last time I tried, I thought I was going to hit the atmosphere WAY too early, so I did a tiny burn towards the zenith and ended up overshooting significantly and running out of fuel trying to slow myself down :-\