why do my orbits suck?

[endl]

so i followed the guides (scott manly) i do the whole 45* turn at 10k i burn at apoapsis just like in the vid but my periapsis never equals my apoapsis its always an elliptical.

is there more to it, am i doing it wrong?

i would like to do a space station at 150k my rocket design is fine its very stable and has plenty of fuel i just feel like my trajectory is some how incorrect. or do orbits not work this way?

Edited August 20, 2014 by endl

[problemecium]

You'll have to do two burns to make a circular orbit.

First, do more or less what Scott Manley told you, although you can feel free to tweak the starting height and aggressiveness of the gravity turn to suit your needs. When the apoapsis is high enough, stop burning (assuming you're above 40km or so. You may need to overdo the apoapsis by just a bit, maybe up to 5km) and just coast up to about 30 seconds before it.

Next, point mostly prograde but just below the horizon (maybe 5 degrees) and throttle up just a bit - I generally thrust at whatever amount gets me about 1 G of acceleration. Continue burning until you start to see a periapsis (this second burn should take place mainly in Map View), and then throttle down to just a smidgen until you get the periapsis close to the height of your apoapsis. You should get a "perfect" orbit every time.

Advanced notes for the curious:

I say just below the horizon because when burning before the apoapsis, burning down will reduce the eccentricity and burning horizontally will cause the apoapsis to rise. Thus burning at a slight angle down will keep the apoapsis in place while accelerating the ship and lifting the periapsis. As you pass the apoapsis, your ship should slowly rotate relative to Kerbin (or you can manually rotate it) so that it goes through the horizon line and starts to point slightly up. At this point you have the opposite effect, i.e. burning up will decrease your eccentricity.

Once you've achieved orbit a few times, you're likely to find that burning at exactly 45 degrees at exactly 10km isn't the optimal solution. Personally, I start slowly tilting eastward at somewhere around 10km and go into Map View so as to watch what my apoapsis is doing. I like to have it reach 45km when I'm tilted to about 15 degrees above horizontal, after which I throttle down so as to reduce wobble from high G-forces and precisely control my trajectory. Once the apoapsis is around 60km or so I find you can just burn straight horizontally and the orbit will start to circularize itself while the apoapsis slowly rises. With some practice, your final insertion burn shouldn't be more than 300 m/s or so (i.e. you should already be at around 1800 m/s or higher when you get to the apoapsis).

Edited August 19, 2014 by parameciumkid

[Kasuha]

Getting periapsis and apoapsis exactly equal is very hard and unnecessary, if they're within 3 km from each other the orbit is circular enough. If your orbit is very elliptical, just coast to apoapsis and burn prograde in map mode, watching what the periapsis is doing and killing the thrust as soon as it gets within reasonable range. If the apoapsis runs away from you, kill the thrust, coast to the new apoapsis and repeat.

There are faster methods but they are not as simple and you'll learn them over time.

[SpeedDaemon]

Don't need to wait until 10k. Start turning gradually as soon as you pass about 130m/s (no more than 5deg off vertical to start), and more or less follow the prograde marker down (you'll probably be totally horizontal somewhere between 30-40km). Burn until your AP is where you want it.

Get something like Kerbal Engineer Redux, so you can see AP and time to AP. When you get to within 20-40s of AP (depending on TWR), start burning at 0deg pitch, and watch the time to AP. If it gets below about 15-20s, throttle up, and if it starts to run away from you throttle down. The closer your PE gets to your AP, you want less time to AP.

[diomedea]

Ships with different TWR and Delta-V at each stage will perform differently, so you can't use one ascent profile for all. The gravity turn may more steep with one, but better if more smooth with another. But probably the single thing you should improve is with the timing for the circularization burn. The initial burn during ascent must be enough to reach apoapsis at the set altitude (e.g., at 100 Km), but once the apoapsis is obtained, you should stop the engines (or, burn almost horizontal). Of course, to know the apoapsis, you have to look at the marker with the map view. So, stop the engines, and set a maneuver node at apoapsis for the circularization: just burn prograde until periapsis is at the same level. If you move the burn too early or too late, you see that the trajectory will turn elliptical; the only correct one is when the burn is exactly at apoapsis. With the node set, try to compute the amount of time needed to perform the burn, and begin burning about half the burn duration before reaching the node. But, keep following the blue node marker for all the burn until all the required Delta-V is done, and you should have a perfect orbit.

[Empiro]

The trick to getting really circular orbits is monitoring your time to apoapsis rather than your periapsis. As suggested above, when burning to circularize, point slightly below the prograde marker toward Kerbin to depress your apoapsis so that it doesn't go up.

Then, once your periapsis is above the surface of Kerbin, slow down your burn. Keep your time to apoapsis at about 5-10 seconds out through a combination of throttle control and pointing either slightly above or below the prograde marker. Do this for as long as you can, and you will end up with an orbit very close to perfectly circular.

[Superfluous J]

If my apoapsis is under 90 and my periapsis is over 70 I call that "circular."

[TythosEternal]

Don't need to wait until 10k. Start turning gradually as soon as you pass about 130m/s (no more than 5deg off vertical to start), and more or less follow the prograde marker down (you'll probably be totally horizontal somewhere between 30-40km). Burn until your AP is where you want it.

Get something like Kerbal Engineer Redux, so you can see AP and time to AP. When you get to within 20-40s of AP (depending on TWR), start burning at 0deg pitch, and watch the time to AP. If it gets below about 15-20s, throttle up, and if it starts to run away from you throttle down. The closer your PE gets to your AP, you want less time to AP.

A word of caution... While 130 m/s is a good threshold, you need to reach that state at a sufficient rate of acceleration, or you will have an unstable pitchover. For example, try to reach it within the first (say) 20 seconds of your flight (though that number is arbitrary, I can't crunch the numbers right now). You could also use T/W as a criteria, but this will require the Engineering add-on.

[Aragosnat]

Don't need to wait until 10k. Start turning gradually as soon as you pass about 130m/s (no more than 5deg off vertical to start), and more or less follow the prograde marker down (you'll probably be totally horizontal somewhere between 30-40km). Burn until your AP is where you want it.

Get something like Kerbal Engineer Redux, so you can see AP and time to AP. When you get to within 20-40s of AP (depending on TWR), start burning at 0deg pitch, and watch the time to AP. If it gets below about 15-20s, throttle up, and if it starts to run away from you throttle down. The closer your PE gets to your AP, you want less time to AP.

Sometimes the design of the rocket kind of forces you to wait till then. As some lower stages have been known to collide with the center stack. As most of the time the decouplers don't have enough. Ohmph to push the stage far enough away. At Least till people realize that they need that little sepatron to help speed boost things away.

[Wanderfound]

so i followed the guides (scott manly) i do the whole 45* turn at 10k i burn at apoapsis just like in the vid but my periapsis never equals my apoapsis its always an elliptical.

is there more to it, am i doing it wrong?

i would like to do a space station at 150k my rocket design is fine its very stable and has plenty of fuel i just feel like my trajectory is some how incorrect. or do orbits not work this way?

Simple version: during ascent, monitor your apoapsis. Once it reaches the height of the orbit you want, kill the engines. Go to map view, set a prograde manoeuvre at your apoapsis that raises periapsis to the same altitude. Wait until it is time to perform that manoeuvre (look at the navball, next to it will be figures for "estimated burn" and "time to node"; divide the burn time by half and start the rockets that long before the node so that the middle of the burn happens at the node) and then do it. Point at the blue symbol on the navball, crank the engines to max, and be ready to shut down as soon as the required ÃŽâ€V (the bar to the right of the navball) hits zero.

Flashy (and more fuel efficient) version: Monitor your "time to apoapsis". Once your apoapsis height starts getting close to where you want it (a fair bit before or you'll overshoot while turning), pull the nose down to the horizon and keep it down until the time to apoapsis figure reduces to 10-20 seconds. Keep burning, adjusting your pitch to hold that figure constant; pitching down reduces it, pitching up increases it. The amount of pitch-up required will reduce as your velocity increases. Continue burning until both apoapsis and periapsis are where you want them, but keep an eye on the apoapsis; at high velocity, you only need to pitch a fraction too high for a short time to hugely increase the apoapsis.

Mods that provide basic in-flight instrumentation (Mechjeb, KER etc) so that you can monitor the needed information without switching to map view make the flashy version much easier to do.

Edited August 20, 2014 by Wanderfound

[Aethon]

Simple version: during ascent, monitor your apoapsis. Once it reaches the height of the orbit you want, kill the engines. Go to map view, set a prograde manoeuvre at your apoapsis that raises periapsis to the same altitude.

I'd also add wait until you're in space ( above 70km- when the music comes on ) before you set your circularization maneuver node. If you're still in atmosphere, you're AP is decreasing and when you get to the node it won't be on your AP, which will make circularizing more difficult.

[Wanderfound]

I'd also add wait until you're in space ( above 70km- when the music comes on ) before you set your circularization maneuver node. If you're still in atmosphere, you're AP is decreasing and when you get to the node it won't be on your AP, which will make circularizing more difficult.

Good advice in general, but problematic if you're aiming for a low 70km-ish orbit; you may not have time to do the burn if you leave it that late, particularly if you used a fairly vertical ascent. By the time you're over 60,000m the drag losses should be slow enough not to matter much.

[KingPhantom]

i hate perfect orbits... try to catch the AP or PE of a perfect orbit in orbital map view... good luck...

[Superfluous J]

i hate perfect orbits... try to catch the AP or PE of a perfect orbit in orbital map view... good luck...

The more your PE and AP jiggle, the less important it is to do your burn right at one of them. The fuel savings of burning at 73,823 km vs 73,825 km is not just negligible, it's laughable.

[Aethon]

...particularly if you used a fairly vertical ascent. By the time you're over 60,000m the drag losses should be slow enough not to matter much.

True, but I like super efficient launches and usually have a 6 min coast to AP.

I used to launch to 70ish km, but found it hard to rendezvous from a lower orbit. When I raised my standard launch height to 85 km I found I sometimes use less dv than to 70km 'cause during the 85km launch, I have time to dip the nose below the horizon, allowing me to be periapsed and nearly circular by the time my AP reaches 85km and M(ain) E(ngine) C(ut)s O(ut) ( usually less than a 1 sec. circ. burn ).