i cant get a good orbit around kerbin

[lightbombs]

i always have a 20,000m or more difference between the apoapsis and periapsis of kerbin! so frustrating

Edited May 2, 2015 by lightbombs

[Red Iron Crown]

Welcome aboard, lightbombs!

It takes practice. I've also found it useful to use very low throttle and patience toward the end of circularization, and an RCS system can be useful for finer adjustments.

[Gaarst]

The best way to easily get a rough circular orbit is wait until your are at the apoapsis and burn carefully prograde. At one point, apoapsis and periapsis point will start to move really fast to invert their mutual position, try to stop burning when they are both 90° from your craft. You can also use manoeuvres to plan this.

Then using RCS you can get a circular orbit within ~10 m with a little practice.

[lightbombs]

guys i have uploaded this vid to show you what i do please tell me what im doing wrong

https://youtu.be/i4SS1JHrbT4

[Gaarst]

guys i have uploaded this vid to show you what i do please tell me what im doing wrong

https://youtu.be/i4SS1JHrbT4

You're going WAY too fast at the beginning !!

First go to the VAB, and limit the thrust of your BACC SRBs to get a TWR of 1.2 to 1.5

Then, ascent profile has changed for 1.0.

Turn ~5° a few seconds after launch, and then turn gradually to follow your prograde icon on the navball. This should make you turn at ~45° at 10km and ~30° at 20km, then air resistance is futile, and you can just go as pre 1.0.

To circularise you orbit as I indicated on my first post, you should first get into orbit, not circular by just burning at the apoapsis until your periapsis is at >70km

Finally, when using a node, you should start burning at half burn time before the node (ie: burn time 1m20, start burning at 40s before) because the node assumes that you get instant acceleration at the point where you put the node. This iis not possible in the game, so you have to start burning before the node so that in average the burn is at the node.

[Danaak]

The simple answer you're looking for is you need to rotate your orbit slightly. On the maneuver node it's the blue circle - at the 12 o'clock and 6 o'clock position in your video. That will allow you to reorient your orbit.

[jarmund]

guys i have uploaded this vid to show you what i do please tell me what im doing wrong

https://youtu.be/i4SS1JHrbT4

A few pointers:

• - You're going too fast too early, wasting fuel fighting drag. I normally try to stay in the 150-200 m/s until I'm above 10000m. Adjust your SRB thrust limiter down, and they'll burn longer and more efficiently instead.
  
• - Your ascent profile is way too steep. It made sense in the pre-1.0 soupmosphere, but now you are wasting a lot of fuel fighting gravity without gaining any orbital velocity. I normally pitch over to 5 degrees pretty much as soon as I clear the pad, and then try to aproximate this curve. It's not perfect, but it allows me to launch to orbit fairly efficiently:
  
  • Pitch to 15 when passing 100 m/s
    
  • Pitch to 30 when passing 200 m/s
    
  • Pitch to 45 when passing 10000 altitude, and start increasing speed.
    
  • Pitch to 60 when passing 20000 altitude, full throttle until AP is where I want it.
    
  • PS: By pitch, I don't mean just pitch, I also mean try to keep your prograde there. Pitch as needed to adjust.
    

  [*]- Missing nosecones on the SRBs. They matter now

  [*]- Loads of speed lost to drag from excessive angle of attack. Try to always keep your nose pointing as close to your prograde as possible. 5-10 degrees AoA max when turning.

In addition, if you have something that tells you "time to apoapsis" (Kerbal Engineer Redux and MechJeb both does this. I'm sure there are other mods too), try to keep that time around 1 minute.

What you're doing once you have an apoapsis of 100km is pretty much correct, except from one thing: Half of your burn should be before hitting the AP, and half after, that's the most efficient way of doing it (otherwise, more of the thrust will be spent stopping the downwards fall). See the burn-time estimate, divide it by 2, and that's how long before the maneuver node you should start your burn. This isn't just for launching into orbit, but maneuvers in general.

Also, Scott Manley has a nice

Edited May 1, 2015 by jarmund

[lightbombs]

i dont know how to find my rocket's twr can you tell me please

[Shpaget]

One more thing.

You placed the node before Ap which caused your initially planned orbit to be elliptical. You can move the node around. When expanded (showing the dragy-aroundy bits) click on the gray circle indicating the node. It will turn a shade lighter, now you can drag it around. Aim to put it as closely to Ap as possible.

[MarvinKitFox]

While all the above comments about overspeeding, no-nose-cones, etc.. are correct , they do not address the OP's problem.

What he is unhappy about is that the maneuver node is giving his a lopsided orbit prediction.

CAUSE: The maneuver node is placed before the apogee,

SOLUTION: zoom in a lot more, *then* place your maneuver node for circularize. It needs to be exactly ON the apogee spot

if it is still a bit lopsided, adjust the other axis of your maneuver node (the lines pointing down/up), until the result is closer to exactly round.

If extreme precision is desired, add another circularization burn, at either perigee or apogee, to do the final nudge

[Habalabam]

You should consider having the Engineer Redux mod. It will show you the realtime data on the orbit. Especially useful during ascent is the height of the apoapsis and the time until you reach it. Your goal is to reach the apoapsis with as much horizontal velocity as you can muster. Once the Apoapsis reaches the desired height, usually about 75K, the your focus should be having adequate speed when reaching it. A good way to achieve this is to find the horizontal burn that will "push" the apoapsis in front of you. If the "time to apoapsis" is counting down, you need more thrust (or slight inclined upwards if at max thrust). The closer it is to you, the closer to orbital speed you need to be in order to avoid it counting down. Usually, the apoapsis can stay about 0.5-1 minute ahead of you. As you pick up speed, you'll find that you can be closer and closer to the apoapsis. As you are close to the apoapsis, only the periapsis will be raised by your thrust, thus achieving a circular orbit.

[Gaarst]

i dont know how to find my rocket's twr can you tell me please

To get the TWR divide the thrust of a stage by the weight of your rocket:

TWR = Thrust / (Mass * 9.8)

Or you can use Kerbal Engineer Redux which does the calculation for you in real time

[lightbombs]

thanks sooooo much to everyone for helping me out! i can do a good orbit around kerbin now (yay)!!

but final question what is a good difference between apoapsis and periapsis for a good orbit?

[Shpaget]

If an elliptical one is not required I go for around <10% Ap Pe difference for parking orbits, and <1% for permanent ones when I want them pretty.

If Pe is above 70 km, it's a good orbit.

In mods like Remote Tech, it is required to achieve multiple very accurately matched launches with as nearly identical orbital periods as possible (eccentricity is not that important). A difference of a few seconds per orbit will quickly accumulate in high warp, and your constellation will fall apart.

[Habalabam]

Please edit your first post in this thread and set the state of the thread to "Answered"

[EtherDragon]

thanks sooooo much to everyone for helping me out! i can do a good orbit around kerbin now (yay)!!

but final question what is a good difference between apoapsis and periapsis for a good orbit?

Depends on what kind of orbit you are looking for. Most people like "circular" orbits that have low eccentricity - aim to have their AP and PE as close in height as possible.

As shpaget mentions, so long as your Pe is above 70km, you have a stable orbit. If you're interested in why all this is like it is check my sig for a good video.