Circularizing my orbit without hassle

[DalisClock]

I feel like I'm missing something really basic here so bear with me. I spent a long time playing on pre 1.0 KSP and that's where I learned to reach and make orbit. Since 1.0 I've finally gotten the hang of thew new gravity turn(I also took a couple months off until now, so that might explain it) but during orbital insertion I've been getting these really lopsided orbits, with Perigee being around 70-80 K but Apogee pushing out to 150-200K. My attempts to circularize end up causing my Perigee to drop back down, sometime below the 70K threshold for a stable orbit. Burning Prograde directly towards the horizon at near my desired altitude creates much the same effect.

Setting up a maneuver node at my 70-80K Apogee during ascent and coasting up to it doesn't seem to work as well as it used it.

I'm not sure if I've forgotten how to space or something in the new updates is messing me up. Any help is appreciated.

Edited September 2, 2015 by DalisClock

[Ripper2900]

I usually launch with a gravity turn, trying to get horizontal at 50-60 km. Then burn horizontal untill apoapsis is 80km. When I reach those 80 km, I burn prograde, raising periapsis to 80 as well. Circular orbit achieved.

It sounds like you're either burning too much during ascent, trying to get you periapsis to 80; this would cause your apoapsis to grow too high. Or you're not burning horizontally AT the apoapsis of 80?

[MarvinKitFox]

...

Setting up a maneuver node at my 70-80K Apogee during ascent and coasting up to it doesn't seem to work as well as it used it.

I'm not sure if I've forgotten how to space or something in the new updates is messing me up. Any help is appreciated.

Absolutely nothing has changed to orbital mechanics, once you are out of the atmosphere.

Any change you experience would be due to pebkac.

(problem exists between keyboard and chair)

[Wolf Baginski]

I've been testing Spaceplanes, which can be a bit different. But, with the right intakes, one per Rapier, I can get over 1400 m/s before the switch to rocket-mode, and not get too much heating. I watch the apoapsis (Kerbal Engineer is good for this) cutting power when it slightly exceeds the target height. Since I'm still deep in atmosphere the apoapsis will drop during the coast upwards, and a short burn after leaving atmosphere will correct any undershoot, and incidentally raise the periapsis a little. The circularisation burn is locked to Prograde with the SAS and it can be done in two burns. Burn 1 raises the periapsis above the atmosphere, while Burn 2 on the next orbit's apoapsis completes the job.

Spaceplanes essentially switch modes with the equivalent of a low gravity turn, dropping their nose at around 20km to get the most out of air-breathing mode. With the new drag models, and the re-balanced thrust from engines, things in atmosphere will be a bit different, Good old-fashioned rockets do better with a steeper climb since the thicker air just wastes delta-V. The thrust and i_SP changes will all have an effect on existing designs.

[mrclucks]

I suggest installing KER so you can see your periapsis and apoapsis without needing to go into map mode. Periapsis should be around -150km or higher. If you're making a manuver node while still in the atmosphere then it will most likely be away from apoapsis once you actually reach it

[Belphegor]

if you are already ahead of your apoapsis (altitude decreasing) and you burn horizontally, it will continue to drop behind you. burn a bit towards the blue side of the navball and try to keep the apoapsis stable or let it 'catch up to you' and then burn horizontally again. note that this is probably less efficient than getting it right the first time

[Stoney3K]

Also, don't ascend too steeply ('up and then right'). If you have an apoapsis of 80km but a very eccentric orbit, you're not going to have enough time to complete your circularization burn before you start dropping back down into the atmosphere.

In the upper atmosphere you need to focus on building horizontal velocity as much as possible to keep your orbital insertion short.

[pandaman]

I have this problem too, more noticeable in the new aero. But I do just put it down to bad piloting on my part during the ascent.

[cantab]

My attitude is an orbit's an orbit. Once established I can worry about making it neater if I want.

[Val]

Also, don't ascend too steeply ('up and then right'). If you have an apoapsis of 80km but a very eccentric orbit, you're not going to have enough time to complete your circularization burn before you start dropping back down into the atmosphere.

In the upper atmosphere you need to focus on building horizontal velocity as much as possible to keep your orbital insertion short.

Agreed!

And with enough TWR and a little aerodynamics it is possible to make an ascent that is pretty much horizontal all the way. It's very dV efficient.

[Desiato]

Maybe think more about your radial when you burn apo. If you are doing anything to disturb the burn like stageing it gets tricky; trying to use an orbital stage to periapse makes timing and pitch control critical (Though you can reduce debris and wasted fuel by taking nothing unnecessary to orbit). If you are nose up after periapsis you are essentially burning a bit radial out, so you are shifting apoapsis higher but moving peri lower as well; vice versa for nose low.It's always cleaner with a high TWR stage at apo., otherwise just be very careful.

P.S. I use Precise node so it's a bit easier, but not withot it's own quirks.

[Empiro]

There's a bunch of tricks you can do to achieve that perfect, circular orbit. I've found that time to AP is the best number to look at and monitor. I use Kerbal Engineer to display my time to AP.

The way I do this is as follows: launch normally, get my AP to about where I want it. Then when I get to about 30 seconds to AP, I point to prograde and fire up my engines. I make sure that my time to AP is slowly decreasing. If it is increasing, then I point slightly below prograde (slightly toward the ground). If it's still increasing, I lower or cut my thrust entirely (and wait until my time to AP is lower). If it's decreasing too quickly (or I somehow fly past it), then I point above prograde.

As you get closer to and closer to circularizing, the time to AP will have been decreasing the entire time, and should be down to 5-10 seconds. At this point it becomes increasingly sensitive, so make sure to use only low thrust. If you keep this process up as long as possible, your orbit should be very close to circular (<100m difference).

Edited September 1, 2015 by Empiro

[problemecium]

Here's what I do:

- burn shallow and gently as you get high (no I mean literally), i.e. when the navball changes from Surface to Orbit mode, you should lower the thrust to about 1 G's worth and aim horizontally.

- be going fast when you achieve apoapsis. When my apoapsis hits 70km, I'm usually going between 1850 and 2050 m/s. This means the circularization burn will be small and provide less opportunity to screw up your orbit.

- achieve orbit in map view. This lets you see your Apoapsis, Periapsis, and general orbit shape without having to use MechJeb or KER.

- the most important part: when BEHIND the Apoapsis, tilt the ship down; when PAST the Apoapsis, tilt up. If you hit the "magic" angle, you'll find that your Apoapsis itself remains unchanged. My circularization burns tend to start about 20 seconds before the Apoapsis with me angled down about 5 degrees, and end about 30 seconds after the Apoapsis with me angled up about 10 degrees. If you were already going fast as mentioned above, this burn will be small, so you can easily center it around the Apoapsis and precisely adjust your trajectory using low thrust without too much risk of sinking back into the atmosphere.

I should probably make a video.