General Question Reducing the eccentricity of an orbit

[thethedev]

Hi There
I downloaded orbiter a couple of days ago and have been slowly getting to grips with it.
I can just about get my ship into orbit at the moment but I'm having some problems.
Occasionally I mess up completely and end up with a very very oval orbit.
I have tried prograde and retrograde burns at Apogee and perigee but this only seems to either increase or decrease the size of the orbit.

How do I make my orbit as close to a circle as possible? Should I be burning normal or antinormal? Because this doesnt seem to work either!

Thanks

 

[cipher169]

prograde raises your orbit on the opposite side, while retrograde lowers it on the opposite side.

in order to achieve low eccentricity, you should try to have your apoapsis and periapsis almost the same(within 10 kilometers is usually fine.

 

[asbjos]

The easy way to make an orbit circular is to burn prograde in apoapsis or burn retrograde in periapsis.

 

[Grover]

ok, with a little thought into the shape of an orbit, you can work out that perfectly circular orbit must have equal highest and lowest points (apoapsis and perapis respectivley), which means that your PeA and ApA should be the same, give or take your own margin of error. 10km is pretty good for starts, but theres nothing to stop you getting it within 1km

as has been mentioned, if you burn PROgrade, you increase your speed, yo the opposite side of your orbit gets higher. and burning RETROgrade reduces your speed, thus lowering the far side of your orbit

so if you burn PROgrade at the highest point of your orbit (Apoapsis, or the hollow circle) you will raise the lowest part, closer to the highest point. take care as you do this to ensure that you dont overshoot

if you want to "circularise" at your lowest point, then burn RETROgrade to lower the highest point of your orbit.

Normal and antinormal are for adjusting the inclination of your orbit, and are critical for intercepting other orbiting objects, so you should learn those once you can stabilise your orbit

for now, just focus on PeA and ApA, and possible ApT and PeT as well (the time to apoapsis and time to perapis respectivley)

 

[statickid]

also, since you are just starting... here's another idea, click the PRJ button on the orbit MFD side panel. It's possible you are perceiving the orbit as eccentric by looking at the ecliptic azimuth instead of your orbit azimuth. I'm only suggesting this because it sounds like you tried the correct operations already and you should have seen it circularize by what you described. Also, read the ECC value on the orbit MFD, it's sixth down on the left. This number move closer to ZERO if you are indeed circularizing your orbit

 

[martins]

Given that an orbit circularisation at the apoapsis distance can be achieved by a prograde burn, and a circularisation at the periapsis distance by a retrograde burn at periapsis, both of which are burns in the same absolute direction, is it possible to achieve an orbit circularisation at any point of the orbit by burning in apoapsis-prograde direction?

In other words, if you add a Delta-v in apoapsis-prograde direction to your velocity vector at an arbitrary point of the orbit, such that the resulting velocity vector is perpendicular to the radius vector, will the magnitude of the velocity be that of the corresponding circular orbit?

 

[thethedev]

also, since you are just starting... here's another idea, click the PRJ button on the orbit MFD side panel. It's possible you are perceiving the orbit as eccentric by looking at the ecliptic azimuth instead of your orbit azimuth. I'm only suggesting this because it sounds like you tried the correct operations already and you should have seen it circularize by what you described. Also, read the ECC value on the orbit MFD, it's sixth down on the left. This number move closer to ZERO if you are indeed circularizing your orbit

Turns out this was the problem :blush:
Thanks for all your help, I almost managed to actually dock with another spacecraft last night!

 

[BruceJohnJennerLawso]

Given that an orbit circularisation at the apoapsis distance can be achieved by a prograde burn, and a circularisation at the periapsis distance by a retrograde burn at periapsis, both of which are burns in the same absolute direction, is it possible to achieve an orbit circularisation at any point of the orbit by burning in apoapsis-prograde direction?

In other words, if you add a Delta-v in apoapsis-prograde direction to your velocity vector at an arbitrary point of the orbit, such that the resulting velocity vector is perpendicular to the radius vector, will the magnitude of the velocity be that of the corresponding circular orbit?

Sounds like a plausible theory, maybe doable with force vectors on, but itll be tricky & require lots of fuel

 

[Ripley]

...is it possible to achieve an orbit circularisation at any point of the orbit by burning in apoapsis-prograde direction?...

Isn't it exactly what IMFD does with its Orbital -> Cir command?

 

[statickid]

Here's a simple way that I tried that works, takes very little fuel and I did it twice at arbitrary points. This may not be optimum or the most efficient but it worked really well sooo... :shrug:

1. at the apoapsis (I orbited the moon because of the low gravity) turn retrograde and identify a star, use planetarium mode if its easier.

2. go to another part of the orbit, arbitrarily

3. point at the star or area on the celestial sphere you were pointing at previously

4. take a note of your degree reading on the orbit HUD, and level your horizontal axis with the orbital plane EXAMPLE: on the orbit HUD you are facing 255 degrees on the orbit HUD AND pointing at the star or point in sky.

5. burn main engines at a LOW setting, to manage your orientation.

6. Simultaneously manage your rotation with the RCS so that your nose reticule maintains its location on the orbit HUD... in other words, rotate your ship so that the reticule stays pointing at 255 degrees, this is easier than it sounds if the engines are burning slow enough.

7. The eccentricity will steadily decrease until it is below .1, for me, it actually lowered to below .05 both times.

8. At this point the periapsis will be near you or actually become your location.

9. turn retro and make fine adjustment

COOL!!!!

I did this with a TRA<180, so it might be different somewhere else