General Question Changing the Periapsis

[greyrogue715]

Could someone explain how to change the longitude/latitude of the periapsis? - or point me to the thread/documents explaining this? Thanks.

Andrew

 

[goaowonk]

Well, I'm not an expert on Orbiter (I'm just starting) but I've understood you wanted to change periapsis' coordinates.
It's my first Aeronautycs degree year, so I only know a few basics on Orbiting Mechanics.

All right, if you want periapsis goes lower, you need to do a retro-grade burn (the cheapest place to do it in terms of fuel consumption is at apoapsis; always change that altitudes at periapsis or apoapsis when changing orbits). That will increase your periapsis speed, and in consequence your periapsis altitude. (If you want to decrease that altitude, just burn retro-grade at apoapsis; and the same if you want to vary apoapsis altitude).

But if you want to vary lattitude and longitude I think you need to:
1.- Change orbiting plane with a normal/anti-normal burn up to your final inclination.
2.- Circulize your orbit (periapsis=apoapsis), slowing or speeding your vessel. Here I suppose it's better going into a circular orbit at final_periapsis altitude (final_apoapsis attitude is also valid, but I don´t like going too far before I've arrived). (*)
3.- When you reach the point in your orbit where your final periapsis is going to be, do a pro-grade burn up to you reach enough speed to final_apoapsis altitude (or slow down from apoapsis attitude and apoapsis point).

(*) I've read it later and I don´t like it very much: that´s suspiuciously easy.

Here are some formulas about what I've said:
- Orbital velocity:

(**)
- Hohmann's transference:

(**)
- Changing orbital plane:

(**)

(**) = all that '3's shouldn't be there... Those're just '1's... Dammed I always mistake it...

I hope it will help you, and I'm sorry because of my English level, I'm Spanish and... well, I know I should improve it.

 

[clive bradbury]

By changing the inclination of your orbit. What are you trying to achieve, precisely?

 

[Urwumpe]

You can also change the argument of periapsis (angle between the crossing the reference plane and your periapsis), at the point where your current orbit intersects with your new orbit - but that is a bit messy to calculate with the standard MFDs.

 

[greyrogue715]

I am trying to match the ISS periapsis to my own. I know sync orbit MFD will work for that, but I am not sure at which point in the orbit to fire thrusters (at my periapsis?). I have attempted the scenario in the 'Checklist' scenario but I guess I am not understanding it. I have successfully rendezvoused and docked a few times.

Basically, if my periapsis is over 'Chicago' for example, how do I move it to another location on the globe? Is it just a simple inclination change maneuver?

Sorry if this is dumb question. I'm a rookie.

Continued thanks,
Andrew

 

[Chris74]

well if u want to match, than better match your apoapsis also, and yout whole orbit, otherwise ur periapsis will be way off again after one full orbit

in an ordinary case, adjusting your PeA along the orbital path is easy. apply downward thrust near PeA, and it will move forward. or vica versa. or constantly zero your vertical speed, and the PeA will "travel" with you. But this will also increase your orbital energy ie. ApA.

adjusting it north or south (...ish) : if its far from the equator, then simple inc. change. if its close to the equator, then change of the longitude of ascending node. But the change will always be perpendicular to your orbital plane. so ull need to adjust again along the orb. path.

 

[Cras]

Try Rendezvous MFD if you want a good way to meet up with an object in orbit.

 

[Urwumpe]

I have also written some stuff in OrbiterWiki about the topic:

http://www.orbiterwiki.org/wiki/Rendezvous

(Again, as usual, feel free to add or tell the world to add information that is missing, we can't look into your head, and if we could, our privacy protection official would stare into our heads)

 

[Felipi1205]

The pictures aren't loading correctly, just the last of the four pictures is correct here...

 

[greyrogue715]

Felipe, if you are talking about the Orbiter Wiki link, I can see all pics fine here. The Math is sadly beyond me right now, which I am also learning (a slow go).

The Rendezvous MFD works great. Thanks for the help guys. I am gonna mess with this stuff today and see if I can't figure this out.

 

[goaowonk]

By changing the inclination of your orbit. What are you trying to achieve, precisely?

At your current orbit, you can't reach any point (at any attitude) which doesn't belong to that orbit plane. So first at all you need to reach a plane which contains that plane.
E.g.: Suppose that your orbit is parallel to Ecuator. That means that you are always above it, and you'll never reach the Poles. If you want to go above North or South Pole, then you'll need to change your plane inclinating it PI/2 rad. Then you'll be in a new plane. That's the importance or changing inclination

 

[greyrogue715]

I made a good launch to orbit, aligned with the Mir, matched PeA, and then synced orbit. I tried it both with and without the Rendezvous MFD. After chasing the station around and wasting all my fuel the first time, I managed to dock the second time, but still used way to much fuel. I am starting to get the hang of it.

(So how far in Math does a guy have to study to start understanding these formulas? Advanced Calc? I wonder how many people here are Math whizzes. I definitely need to improve my skills and I am considering getting back into college, just so I can understand Orbiter better!)

Thanks for your continued tolerance and help, :thumbup:

Andrew

 

[Urwumpe]

A bit of trigonometry isn't bad, otherwise, you rarely need advanced math there. The front cover functions are enough

 

[greyrogue715]

I found this basic video helpful also:

---------- Post added 05-02-12 at 03:01 AM ---------- Previous post was 05-01-12 at 03:30 PM ----------

So could someone clarify to me why the LAN does not change on every orbit. If LAN is the point where the orbit intersects the equator heading north, how come it stays the same with every orbit when I cross the equator heading north at a different point every orbit? I'm not getting it.

 

[Cras]

I made a good launch to orbit, aligned with the Mir, matched PeA, and then synced orbit. I tried it both with and without the Rendezvous MFD. After chasing the station around and wasting all my fuel the first time, I managed to dock the second time, but still used way to much fuel. I am starting to get the hang of it.

(So how far in Math does a guy have to study to start understanding these formulas? Advanced Calc? I wonder how many people here are Math whizzes. I definitely need to improve my skills and I am considering getting back into college, just so I can understand Orbiter better!)

Thanks for your continued tolerance and help, :thumbup:

Andrew

How I tend to use Rendezvous is start with a time of 2400 seconds, and adjust from there. If the end velocity (relative to the target) is manageable, and this will change from ship to ship, or instance the XR-2 can handle a far larger amount than say the Space Shuttle, I will then go for that path. But I also pay attention to what that path will be. I could just go with that solution regarless, but I like to not dip down in altitude too much, and want very much to approach under the target, not loop around it, or end up being in front of it.

To get a good solution, as I get closer and close, I will slowly raise my orbit higher and higher until I am fairly close to the target's orbit (helps alot of the target's orbit has nearly 0 eccentricity.)

Then odds are Rendezvous will provide me a solution that will be a nice path to under the target with a nice small relative velocity than can be nulled out.

Or you can use IMFD. Which is probably easier.

Or you can use Guidance MFD, which will do the whole thing for you. Which is the easiest. And not much fun.