ISS orbit sync problems

[Wintermute]

I'm trying to sync my orbit with the ISS for docking, but here's what I'm encountering.

I record the altitude of the ISS as it passes through my periapsis, then burn prograde at my apoapsis to adjust accordingly. The thing is, after the burn, my per and ap start shifting back and forth (I can see the dots sort of swinging left and right on the orbit MFD. When I burn again at the periapsis to adjust my orbit duration, the DtMin starts climbing back up from zero as I continue orbiting, and I end up passing the ISS at about 9km.

I'm using the ISS docking tutorial from Go Play in Space and everything seems to work fine when I let it play out by itself. As far as I can tell, I'm doing everything exactly the same, but it goes all screwy on me.

Any ideas?

 

[Ark]

9 KM is plenty close to match speeds and thrust towards it.

 

[Hmuda]

9 KM is plenty close to match speeds and thrust towards it.

Was about to say that. My personal best was a 4km pass (I'm an Orbinaoob) but I'm plenty happy with pretty much anything that's lower than 50km.

 

[ar81]

[ame="http://www.orbithangar.com/searchid.php?ID=3074"]Tutorial: DG to ISS[/ame]

 

[Wintermute]

In the tutorial I'm using it has you turn toward V(ISS) and burn to slow your relative velocity at around 5km. How would I match speeds from 9-10km? Same way? Cuz I tried that and ended up further away.

Or do I retrograde burn right at the periapsis, something like that?

 

[george7378]

The thing is, after the burn, my per and ap start shifting back and forth (I can see the dots sort of swinging left and right on the orbit MFD.

Disable 'Nonspherical gravity sources' in parameters.

 

[Wintermute]

Disable 'Nonspherical gravity sources' in parameters.

Ah ha. That was it, George. Thanks!

 

[ar81]

In the tutorial I'm using it has you turn toward V(ISS) and burn to slow your relative velocity at around 5km. How would I match speeds from 9-10km? Same way? Cuz I tried that and ended up further away.

Or do I retrograde burn right at the periapsis, something like that?

You do not need to match speeds. You can use your inertial velocity to reach ISS. If you read the tutorial you will see.

Or you may use [ame="http://www.orbithangar.com/searchid.php?ID=1199"]Rendezvous MFD 050621[/ame]
But I advise you to try doing it as DG to ISS tutorial says, so you understand the logic behind it. With space shuttle flights you will not have enough fuel to spend.

 

[george7378]

Basically, there are 5 things you need to do to dock, none of which require you to do maths or anything complicated. Here they are in order:

- Align planes (use the map and align planes MFD burning normal + or - at orbit intersections until RInc reaches close to zero).

- Take note of the altitude at which the ISS passes your periapsis, and make sure that your periapsis matches that value. Remember to set the ISS as a target in the Orbit MFD to see (Sorry about the basic teachings, I don't know how much of a newbie you are )

- Sync orbits (burn (usually prograde) at your periapsis until the DTmin reaches close to zero.) You can make minor adjustments with LIN RCS mode, at any point in your orbit. Do it close to the periapsis for maximum efficiency.

- Wait until the top two lines of numbers in the top right of the Sync MFD are yellow. The countdown tells you how long until you meet.

- Use the relative velocity and distance indicators on the sync MFD, as well as LIN RCS and main thrusters to match speed roughly, and simply manouvre towards it.

It is useful to press F9 to enter planetarium mode as you get close to the ISS, so that you can centre it in your windshield (after the speed-matching retro burn) and use LIN thrusters (numpad 1, 2, 3, 6, 8, 9) to keep it so that it isn't moving across your windshield. Then, simply move towards it.

You can then use the docking MFD, but I prefer to dock without. Much less to worry about.

 

[martins]

Disable 'Nonspherical gravity sources' in parameters.

Turning off the laws of nature often helps, although some may call it cheating ...

 

[the.punk]

You could also use TransX.
With it I make one little burn at the right time by tweaking date and vel. Then I turn off the maneuver mode and see how my clostest approach is. With the RCS I put it down to 1 or 2km and wait until I'm there.
That works good.

 

[Sam]

I'm trying to sync my orbit with the ISS for docking, but here's what I'm encountering.

I record the altitude of the ISS as it passes through my periapsis, then burn prograde at my apoapsis to adjust accordingly. The thing is, after the burn, my per and ap start shifting back and forth (I can see the dots sort of swinging left and right on the orbit MFD. When I burn again at the periapsis to adjust my orbit duration, the DtMin starts climbing back up from zero as I continue orbiting, and I end up passing the ISS at about 9km.

If you end up a few km above or below your target when you null your relative velocity, as others have said you can always just burn straight toward the target and get there that way. You'll have to make a few corrective burns on the way in and it's not the most fuel-efficient way to do it, but if you're ok with that there's nothing wrong with it.

Now if you don't mind having that kind of result, here's a little trick. In your example you've got your apoapse above ISS and are trying to rendezvous at periapse. Set your periapse a few km below where you expect ISS to be. That way instead of trying to aim for a rendezvous at one point, you've got a wider arc of ISS's orbit in which you'll be at about its radius, +/- a few km. Even if the rendezvous point gets rotated further along or backwards in the orbit by non-sph. grav., perturbation, etc., it'll probably still be within that arc and you're likely to be near ISS's radius when you get to wherever the rendezvous ends up happening.

Similarly, if you end up putting your apoapse at ISS and dropping your periapse (maybe you need to catch up just a little), put your apoapse slightly above where you expect ISS to be.

SAM

 

[bryanfuller]

Nice Post

Hmuda says right

 

[jinglesassy]

Turning off the laws of nature often helps, although some may call it cheating ...

i dont call it cheating i call it changing the unchangable

 

[insanity]

A good quick and dirty way to dock with the ISS is to get a nice circular orbit below the orbit of the ISS. Align planes and then switch to the sync mfd and perform a burn until you have a point of intersection (intersect 1 or 2). Wait until you are at the point of intersection and burn prograde for a bit. While burning prograde, switch the mode to ship Perapsis until DTmin is close to zero and you are satisfied with the time enroute (keep burning to increase or decrease the number of orbits until intersection, but remember that when you are finally close to the ISS you are going to have to use fuel to match its orbit and fly in). Make small adjustments if necessary with the RCS system to account for any changes that skew the transfer solution. Flying this method I can get to the ISS in 3.5 hours or less from the surface.