Flight Question Docking the Iss with nonspherical gravity source and and gravity-gradient torque

[LuckyHendrix]

Until now I used to play with all the perturbation settings unchecked. But now that I have activated them I'm having trouble docking to the ISS with a reasonlable amount of fuel left (i'm using either soyouz, progress or the shuttle.)
This is how I operate :
-After a successfull launch, when I reach my Apopasis I make a prograde burn that will raise my Periapsis ( soon to become apopasis) to the height of iss's orbit.
-then I perfect my plane alignement to have a Rinc of 0.00
-When I reach again my apopasis I make a prograde burn until I see on the Sync orbti MFD that I will have a DTmin of 0 in 20 orbits.

But over time my DTmin increase a lot and I have to do several burn in order to keep it near 0. And my plane alignement with ISS increases too and I have to re-align it several times. This causes me to lose a lot of fuel and eventually i don't have enough to come back after docking with ISS.

How do you do it ? any tips or advices ?

 

[Urwumpe]

Instead of doing single large corrections, do smaller steps. And remember to correct your plane at the apoapsis, when the velocity is lower.

 

[Usquanigo]

Instead of doing single large corrections, do smaller steps. And remember to correct your plane at the apoapsis, when the velocity is lower.

But aren't you supposed to get better results when doing it at the Nodes (Ascending and Descending)? Align Planes MFD and some tutorials say to do it then ("AN to AN" - Anti-Normal to Ascending Node, and all that).

 

[Urwumpe]

But aren't you supposed to get better results when doing it at the Nodes (Ascending and Descending)? Align Planes MFD and some tutorials say to do it then ("AN to AN" - Anti-Normal to Ascending Node, and all that).

Yes, of course you do so at the intersection, but at the one that is further away from Earth.

 

[Usquanigo]

Yes, of course you do so at the intersection, but at the one that is further away from Earth.

Gotcha.

 

[Urwumpe]

ideally, you do your phasing burn so, that you have the apoapsis exactly at the intersection...but without proper orbit analysis this is not easy.

Better use small steps...first raise about 30 km below the ISS, and have the apoapsis at below the apoapsis of the ISS, then do a few orbits before you raise the periapsis so it is also 30 km below the ISS periapsis, then wait a moment before raising into the ISS orbit, etc.

I had written a bit about the correction maneuvers that you do for getting to the ISS, the Shuttle does that as well.

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

By aiming a bit behind the ISS before you do the final correction, you need less fuel.

 

[Andy44]

Also, with gravity gradient, you're going to have to make sure the ISS and the orbiter are in the proper attitudes, or else you're going to blow more fuel when you get there just to stay aligned with the docking port.

 

[LuckyHendrix]

another question , the orbiter's OMS are pointing 15° up right? do I have to account to for all my orbital manouvre ?

 

[N_Molson]

Yes, that's why there is an additional horizontal line on the HUD. Take it as your horizon reference.

 

[LuckyHendrix]

This time (with progress) I stayed on a lower orbitthan ISS (250km*195km) untill my the ISS reched my ApA at the same time than I did.Then I raised my ApA to the ISS's orbit height, this allowed me to meet the ISS in 4 orbits. Once I was 700km away form it I used rendez-vous MFD, it's very usefull :thumbup:.
I managed to dock with ISS with 25% fuel left wich I think is reasonable for a Progress.
Thanks for your advices.

Yes, that's why there is an additional horizontal line on the HUD. Take it as your horizon reference.

I don't have this line with the shuttle fleet only with stock Atlantis. Do I still have to make the corrections ?

 

[Andy44]

Yes, you still have to make the corrections with shuttle fleet. I think the angle is actually 13.5 deg.