Scenario ISS Rendezvous

[Alexrey]

Hi guys,

I've been following a tutorial on how to rendezvous with the ISS in the Delta Glider but am having some trouble. I align planes, then set my perigee to that of the ISS's altitude when it passes my perigee, and then try and get DTmin to 0.00 by applying appropriate prograde burns at perigee to raise my apogee using the main engines and RCS. Now, in the video ([ame="http://vimeo.com/4718661"]DG4 to ISS: Orbiter Tutorial on Vimeo[/ame]), Tex is able to get DTmin down to zero and it subsequently stays at zero from then on. When I try this though, I get it down to about 0.05, but the damn number keeps changing up and down constantly so I can never get it down to zero. Then, as I wait for the rendezvous orbit to come around, I notice that DTmin fluctuates drastically at different points in my orbit, but then comes back down to within 5 seconds once I reach my intercept point (perigee) again. Now a DTmin of 5 seconds is far too high because of the massive distances covered by such high velocities of the ISS and DG, so I end up about 45km from the ISS on the intercept orbit!

I don't know how the heck Tex managed to make DTmin stay at exactly 0.00 for 3 orbits whereas my value jumps around constantly. Did he have orbital perturbations (non-spherical gravity sources, radiation pressure, gravity-gradient torque) turned off, or am I just doing something wrong?

 

[garyw]

45km isn't that far really. I tend to ignore DTMin once I'm that close and instead switch to the docking view on the HUD and tune in the XPDR of the ISS.

I also tend to keep one part of my orbit just below that of the ISS. This way, the lower, faster orbit will continue to catch up to the ISS.

Once I'm at about 10KM I null out the differences in velocity and start the approach to the ISS for docking.

However, that's just my way of doing the approach.

 

[asbjos]

Did he have orbital perturbations (non-spherical gravity sources, radiation pressure, gravity-gradient torque) turned off, or am I just doing something wrong?

I guess that he had all of those disabled. If you're in a normal orbit (not dropping into the atmosphere or below approx. 200 km) your orbit should be fairly stable. All that you have described here seems like what happens with orbital perturbations activated, especially non-spherical gravity and radiation pressure. If you disable those you would also get a stable and predictable orbit.

 

[meson800]

You may be able to use small bursts of linear RCS to reduce DTMin even when you are not at perigee.

However, I use a method similar to garyw, and I don't worry about the initial intercept distance that much. I normally end up around 20-30km from the ISS on the first intercept, then I use the Docking MFD and HUD for the rest.

 

[Alexrey]

Well I managed to dock with the ISS last night which gave me a smile for quite a while afterwards , but I did so from quite a distance away and have a few questions regarding this.
I got to about 45km from the ISS before the distance started increasing again, so using the Docking HUD I set my velocity relative to the ISS to close to zero (by pointing my nose at the "relative retrograde indicator" (circle with dot in centre)). I then turned by nose towards the ISS and gave a few seconds of forward main engine thrust which pushed me slowly towards the ISS. After some time the ISS started drifting to my right so I again set my relative velocity to near zero by finding the relative retrograde indicator again, then turned my nose to point at the ISS and gave some more forward thrust. I had to do this one more time before I was close enough to be able to just use little amounts of main engine thrust and RCS to align myself properly (instead of having to find the relative retrograde indicator again). My method seems a bit wasteful in terms of fuel usage and I was wondering how some of you manage to perform efficient rendezvous from similar initial distances to mine. Should I have not pointed directly at the ISS, but instead a few tens of degrees to the right, when performing the main engine thrusts?

 

[N_Molson]

Should I have not pointed directly at the ISS, but instead a few tens of degrees to the right, when performing the main engine thrusts?

Ideally, the best method would be to predict the relative motion of the ISS over time, but that would require a special MFD. Not sure it exists.

Else, rather than "stopping", turning and accelerate again, it would be more efficient to turn your ship 90° (roughly) in the required direction, then engage main thrusters. That will "bend" your trajectory. That way you will use less fuel. I'm pretty sure there are MFD that can help you to do that.

And BTW, congrats for your first ISS docking. Always a great moment. :thumbup:

 

[astrosammy]

:thumbup:

If you want a special tool, there's Rendezvous MFD, developed for Orbiter 2005, still working great in 2010 with minor quirks, it helped me a lot on final approach.
It shows you your trajectory relative to the target and the required trajectory to reach the target at the selected time.

[ame="http://www.orbithangar.com/searchid.php?ID=1199"]Rendezvous MFD 050621[/ame]

If you want to use default MFDs, I'd say turn into the drifting direction of ISS, bringing DTMin to zero while burning. That should bring you quite close.

There are almost as many ways to do it as there are Orbinauts .

 

[garyw]

There is also this beauty...... [ame="http://www.orbithangar.com/searchid.php?ID=6221"]RV Orientation[/ame]

 

[Alexrey]

All I can say is WOW, Rendezvous MFD is amazing! Is this the Hill equation that the author was refering to: http://en.wikipedia.org/wiki/Hill_differential_equation? (I'm guessing he uses a system of 3 equations, or possibly 2 since you're in the same plane as the target spacecraft)

 

[ADSWNJ]

@Alexrey ... all RV's are cool, especially your first one. If you want to get your head around something crazy, have a try to go from Brighton Beach (moon base) to Luna (a space station in lunar orbit). The special thing is that you are docking onto a rotating port in the hub of a catherine wheel. (It reminds me of Elite, but that would be dating myself).

On RV in general, when you fly in more realistic ships (read: 10x to 30x less thrust), you need to really look at the shape of your orbit relative to the target. Think of it like trying to match an elliptical target orbit, so not only is your inclination the same, but the argument of periapsis, the apoapsis and periapsis altitudes, and so on. It's like reeling in a fish - in that you run under or over the target orbit to reel it in, and as you get 1-2 orbits from closest approach, you tune one of the orbital elements to get closer to the target shape (e.g. come up from 100km below the periapsis to 30km below). This will put you back to say 6 orbits to go, but the encounter dV becomes say 10m/s or 8 m/s instead of say 100 m/s. In this regard, you will get a silky smooth encounter on target.

(Then use RV Orientation, of course!)