• FlightGear - Visiting the ISS

    Docking Along The Rbar

    Once in the rbar, we rotate the Shuttle again such that the ISS becomes visible in the overhead windows and put the spacecraft into local horizon attitude holding mode (the ISS is stabilized with respect to the local horizon as well).

    Now we need to go 'up' till we reach the docking port. Well, it doesn't actually feel like up, because looking out of the overhead windows it feels more like forward. But then, rolling the Shuttle feels like yaw, and yawing like roll. If you imagine having to fly your favorite airplane that way, it could make your head spin! But this is the Space Shuttle - there's a 'control sense' switch which can be flicked, and the avionics makes it easy - every control input will be interpreted as if we're really flying forward toward ISS rather than upward.

    As we've said before, the spacecraft isn't stable on the rbar, so simply holding it there will ultimately mean the Shuttle moves down, towards Earth and away from the ISS. But this in fact is good - because if we use thrusters to move upward, there'll be a braking force provided by orbital mechanics that reduces upward speed - we never need to use any thrusters to slow down with an rbar approach if we do it right!

    Thus, the last remaining task is to gradually nudge the Shuttle up (or forward as it looks from the overhead windows) towards the docking port while nulling the position and velocity errors on the other axes. That is not as easy as it might seem, because the Shuttle's Reaction Control System (RCS) thrusters are never clean translations or rotations - any firing to hold attitude will also give a small translation force, any forward firing will require subsequent attitude-hold firings --- there is plenty of mode-mixing. Slow and careful does the trick, and in fact a full 45 minutes later, just as another dusk begins over the Atlantic ocean, the task is done and the docking is made.

       

    Now the crew can take a much needed breather and enjoy the display of the Northern Lights around the pole. Later when crossing over central Asia, the connection will be prepared so as to enter the interior of ISS.

    Which is not simulated... yet.

    More information:


    2 Comments
    1. zswobbie1's Avatar
      zswobbie1 -
      Wow, thanks for a most interesting review of the shuttle docking.
      I had not realised how detailed the shuttle was, and still a beta!
    1. ThorstenRenk's Avatar
      ThorstenRenk -
      Quote Originally Posted by zswobbie1 View Post
      Wow, thanks for a most interesting review of the shuttle docking.
      I had not realised how detailed the shuttle was, and still a beta!
      Thanks.

      I'd guess that outside of what NASA had, it's the most detailed Shuttle simulation you can find - you can do things like cause multiple electric failures during ascent, go through the corresponding NASA checklists and 80% of the items there will be working in the sim while you fly your abort procedure. I think right now 90% of the switches in the cockpit do what they're supposed to do and we've started on the circuit breakers.

      The largest gaps really are things which are impossible for us to implement - like the ability to patch the software on orbit (of course what's underneath the displays is not the original NASA code...)
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