• FlightGear - Visiting the ISS

    Arrival At The ISS

    As already mentioned above, a Lambert solver provides a sequence of two burns, of which the first brings the Shuttle to a specified location, and the second nulls the relative velocity with the target once there. It is now time to do the second burn in the sequence, and the blue marble of Earth is seen through the flightdeck windows as the spacecraft moves into burn attitude.

    With the ISS already visible in the distance, the braking burn is executed with the left OMS engine firing - the burn attitude automatically computed to put the thrust vector through the Shuttle's center of gravity.

    Rotating the spacecraft, we can now see the ISS through the overhead windows some 1.6 miles away, pretty much at the same altitude. This is the so-called 'vbar' - we are located along the velocity vector of the ISS, which in fact means we are in the same orbit, just a tad behind the station. The vbar is the only stable position relative to ISS - if we were, say, above the station (along the radius vector or 'rbar'), our orbit would have a lower orbital velocity, and we'd gradually lag behind, so regular thruster firings for 'station-keeping' would be required.

    On the vbar, we could stay for a long time, but daylight won't last for more than 45 minutes in this orbit, so if we want to dock soon, we need to get moving.

       

    The Vbar Approach

    The first task is to get an accurate range to the station. So far everything has been computed based on what the navigation system of the Shuttle has computed for the position of the ISS relative to the Shuttle (the position of which is based on inertial navigation). That might easily be off a hundred feet or more - clearly it is not accurate enough for docking. Thus first the Shuttle is rotated such that the payload bay faces the ISS, then the Ku-band antenna is reassigned from keeping ground communications via the TDRS satellite network, to acting as a radar ranging device.

    Making the payload bay face the station will be the attitude at which the whole set of maneuvers (or proximity operations) is now flown. This has a couple of advantages: First, this is the attitude into which the docking collar actually points, so ultimately we need to approach the ISS that way to dock. Second, from this attitude the radar ranging antenna has a clear and unobstructed view at all times. And finally, it is the only attitude in which the Shuttle can fire thrusters to brake close to the target without pointing thruster exhausts directly at the station and damaging it.

    The reason for the latter is the so-called 'low-Z' mode in which forward and backward pointed thrusters fire together. Since they're mounted at an angle, the net force is downward (i.e. reducing motion along the up direction out of the payload bay), although this combination of thrusters is not exactly fuel-efficient. As we'll see later, there is an even better way to brake.

    For the time being, we fire thrusters to move towards the ISS and aim slightly below the station - this is a vbar approach.

    Now, orbital mechanics being orbital mechanics, we can't 'just' fly over this way. By thrusting towards the station, we increase orbital speed, so the Shuttle gradually starts to go higher and then get slower - which means we have to actively prevent that, so as we approach along the vbar, we need to fire thrusters regularly to stay level with the station (or in fact slightly lower) - till we finally arrive about 300 feet below the ISS in the rbar position.

       


    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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