posted 06-28-2005 05:03 PM               

The "RPM", or rendezvous pitch maneuver takes place 600 feet below the station on what is called the "RBAR", or radius bar (extending from the ISS downward toward the earth).

Old method for prox ops (proximity ops) phase of rendezvous: During the approach from below and behind the ISS, we used to null our closure and rates during rendezvous as we reach this imaginary location (RBAR 600'). At this point the shuttle is in a nose forward/payload bay up attitude. Then we would start an ascending spiral/pitch up maneuver that would put us forward of the station at the same altitude as the station but around 250' out, and now with the nose pointed up and the payload bay still facing the station. From there we fly down the VBAR and dock with the station's docking port which is oriented toward station forward.

New method (with RPM): Everything is the same, except when we arrive at 600' on the RBAR, we now null out the rates then initiate a back flip-like maneuver, pitching the nose up 360 deg over the span of 5 or 6 minutes. When the orbiter is flipped upside down (~180 deg pitch), the station crew uses handheld and external video cameras to photograph the belly of the shuttle. What we're looking for here is gouges from foam strikes during ascent. The tiles are brilliant white, with a thin hard black coating. Any gouges will be very evident as bright white streaks or spots on the black background. After the 360 deg or back flip, we null the rates again and press on with the transition to the VBAR as above.

The shuttle shouldn't get any closer than 600 ft (CG-to-CG) from the station (maybe 500 ft considering the closest structure on each vehicle). It's a really easy and controlled operation, that we've simmed many times. The beauty of doing this on the RBAR is that orbital mechanics forces the vehicles to continually open, that is you can't maintain a fixed distance on the RBAR without constantly inputting jet firings to push you in. Typically when the pitch maneuver is complete, you're 50-80 feet farther away and falling away faster and faster. The VBAR is different - if we were to do this there, I think there'd be concern about potentially getting closer than you want to be.