On Wednesday, Discovery will undock from the ISS and perform a flyaround of the station before departing and loitering on orbit before returning to earth two days later.  It’s an interesting story as to why the shuttle performs a flyaround and who flies the shuttle during the event.  Here’s the story, which goes back to the mid-1990’s.

I was a young(er) engineer working my first big assignment - as the lead Rendezvous Guidance and Procedures Officer (Mission Control call sign “Rendezvous”) for STS-63, the first shuttle mission to visit the Russian Space Station Mir.  My task as Rendezvous was to design the procedures for piloting the shuttle during rendezvous, docking, and departure, then monitor the crew as they performed these tasks from Mission Control.

Somewhere in the NASA and Russian Space Agency hierarchy, someone got the idea of having the shuttle perform a flyaround of Mir as part of the rendezvous, approach, and departure operations on STS-63.  It gained traction as a photography opportunity as well as somewhat of a piloting stunt (although no one claims the latter).  In any case, a team of shuttle mission designers came up with the basic technique, and it was up to me to design the procedures and help the training team work with the crew so that they could execute it.

The challenge of the flyaround is that for photography purposes, we wanted the flyaround to be at a constant range. The photography experts determined a range that would be acceptable based on the lenses they provided, and from there they asked that the range be held constant.  This is a challenge in low earth orbit, since due to orbital mechanics effects, objects up high travel slower than those lower, which tends to elongate a circular path into a football shape.  The design team came up with a clever trick to execute the flyaround at twice orbital rate.  Since it takes the shuttle about 90 minutes to complete one orbit of the earth, the flyaround was to be execute in 45 minutes per lap. Thus, it gained the name “Twice Orbit Rate Flyaround”, or TORF in the NASA love for acronyms.

To execute the flyaround, the shuttle onboard computers are commanded to pitch the shuttle up automatically.  The crew’s job is to translate the shuttle forward to keep the center target in view of the overhead windows.  Through the action of the computers pitching the shuttle up and the crew manually translating the shuttle forward, a flyaround of the center would be executed.  They key feature of the TORF was that no jets on the shuttle had to be fired towards Mir to round the football into a circular shape, in principle.  It was harder to do than it appeared and on several instances in the simulator, the crew had to fire jets at Mir in order to hold to the constant range circular shape of the flyaround.  During one training session, Astronaut Mike Foale turned to me, and in either a flash of insight or brilliance (or both in his case) said to me, “We ought not to make any corrections as we are crossing the plus V-bar and minus V-bar.”  The V-bar is an imaginary line extending from the center point (in this case, Mir) and pointing in the orbital direction (or opposite, for the minus side).  Hmm, I didn’t know if he was right or not, so after the training session I decided to prove it to myself through some analysis of orbital mechanics.  The result you can find here, which is heavy on the mathematics.  Turns out what Mike said was dead on.  If the crew made no inputs while crossing the plus or minus V-bar and only made inputs near the top and bottom on the flyaround, it would be successful.

After that realization, the crew incorporated the changes and were able to fly the flyaround repeatedly in the simulator without firing jets towards Mir.  They grew so comfortable with it that the commander, Jim Wetherbee, allowed his rookie pilot, Eileen Collins, to fly the flyaround during the mission itself.  Jim is a humble guy who never sought the limelight for himself, and it would take such a person to hand over the stick to his pilot.  During the actual mission, Eileen flew the flyaround flawlessly.  Jim went on to command several other shuttle missions to Mir and ISS before retiring from the astronaut corps.  Later, Eileen became the first female to command a shuttle mission, and commanded several before retiring.  Mike Foale went on to do extended tours onboard Mir and ISS, and I still see him in the hallways (so to speak) on occasion. Each time I see him, I am reminded of that conversation we had, one that has had such a lasting impact on human spaceflight.

As for the flyaround, it was incorporated into every subsequent mission to Mir and to the ISS, and the commander hands over the stick to his/her pilot to fly it to this day.