Rendezvous Part 7 – Fine Tuning the Approach

In my book Shuttle, Houston: My Life in the Center Seat of Mission Control, I bring readers deep into the middle of how Mission Control worked during the Space Shuttle era. I give readers a glimpse of both the technology and the humanity required to put humans in space on a routine basis. There are many parts to this thirty-year story, and only a small portion of the tales fit into the book – so here is an excerpt from an unused chapter on the process of rendezvousing the shuttle with other objects in orbit. 

Back to the rendezvous – on a good day. Once you were inbound from Ti, the crew got busy targeting the various Mid-Courses using onboard sensor data. The ground backed them up with its own targeting, but the onboard was prime. Now something I have to point out is that while computers do a marvelous job of calculating the mathematics and physics to ridiculous decimal values, the truth is that those calculations are only as good as the sensor data that determines the relative positions of the two spacecraft. They are also dependent on the knowledge of the Orbiter’s own velocity and velocity changes, sensed by the inertial measurement units, and integrated over time to determine position. So while you might get what appears to be a very precise answer to “where am I, and how am I moving’, the truth is that you only know those things to within a small error ellipsoid.

When we did correction burns, we could see the Dv in all three spacecraft axes – but we really could only measure the changes down to about .2 feet per second. Burning to get less than that was futile, because you might be burning away from a perfect condition “0“ and not actually know it. So, you burned X, Y, and Z to 0.2, and left them there. Then you went on to target the next burn, and any residual errors would hopefully be rolled into the next round of tracking. These were the slight undulations in the green affecting each putt as you moved closer to the hole. The goal was to come out of the four mid-course burns with a trajectory good enough to allow the CDR to manually fly the rest of the rendezvous using sensors and eyes out the window. And this worked surprisingly well. Most mid-course corrections were in the neighborhood of tenths of fps in each axis, and the crew had a good view of the target by then – by reflection in daylight, or with docking lights at night.

From the control center, a Rendezvous involved primarily the Trench (FDO and RNDZ) along with the various systems folks that supported them with hardware and software – PROP, GNC, and DPS primarily. Of course, during a sim, we had a full team, and we didn’t want to waste the time of any flight controllers, so while the rendezvous (and its associated failures) was going on, it was not uncommon to be working some cabin leaks, cooling problems, some instrumentation failures – and maybe develop an APU leak, as well. At times, the Flight Director felt like he or she was swatting at a horde of gnats while trying hard to concentrate on getting through the jungle.

The key was always to set priorities, and remember just what you needed to accomplish a docking – and which items were absolute show stoppers that would call for a “break-out” – abandoning the rendezvous and docking for the day – or the mission. I personally found that the more failures we encountered, the easier things got – because there were fewer things that could still go wrong, and you’d be “GO”. Taking away options simplified the problem, in other words. The really fun cases were those that intertwine failures in different systems and disciplines in a way that was new – finding failure cases that have never been documented before that could cause problems in  ways that hadn’t been anticipated. That was where creative thinking and careful analysis came together, and it was where a good flight control team could shine.


We’ll bring you the next portion of “Rendezvous” next week right here! If you enjoyed this look inside the Shuttle program, you can find many more details and stories in the book – look for it wherever you buy your paper books, or add it to your favorite E-reader or audio book account.

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Christmas is coming!

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