Rendezvous Part 5 – Terminal Phase Initiation

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. 

TI was the first burn that actually targeted an intercept. If you hit it exactly right, you’d have a hole in one – you’d fly right into the target’s center of mass. In reality, we never knew positions and actual burns that precisely – but the idea was that inside TI, everyone got serious. That made the “Go/NoGo” call for TI a critically important point in the day. There were flight rules and checklists that listed what equipment was required inside TI, what the trajectory parameters had to be, and what kind of fault tolerance you had to have to burn TI. This is where the sim team made their money during integrated training – putting in cases to test the MCC team’s ability to make good calls and execute the TI – or execute what was called a TI Delay – a burn that would bring you back around to the same place one rev later to try again – after solving whatever issue might have come up.

You didn’t want to end up burning TI, and then have no way to successfully complete the rendezvous because of additional failures – or something lurking that you missed. If you did nothing, you flew right past, and coming back later was difficult – remember, once you are out front, you are going to have to burn retrograde, which is a terribly inefficient way of using propellant. Burning a TI Delay used about ten fps of gas but brought you right back to the same point relative to the station. This was useful if you were working to solve a problem that you figured could be taken care of to salvage the day’s rendezvous, but you weren’t sure you’d get there before the nominal TI.

A few good examples might be jet failures, or problems with a manifold of jets. We needed a certain level of redundancy in the myriad of jets that maneuvered the vehicle in order to make sure we didn’t lose control in close to the target. Another example would be computer redundancy – while we usually flew the approach with 3 GPC’s set up for guidance and navigation, we only needed two to complete the rendezvous. If you went through TI with only two, and then had another one fail, it could be very difficult to recover in time to still make the rendezvous – leading to a breakout. Better to burn a delay, fix the problem in the 90 minutes you bought yourself, and then come back with the necessary redundancy.

Now once you had burned TI, if you did it perfectly, you didn’t need any other burns to get you to the terminal phase – essentially a spot 60 feet below the target, ready to move in on the radial vector, then transition out to a spot in front so that you could back in at the same altitude. That’s theoretically. The truth is that we never knew our position perfectly, and burns were never perfect in any axis – there was too much asymmetry in the mounting of jets and the jet burns themselves. So, we planned a series of mid-course corrections – four of them, in fact. MC1 through MC4 were tiny burns, each targeted onboard using sensor data from the Shuttle itself – radar range and range rate, and star tracker sightings of the target. Mid-course burns were little puffs of gas, usually less than 2 fps in any axis, designed to bring you closer and closer to the ideal trajectory. To use the old gold analogy, TI was the approach shot and the MC’s were the putts on the green.


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!