This posting doesn’t relate to the Columbia accident, but it addresses a question I get on my monthly KSC tours offered through the Kennedy Space Center Visitor’s Center.
Why were the launch windows for flights to the International Space Station 10 minutes long, while others—especially early in the program—were 2.5 hours long? The answer is Rendezvous.
Without getting into a mini-course on orbital mechanics, for ISS missions we had to get to a specific point in orbit while the station was there. That’s pretty clear. The trick is that the Shuttle had to do it with the available fuels on board—those in the Solid Rocket Boosters, the Shuttle Main Engines, and the Orbital Maneuvering System and thrusters. Simplistically, that total is called the ‘performance’ of the entire system. The launch window was in large part defined by it. It said we had to launch at a time that when after the Shuttle reached orbit it was close enough to ‘catch’ the ISS with the least amount of maneuvering fuels necessary. If too far away, more fuel would have been needed than was available. Rendezvous would have been impossible. Mission failure. So that permissible distance in orbit defined exactly when launch had to occur. And the least fuel would be used if we launched at that precise time, called the ‘preferred time’. The available maneuvering distance once in orbit said rendezvous could still happen if we launched about 5 minutes before or 5 minutes after the preferred time, again, all based on fuel usage. Thus the 10 minute launch window.
In contrast, for missions not requiring rendezvous, like deploying a satellite or STS-107’s Spacehab mission, just getting to the proper orbit was the main objective. It didn’t matter quite as much when—just that the Shuttle got there. The length of the window was defined by other parameters. Usually the most constraining one was the length of time the astronauts could spend on their backs strapped in the Orbiter awaiting launch. This was 5 hours 15 minutes. Given the time the first astronaut (the Commander) got into the shuttle before launch was usually 2 hours 45 minutes before the scheduled T-0, it resulted in a 2.5 hour launch window. There you go!
But what about ‘launching early’? How can that be?
Flash back to August 10, 2001, Discovery’s launch day for STS-105. The mission was to take the Leonardo Multi Purpose Logistics Module (MPLM) to the International Space Station. Our launch attempt on the previous day was scrubbed due to weather. Today, all was going well in the Firing Room for launch, but a mid-summer storm was pounding Lake Okeechobee, approximately 110 miles south of the launch pad, and the storm was moving north. Heavy rain and lightning were heading at us with no expectation to dissipate. Time of arrival in our area was calculated to be about T-0. Weather launch commit criteria were going to be violated—no way around it. If we scrubbed today, we would have a several-day turnaround before the next launch attempt, as we would have to top off the cryogenics aboard the Shuttle. The only hope was to launch as soon as possible and beat the storm’s arrival.
By rule and practice, how the available launch window was used was at the Launch Director’s discretion (within reason of course!). I called the Flight Director in Houston and discussed my idea to launch at the beginning of the 10-minute window rather than in the middle as we had planned. He liked the idea, so we accelerated countdown beginning about 2 hours prior to liftoff, to save the 5 minutes.
All worked out well. We launched at the opening of the window, 5 minutes early, about 20 minutes before the lightning sensors went out of limits. We beat the storm!
Launching early worked. It was never done before or after. STS-105 holds the distinction as the only manned mission ever to ‘launch early’.