Aiming for an Open Window

Why does NASA sometimes schedule a rocket launch for the middle of the night, or aim for a liftoff time when weather is notoriously unlikely to cooperate?

The simplicity of the question belies the complexity of the answer. The best time to start a mission is based on a blend of factors: the flight's target and goals, the needs of the spacecraft, the type of rocket, and the desired trajectory, which refers to the path the vehicle and spacecraft must take to successfully start the mission. Not only do these variables influence the preferred launch time the ideal time of departure but the overall length of the launch window, which can vary from one second to several hours.

The dynamics change from mission to mission, and determining the launch window is an important part of the overall flight design.

"The interesting thing about our job is each mission is almost completely different from any other mission," said Eric Haddox, the lead flight design engineer in NASA's Launch Services Program (LSP), based at Kennedy Space Center in Florida.

Haddox leads the team of agency and contractor personnel overseeing and integrating the trajectory design efforts of the spacecraft team and launch service contractor for each LSP mission. Once the spacecraft team identifies its needs, a rocket is selected, and the work of hammering out the best launch window and trajectory begins. Ultimately, the launch window and preferred liftoff time are set by the launch service contractor.

"We help everybody understand the requirements of the spacecraft and what the capabilities are of the launch vehicle, and try to mesh the two," Haddox explained.