From: NASA Blogs
Posted: Wednesday, October 28, 2009
One question that comes up a lot is why Ares I-X has a four-hour launch window. After all, unlike the Space Shuttle, it doesn't have to rendezvous with the Space Station, so what's the challenge? Actually, there are several.
First, the Eastern Range typically allots 4-hour launch windows. Given the duration of Ares I-X (about seven minutes from liftoff until the final pieces splash down), more time is not required. As was demonstrated on the first launch attempt, the rocket can be reset quickly, so four hours was considered plenty of time to wait out weather and technical challenges.
Next, there are human limitations. Console operators in the Launch Control Center must be at their consoles at least 7 hours before the planned launch. When you add the 4-hour window this means that operators may have to be on station for 11 hours before launch. There is also a lot of work to do after launch or after a scrub.
Additionally, anyone familiar with Florida weather understands that winds typically pick up later in the day as the atmosphere heats up and interacts with evaporation from the ocean. Central Florida's "afternoon thunderstorms" produce a terrific number of lightning strikes. High winds are a problem for any launch. Because of its experimental nature, Ares I-X has very conservative wind constraints--20 knots (nautical miles per hour--about 23 statute miles per hour) as opposed to the Space Shuttle, which can fly in winds up to 30 knots (34.5 miles per hour).
A 4-hour window, gives the team the ability to complete all the preparation work, wait for the right combination of winds, weather and clouds and then go. Following the LCC guidelines gives Ares I-X the best chance to collect the important data that we need for next exploration steps we take.
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