NASA shortens shuttle launch windows
BY WILLIAM HARWOOD
Posted: August 18, 2000
The five-minute launch windows typical of shuttle missions to the international space station likely will be shortened to as little as two-and-a-half minutes for all upcoming flights to improve safety and the odds of carrying out a successful mission.
After reviewing previous launch rules and safety contraints, senior shuttle program managers met last week "and decided the safest in terms of maximizing our capability is to launch at the optimum in-plane time," Hale said.
He was referring to the moment when Earth's rotation carries the shuttle launch pad into the plane of the space station's orbit. Launching at that instant requires the least amount of rocket fuel to reach the desired target.
The recommendation to shorten future shuttle-station launch windows is under review, but approval is expected shortly.
For previous station missions, the shuttle launch period extended five to 10 minutes with the ideal in-plane time in the middle of the window. The shuttle does not carry enough fuel to reach the station if it launches more than five minutes or so to either side of the in-plane time.
"All the points in (the old five- or 10-minute windows) are safe," Hale said. "But getting ready for the next worst thing that can happen to you, there are pros and cons. This has been debated ever since we started doing these short launch windows."
In so doing, a shuttle crew will not waste any rocket fuel steering into the station's orbital plane and will have more on-board reserves to handle possible main engine performance problems.
During shuttle mission STS-93 in August 1999, for example, one of the shuttle Columbia's main engines leaked hyrogen fuel from a damaged nozzle, causing the ship to reach orbit with less than the desired engine cutoff velocity.
The new launch window philosophy increases the odds an under performing shuttle can reach the altitude necessary to catch up with the space station.
"On a day all the engines run but not exactly in a well-tuned mode, that some minor problem happened to them, that gives you the greatest probability of making your mission and thereby avoiding having to refly a payload," Hale said.
It also improves the odds a shuttle crew could reach an East Coast landing site in the event of multiple engine failures during the climb to space.
The shuttle was designed to make an intact landing in the event of a single engine failure at any point during ascent.
During the first two-and-a-half minutes or so of flight, the shuttle would be forced to return to the Kenned Space Center in the event of an engine failure. This is known as a "return to launch site abort," or RTLS.
Between two-and-a-half minutes or so and about five minutes, the shuttle could reach an emergency runway in Spain or Africa if a single engine failed, a so-called "trans-Atlantic abort," or TAL. After that, the shuttle could limp into a lower-than-planned orbit on just two main engines. That scenario is known as an "abort to orbit," or ATO.
In addition, NASA has agreements with nine landing facilities along the U.S. and Canadian east coast -- from Myrtle Beach, S.C., to Gander, Newfoundland -- to protect crews in the event of multiple engine failures that might prevent a shuttle from making it back to Florida or reaching an overseas runway.
By launching at the optimum in-plane time, the trajectory of shuttles launched to the space station will carry the orbiters slightly closer to the east coast, improving a crew's chance of making a safe landing.
"It doesn't guarantee it, there are still gaps, but it improves the odds of making one of the runways we have up the east coast," Hale said.
An in-plane launch also shortens the time required for a shuttle to reach a point where it could carry out an abort to orbit.