Waiting for discovery
By Gayle Putrich
After a week's worth of aborted attempts to send the Space Shuttle Discovery on its way to the International Space Station, NASA now says the earliest it will launch is 30 November. But rescheduling a Shuttle launch is no mean feat.
The soonest Discovery can be launched is 4:05 EST on 30 November, NASA says, and the next window will last until 5 December.
One major hurdle to overcome in launch planning is timing the rendezvous with the ISS. Before the space station became the primary focus of Shuttle missions, the US space agency had 2h launch windows, providing more options for fixing last-minute problems and waiting out bad weather. But when the Shuttle has to meet with the space station, NASA gets only one 10min launch window every day.
To keep manoeuvring - and therefore propellant consumption - to a minimum as the Shuttle races to meet the space station in low Earth orbit, NASA tries to launch as close to the middle of the 10min window as possible, when the Shuttle and the space station are "in the same lane," the US space agency says.
The ISS's low Earth orbit also comes into play, limiting the amount of time the Shuttle has to adjust its attitude in flight. Missions to farther-flung objects such as the Hubble telescope get longer launch windows.
The space station flies over the Kennedy Space Center in Florida twice a day, though the astronauts' sleep shifts really only allow for one launch attempt a day, NASA says. Shuttle launch times are set to when the angle of Earth's rotation will allow the Shuttle to slip into the same plane in which the space station is flying.
Those times shift about 22min earlier each day, moving what was meant to be a 16:40 flight on 1 November to a 15:04 attempted launch on 5 November - and ultimately a launch planned for 4:05 at the end of the month.
LEAKING GAS LINES
The planned 1 November take-off was repeatedly scrubbed, first to repair leaking helium and nitrogen gas lines on the Shuttle's orbital manoeuvring system pod, which took longer than expected. Then, due to electrical problems during a routine engine power-up and check-out and finally because of weather. The last straw came on 5 November when a hydrogen gas leak was detected while filling the external fuel tank.
The leak was at the Ground Umbilical Carrier Plate (GUCP), an attachment point between the external tank and a pipe that carries gaseous hydrogen safely away from Discovery to the flare stack, where it is burned off. Two previous Shuttle missions in 2009 - Discovery's STS-119 mission and Endeavour's STS-127 - had similar problems, NASA says.
While emptying the fuel tank, engineers also discovered a foam crack on the external fuel tank's liquid oxygen intertank flange.
"We always place safety first," says NASA associate administrator for space operations Bill Gerstenmaier. "It is essential we repair this hardware before we fly the mission, and we will take the time to properly understand and fix the failure before we launch."
NASA does not expect to need all of November to fix Discovery's latest ailments, but there are more scheduling hurdles than just selecting a time when it comes to launches.
The new 30 November to 5 December launch window is "being driven almost exclusively by the solar beta cutout" that began 8 November and will last until 23 November, NASA says. The angle of the Sun relative to the space station changes every day. While not a problem for the station itself, it becomes a thermal problem for the Shuttle when it is docked with the space station, NASA says.
When the orbiter is flying at a beta angle greater than 60°, being hit with direct sunlight, it goes into "rotisserie" mode, rotating slowly around its X-axis. But making large rotations of the Shuttle is complicated when it is docked at the back of the ISS - the preferred docking location since the loss of Columbia in 2003, where the Shuttle is most protected from micro asteroids and debris.
Other traffic to and from the space station must also be taken into account. A Soyuz cargo flight is to depart on 29 November, and NASA prefers to not have multiple vehicles coming and going at the same time for safety reasons.
If the new, later launch window is not viable, the Shuttle programme will face even more scheduling challenges as it wraps up it final flights. Another solar beta cutout is due at the start of 2011, and before that, another Soyuz will arrive with three new crew members around 17 December. European and Japanese partners are also expected to make ISS trips early in the year. The combination of events would put the next opportunity to launch at the end of February, NASA says. But that slot has been promised to Endeavour.